Plural generators

Multi-mode digital enciphering system

4169212

Abstract

The specification discloses an electronic digital enciphering system which is selectively operable in a plurality of different modes. The system may be operated in an off-line mode for preparing enciphered message tapes prior to transmission via teleprinter message circuits. The system may also be operated in an on-line asynchronous mode wherein a sending station may asynchronously transmit enciphered data on-line to a remote receiving unit, synchronization being provided by the start/stop pulses of the individual digital characters. The system may also be operated in an on-line synchronous mode wherein a sending and a receiving unit are automatically synchronized in time by local self-contained clocks in order to bridge fades or interference in a transmission medium such as radio telegraph, or the like. The system may be utilized on either half-duplex or full-duplex communication channels.


Claims

What is claimed is:

1. Correlation circuitry for detecting a predetermined digital synchronization pattern in a digital code system, comprising:

means for receiving digital patterns including a digital synchronization pattern in which a predetermined number of bits are at a predetermined level,

counting means for counting the number of bits at the predetermined logic level in said digital patterns received by said means for receiving,

a threshold detector responsive to said counting means for generating an output threshold signal if said number of bits counted at the predetermined logic level is greater than a predetermined number but less than the previously detected number, and

means responsive to said output threshold signal for generating a peak signal to indicate the reception of said predetermined digital sychronization pattern.

2. The correlation circuitry of claim 1 wherein said predetermined logic level comprises a logic one.

3. The correlation circuitry of claim 1 wherein said counting means causes the circulation of digital bits within said receiving means and counts the number of said bits at said predetermined logic level during said circulation.

4. The correlation circuitry of claim 1 wherein said means for receiving comprises a shift register.

5. The correlation circuitry of claim 4 wherein said counting means comprises:

circuitry for connecting said shift register in a circulating mode and including circuitry for counting the number of bits at logic one stored in said shift register during one circulation of the contents of said shift register; and

a holding register for storing the number of bits at logic one counted by said counting means during one circulation of said shift register.

6. The correlation circuitry of claim 5 wherein said threshold detector compares the number of bits at logic one presently counted by said counting means with the number of bits at logic one previously stored in said holding register to detect a negative transition.

7. Synchronization circuitry for a digital code system comprising:

means for receiving digital signals including a correlation pattern, said correlation pattern comprising a known plurality of predetermined digital bit patterns,

means for generating output signals upon the occurrences of the predetermined digital bit pattern,

means for detecting said output signals during prescribed time windows,

means for counting the number of output signals detected during a predetermined time interval and for generating a count signal, and

synchronizing means responsive to said count signal for generating a synchronization signal to initiate synchronization of said digital code system when the known plurality of predetermined digital bit patterns is counted indicating the presence of the correlation pattern.

8. The synchronization circuitry of claim 7 wherein said correlation pattern comprises a plurality of digital representations of a character followed by an inverse digital representation of the character.

9. The sychronization circuitry of claim 8 and further comprising:

means for inverting the input to said means for generating prior to reception of said inverse digital representation.

10. The synchronization circuitry of claim 7 wherein said means for generating output signals comprising:

means for detecting the number of bits at a predetermined logic level contained in said digital pattern,

a threshold detector responsive to said detecting means for generating an output threshold signal if said number of bits at a predetermined logic level is greater than a predetermined number but less than the previously detected number, and

means responsive to said output signal for generating a peak signal to indicate the reception of said predetermined digital pattern.

11. The synchronization circuitry of claim 7 and further comprising:

a miss counter for counting the number of times an output signal is not detected during a predetermined time interval,

logic means responsive to said counting means and said miss counter for determining whether a predetermined number of misses have occurred within said predetermined time interval and for generating an inhibit signal in response thereto, and

means responsive to said inhibit signal for preventing the generation of said synchronization signal for another predetermined time interval.

12. In a synchronous digital deciphering system, the combination comprising:

a free-running receive synchronizer for generating timing pulses in synchronism with a remote digital ciphering unit,

means for receiving enciphered digital data and timing bits including received start bits from the remote enciphering unit,

means responsive to said timing pulses for deciphering said enciphered digital data,

said free-running receive synchronizer generating an internal start bit in synchronism with the received start bit,

first means for generating a first signal in response to the occurrence of the received start bit prior to the occurrence of the internal start bit,

second means for generating a second signal in response to the occurrence of the internal start bit prior to the occurrence of the received start bit, and

means for varying said receive synchronizer in response to said first and second signals to maintain said receive synchronizer in synchronism with the remote enciphering unit.


Description

FIELD OF THE INVENTION

This invention relates to digital encryption, and more particularly relates to electronic apparatus for automatically enciphering and deciphering digital data.

THE PRIOR ART

Electronic systems for automatically enciphering and deciphering digital data have been heretofore developed. Such prior systems have included "off-line" systems wherein a punched tape or the like is enciphered and then transmitted to a remote location for deciphering. An example of such an "off-line" system is described in U.S. Pat. No. 3,781,472, entitled "DIGITAL DATA CIPHERING TECHNIQUE" issued Dec. 25, 1973, and assigned to the present assignee.

Another type of electronic enciphering device is known as an "on-line" system and may be utilized to automatically encipher and decipher digital data being input on a real-time basis through an input terminal such as a teleprinter or the like. On-line operation may be conducted either asynchronously, wherein the synchronization between the transmitting and receiving unit is maintained on a character by character basis, or in the synchronous mode, wherein the transmitting and receiving units are independently maintained in synchronization by independent clocks. An example of an electronic system which may be operated in either an off-line mode or in the asynchronous on-line mode is disclosed in U.S. patent application Ser. No. 299,387, filed Oct. 20, 1972, and entitled "DIGITAL CRYPTOGRAPHIC SYSTEM AND METHOD" and assigned to applicant.

Although the previously developed electronic encryption systems have worked well in practice, the prior systems have often been limited to one or two operating modes, and to operation with a particular digital character set. Moreover, prior encryption systems adapted for operation in the on-line synchronous mode with a full duplex communications channel have required resynchronization of the system where it was desired to reverse the direction of data transmission.

A need has thus arisen for an improved electronic encryption system which is selectively operable in the off-line mode, the on-line asynchronous mode or the on-line synchronous mode. Moreover, a need has risen for a multi-mode digital encryption system which is operable with a variety of character sets, at a variety of baud rates and with a variety of input and output devices and over a variety of communications lines.

Prior enciphering systems have generally utilized conventional modulo-2 addition of plain text with a pseudorandom key stream to provide encryption. In addition, other techniques have been developed wherein a plain text digital word is loaded into a binary counter which is then clocked for a number of steps determined by a randomized digital key word. An example of such encoding circuitry is described in U.S. Pat. No. 3,781,472, issued Dec. 25, 1973, and entitled "DIGITAL DATA CIPHERING TECHNIQUE", and assigned to the present assignee. Such previously developed enciphering techniques have generally worked well, but require a substantial amount of circuitry and time to accomplish the required clocking steps. A need has thus arisen for an enciphering technique which may be accomplished with a minimum of circuitry and at a high speed, while providing maximum security.

SUMMARY OF THE INVENTION

In accordance with the present invention, an electronic digital encryption system is provided which substantially eliminates and reduces the problems and inadequacies found in devices heretofore developed. The present invention provides a multi-purpose digital enciphering system for selective operation of an off-line mode, an asynchronous on-line mode or in a synchronous on-line mode.

In accordance with a more specific aspect of the invention, a multi-purpose digital cipher system is provided for use with a teleprinter having a keyboard and a printer. A keyboard synchronizer is operable in an asynchronous mode to clock in digital characters input from the keyboard. Circuitry enciphers the digital character clocked in by the keyboard synchronizer. An output synchronizer is operable in either an asynchronous or a synchronous mode for outputting enciphered digital characters from the enciphering circuit. An off-line switch may be operated to interconnect the output synchronizer in an asynchronous mode such that the enciphered digital characters are asynchronously shifted to the printer. An asynchronous on-line switch may be operated to interconnect the output synchronizer in an asynchronous mode such that the enciphered digital characters are asynchronously shifted to a communications channel. A synchronous on-line switch may be operated to interconnect the output synchronizer in a free-running synchronous mode such that the enciphered digital characters are synchronously shifted to a communications channel.

In accordance with another aspect of the invention, a synchronous on-line digital cipher system includes a first cipher unit having a first frame sync generator for transmitting a prescribed sequence of digital synchronization patterns. A second cipher unit is provided to receive the sequence and includes a correlation circuit for detecting a predetermined number of the digital synchronization patterns within a set time interval in order to generate a sync signal to synchronize the second cipher unit to the first cipher unit. Circuitry in the second cipher unit is operable upon generation of the sync signal for retransmitting the prescribed sequence of digital synchronization patterns to the first cipher unit. Circuitry in the first cipher unit is responsive to the prescribed sequence for synchronizing the first cipher unit to the second cipher unit. Thus, ciphered data may be transmitted synchronously between the first and second cipher units in either direction.

In accordance with yet another aspect of the invention, correlation circuitry is provided to detect a predetermined digital synchronization pattern in a digital code system. The correlation circuitry includes circuitry for receiving a digital pattern. Circuitry detects the number of a predetermined logic level contained in the digital synchronization pattern. A threshold detector is responsive to the detecting circuitry for generating an output threshold signal if the number of a predetermined logic level is greater than a predetermined number, but less than the previously detected number. Circuitry is responsive to the output signal for generating a peak signal to indicate the reception of the predetermined digital synchronization pattern.

In accordance with another aspect of the invention, synchronization circuitry for a digital code system includes circuitry for generating output signals upon the occurrences of a predetermined digital pattern. Circuitry detects the output signals during prescribed time windows. Circuitry counts the number of output signals detected during a predetermined time interval. Circuitry is responsive to the counting circuitry for generating a synchronization signal to initiate synchronization of the digital code system.

In accordance with another aspect of the invention, prime circuitry for a digital cipher system includes a key generator for generating a plurality of random digital prime bits. Circuitry transmits the prime bits to a remote cipher unit and simultaneously to a storage unit. Circuitry accesses the storage unit for transmitting the prime bits a predetermined additional number of times to insure correct reception of prime by the remote cipher unit. At the remote unit, the received digital prime bits are stored and are subsequently accessed according to a predetermined algorithm in order to determine a single prime word.

In accordance with another aspect of the invention, a code system for digital data representing a set of characters includes circuitry for receiving a first digital word representative of one of the characters. Circuitry randomly generates a digital key word for a predetermined set of digital key words. A read only memory stores digital representations of all of the characters grouped in addressable subsets according to the generated first digital word and the generated digital key word. Circuitry is responsive to the first digital word and the digital key word for generating an address representative of one of the digital representations stored in the read only memory. Circuitry detects the addressed one of the digital representations. Buffering circuitry is provided to selectively buffer the addressed one of the digital representations when the system is operating in a synchronous mode. Circuitry is also provided to selectively buffer the digital key word when the system is operated in a synchronous mode with a full duplex channel. Circuitry is also provided to operate as a digital phase lock loop in order to maintain the system in synchronization when operating in the synchronous mode.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and for further objects and advantages thereof, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a somewhat diagrammatic illustration of the present system connected in the off-line mode;

FIG. 2 is a somewhat diagrammatic illustration of the present system connected in an on-line half-duplex mode;

FIG. 3 is a somewhat diagrammatic illustration of the invention connected in an on-line full-duplex mode and operable in a synchronous manner;

FIG. 4 is a somewhat diagrammatic illustration of the present invention connected in an on-line synchronous mode utilizing a radio telegraph communications system;

FIG. 5 is a front view of the control panel of the present system;

FIG. 6 is a view of the universal thumbwheel switches utilized to insert a particular code into the system;

FIG. 7 is an illustration of the custom coding rotary switches utilized to set another degree of code complexity into the invention;

FIG. 8 is a block diagram of the front panel, rear panel and key variable circuits of the invention;

FIG. 9 is a block diagram of the code generator and prime data circuits of the invention;

FIG. 10 is a block diagram of the alarm and lamp driver and Scrambler I circuits of the invention;

FIG. 11 is a block diagram of the data switching and Scrambler II circuits of the invention;

FIG. 12 is a block diagram of the master controller and clock and receive synchronizer of the invention;

FIG. 13 is a block diagram of the sequence detector and output synchronizer of the invention;

FIG. 14 is a block diagram of the correlator controller and the keyboard synchronizer of the invention;

FIG. 15 is a block diagram of the correlator of the invention;

FIG. 16 is a block diagram of the power supply of the invention;

FIG. 17 is a schematic diagram of the prime data circuit of the invention;

FIG. 18 is a schematic diagram of the Scrambler I circuit of the invention;

FIG. 19 is a schematic diagram of the Scrambler Ii circuit of the invention;

FIG. 20 is a schematic diagram of the receive synchronizer of the invention; and

FIG. 21 is a schematic diagram of the correlator circuit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a block diagram of the present encryption device connected in an off-line mode is illustrated. The encryption system includes a first encoder/decoder unit 10 interconnected with a conventional teleprinter 12. Digital data may be transmitted from the teleprinter 12 through conventional teleprinter communication channel 14 to a remote teleprinter 16. A second encoder/decoder unit 18 in accordance with the present invention is connected off-line with the teleprinter 16.

In operation of the system shown in FIG. 1, it will be assumed that it is desired to encode a digital message at the teleprinter 12 and to transmit the enciphered message via the communications channel 14 to teleprinter 16. A clear or plain text (uncoded) punch tape is prepared on the teleprinter 12 in the conventional manner. The teleprinter 12 is placed in the LINE position and the tape punch is turned on. The encoder/decoder unit 10 is placed in the encoding mode and a second encoded tape is punched by reading the clear tape at full speed into the teleprinter 12. The encoded tape is then transmitted through the communications channel 14 to the remote station, or alternatively, may be physically handcarried to the remote teleprinter 16. To decode the cipher message, the cipher tape is placed in the teleprinter reader and the encoder/decoder unit 18 is properly set for decoding operation. The decoded message will then be printed out by the teleprinter 16. For a more detailed description of the operation of a system in the off-line mode, reference is made to the previously noted U.S. Pat. No. 3,781,472.

FIG. 2 illustrates the present encoder/decoder system connected in an on-line configuration. In this embodiment, the encoder-decoder unit 10 is connected between the teleprinter 12 and the teleprinter communications channel 14. Similarly, the encoder/decoder unit 18 is connected between the communications channel 14 and the teleprinter 16. The printer coil 20 is connected to receive data from the encoder/decoder, while the keyboard switch 22 is connected to generate digital data to the encoder/decoder. Similarly, the printer coil 24 of teleprinter 16 is operable to print out digital data received from the encoder/decoder 18, while the keyboard switch 26 is operable to generate digital data for transmission to the encoder/decoder 18. The system illustrated in FIG. 2 utilizes a two wire transmission channel and the teleprinters 12 and 16 are connected in a half-duplex mode. When the teleprinters are connected in half-duplex, off-line operation is not possible. However, the encoder/decoder units 10 and 18 may be operated in either the on-line asynchronous or on-line synchronous modes when configured as shown in FIG. 2.

In operation of the system shown in FIG. 2, clear or plain text (uncoded) digital data is entered through the keyboard of the teleprinter 12 and is applied to the encoder/decoder 10, whereupon it is enciphered. The enciphered digital data is applied through the communications channel 14 to the encoder/decoder 18, whereupon it is decoded. The decoded clear text digital data is then applied to the printer coil 24 and is printed out at the teleprinter 16. Upon proper reinitialization and synchronization of the systems, data may be transmitted from teleprinter 16 to teleprinter 12 in the same manner. Due to the enciphering of the digital data, unauthorized tapping of the communications channel 14 will result in an unintelligible garble of digital bits, thereby rendering the transmitted information secure.

FIG. 3 illustrates the enciphering system connected in a four line full-duplex system. The encoder/decoder 10 in this configuration is connected separately to the printer coil 20 and to the keyboard switch 22. The communications channel 14 comprises a two-wire pair 30 for transmitting data and a two wire pair 32 for receiving data. The encoder/decoder 18 is separately connected to the printing coil 24 and to the keyboard switch 26 of the printer 16. In this configuration, the system may be operated in either the synchronous or asynchronous modes.

In operation of the system shown in FIG. 3, clear text data is entered into the keyboard of the teleprinter 12 and the keyboard switch 22 transmits clear text digital data to the encoder/decoder 10, which enciphers the digital data. The enciphered digital data is transmitted via the wire pair 30 to the remote station. Any tapping of the wire pair 30 will result in an unintelligible digital garble, thereby rendering any transmitted data secure. The enciphered digital data is received by the encoder/decoder 18 and is decoded. The decoded digital data is applied to the printer coil 24, and the clear text is printed at the printer of teleprinter 16. When the system is operating with a full-duplex channel in the synchronous mode as shown in FIG. 3, data may be typed into the keyboard of teleprinter 16 at any time and transmitted to the printer of teleprinter 12, without the need for resynchronization of the system.

FIG. 4 illustrates the use of the present enciphering system in a radio telegraph communications channel. The teleprinter 12 may be connected in either the half-duplex or full-duplex mode to the encoder/decoder 10 in the manner previously described. The encoder/decoder 10 is connected to a frequency shift key (FSK) keyer 34 which converts telegraphic data into tones for transmission, and vice-versa. The tones are applied to a radio transceiver 36 which transmits radio waves to a remote transceiver 38. The output of transceiver 38 is applied through an FSK keyer 39 which generates digital data to the encoder/decoder 18. The encoder/decoder 18 is connected to the teleprinter 16 in either a half-duplex or full-duplex mode as previously described. With the configuration shown in FIG. 4, digital data may be encoded in the manner previously described and transmitted via the radio telegraph system in either the asynchronous mode or the synchronous mode.

The synchronous on-line mode is particularly designed for use with the system shown in FIG. 4, as the radio-telegraph system is susceptible to interference or fades and often has high error rates. The present enciphering system enables both systems to be synchronized and to individually maintain the synchronization by clocks at each location. Once synchronization is established, the two stations remain in synchronization independent of channel conditions. In the half-duplex radio channel mode, the encoder/decoder units must be reinitiated each time the traffic is reversed. However, if a full-duplex channel exists, this system has the capability of synchronizing themselves in both directions simultaneously. Once this is done, half-duplex traffic may be utilized without having to reinitiate each time the direction of traffic is reversed. If the system is used in either of the two on-line modes, the system will automatically switch from encipher to decipher upon the reversal of the traffic.

An important aspect of the present invention is that the present system has the capability of being interconnected in any of the configurations shown in FIGS. 1-4, merely by switching control switches and by correctly interconnecting the devices to the teleprinters and to the communication channels. The system may be utilized with single (neutral) or double (polar) currents, and with either an internally or externally supplied battery. The system has a plurality of selectable baud rates, and is operable with a five level Baudot (CCITT2) code, or the equivalent. The system may be utilized with a full character set (all 32 Baudot characters) or the Telex character set (29 characters) in the enciphered mode.

FIG. 5 illustrates a view of the front panel of the encoder/decoder unit 10, which is identical to the encoder/decoder 18. The front panel 40 is removably attached to a housing 42 which contains the circuitry of the invention. Front panel 40 is hinged to be opened in order to expose switches to enable selection of the desired code, as will be subsequently described.

A power on/off switch 44 may be operated to control the mains power to the system. Power is applied to the system by connection to the mains AC power receptacle 46. The unit is capable of handling nominal voltages from 100 to 260 volts AC (selectable) and from 50-60 Hz.

The meter select switch 48 is operable between five on positions in order to select one of five circuits for the meter 50 to monitor. Four of the circuits to be monitored are teletype loops and the fifth is the mains voltage. While monitoring one of the teletype loops, a MARK is indicated by deflection on the needle to the right. A SPACE is indicated by no deflection in the case of neutral current or deflection to the left in the case of polar current. The indication of the teletype loop currents varies from zero to 60 milliamperes to either side of center.

The four teletype loop circuits for monitoring by switch 48 are designated as:

(1) KYBD which monitors the current in the keyboard contacts;

(2) PTR, which monitors the current driving the local printer coil;

(3) RX, which denotes the current in the receive (incoming data) communications line; and

(4) TX, which denotes the current in the transmit (outgoing data) communication line.

When the meter select 48 is placed in the MAINS position, the meter monitors the output of the internal DC voltage which directly represents the condition of the mains voltage.

The local receptacles 52 provide a connection to the keyboard contact and printer of the local teleprinter. The line receptacles 54 provide a connection to the receive line and to the transmit line of the communications line. In the case of a half-duplex system, only two terminals to the line terminals are utilized (and two are strapped together), while in the case of a full-duplex, four of the line terminals are connected.

A universal key panel 56 includes a mechanical lock 58 which may be unlocked by a suitable mechanical key. Upon the unlocking of the lock 58, the panel 56 may be removed from the front panel 40 to expose seven octal switches numbered zero through seven, and an eighth switch containing the letters "U" and "C" to denote the Universal and Custom keys. As will be subsequently described, operation of the octal switches enables any of a plurality of codes to be mechanically set into the unit 10. Unit 10 must always have the same code entered therein as does the mating unit 18.

The mode select switch 60 comprises a three positioned rotary switch to enable switching between the OFF-LINE, ASYN (asynchronous) and SYN HD (synchronous) modes.

An ALARM RESET momentary push button switch 62 acts as a master reset and should be depressed each time the mode is changed to insure proper initialization of the equipment or to clear any alarm condition. The alarm reset button 62 should also be depressed to reset a break condition sent from a remote location. When the push button switch 62 is illuminated, it indicates that some fault or illegal operation has occurred in operation of the system. In such a case, the printer and the line will be inhibited. Once the push button switch 62 has been pressed, the illumination of the light should be extinguished. This push button switch also serves as a lamp test, and when the button is depressed, all the other lamps on the front panel and the audible alarm will be energized.

The BREAK momentary push button switch and indicator 64 may be depressed to perform a function similar to the break key on a teleprinter, thereby placing a "space" on both the local teleprinter and the transmit line. This break will be detected by the remote station and cause an audible alarm and a blinking break light at the remote station to alert the operator of the condition. When a break is received by the unit 10, the break indicator lamp 64 will blink, in combination with the generation of an audible alarm for alerting the operator.

The ENCIPHER push button switch and indicator 66 may be depressed to place the unit 10 in the encipher mode when operating in the off-line mode. When the unit is operating in either of the on-line modes, the indicator 66 is illuminated to indicate that traffic is being transmitted.

The DECIPHER push button switch and indicator 68 may be depressed to place the machine in the decipher mode when the unit is operating in the off-line mode. When the system is operating on-line, the indicator 68 will be energized to denote that the unit is receiving.

The PLAIN alternate action push button switch and indicator 70 may be depressed to place the system in the bypass or normal mode. When the indicator 70 is continuously illuminated, it denotes that the system is not in an enciphering or deciphering state. When in the off-line or ASYN mode, illumination of the indicator 70 denotes that the plain-to-crypto sequence has not been sent. If the indicator 70 is blinking, it denotes that the bypass mode has been placed in operation. When the bypass mode is selected, an audible beeping tone is heard once every four seconds to warn the operator that the system must be returned to the normal mode before transmitting secure traffic.

A CRYPTO illuminator 72 comprises a lamp indicator which denotes that the machine is either enciphering or deciphering. In either the OFF-LINE or ASYN mode, the crypto illuminator 72 will be illuminated only after the proper character sequence is recognized. The crypto illuminator 72 will be extinguished and the plain indicator 70 will be illuminated after another predetermined character sequence is recognized. In the SYN HD mode, the indicator 72 will always be illuminated.

The INITIATE push button switch and indicator 74 comprises a momemtary push button switch which is employed only when the system is in the SYN HD mode. When the SYN HD mode is first entered by changing the mode select switch 60, the indicator 74 begins to blink. Once the initiate push button switch 74 is depressed, and the synchronizing sequence of the invention has been transmitted, the indicator 74 will be continuously illuminated, thereby indicating that the machines are synchronized with one another. During synchronization, the indicator 74 is extinguished.

The SLAVE indicator 76 is illuminated only when the system is in the SYN HD mode. The indicator 76 is extinguished during the OFF-LINE and ASYN modes. When in the SYN HD mode and a synchronizing sequence has been received by the unit, the indicator 76 will be illuminated denoting the receiving unit as the slave unit.

A number of connections, not shown, are provided on the rear of the unit 10 shown in FIG. 5 to enable alternate AC power input and alternate local or line connections. In addition, switches, not shown, are provided internal to unit 10 to allow the user to select one of six main voltages to be utilized, or to select an internal battery voltage for driving the teleprinter loop circuits as desired. Switches, not shown, are also provided to enable selection of five baud rates, and three baud lengths. In addition, switches are provided within the unit, not shown, to enable either the full character set or a Telex character set to be utilized with the system.

If the full character set (FULL SET) is selected, all thirty-two Baudot (CCITT #2) characters appear in the encrypted output text of the unit, and the unit must thus be utilized on radio-teleprinter or hardwire circuits wherein the transmission path is transparent to the transmitted text. If the Telex character set (TELEX) is selected, the encrypted output text of the unit is fully compatible with TELEX and other switched five level networks, and encrypted text can be sent via all standard international message characters. Switches are also provided within the unit to allow connection of the system to either a two wire (half-duplex) circuit or a normal four wire (full-duplex) circuit.

Asynchronous Mode Operation

In operation of the unit 10 shown in FIG. 5, the power switch 44 is operated to turn the unit on. At this time, at least two of the front panel indicator lamps should be illuminated. To place the system in the asynchronous (ASYN) mode, the operators at both ends of the link should first set the identical code in both units, set the mode select switch 60 to the ASYN position and then press the alarm reset indicator 62 to insure proper initialization of the machines. From this point on, either operator can initiate typing on the teleprinter and traffic will be plain text, or unciphered, until one of the operators types in the character sequence LTRS QQ, followed by any five additional characters. This character sequence switches the unit at both the sending and receiving ends into the crypto mode.

Secure or encrypted traffic can then be transmitted back and forth alternately between either end of the communications link until either of the operators types the character sequence "CR,LF,LTRS,QK". This sequence returns both machines to plain or unenciphered operation. An additional feature which may be utilized when using the ASYN mode is the use of the "remote clear" sequence. This sequence consists of four "CR" characters typed in succession. Even if cryptographic synchronization is lost, the typing of the four "CR" characters forces both the sending and receiving units to the plain mode. Once both machines are in a plain mode, the "LTRS QQ" sequence is typed to return the unit to the crypto mode to reestablish cryptographic synchronization.

The asynchronous mode may be utilized for conversational traffic between two operators, as well as to send prepared plain text tapes which have the proper character sequences already punched in the tape. Once transmission begins from the tape reader, the unit switches back and forth between the plain and crypto modes as these control character sequences are encountered. The switching of the unit between the encipher and decipher modes is done automatically by the logic within the machines which senses traffic direction. When the unit is sending in the ASYN mode, the encipher indicator 66 will be illuminated. After the operator has typed "LTRS QQ" and five more characters, the crypto indicator 72 will be illuminated, verifying that the traffic is being enciphered.

When initiating a message, the operator will observe five random characters printed on the page printer of the teleprinter immediately after the "LTRS QQ" sequence, regardless of the character being typed on the keyboard. This is the automatic random prime being generated by the unit, as will be subsequently described. At the receiving end, the five characters which immediately follow the "LTRS QQ" sequence will be the "space" character. All other printout at the local teleprinter will be plain text, regardless of the operating state of the unit.

SYNCHRONOUS MODE OPERATION

With the synchronous (SHY HD) mode of operation, the unit has the capability of maintaining cryptographic synchronization over communications channels that are subject to severe fading and interference. This mode is thus especially useful over radio teletype networks, over long distances or under adverse communication channel conditions.

The unit 10 is a half-duplex unit, thus being able to either encipher (transmit) or decipher (receive) at a single time, but not both simultaneously. The equipment can be utilized over either a half-duplex (two-wire) or full-duplex (four-wire) communications channel, and has expanded capability in this mode with a full-duplex channel.

Before traffic can be initialized in this mode, the transmitting and receiving machines must be synchronized. The unit initiating the synchronization process is designated the "master" and the terminal receiving synchronization is termed the "slave".

To place the unit in the synchronous mode on half-duplex communications channels, the mode select switch 60 is placed in the SYN HD position and the internal switches in the unit are placed in the HD (half-duplex) position. The alarm reset push button switch 62 is depressed to insure proper initialization of the machine. The operator will note that the initiate light 74 is blinking at this stage, the crypto indicator 72 is illuminated, and the local printer of the teleprinter is inhibited.

The synchronization sequence for this mode can be initiated by two methods. First, the initiate push button switch 74 may be depressed. Alternatively, the sequence "QZQZ" may be typed on the teleprinter keyboard. After the synchronization sequence is initiated, the local teleprinter will begin printing a sequence of approximately 50 characters. The operator should notice that the initiate indicator 74 is extinguished at this point, and as soon as the indicator 74 returns to an illuminated state and the teleprinter stops, secure traffic may commence.

The traffic should commence by typing a "LTRS" character on the teleprinter to insure that the machine on the receiving end is in the LTRS (lower) case. All copy of the local teleprinter after the synchronizing sequence is in the plain text, but all data transmitted over the communications line is enciphered. The synchronizing sequence may be reinitiated at any time by typing the character sequence "QZQZ" on the local keyboard or by pressing the initiate push button switch 74.

When the direction of traffic is to be reversed, the unit which wishes to send the message must become the master unit. A slave unit may become the master at any time by typing the character sequence "QZQZ" or pressing the initiate push button switch 74. The synchronous mode is not subject to any plain-to-crypto or crypto-to-plain control character sequences from the teleprinter. The only useable sequence is "QZQZ" which establishes synchronization. In the synchronous mode, the unit is always in crypto, unless the bypass is utilized.

When operated over a full-duplex communications channel, the unit has the capability of establishing cryptographic synchronization in both directions simultaneously, compensating automatically for communications channel time delay and permitting conversational communications. The direction of traffic can be reversed without having to resynchronize each time.

To begin operation in this mode, the internal switches in the unit must be in the FD (full-duplex) position and the mode select switch 60 must be set to SYN HD. The operator should press the alarm reset push button switch 62 to insure proper initialization of the machine. At this time, the crypto indicator 72 should be illuminated and the initiate light 74 should be blinking.

Synchronization may be accomplished by the same two methods previously described. While the sequence is being transmitted, the initiate light 74 is not illuminated. Once synchronization has taken place, the initiate light 74 is illuminated and secure traffic can be transmitted. At this point the slave light 76 is not illuminated, indicating that the transmitting station is the master station.

On the receiving end of the full-duplex communications channel when in the synchronous mode, the operator should prepare his machine by placing the mode select switch 60 in the SYN HD position, switching the system to a FD position and pressing the alarm reset push button switch 62. The initiate light 74 should be blinking at this point until the synchronizing sequence is received from the master station. When the synchronizing sequence is being received, the initiate light 74 will be extinguished and the slave light 76 will be illuminated. As soon as the synchronization has taken place, both the initiate light 74 and the slave light 76 will be illuminated and secure traffic can be received. However, in this situation, the slave terminal can transmit traffic without becoming the master. In this mode, it is not necessary for the operator to press the encipher push button switch 66 or the decipher push button switch 68, since enciphering and deciphering is done automatically by sensing the direction of traffic flow, thus facilitating conversational traffic.

OFF-LINE MODE OPERATION

To operate the system in the OFF-LINE mode, the local teleprinter should be switched to LINE for preparation of tape according to the "two-step" method. The mode select switch 60 should be set to the OFF-LINE position and the alarm reset push button switch 62 should be depressed. The plain push button switch 70 should then be depressed to place the unit in bypass operation, and the plain indicator 70 will begin blinking accompanied by a beep from the audible alarm to denote the bypass mode. The tape punch of the teleprinter should be turned on and the "LTRS" characters should be typed in several times for a leader. The header of the message to be sent should be typed in the teleprinter, and then the character sequence "LTRS QQ" and five "Q's" should be typed in, which is the required control sequence to switch to the crypto mode. The private portion of the message is then typed in the teleprinter. The control sequence "CR LF LTRS QK" is typed in order to return the unit to the plain mode. Any necessary trailer should then by typed into the teleprinter. The clear text tape thus prepared should be removed from the punch, thereby completing the first pass.

The clear text tape previously prepared in the first pass should be placed in the tape reader. If the key is properly set in the unit, the secure unit 10 should be placed in the normal mode by pressing the plain push button switch 70. The mode select switch 60 should be maintained in the OFF-LINE position and the alarm reset push button switch 62 should be depressed. The encipher push button switch 66 should then be depressed. The tape punch should then be turned on and the teleprinter placed in the line position. The tape reader is then started, and the crypto indicator 72 should be on during the private portion of the message, while the plain indicator 70 should be on during the header and the trailer bearing the clear text message. This completes the second pass and a tape bearing enciphered data has thus been prepared. The tape bearing the enciphered data is then transmitted through the teleprinter system as usual. The enciphered data is received by the recipient who has his tape punch turned on for the transmission, whereupon the tape is received and deciphered in the reverse manner.

OPERATION IN BYPASS MODE

The bypass feature in the unit 10 causes the system to be locked in the plain state and to be transparent to all data sent to it by the local teleprinter or by the line. The operation of the unit in all three modes previously described is unchanged, except that the enciphering/deciphering circuitry of the unit is inhibited. In this mode, no enciphering or deciphering can take place regardless of commands from the keyboard or the state of the crypto indicator.

The bypass feature is activated by pressing the plain push button switch 70. An audible alarm is heard every four seconds and the plain indicator 70 begins to blink to prevent inadvertent transmission of clear text. The plain push button switch 70 is again depressed to deactivate the bypass feature, thereby placing the unit in its regular enciphering/deciphering mode, which is referred to as the normal mode.

The bypass feature may be utilized while in either the on-line, asynchronous or synchronous modes. For example, it is sometimes convenient to activate the bypass momentarily for giving call signs in the clear, without disturbing cryptographic synchronization. The bypass mode can also be useful for the routine of preparing clear text tape off-line where the teleprinter does not have its own local mode.

FIG. 6 illustrates the seven octal switches 80 and the select switch 82 which are revealed when the panel 56 (FIG. 5) is removed from the front panel 40. Each of the seven switches 80 may be selectively positioned in any one of eight positions, and switch 82 may be set in either the Universal or Custom position. When switch 82 is set in the Universal position, the switches 80 may be manually set to provide any one of two million Universal key settings into the present system. Both the transmitting and receiving units must be set with the Universal key having identical settings of switches 80 before communication can be established between the stations. The settings of the switches shown in FIG. 6 sets a predetermined code into the random code key generator of the invention, which is described in greater detail in U.S. Pat. No. 3,781,473, entitled "RANDOM DIGITAL CODE GENERATOR", issued Dec. 25, 1973, to the present assignee.

FIG. 7 illustrates another set of Custom code setting switches which are located inside the chassis behind the front panel 40 of the present unit. These switches enable a selection of any one of sixteen million Custom keys, which in combination with the Universal keys provide a total of 32,000,000,000,000 possible keys for the present unit. The Custom keys comprise eight rotary switches 84 which are octally coded so that digits 0-7 are available. To operate switches 84, the switch 82 (FIG. 6) is set to the "C" position and the front panel 40 of the unit is unlocked and swung forward to provide access to switches 84. Each switch 84 is set by rotating the center section of the switch in either direction until the pointer on the switch is adjacent the desired number. Prior to operation of the system in any of the three available modes, each unit must have a complete and identical Universal and Custom key set therein to provide proper encryption operation.

DEFINITION OF CIRCUIT SIGNALS

FIGS. 8-21 illustrate in detail the electronic circuitry of one of the encoder/decoder units of the present invention. A plurality of data, key control, timing and switch signals are utilized in the following figures, and to assist in a better understanding of the description, the following definition of signals is provided: 

    __________________________________________________________________________
             PVT    private
             FC2    fast clock phase 2
             IP     initiate prime
             FC1    fast clock phase 1
             PLC    priming complete
             RP     receive prime (tells key generator
                    to load prime in)
             RK     request key
             ENDW   end wide - timing signal for key
                    generator
             KEY    key
             CGD    code generator data (prime data
                    loaded by RP)
             PD     priming data (actual prime
                    transmitted)
             K3CNT  timing signal used by key
                    generator to prevent forbidden
                    character from appearing in
                    prime
             FULLSET
                    switch which selects either full
                    set or Telex cipher set
             P35X   timing signal - denotes 3-5
                    repetition of prime
             BP0/2  clock to drive correlator
                    (8 .times. bit rate)
             OEND   timing signal from output
                    synchronizer denoting end of
                    character
             INITSW initiate switch - press to initiate
                    in synchronous mode
             ILGSEQ illegal sequence - denotes that
                    sequences NNNN or ZCZC have
                    occurred in cipher text
             RXRAW  unaltered received data
             SYN    denotes in synchronous mode
             SHD    switch output denoting synchronous
                    mode
             ENC    encipher mode
             CHECK  denotes alarm check circuit has
                    detected simulated fault during
                    priming
             ENC    not in enciphering
             ENC*   unbuffered encipher signal
             COMP   compare - compares cipher text
                    and plain text to drive check
                    circuit
             FIFOV  FIFO overflow - denotes data
                    FIFO have overflowed-probably
                    because transmitter receiver
                    sending simultaneously
             BYPASSW
                    bypass switch from front panel -
                    cuts off all enciphering
             CR     carriage return - denotes CR has
                    been detected-only in Telex mode
             CRFF   CR flipflop - remembers that CR was
                    detected--allows next character
                    to go out in clear
             MCLR   Master clear - clears everything
                    (from power on or reset button)
             KEND.uparw.
                    timing signal - denotes leading edge
                    of end pulse of character from
                    keyboard
             MCLK   master controller clock - any state
                    changes gives this clock
             PRIMEX synchronized signal denoting in
                    prime state
             SLAVE  denotes slave unit
             FRAME  state of machine occurring only in
                    synchronous mode and denotes sending
                    frame synchronizing pattern
             KEND.dwnarw.
                    timing pulse from keyboard sync -
                    denotes trailing edge of end of
                    character pulse
             P67    timing pulse - prime being sent for
                    6th and 7th time
             P35X   timing pulse - prime being sent for
                    3rd through 5th time
             NULL   denotes null character detected -
                    Telex mode only
             BREAKSW
                    panel switch - push to cause break
                    in line signal - to interrupt
                    transmitter
             RENDW  timing signal - denoting end pulse
                    on receive synchronizer
             ALARM  denotes 1 of 6 alarm conditions have
                    occurred
             KEND   keyboard sync end pulse
             FIFOEMP
                    key FIFO empty
             FIFOFULL
                    key FIFO full
             REND   receive synchronous end pulse
             ASYN   front panel switch denoting
                    selected on-line asynchronous mode
             SHD    front panel switch denoting selection
                    of synchronous half-duplex mode
             ILGCOM illegal command - only in Telex
                    mode - tried to put machine in
                    crypto state when already in it
             NSBYP  non sync bypass - allows previously
                    encrypted tapes to be passed in
                    bypass mode
             ALL1   denotes key word is all "1's"
                    used in alarm circuit in case key
                    generator gets stuck in all "1's"
             CRYPTO denotes crypto mode (private mode)
             PRIME  denotes machine in prime state
             OPR    denotes machine in operate state
             REND.dwnarw.
                    receive character end pulse -
                    trailing edge
             IDLE   denotes machine in idle state
             P37    timing pulse denoting prime sent
                    3rd through 7th time
             P35    prime send 3rd through 5th time
             PD*    reconstructed priming data from
                    prime data card (A6) where it is
                    stored
             ORK    output request for key (timing
                    signal from output synchronizer)
             KRK    output request for key from keyboard
             XPRIME timing pulse denoting synchronizer
                    transfer to priming state
             GKSHIFT
                    gates shift pulse from keyboard
                    synchronizer
             RRK    request for key from receive
                    synchronizer
             KDRI   keyboard data register input - used
                    as synchronized keyboard data signal
             RXDAT  received data from line
             GRSHIFT
                    gated shift pulse from receive
                    synchronizer
             RVALID signal from receive synch just
                    prior to center of start bit -
                    sampled to make sure character
                    is valid
             OFFLINE
                    front panel mode select switch -
                    offline mode
             RDRI   received data register input
             ID1 through
                    denote certain bit of parallel
             ID5    input data word
             KSTART denotes start bit from keyboard
                    synchronizer
             KSTOP  stop bit from keyboard synchronizer
             OSHIFT shift pulse from output synchronizer
             KSHIFT shift pulse from keyboard
                    synchronizer
             OEND.uparw.
                    leading edge of end pulse from
                    output synchronizer
             REND.uparw.
                    leading edge of end pulse from
                    receive synchronizer
             INV    signal to data FIFO denoting start
                    and stop should the inverted (for
                    frame synch pattern)
             SK1-5  parallel selected key - applied to
                    either ROM or modulo-2 adders as
                    key word
             RREAD* timing pulse from receive
                    snychronizer - RESPONSE timing signal used to tell
                    printer
                    when to print response from slave
                    signal (when to stop)
             FSC    frame sync complete
             PRTRESS
                    timing signal to tell printer when
                    to start
             KBDAT  keyboard data
             KAR    timing pulse from keyboard
                    synchronizer
             RSHIFT shift pulse from receive synchronizer
             KSTB   strobe pulse from keyboard synchro-
                    nizer used to load data FIFO
             RAR    timing pulse from receive synchro-
                    nizer used to strobe ROM - enciphers
                    in Telex mode
             OD1-5  parallel output data word
             ODRO   output data register output
             ENFIFO tells when to enable FIFO
             RSTB   strobe pulse from receive synchro-
                    nizer used to load data FIFO
             FIFOMR FIFO master reset
             GFSD   gated frame sync detect
             QQ     denotes sequence LTRS QQ detected
             4CR    denotes 4 CR characters have been
    Only in off-line
                    detected - used to remotely clear
    and asynchron-  machine in Telex or off-line mode
    ous mode        even if crypto sync lost
             QK     denotes sequence CR LF LTRS QK
                    detected - switches from crypto
                    back to plain
             QZQZ   denotes sequence QZQZ detected -
                    causes machine to be initiated in
                    synchronous half-duplex mode
             CP0/2  phase 2 of a 2 phase clock (basic
                    system timing)
             CP0/1  phase 1 of a 2 phase clock (basic
                    system timing)
             KSYNEN denotes keyboard synchronizer
                    enabled
             RSTART denotes start bit from receive
                    synchronizer
             PRINT  denotes incoming character from
                    line had valid start bit and
                    character should be printed
             FSD    frame synch detect - denotes
                    correlator has detected frame
                    synch from line-switch to load
                    prime mode
             EN45 EN57
                    signals from baud rate select switch
                    located on power supply module
             EN50 EN100
                    A21 - select baud rate
             OSTOP  denotes stop bit from output
                    sychronizer
             O3CNT  denotes 3rd bit of output sync -
                    used on data switching card to
                    generate space character-used
                    on printer when loading prime
             OSYNEN output synchronizer is enabled
             KVALID similar to RVALID - pulse to
                    validate start bit (from keyboard)
             C2RESET
                    insures correlation reset
                    immediately after frame synch
                    detected
             C2     state of correlator controller
                    denoting it is looking for final
                    inverted pattern
             BP0/1  20 clock - operates correlator
                    (8 .times. bit rate)
             CIRC   denotes correlator should
                    circulate contents of register
                    to count the number of ones
             PEAK   denotes correlator has detected a
                    peak
             INITSW initiate switch
             BYPASSW
                    bypass switch
             DECSW  decipher switch



             ENCSW  encipher switch
             BREAKSW
                    break switch
             RESETSW
                    reset switch
             BREAKLP
                    break lamp
             ALARMLP
                    alarm lamp
             INITLP initiate lamp
             SLAVELP
                    slave lamp
             PLAINLP
                    plain lamp
             CRYPTOLP
                    crypto lamp
             ENCLP  encipher lamp
             DECLP  decipher lamp
             CORP   signal from front panel switch
                    denotes in custom mode - 32 trillion
                    keys
             LINEFD denotes line switch is set to full-
                    duplex position
             HORN   denotes signal that drives audible
                    alarm
             KBRAW  unaltered keyboard data
             TXDAT  transmit data (from logic to
                    interface circuitry)
             RXRAW  unaltered receive data
             FANAC  115 VAC from power supply to fan
             BAUD7.5
                    denotes baud length set to either
                    7.42 or 7.5
             LOCALHD
                    local switch in half-duplex
                    position
             MAINSMON
                    mains monitor
    __________________________________________________________________________


PANEL CIRCUITRY

Referring to FIG. 8, a block diagram of the rear and front panel switch circuitry is illustrated. Like numerals are utilized for like and corresponding parts of the various drawings. The switches and lamps on the front panel shown in FIG. 5 are represented by the block 90, with the lamp energizing signals and switch output signals being applied thereto. The mode select switch 60 is also illustrated. Power from a suitable alternating current source is applied to the mains filter 92 and to a mains filter 94. The select mains switch 96 selects the mains to be utilized. Telegraphic loop currents from the power supply are applied to local terminals 98 and line terminals 100 to enable interconnection of the system in the various modes. The output from the power supply is also supplied to the local terminals 52 and line terminals 54 (FIG. 5). The meter 50 is operated in accordance with the setting of the monitor select switch 48. The power switch 44 is also operated from the front panel as previously described. The Universal thumbwheel switches 80 are operable upon the removal of the panel 56 in the manner described with respect to FIG. 6. AC power is applied to the rear panel to operate a fan 102 for cooling off the housing.

RANDOM CODE GENERATOR

The randomized digital keystream which is the basis for enciphering and deciphering with the present system is generated by the key variable circuit shown within the dotted line 104 in FIG. 8 and in the dotted line 106 in FIG. 9. Referring to FIG. 8, the outputs from the Universal key switches 80 are applied to a Universal key register 110. The Custom key switches 84 (FIG. 7) are applied to a Custom key register 112. The register control and timing circuitry 116 controls the operations of the registers 110 and 112. The outputs of the registers are applied to a nonlinear combining circuit 118, the output of which is applied to the random key generator shown in the dotted line 106 in FIG. 9.

Referring to FIG. 9, a prime generation and loading circuit 120 receives timing and loading instructions in order to control the loading of the main register group 122. A register configuration and control circuit 124 controls the interconnections of the main register group in the desired manner. The output of the register group 122 is applied to a program storage circuit 126 which is interconnected with a random stepping and timing circuit 128. The output of the main register group 122 is applied to a nonlinear combining circuit 130, the output of which is applied to control the register configuration and control circuit 124. The resulting configuration and stepping of the main register group 122 results in a randomized digital signal which is applied to a key flipflop 132. The flipflop 132 generates a long stream of randomized digital signals for use in enciphering and deciphering in the present invention.

The random code generator of the system shown in FIGS. 8 and 9 comprises a group of registers 122 which may be combined in various configurations and stepped a number of random steps in order to generate a single bit of key data. The request for key (RK) signal is applied to the prime generation and loading circuit 120 in order to cause the main register group 122 to connect themselves in a random fashion, take a random number of steps, and generate a single bit of key which is stored in the key flipflop 132. Each character typed on the keyboard generates five requests for key and generates five bits of key. These five bits of key are accumulated serially in the key work register to be subsequently described. The key word register governs how the particular character is to be enciphered or deciphered.

Two other principal signals controlling the key generator circuit are the initiate prime (IP) and receive prime (RP). The first five characters after the system has been commanded into the crypto mode are termed priming characters. When enciphering, the signal IP instructs the key generator to generate and supply five random characters (25 bits of random data) to the prime generation and loading circuit 120 to generate the priming data (PD).

When deciphering, the signal RP instructs the key generator to accept the five characters applied as a signal CGD in order to load the characters into the main registers 122. At the same time that prime is being received or generated, the crypto variables located on the Universal key switches 80 and the Custom key switches 84 are being loaded in parallel into their respective registers 110 and 112. Once these variables have been loaded and the prime has been received or generated, the key generator begins to generate key bits as requested by the signal RK. The signal CORP instructs the key generator to go into the Custom mode which offers the user the full 32 trillion key variable combinations.

Just as there are forbidden characters in the cipher text, the prime characters which are generated by the key generator must also be free of forbidden characters if the Telex cipher set is to be selected. The two signals K3CNT and the signal FULLSET prevent the generation of forbidden characters in the prime data if the system is set to the Telex mode. The timing for the key generator is supplied by the two phase high speed clocks termed FC1 and FC2.

For a more detailed description of the construction operation of the random code generator of the invention, reference is made to U.S. Pat. No. 3,781,473, entitled "RANDOM DIGITAL CODE GENERATOR", issued Dec. 25, 1973, and assigned to the present assignee.

PRIME DATA CIRCUIT

The circuit for generating the prime data for use with the present system is illustrated in FIG. 9. This circuitry accumulates, checks and transmits the five random priming characters that precede each enciphered transmission by the system. In both the off-line and asynchronous modes of operation, the priming data from the key generator previously described (termed PD) is routed immediately from the key generator to the controller for transmission on-line or routed to the local teleprinter for off-line operation.

In both of these modes, the prime data received from decoding a tape off-line or from receiving an enciphered message on-line, is collected by a receive prime control circuitry 134 and sent to the key generator as the signal CGD. When enciphering in both the off-line and asynchronous modes of operation, the five priming characters (or 25 priming bits) are routed from the key generator to the printer or line immediately, with no modification supplied by the prime data circuit shown in FIG. 9. When deciphering, the five prime characters are routed from the receive data line or keyboard immediately to the key generator, with again no modification from the prime data circuit shown in FIG. 9.

However, in the synchronous mode (SYN HD), the prime data is transmitted five times. This amount of redundancy is required because correct reception of prime is essential in cryptographic synchronization in the synchronous mode. When the prime state is entered in the synchronous mode, the key generator is requested to deliver 25 bits of priming information just as in the other two modes. This priming data, PD, is now transmitted and simultaneously routed to a random access memory (RAM) input select circuit 136 to the input of a random access memory (RAM) 138 for storage. The prime is then retransmitted four additional times from the RAM 138 through a transmit prime control circuit 140 to the controller as the signal PD*. This results in the transmission of prime five times and also allows the receiving unit to make a three-out-of-five decision as to the correct reception of prime.

A prime bit counter 142 keeps track of which bit of prime is being acted upon by the RAM 138 and provides the address for the RAM 138. A prime repeat counter 144 keeps track of how many times the 25 bits of prime has been transmitted. Counter 144 is decoded by a prime repeat decode 146 and generates signals such as signal P5 which indicates the fifth transmission of the prime information.

When receiving the priming sequence in the synchronous half-duplex mode (SYN HD), the first four priming sequences are stored in the RAM 138. As the fifth priming sequence is being entered, address scan circuitry 148 rapidly accesses the corresponding bits from the first four priming sequences and a three-out-of-five prime voting circuit 150 performs a three-out-of-five prime vote to determine what the correct priming bit should be. In other words, the logic level of the five priming bits are detected and the final level is determined by the largest number of a particular logic level in the detected five levels. Once the decision has been made, the three-out-of-five prime voting circuitry 150 routes the results to receive prime control 134 in order to be loaded into the key generator as valid priming information CGD.

Timing and control for the circuitry is provided by timing and control 152. A read/write control circuit 154 operates to control the reading or writing into the RAM 138. The request for key (RK) logic 156 generates the request for key (RK) for the code generator. Depending upon which mode of operation in which the system is operating, the key generator is under the command of either the receive, output or keyboard synchronizers, as will be subsequently described. A request for key logic 156 determines which synchronizer has control and generates the request at the proper time, in a manner to be subsequently described in greater detail.

ALARM CIRCUIT

FIG. 10 illustrates the alarm and lamp driver circuitry of the invention which drives the eight front panel lamps previously described. The circuitry also drives an audible alarm by the generation of the signal HORN.

The alarm and lamp driver circuit contains all of the lamp driver circuits and logic to drive the eight front panel lamps. A blink generator 160 and the beep generator 162 generate timing signals for the flashing lights and the periodic beep of the audible alarm. The generators 160 and 162 are connected to the lamp driver circuit 164, which generates lamp driving signals.

A break detector circuit 166 senses an open condition on the receive data line labeled RXRAW. If an open condition lasts for approximately two characters duration, a break flipflop 168 is set and the break lamp will begin flashing. The power on preset circuit 170 provides a clearing pulse on the master reset line MCLR to insure that the system is in a cleared condition after the application of power. The signal MCLR is also generated any time that the alarm reset switch 62 (FIG. 5) is depressed. The timing and control for the circuitry is provided by the timing and control circuitry 172.

The key generator alarm and check circuit 174 constantly monitors the cipher text and plain text being generated within the system. Should a fault occur in the key generator, such as the key being stuck at a logical "0" or logical "1", the cipher text and plain text will become equal or complements of each other. Should this condition occur for five consecutive characters, the key generator alarm and check circuit 174 will detect the malfunction and generate the signal KGFAULT. This signal is applied to the alarm detect circuit 176. The key generator alarm and check circuitry is self checking. Each time the system is commanded to the crypto mode, a simulated fault condition is applied to the key generator alarm and check circuitry 174 during the priming sequence, and a signal called CHECK is required from the circuit before the machine can switch to the crypto mode. Once the check signal has been generated by the key generator alarm and check circuit, and the machine has entered the crypto mode, any subsequent malfunction will be considered a key generator fault.

The only time that the key generator alarm and check circuit is disabled is during a Carriage Return, Line Feed, or a Null character. These characters are passed unciphered and the alarm circuit should be disabled when they occur. These three conditions are denoted by the signals Cr, CRFF, and NULL. The signal COMP denotes that the ciper text and plain text have compared and is one step toward an alarm condition. Likewise, the signal ALL1 denotes that the key word generated is all logical ones. Five consecutive characters with either of these conditions being true will cause the generation of an alarm.

The key generator FAULT is only one of six alarm conditions. For a more complete description of alarm and check circuitry, reference is made to U.S. Pat. No. 3,781,472, entitled "DIGITAL DATA CIPHERING TECHNIQUE", issued Dec. 25, 1973, and assigned to the present assignee.

The remaining alarm conditions include the typing in of an illegal command. If the system is being used in the OFF-LINE or ASYN mode, the character sequence LTRS QQ is typed while the system is already in the crypto mode, this is termed an illegal command (ILGCOM) and will result in the generation of an alarm. To recover, the alarm reset indicator is pressed and the message is reenciphered from the beginning.

On rare occassions, such as once every 350,000 characters, the sequences NNNN or ZCZC will occur in the cipher text. Since these should not appear in the cipher text, an alarm condition (ILGSEQ) is indicated if either of these sequences occur. This alarm is used primarily in the off-line mode when preparing enciphered data to be transmitted over a Telex network. To recover from this alarm situation, the alarm reset indicator is depressed and the message reenciphered.

Additionally, the data buffer may exceed its capacity due to an internal fault. If this fault occurs (FIFOV), an alarm is also generated. To recover, the alarm reset button is depressed.

In the SYN HD mode with full-duplex channel, the key buffer may become empty (FIFOEMP) due to an internal fault, if this condition exists, an alarm is generated. To recover, the alarm reset indicator is depressed.

The last alarm condition is caused by the failure of the master unit to receive the acknowledge from the slave unit when in the SYN HD mode with full-duplex channel. If this slave station has its LINE switch in the HD position while operating in the SYN HD mode, the slave station will fail to give a suitable response to the master unit. This lack of response will cause the master unit to enter the alarm condition (FIFOFULL) approximately ten seconds after the synchronization process is complete. To recover from this condition, the operator of the slave unit must be informed to place his LINE switch in the FD position.

The six alarm conditions are OR'ed together to generate the signal ALARM by the alarm detect circuit 176.

SCRAMBLER I

FIG. 10 also illustrates the Scrambler I circuit which includes most of the key word handling circuitry. As the key data is requested by the prime data circuit, it is accumulated serially in the key word register 180. In the synchronous mode, it is necessary to hold the key data in the key holding register 182, in order to synchronize the key (which is being generated synchronously) with the data which can be entered from the keyboard in an asynchronous fashion. In both the off-line and asynchronous modes, the key data is accumulated in the key word register 180 and then routed in parallel through the key first in-first out (FIFO) input select circuitry 184, around the key FIFO register 186 and out through the key select circuitry 188. The selected key word appears in parallel labeled SK1 through SK5. In the synchronous mode, the key is accumulated in the key word register 180 and transferred in parallel into the key holding register 182. It is then routed through the key FIFO select circuitry 184 to the key FIFO register 186. The key FIFO register 186 is a first in-first out register which stores the key word for use in the synchronous mode with a full-duplex channel.

In the SYN HD mode, if cryptographic synchronization is to be established in both directions simultaneously, the enciphered data at the master station must be enciphered with real time key. However, the received enciphered data must be deciphered with key that was generated at an earlier time. When the unit is operating as a master, the key FIFO register 186 holds the key generated and compensates automatically for the transmission delays encountered through the channel. The signal inputs labeled PRTRESS and RESPONSE are timing signals which enable the master unit to establish the correct time at which the response from the slave unit should be printed.

The all one's detect circuit 190 determines when the key accumulated in the key word register 180 is in the all one's condition. This is used for alarm check purposes in the alarm circuit. The FIFO status circuit 192 is used to determine whether or not the key FIFO register 186 is empty or has overflowed. Both of these conditions constitute an alarm. The frame sync counter 194 and the frame sync generator 196 are utilized during the preamble to the synchronization sequence wherein a series of characters are transmitted which are recognized by the remote station as the frame sync pattern. The frame sync counter 194 keeps track of the number of characters transmitted in this sequence and the frame sync generator 196 generates the proper character in the sequence. This series of characters will be subsequently described in greater detail.

The data from the keyboard is accumulated serially in the keyboard data register 198. The input to the keyboard data register 198 can be any one of three inputs; PD*, KBDAT, and FSG. Since the data accumulated in the keyboard data register 198 represents the data to be transmitted, it normally consists of the keyboard data, KBDAT. However, during the priming sequence, the prime data from the key generator must also be supplied to the keyboard data register for subsequent transmission. This is denoted by the signal PD*. In the synchronous mode, the keyboard data register 198 must also be loaded with the output of the frame sync generator for transmission of the frame sync pattern.

The signals KSTART and KSTOP are applied to the keyboard input select 200 in order to attach a proper start stop bit to the key generator priming date, PD*. Since these are random information bits from the key generator, they must be converted to legal telegraph characters before transmission. The signal KDRI is a synchronized signal representing the keyboard input.

Data from the receive line, RXDAT, can also be selected by the RX register input select 201 and accumulated serially in the RX data register 202. The selection of which register to use is made by the register select 204, depending on whether or not the system is enciphering or deciphering. This parallel selection is then routed through to the enciphering circuitry in the form of five input data lines labeled ID1 through ID5. The signal RDRI is a synchronized serial data signal representing the received input data. Timing for the Scrambler I circuit is provided by the timing and control circuit 206.

SCRAMBLER II

FIG. 11 illustrates the Scrambler II circuit which consists of the system enciphering/deciphering circuitry as well as the data FIFO buffer. The selected key bits SK1 through SK5, as well as the selected input data words ID1 through ID5, are applied to a read only memory (ROM) 210, as well as the modulo-2 adders 212. Both of these circuits perform the enciphering/deciphering algorithm. The ROM 210 generates a telex compatible cipher-set of 29 characters, which has eliminated the three forbidden Telex characters, in a somewhat similar manner as that described in previously noted U.S. Pat. No. 3,781,472. The other enciphering circuitry, the modulo-2 adders 212, provide a full 32 character output. This is known as the full cipher set. The selection of which set is to be used is performed by the FULL/TELEX select 214. This select circuit 214 is actuated by the SET switch located on the power supply module.

When using the Telex-set, the output of ROM 210 must be modified in some instances. Whenever a line feed (LF) character results from a decipher operation, the previous character must be examined to determine whether or not it was a carriage return (CR). If the previous character was not a CR as determined by the circuit CR F/F 220, the LF-to-FIGS. conversion circuitry 222 converts the LF to a FIGS. character. In order to set the CR F/F 220, a character decode circuit 224 determines whether or not the output of the ROM 210 is a CR, a LF or a NULL character.

The output of the Full/Telex select 214 is applied to comparators 226 as well as the Plain/Crypto select circuitry 228. The comparators 226 compare the enciphered data with the unenciphered input data and the result, COMP, is used in the alarm circuit. The Plain/Crypto select circuitry 228 selects the unciphered input data, ID1 through ID5, or the enciphered circuitry from the Full/Telex select 214. This selection depends upon the mode of the system. For instance, if in the off-line or asynchronous mode, the input data would be selected prior to the plain-to-crypto sequence and the output of the Full/Telex select 214 would be selected after the sequence. In the synchronous mode, the enciphered data is always selected, except when the BYPASS switch has been activated.

The output of the Plain/Crypto select 228 is applied to the data FIFO buffer 230. Buffer 230 is essential in smoothing out the timing differences in the synchronous mode. In the synchronous mode, the data is transmitted under the control of the highly accurate clock. Each character is located precisely within a timing pattern. Since the data from the keyboard is not in synchronization with the transmission line, the data FIFO buffer 230 automatically holds the character from the keyboard until such time as the line is ready for a new character. When the line is ready for a new character, the output of the data FIFO buffer 230 is transferred in parallel to the output data register 232. The output data register 232 is then shifted serially through the pad flipflop 234 to the data switching circuitry in the form of the signal ODRO. The status of the FIFO buffer is detected by FIFO status 235.

If the line is ready for a new character to be transmitted and none is present in the data FIFO buffer 230, the pad flipflop 234 is set which puts a "mark hold" on the line precisely equal to one character in length. Since the transmission line is running slightly faster than the maximum normal keyboard input rate, approximately every 10 to 15 characters, the pad flipflop 234 will be set to allow the keyboard to catch up with the line. The FIFO status flipflop 235 determines whether or not the data FIFO buffer 230 has overflowed to provide one of the alarm conditions. Timing for the Scrambler II circuit is provided by the timing and control 238.

An important aspect of the present invention is the encoding and decoding provided by the ROM 210. A suitable ROM for use with the present invention is the 4854 Read Only Memory manufactured and sold by Electronic Arrays, Inc. of Mountainview, California, which has a 2048.times.8 memory capacity.

Briefly, for enciphering, the ROM 210 contains enciphered digital representations of all of the characters in the Telex cipher set, the enciphered representations being grouped in addressable subsets according to the generated clear text digital word and the generated digital key word. Therefore, for a particular clear text character to be enciphered and for a particular key word, an address signal is generated in order to access a particular encoded digital word. This digital word is then output as an encoded character. The three forbidden characters are not stored within the ROM 210 and are thus not generated. The binary digital representations stored within the ROM 210 are represented in an octal form in order to simplify the description of the ROM.

Similarly, for deciphering, decoded binary representations are stored in the ROM in order to generate clear text characters in response to specific combinations of an enciphered digital character and a random key word. These digital representations are also represented in octal form.

Tables I and II set forth below are representative of portions of the stored data within the ROM 210. Table I represents a portion of the stored data for encoding, while Table II represents a portion of the stored data for decoding. 

                                      TABLE I
    __________________________________________________________________________
    Plain Text Characters
        Data
           NULL
               E  LF A  SP S  I  U  CR D  R  J   N  F  C  K
    Key.dwnarw.
        .fwdarw.
           T   Z  L  W  H  Y  P  Q  O  B  G  FIGS
                                                 M  X  V  LTRS
    __________________________________________________________________________
    K=0 0000
           000 001
                  002
                     003
                        004
                           005
                              006
                                 007
                                    010
                                       011
                                          012
                                             013 014
                                                    015
                                                       016
                                                          017
        0020
           020 021
                  022
                     023
                        024
                           025
                              026
                                 027
                                    030
                                       031
                                          032
                                             002 034
                                                    035
                                                       036
                                                          037
    K=1 0040
           000 021
                  022
                     023
                        024
                           025
                              026
                                 027
                                    010
                                       031
                                          032
                                             007 034
                                                    035
                                                       036
                                                          037
        0060
           030 011
                  012
                     013
                        014
                           015
                              016
                                 017
                                    004
                                       005
                                          006
                                             022 002
                                                    003
                                                       001
                                                          020
    K=2 0100
           000 011
                  012
                     013
                        014
                           015
                              016
                                 017
                                    010
                                       005
                                          006
                                             027 002
                                                    003
                                                       001
                                                          020
        0120
           004 031
                  032
                     007
                        034
                           035
                              036
                                 037
                                    024
                                       025
                                          026
                                             012 022
                                                    023
                                                       021
                                                          030
    K=3 0140
           000 031
                  032
                     007
                        034
                           035
                              036
                                 037
                                    010
                                       025
                                          026
                                             017 022
                                                    023
                                                       021
                                                          030
        0160
           024 005
                  006
                     027
                        002
                           003
                              001
                                 020
                                    014
                                       015
                                          016
                                             032 012
                                                    013
                                                       011
                                                          004
    K=4 0200
           000 005
                  006
                     027
                        002
                           003
                              001
                                 020
                                    010
                                       015
                                          016
                                             037 012
                                                    013
                                                       011
                                                          004
        0220
           014 025
                  026
                     017
                        022
                           023
                              021
                                 030
                                    034
                                       035
                                          036
                                             006 032
                                                    007
                                                       031
                                                          024
    K=5 0240
           000 025
                  026
                     017
                        022
                           023
                              021
                                 030
                                    010
                                       035
                                          036
                                             020 032
                                                    007
                                                       031
                                                          024
        0260
           034 015
                  016
                     037
                        012
                           013
                              011
                                 004
                                    002
                                       003
                                          001
                                             026 006
                                                    027
                                                       005
                                                          014
    K=6 0300
           000 015
                  016
                     037
                        012
                           013
                              011
                                 004
                                    010
                                       003
                                          001
                                             030 006
                                                    027
                                                       005
                                                          014
        0320
           002 035
                  036
                     020
                        032
                           007
                              031
                                 024
                                    022
                                       023
                                          021
                                             016 026
                                                    017
                                                       025
                                                          034
    K=7 0340
           000 035
                  036
                     020
                        032
                           007
                              031
                                 024
                                    010
                                       023
                                          021
                                             004 026
                                                    017
                                                       025
                                                          034
        0360
           022 003
                  001
                     030
                        006
                           027
                              005
                                 014
                                    012
                                       013
                                          011
                                             036 016
                                                    037
                                                       015
                                                          002
    K=8 0400
           000 003
                  001
                     030
                        006



                           027
                              005
                                 014
                                    010
                                       013
                                          011
                                             024 016
                                                    037
                                                       015
                                                          002
        0420
           012 023
                  021
                     004
                        026
                           017
                              025
                                 034
                                    032
                                       007
                                          031
                                             001 036
                                                    020
                                                       035
                                                          022
    K=9 0440
           000 023
                  021
                     004
                        026
                           017
                              025
                                 034
                                    010
                                       007
                                          031
                                             014 036
                                                    020
                                                       035
                                                          022
        0460
           032 013
                  011
                     024
                        016
                           037
                              015
                                 002
                                    006
                                       027
                                          005
                                             021 001
                                                    030
                                                       003
                                                          012
    __________________________________________________________________________
     ##STR1##


                                  TABLE II
    __________________________________________________________________________
    Enciphered Characters
        Data
           NULL
               E  LF A  SP S  I  U  CR D  R  J   N  F  C  K
    Key.dwnarw.
        .fwdarw.
           T   Z  L  W  H  Y  P  Q  O  B  G  FIGS
                                                 M  X  V  LTRS
    __________________________________________________________________________
    K=0 2000
           000 001
                  002
                     003
                        004
                           005
                              006
                                 007
                                    010
                                       011
                                          012
                                             013 014
                                                    015
                                                       016
                                                          017
        2020
           020 021
                  022
                     023
                        024
                           025
                              026
                                 027
                                    030
                                       031
                                          032
                                             002 034
                                                    035
                                                       036
                                                          037
    K=1 2040
           000 036
                  034
                     035
                        030
                           031
                              032
                                 013
                                    010
                                       021
                                          022
                                             023 024
                                                    025
                                                       026
                                                          027
        2060
           037 001
                  002
                     003
                        004
                           005
                              006
                                 007
                                    020
                                       011
                                          012
                                             034 014
                                                    015
                                                       016
                                                          017
    K=2 2100
           000 016
                  014
                     015
                        020
                           011
                              012
                                 023
                                    010
                                       001
                                          002
                                             003 004
                                                    005
                                                       006
                                                          007
        2120
           017 036
                  034
                     035
                        030
                           031
                              032
                                 013
                                    037
                                       021
                                          022
                                             014 024
                                                    025
                                                       026
                                                          027
    K=3 2140
           000 026
                  024
                     025
                        037
                           021
                              022
                                 003
                                    010
                                       036
                                          034
                                             035 030
                                                    031
                                                       032
                                                          013
        2160
           027 016
                  014
                     015
                        020
                           011
                              012
                                 023
                                    017
                                       001
                                          002
                                             024 004
                                                    005
                                                       006
                                                          007
    K=4 2200
           000 006
                  004
                     005
                        017
                           001
                              002
                                 035
                                    010
                                       016
                                          014
                                             015 020
                                                    011
                                                       012
                                                          023
        2220
           007 026
                  024
                     025
                        037
                           021
                              022
                                 003
                                    027
                                       036
                                          034
                                             004 030
                                                    031
                                                       032
                                                          013
    K=5 2240
           000 032
                  030
                     031
                        027
                           036
                              034
                                 015
                                    010
                                       026
                                          024
                                             025 037
                                                    021
                                                       022
                                                          003
        2260
           013 006
                  004
                     005
                        017
                           001
                              002
                                 035
                                    007
                                       016
                                          014
                                             030 020
                                                    011
                                                       012
                                                          023
    K=6 2300
           000 012
                  020
                     011
                        007
                           016
                              014
                                 025
                                    010
                                       006
                                          004
                                             005 017
                                                    001
                                                       002
                                                          035
        2320
           023 032
                  030
                     031
                        027
                           036
                              034
                                 015
                                    013
                                       026
                                          024
                                             020 037
                                                    021
                                                       022
                                                          003
    __________________________________________________________________________
     ##STR2##


Referring to Table I, it will be seen that the data stored in the ROM 210 comprises seventeen vertical columns which are broken up into a plurality of pairs of horizontal rows. The first vertical column is representative of the beginning octal address for the characters stored in the corresponding row. The remaining vertical columns represent the plain text characters corresponding with the encoded octal numbers in the corresponding column position. For example, referring to the second vertical column, the octal number "000" corresponds to the plain text character NULL. The octal number "020" corresponds to the character T.

Each pair of horizontal rows corresponds to a particular five bit random key word, of which there are a total of thirty-two possible characters. Each of the key characters has been provided with a number from 0-31. For example, key "0" is equal to the five digital bits "00000", while key word "5" is equal to the digital word "00101". Table II is constructed in the identical manner, except plain text data is stored within the columns and rows.

In order to further describe the operation of the ROM 210, it will be assumed that it is desirable to encipher the character "Q" with a random code word number "5". An address signal ID1-ID5 is thus generated to represent the input clear data character "Q". The signal ID5-ID1 would thus comprise the digital word "10111". Similarly, a key word is generated from the random code generator previously described and is applied as signals SK1-SK5 to the ROM 210. In the particular example, the key word number "5" is represented by the digital word "00101". A digital bit "0" is also generated by the circuit to denote it is desired to encipher with the ROM 210.

Thus, the digital word "00010110111", comprising the eleven digits previously described, is applied to the ROM 210. Converting this digital word into octal, beginning with the least significant bit, provides the octal number "0267". Looking for the octal number "0267" in Table I, it will be found that the octal word "004" appears at the intersection of the column corresponding to the character "Q" and the row corresponding to "K-5". Converting the resulting octal number "004" to binary provides the binary number "00000100". The last five bits "00100" correspond to the SPACE character which is the enciphered character which is then output by the system. In operation, the address provided by the signals ID1-ID5 and SK1-SK5 are applied to the ROM 210, the ROM 210 is strobed and the SPACE character would be generated to an output latch.

Utilizing the reverse of the previous example, it will now be assumed that it is desirable to decipher the character SPACE utilizing the key word number "5". The signals ID5-ID1 for the SPACE character comprise "00100". The signals SK1-SK5 for key word number "5", as before, is "00101". A "1" bit is generated to denote that it is desired to decipher. The resulting digital word comprises "10010100100". The resulting octal address for this number is "2244". At that address in the data shown in Table II, the stored plain text data is "027". The resulting binary number for the octal number "027" is "00010111". The last five bits "10111" correspond to the character "Q". The character "Q" is then output by the ROM 210 as the clear text word corresponding to the encoded character.

DATA SWITCHING CIRCUIT

FIG. 11 also illustrates the data switching circuitry which selects the routing of the data paths within the system. The two main data paths are the printer data and the transmit line data. The receive data, labeled RXRAW, and the keyboard data, labeled KBRAW, are also routed through the data switching circuitry. The primary influence on the selection and routing of the data is the mode of operation indicated by the three signals OFFLINE, ASYN, SHD. These signals denote the three basic operating modes of the system and are applied to control the printer data select 239 and the transmit data select 241.

Each of the lines PRDAT and TXDAT can be forced to a logical "0" by the actuation of the BREAK switch as indicated by the signal BREAKSW applied to the alarm inhibit and break circuits 246 and 248. Both can be forced to a logical "1" or marking condition by the occurrence of an alarm condition.

The printer inhibit 240 and keyboard inhibit 242 circuits are activated when a local half-duplex teleprinter is used. When employing a half-duplex teleprinter, the KY and PR ports must be connected in series. For this reason, it is necessary to inhibit the printer data when the keyboard is active. It is also essential that the keyboard circuit be inhibited when the receive data is active.

Likewise, the receive data inhibit 244 inhibits the RX data port whenever a signal is being transmitted in the two-wire line configuration. When the two-wire or half-duplex line is used, the RX and TX ports must be connected in series. To prevent the outgoing character from being reflected back into the RX port, the receive data inhibit 244 blocks out the RX port for one character time.

The probe test logic 250 is for diagnostic purposes and allows the printer data to be connected to the PROBE test point. The space generator circuit 252 is used to provide the space (SP) character supplied by the printer when the receive prime is being loaded into the key generator. The timing and control circuitry 254 is used for timing of the data switching circuitry while the timing circuitry 256 is used primarily to aid in the control of the digital corrrelator during the SYN HD mode.

MAIN CONTROLLER

FIG. 12 illustrates the main controller circuit which provides two primary functions. It generates all of the control sequences that determine the state of the system, as well as generating all of the clock signals for the timing. The heart of the controller consists of the shift register 260. The four states of the machine are IDLE, FRAME, PRIME and OPR. The IDLE mode indicates that the system has not yet initiated a priming sequence.

In the off-line and asynchronous mode, the IDLE mode means that the plain-to-crypto sequence has not been encountered or the crypto-to-plain sequence has returned the machine to the IDLE state. The normal next state from IDLE is the PRIME state. This state causes the system to either generate or accept priming information either to or from the key generator. The OPR, or operate state, denotes that the system is actually enciphering or deciphering a message. The fourth state, FRAME, is entered only in the SYN HD mode and denotes that the system is sending the frame sync pattern.

The controller is initiated by the power on preset or the ALARM RESET button to the IDLE state. From this point, the normal sequence of states, IDLE, FRAME, PRIME and OPR, are actuated by the state transfer logic and clock control circuitry 262. This logic consists of merely shifting the "1" bit set in the shift register 260 to the next state when a sequential state transfer occurs. However, if a jump state is required, the controller register 260 must be preset to the new state by the jump state steering logic 261.

Several sub-control functions are also contained in the master controller circuitry. The plain/crypto flipflop 264 determines whether or not the system is to encipher a particular message or pass it unciphered. The flipflop 264 is set and reset by the control sequences.

The master/slave flipflop 266 is affected only during the SYN HD mode. If the operator presses the INITIATE switch or types the sequence QZQZ to initiate the framing and priming sequence, the flipflop 266 will be reset to indicate that the machine is a master. If the frame sync pattern has been received and detected by the correlator circuit, flipflop 266 will be set to indicate that the unit is a slave.

The ENC/DEC flipflop 268 denotes whether the machine is enciphering or deciphering. When off-line, the flipflop 268 is directly set or reset by the ENCIPHER and DECIPHER switches respectively. However, in the on-line modes (ASYN or SYN HD), the flipflop 268 is set under the control of the keyboard synchronizer and the receive synchronizer, to be later described. Timing is provided by timing and control circuitry 269.

All of the timing signals and clock pulses for the entire system are generated by a frequency divider 270, which divides the highly stable temperature compensated crystal oscillator (TCXO) 272. The output of this divider is fed into two two-phase clock generator circuits 274 and 276. Circuit 274 generates the high speed clocks FC1 and FC2 and the circuit 276 generates a slower speed two-phase clock labeled CP.phi.1 and CP.phi.2.

RECEIVE SYNCHRONISER

FIG. 12 also illustrates the receive synchronizer. This synchronizer is almost identical to the output and keyboard synchronizers subsequently shown in FIGS. 13 and 14. The three synchronizers provide the timing necessary for data handling and transfer, as well as register loading and control. All three synchronizers operate in basically the same manner. The only differences in operation are the signals which control the synchronizers, the clock which drives them, and the number of timing signals derived. The keyboard synchronizer (FIG. 14) controls the handling of the data entered from the keyboard. In the off-line and asynchronous modes, the synchronizer operates in a start/stop fashion and is actuated by the data from the keyboard. In the SYN HD mode, the keyboard synchronizer also operates in the free-run mode temporarily while the frame sync pattern and priming data are being transmitted.

The output synchronizer (FIG. 13) controls the output of the system, either to the line or the local printer. In the off-line and asynchronous modes, the output synchronizer operates in start/stop fashion and is actuated by the KEND pulse from the keyboard synchronizer. In the SYN HD mode, the output synchronizer operates in the free-run mode as soon as the priming information has been transmitted. The output synchronizer controls the key generator in the SYN HD mode if the machine is a master.

The receive synchronizer shown in FIG. 12 controls the handling of the data from the RX port in either of the on-line modes. In the asynchronous mode, the synchronizer operates in start/stop fashion and is activated by the received data, RXDAT. In the SYN HD mode, the synchronizer free-runs as soon as the correlator recognizes the frame sync pattern from the sending unit.

In a slave machine, the receive synchronizer controls the key generator. The only time the receive synchronizer is active in a master unit is when the LINE switch is set to FD. The unit is then expecting a response from the slave unit so that synchronization can be established simultaneously in both directions. In this situation, the receive synchronizer does not control the key generator, but does control the output of the key words stored in the key FIFO register 186 (FIG. 10).

As shown in FIG. 12, the clock labeled CP.phi.2 is divided by a multi-modulus baud rate counter 280, to provide a clock signal equal to 16 times the baud rate. The amount by which the baud rate counter 280 divides the CP.phi.2 signal depends upon the baud rate selected on the BAUD RATE switch located on the power supply module. The output of this switch is indicated by the signals EN45, EN50, EN57 and EN100. The output of the baud rate counter 280 is further divided by sub-bit counter 282 and decoded by decode 284 to provide timing signals equal to 1/16 of a bit period. The signal is further divided by the bit counter 286 and decoded by decode 288 to provide timing signals which correspond to each individual bit of the data character. Both of these decode circuits are further decoded by decode 290 to provide all of the timing pulses necessary to operate the system.

In both the off-line and asynchronous on-line modes, the receive synchronizer is enabled by the appearance of a start bit on the RXDAT signal as controlled by the start/stop flipflop 292. The synchronizer is enabled and all of the timing pulses are generated until the REND pulse is generated which resets the start/stop flipflop 292. The synchronizer is then idle until the next start bit occurs on the RXDAT line. The receive synchronizer free-runs during certain modes of operation. This free-running nature of the receive synchronizer is controlled by the free-run logic 293 and is governed by the signals SYN, PRIME and OPR.

An additional feature of the receive synchronizer is the late flipflop 294 and the early flipflop 296. Whenever the synchronizer is free-running its effective baud length is exactly 6.75. When the start bit from the RXDAT line perfectly coincides with the start bit within the synchronizer, neither the late nor the early flipflop will be set. However, should the data and the synchronizer drift slightly, either of these flipflops 294 and 296 will be set, depending on the direction of the drift. Appropriate action is taken within the decode timing pulse logic 290 to correct the synchronizer to keep it aligned with the incoming data. This constitutes a digital phase-lock-loop which keeps the receive synchronizer fine tuned to the incoming RXDAT signal. As long as the RXDAT signal is present, the receive synchronizer should be able to track the incoming data as long as the slew rate is not more than 1/16 of a bit per character. The capture range of the digital phase lock loop is plus or minus one-half bit.

A final feature of the receive synchronizer is the print flipflop 298 which determines if an incoming character had a start bit in the proper location. If not, the print flipflop 298 will be reset and the corresponding character will not be deciphered and applied to the printer. This prevents unwanted characters from being printed which were generated by noise on the line thus creating false characters. Timing for the receiving synchronizer is provided by the timing and control circuit 300.

OUTPUT SYNCHRONIZER

FIG. 13 illustrates the output synchronizer which controls the output of the system, either to the line or to the local printer. In the off-line and asynchronous modes, the output synchronizer operates in start/stop fashion and is actuated by the KEND pulse applied from the keyboard synchronizer to the asynchronous start/stop flipflop 310. When a character is entered, the keyboard synchronizer shifts the character into the KY register 198 (FIG. 10) and loads the enciphered character into the data FIFO buffer 230 (FIG. 11). The output synchronizer is then activated to provide the timing to shift the character out from the data FIFO buffer 230 to the line or printer. In the SYN HD mode, the output synchronizer shown in FIG. 13 operates in the free-running mode by operation of the free-run logic 312, as soon as the priming information has been transmitted. The output synchronizer controls the key generator in the SYN HD mode if the machine is operating as a master.

The clock signal CP.phi.1 is divided by a multi-modulus baud rate counter 314, in dependence upon the baud rate selected by the baud rate switch located on the power supply module. The output of counter 314 is further divided by a sub-bit counter 316 which is decoded by a sub-bit decode circuit 318. The clock signal is further divided by a bit counter 320 which is decoded by a bit decode 322 to provide timing signals corresponding to each individual bit of the data character. Both of the decode circuits are further decoded by a timing pulse decode 324 to provide the timing pulses necessary to operate the circuit.

SEQUENCE DETECTOR

FIG. 13 also illustrates a sequence detector circuit which is used to detect the control characters required to switch the system from the plain to crypto and from crypto to plain. Each of these characters is decoded by the character decode 330 from either the input data labeled ID1 through ID5 or the output data, OD1 through OD5. This selection is made by select circuit 332 and is dependent primarily upon whether or not the machine is enciphering or deciphering. Once the data has been selected by select 332 it is applied to the character decode logic 330, where the six required characters are decoded and applied to the sequence detectors 334. The sequence detector generates four signals labeled 4 CR, QZQZ, QK and QQ. These represent the four sequences used to control the system.

In addition, another character decode 336 and a sequence detector 338 are constantly monitoring the cipher text (OD1 through OD5) to determine if the sequences NNNN or ZCZC have occurred. Both of these sequences are invalid as data in the Telex network. Should either of these signals occur during the enciphering of a message in the Telex mode, either off-line or in the asynchronous on-line mode, an illegal sequence (ILGSEQ) signal will be generated by sequence circuit 340 and result in an alarm condition. Likewise an illegal command situation can be encountered whenever the system is commanded to switch to the crypto mode when it is already in the crypto mode. This can occur only in the off-line or asynchronous mode. When the system is in the SYN HD mode, it is always is crypto and no sequences are required. Illegal commands are detected by detector 342 to generate the signal ILGCOM. Timing and control of the circuit is provided by timing and control 344.

KEYBOARD SYNCHRONIZER

FIG. 14 illustrates the keyboard synchronizer which controls the handling of the data entered from the keyboard. This synchronizer operates in the start/stop fashion in the off-line and asynchronous modes, and is actuated by the data from the keyboard. In the SYN HD mode, the keyboard synchronizer operates in the free-running mode temporarily while the frame sync pattern and priming data are being transmitted.

Referring to FIG. 14, the asynchronous start/stop flipflop 350 is actuated by the keyboard data KBDAT in order to start and stop operation of the keyboard synchronizer. During the SYN HD mode, the free-run logic 352 is actuated in order to place the system in the free-running mode. The CP.phi.1 clock is applied to a baud rate counter 354 which divides the signal in dependence upon the selection of the baud rate switch located in the power supply module. The output of the counter 354 is further divided by a sub-bit counter 356, the output of which is decoded by a sub-bit decode 358. The output of counter 354 is also further divided by a bit counter 360, the output of which is decoded by a bit decode 362. The output of decodes 358 and 362 are further decoded by a timing pulse decode 364 in order to generate timing pulses for operation of the system.

It will thus be seen that the receive synchronizer, output synchronizer and keyboard synchronizer operate in conjunction with one another in the different modes of operation of the system. In the asynchronous mode, the receive synchronizer is connected in an asynchronous configuration to receive enciphered digital data from a communications channel and to direct the enciphered digital data to the deciphering circuitry. In this mode, the output synchronizer is connected in an asynchronous configuration to the output of the deciphering circuitry to direct the deciphered digital data to the printer. When it is desired to encipher plain text data in the asynchronous mode, the digital characters are clocked in by the keyboard synchronizer to the enciphering circuitry. The output synchronizer is operable in an asynchronous configuration such that the enciphered digital characters are asynchronously shifted to the communications channel in an on-line configuration, or to the printer in an off-line configuration.

In the synchronous mode, the keyboard synchronizer clocks in the digital characters from the keyboard. The digital characters are enciphered, and the output synchronizer is connected in a free-running synchronous mode to synchronously shift the enciphered digital characters to the communications channel. In the receive on-line synchronous mode, the receive synchronizer is connected in a synchronous free-running configuration to synchronously receive enciphered data from the communications channel. The receive synchronizer then synchronously directs the enciphered digital data to the deciphering circuitry. The output synchronizer is connected in a synchronous mode to the output of the deciphering circuitry to synchronously direct the deciphered digital data to the printer.

CORRELATOR CONTROLLER AND CORRELATOR

The correlator controller shown in FIG. 14 operates with the correlator shown in FIG. 15 to recognize the frame sync pattern sent from the master to the slave unit in the SYN HD mode. A correlation technique is used to recognize the frame sync pattern. The actual correlation is done in the circuitry shown in FIG. 15, while the control, timing and counting circuitry which accompanies the correlator is shown in FIG. 14.

The correlation pattern consists of 15 characters. The Baudot character with the best cross correlation properties is the character "B". The frame sync pattern thus consists of three inverse B's, followed by eleven normal character B's, followed by one final inverted B (B) character. The inverted B is truly the inverse of the normal character B, including the start and stop bits which have also been inverted. This is required to give a true inverse correlation pattern.

Each character, B or B, is to be recognized by the correlation circuitry shown in FIG. 15 and a correlation pulse shown as PEAK is generated. If a correct number of peaks occur consecutively, frame sync will be established. The first three B characters are for preconditioning the correlator. This is to ensure t