Electronic mail filtering by electronic stamp5999967Abstract An improved method and apparatus for filtering electronic mail. The method and apparatus allows the receiver of electronic mail to make a decision to accept, reject, prioritize, or expedite delivery of a piece of electronic mail based on the value of an attached electronic stamp. The apparatus consists of two complimentary parts: a sender side part with responsibility for attaching the electronic stamp; and a receiver side part with responsibility for removing the electronic stamp and filtering the electronic mail based on the value of the electronic stamp. The method and apparatus are used in a system for reducing or eliminating the amount of junk electronic mail in the electronic mail system. Claims I claim: Description BACKGROUND
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List of Reference Numerals
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10 Mail Device 26 Receiver Side Output
10A Sender Mail Device
30 Sender Side Input
10B Receiver Mail Device
31 Control Module
11 User Agent 32 History Log
11A User Agent 33 Encryption Module
11B User Agent 34 Assembly Module
12 Mail Transport Agent
35 Sender Side Output
12A Mail Transport Agent
40 Serial Number Field
12B Mail Transport Agent
41 Value Field
13 Network 42 Date Field
20 Receiver Side Input
43 Hash Field
21 Disassembly Module
44 Receiver Address Field
22 Decryption Module
45 Sender Address Field
23 Analysis Module 46 Identification Field
24 Preferences Record
47 Signature Field
25 History Log 48 Token Field
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SUMMARY This invention provides an improved method and apparatus for filtering electronic mail. DESCRIPTION OF THE PREFERRED EMBODIMENT The following description is divided into four sections. The first section describes an electronic mail system and shows where this invention fits. The next two sections describe the two major parts of this invention. One part resides on the receiver's side of the electronic mail system. It is described first. The other part resides on the sender's side of the electronic mail system. It is described last. The final section describes the electronic stamp used by this invention. The Electronic Mail System An electronic mail system typically supports a plurality of users. Users involved in an exchange of electronic mail assume either the role of a sender or a receiver. Senders create and send electronic mail. Receivers receive and read electronic mail. Roles are dynamic. A user may be a sender during one exchange and a receiver during the next. Every user has at his or her disposal a device to assist with sending and receiving electronic mail. This device most often takes the form of a software program or suite of software programs. FIG. 1 shows a block diagram of such a device. A Mail Device 10 comprises two parts. It will be understood that Mail Device 10 may include other parts, not shown in FIG. 1, whose existence current discussion. A User Agent 11 provides a (hopefully) user friendly front-end to the electronic mail system. The user interacts exclusively with User Agent 11. User Agent 11's primarily concern is with electronic mail content issues. A Mail Transport Agent 12 connects User Agent 11 to a Network 13. Mail Transport Agent 12 knows how to route electronic mail so that it reaches its intended receiver. Mail Transport Agent 12's primarily concern is with routing issues. The separation between these two parts, more well-defined in the past, has blurred in recent years. It will therefore be understood that the use of such separation in this description is meant to emphasize functional differences between the parts. In practice, no such separation is implied nor is it necessary. Network 13 isn't part of the device, as such. Rather it is the medium by which each device is connected to others and over which electronic mail flows. The combination of a network and a plurality of these devices creates an electronic mail system. FIG. 2 shows an electronic mail system consisting of only one sender side device and only one receiver side device. A device functions differently depending on whether it is being used by the sender or by the receiver. The following discussion illustrates the differences. On the sender side, a user interacts with a sender side User Agent 11A in order to compose her electronic mail. User Agent 11A provides such tools as are necessary to accomplish this task, including text processing, spelling correction, and local storage. A sender side Mail Transport Agent 12A sends electronic mail to the receiver via Network 13. On the receiver side, a receiver side Mail Transport Agent 12B receives and stores electronic mail as it arrives from the sender. It holds the electronic mail until the user is ready to read it. A user interacts with a receiver side User Agent 11B in order to read his electronic mail. User Agent 11B also provides tools for storing and replying to electronic mail. This invention is meant to be used in an electronic mail system similar to the system described above. Like the electronic mail system described above, this invention comprises two parts--one resides on the sender's side of the electronic mail system and the other resides on the receiver's side of the electronic mail system. The sender side part prepares an electronic stamp and attaches it to a piece of electronic mail. The receiver side part removes the electronic stamp and decides whether or not to accept the piece of electronic mail based on its value. A key component of this invention is the electronic stamp. Like a postage stamp, an electronic stamp must be attached to a piece of electronic mail before the receiver side will accept it. If the electronic stamp is not present, the receiver side will automatically reject the piece of electronic mail before the receiver ever sees it. There are three common ways in which this invention can be added to the electronic mail system described above. First, the two parts of the filtering apparatus are embedded within User Agent 11A and User Agent 11B respectively. Second, the two parts of the filtering apparatus are embedded within Mail Transport Agent 12A and Mail Transport Agent 12B respectively. Third, the two parts of the filtering apparatus are embedded between User Agent 11A and Mail Transport Agent 12A, and User Agent 11B and Mail Transport Agent 12B respectively. Both the description of the preferred embodiment and the description of an alternate embodiment assume the second of the three configurations. The other two can be substituted with no loss of functionality. Since this invention's stated domain is electronic mail filtering, this description does not cover electronic mail composition, handling, transport, storage, display, or other related functions; nor will it describe the methods and protocols required to carry out these functions. Such information is already widely available and understood by persons educated in the art. The Receiver Side Apparatus FIG. 3A is a diagram of the receiver side of the filtering apparatus. Because the primary function of the apparatus, filtering, occurs on the receiver side of the apparatus, I describe it first. The sender side of the apparatus is best viewed as playing a supporting role. The receiver side of the apparatus removes the electronic stamp from a piece of electronic mail and decides whether or not to accept the piece of electronic mail based on its value. It comprises: A Receiver Side Input 20 connected to Mail Transport Agent 12A. Receiver Side Input 20 receives electronic mail from Mail Transport Agent 12A. A Disassembly Module 21 connected to Receiver Side Input 20. Disassembly Module 21 removes the electronic stamp from electronic mail. A Decryption Module 22 connected to Disassembly Module 21. Decryption Module 22 removes encryption, if present, from the electronic stamp. Any secure symmetric encryption algorithm may be used. Algorithms include (but are not limited to) DES, LOKI, RC2, RC4, and IDEA. An Analysis Module 23 connected to Decryption Module 22. Analysis Module 23 validates the electronic stamp. It also decides whether to accept, reject, or otherwise handle electronic mail based on the value of its electronic stamp. A Preferences Record 24 connected to Analysis Module 23. Preferences Record 24 holds user preferences. In particular, it holds the amount of compensation the sender desires for each piece of electronic mail received. Analysis Module 23 uses this information when deciding how to handle electronic mail. A History Log 25 connected to Analysis Module 23. History Log 25 holds previously received electronic stamps for record-keeping as well as billing purposes. A Receiver Side Output 26 connected to Analysis Module 23. Receiver Side Output 26 returns electronic mail to Mail Transport Agent 12A. The Sender Side Apparatus FIG. 3B is a diagram of the sender side of the filtering apparatus. The sender side of the apparatus attaches the electronic stamp to a piece of electronic mail. It comprises: A Sender Side Input 30 connected to Mail Transport Agent 12B. Sender Side Input 30 receives electronic mail from Mail Transport Agent 12B. A Control Module 31 connected to Sender Side Input 30. Control Module 31 creates the electronic stamp. A History Log 32 connected to Control Module 31. History Log 32 holds previously created electronic stamps for record-keeping purposes. An Encryption Module 33 connected to Control Module 31. Encryption Module 33 adds encryption, if necessary, to the electronic stamp. Any secure symmetric encryption algorithm may be used. Algorithms include (but are not limited to) DES, LOKI, RC2, RC4, and IDEA. An Assembly Module 34 connected to Encryption Module 33. Assembly Module 34 attaches the electronic stamp to electronic mail. A Sender Side Output 35 connected to Assembly Module 34. Sender Side Output 35 returns electronic mail to Mail Transport Agent 12B. The Electronic Stamp FIG. 4 is a diagram of the electronic stamp. The electronic stamp comprises the following fields: A Serial Number Field 40. Serial Number Field 40 holds the serial number of the electronic stamp. This number is issued by the sending system. A serial number must never be reissued. The simplest serial number generator is a counter that is incremented for each electronic stamp generated. A Value Field 41. Value Field 41 holds the amount the sender agrees to pay the receiver if the receiver accepts the electronic mail. The receiver must bill the sender for the agreed amount if he accepts the electronic mail. A Date Field 42. Date Field 42 holds the creation date of the electronic stamp. A Hash Field 43. Hash Field 43 holds a value computed from the contents of the body of the electronic mail. Any secure one-way hashing algorithm may be used. Algorithms include (but are not limited to) MD5, SHA, and RIPE-MD. A Receiver Address Field 44. Receiver Address Field 44 holds the address of the receiver of the electronic mail. A Sender Address Field 45. Sender Address Field 45 holds the address of the sender of the electronic mail. The values in Hash Field 43, Receiver Address Field 44, and Sender Address Field 45 serve to bind the electronic stamp to a piece of electronic mail. Without this information, a malicious party could intercept the electronic mail, remove the electronic stamp, and place it on another electronic mail. An Identification Field. Identification Field 46 holds the identification number of the sending system. Each sending system will be issued a unique identification number. A Signature Field 47. Signature Field 47 holds the digital signature of the electronic stamp. The digital signature may be computed using any secure digital signature algorithm. Algorithms include (but are not limited to) El Gamal and DSA. Once it assembles the values in Serial Number Field 40, Value Field 41, Date Field 42, Hash Field 43, Receiver Address Field 44, Sender Address Field 45, and Identification Field 46, the sending system generates the digital signature of the electronic stamp and signs the electronic stamp with it. Without this digital signature, a malicious party could forge an electronic stamp claiming to be from any sender in the electronic mail system. OPERATION OF THE PREFERRED EMBODIMENT Receiver Side Operation The operation of the receiver side of the apparatus in its preferred embodiment comprises the following steps: In step (a) Mail Transport Agent 12B delivers electronic mail to Receiver Side Input 20. Delivery of electronic mail awakens the receiver side apparatus and starts receiver side processing. Receiver Side Input 20 hands the electronic mail to Disassembly Module 21. In step (b) Disassembly Module 21 removes the electronic stamp from the electronic mail. Disassembly Module 21 hands both the electronic stamp and the electronic mail to Decryption Module 22. In step (c) Decryption Module 22 decrypts the electronic stamp. Decryption Module 22 hands both the electronic stamp and the electronic mail to Analysis Module 23. In step (d) Analysis Module 23 reads the identity of the sending system from Identification Field 46 and the digital signature from Signature Field 47. It verifies the identity of the sending system and the integrity of the electronic stamp by testing the digital signature. If the digital signature fails the test, this is a sure indication that either the electronic stamp has been corrupted or tampered with, or someone other than the sender created the electronic stamp. In either case, the electronic stamp and the associated electronic mail should be rejected. In step (e) Analysis Module 23 reads the hash value from Hash Field 43. It compares this value with the value it calculates from the body of the electronic mail. If the comparison fails, this is a good indication that either the electronic mail has been corrupted or the electronic stamp is no longer attached to the original piece of electronic mail. In either case, the electronic stamp and the associated electronic mail should be rejected. In step (f) Analysis Module 23 reads the address of the receiver from Receiver Address Field 44, and the address of the sender from Sender Address Field 45. It then compares these values with the appropriate parts of the electronic mail. If the comparison fails, this is a good indication that either the electronic mail has been corrupted or the electronic stamp is no longer attached to the original piece of electronic mail. In either case, the electronic stamp and the associated electronic mail should be rejected. In step (g) Analysis Module 23 reads the serial number from Serial Number Field 40. It then checks History Log 25 to see if this electronic stamp has been received before. If the electronic stamp is found in History Log 25, this is a good indication that the electronic mail has been delivered multiple times, either due to a fault in the electronic mail system or due to malicious intent. In either case, the electronic stamp and the associated electronic mail should be rejected. In step (h) Analysis Module 23 reads the date from Date Field 42. If the electronic stamp is older than when History Log 25 was last truncated (truncating being of practical necessity) then the electronic stamp may have been received before--it's impossible to know. The electronic stamp and the associated electronic mail should be rejected. In step (i) Analysis Module 23 reads the value of the electronic stamp from Value Field 41. It compares this value with the desired compensation value found in Preferences Record 24. If the value of the stamp is greater than or equal to the desired compensation value, the electronic stamp and the associated electronic mail should be accepted. In step (j) Analysis Module 23 adds the electronic stamp to History Log 25. In step (k) Analysis Module 23 hands the electronic mail 26. In step (k) Receiver Side Output 26 returns the electronic mail to Mail Transport Agent 12B. Sender Side Operation The operation of the sender side of the apparatus in its preferred embodiment comprises the following steps: In step (a) Mail Transport Agent 12A delivers electronic mail to Sender Side Input 30. Delivery of electronic mail awakens the sender side apparatus and starts sender side processing. Sender Side Input 30 hands the electronic mail to Control Module 31. In step (b) Control Module 31 generates a serial number one greater than the last serial number generated. It places the serial number in Serial Number Field 40 of the electronic stamp. In step (c) Control Module 31 places the date in Date Field 42 of the electronic stamp. In step (d) Control Module 31 places the value of the electronic stamp in Value Field 41 of the electronic stamp. This value indicates how much the sender is willing to pay the receiver if the receiver accepts the associated electronic mail. In step (e) Control Module 31 examines the body of the electronic mail and calculates a hash value based on the information contained therein. It writes the calculated hash value to Hash Field 43. In step (f) Control Module 31 takes the address of the receiver of the electronic mail and writes it to Receiver Address Field 44. It takes the address of the sender of the electronic mail and writes it to Sender Address Field 45. In step (g) Control Module 31 writes the identity of the sending system to Identification Field 46. Control Module 31 writes the digital signature of the electronic stamp to Signature Field 47. The digital signature verifies the identity of the sending system and the integrity of the values comprising the electronic stamp. In step (h) Control Module 31 adds the new electronic stamp to History Log 32. In step (i) Control Module 31 hands both the electronic stamp to Encryption Module 33. In step (j) Encryption Module 33 encrypts the electronic stamp. The electronic stamp is encrypted because the networks, phone lines, and other pathways that make up Network 13 are often insecure. Encryption prevents others from tampering with the electronic stamp while the electronic mail is in transit. Encryption Module 33 hands both the electronic mail and the electronic stamp to Assembly Module 34. In step (k) Assembly Module 34 appends the electronic stamp to the end of the body of the electronic mail. Assembly Module 34 hands the electronic mail to Sender Side Output 35. In step (l) Sender Side Output 35 returns the electronic mail to Mail Transport Agent 12A. AN ALTERNATIVE EMBODIMENT The invention in its preferred embodiment places upon the receiver a significant accounting burden. The receiver must maintain records, bill each sender, and collect payments. While the system itself can assist with record-keeping, in an electronic mail system with a multitude of senders and receivers the work can quickly become overwhelming. In order to correct this deficiency, the following alternative embodiment is presented. This embodiment requires the services of a third party called a bank. The bank distributes electronic tokens. Electronic tokens are digital information and can be manipulated electronically. Every electronic token is unique. The electronic tokens represent money being held in accounts with the bank. Both the sender and the receiver have accounts with the bank. When the time comes to create an electronic stamp, the sender side apparatus withdraws an electronic token from the sender's bank account. This action decreases the sender's account balance. The sender side apparatus encodes and encrypts the electronic token and attaches it to the electronic stamp. As before, the sender side system attaches the electronic stamp to the electronic mail and sends both to the receiver. The receiver side apparatus does exactly the reverse. It removes the electronic stamp from the electronic mail, and then decrypts and decodes the electronic token. It deposits the electronic token in the receiver's bank account. This action increases the receiver's account balance. The bank takes care of actually transferring the money between the two accounts. FIG. 5 is a diagram of the electronic stamp. The electronic stamp in FIG. 5 differs from the electronic stamp in FIG. 4 in only one way. Instead of Serial Number Field 40, the electronic stamp has a Token Field 48. Token Field 48 holds the electronic token from the bank. The description and operation of the apparatus in this embodiment are identical to the description and operation of the apparatus in its preferred embodiment, except for the following differences: In step (g) of the operation of the receiver side apparatus, Analysis Module 23 reads the electronic token from Token Field 48. It then checks History Log 25 to see if this electronic token has been received before. If the electronic token is found in History Log 25, this is a good indication that the electronic mail has been delivered multiple times, either due to a fault in the electronic mail system or due to malicious intent. In either case, the electronic stamp and the associated electronic mail should be rejected. In step (b) of the operation of the sender side apparatus, Control Module 31 requests an electronic token from the bank. It places the electronic token in Token Field 48 of the electronic stamp. The electronic token can be implemented with any one of the several electronic cash systems currently available. Systems include (but are not limited to) Ecash.TM. by DigiCash, Inc. and CyberCoin.TM. by CyberCash, Inc. CONCLUSIONS, RAMIFICATIONS, AND SCOPE The foregoing description has disclosed to those skilled in the computer and networking arts how an electronic stamp is generated and used. It has disclosed how it is attached to and sent along with a piece of electronic mail. It has disclosed how it is removed from and used to filter a piece of electronic mail. The foregoing description has shown how the above methods are used as part of a novel electronic mail filtering apparatus for filtering junk electronic mail. While the foregoing description included many specificities, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of a preferred embodiment thereof. Many additional variations are possible, including the following: 1. The invention in its preferred embodiment uses symmetric key cryptography to protect the electronic stamp while the electronic mail is in transit. In environments where access to the network or transport medium is limited to trusted parties, this is sufficient. However, in an environment where the trustworthiness of the parties having access is unknown, public key cryptography would be a more appropriate solution. 2. If the invention is part of a system where the network or transport medium is already secure, or information sent across the medium is transparently encrypted, then the encryption steps in the invention are redundant and can be removed. 3. The invention in its preferred embodiment is designed to reduce or eliminate junk electronic mail. The invention is also suitable for use in any electronic messaging system or medium in which it is desirable to reduce or eliminate junk. Such systems include (but are not limited to) FAX, audio, video, USENET, or BBS systems. 4. The invention in its preferred embodiment simply discards rejected electronic mail. In some cases it might be better to notify the sender when this occurs, or to return the original electronic mail and attached electronic stamp. 5. The invention in its preferred embodiment passes acceptable electronic mail on to an external mail transport system for delivery and discards unacceptable electronic mail. The system could also prioritize and/or expedite acceptable electronic mail. The invention could be extended to assign a priority to a piece of electronic mail based on the value of the associated electronic stamp. User agent software could then present the electronic mail to the user sorted according to that priority or otherwise expedite the delivery of the electronic mail. 6. The invention in its preferred embodiment utilizes electronic stamps that are based on money. Electronic stamps, however, do not have to be based on money. They can be based on any limited resource with some perceived value to both the sender and the receiver. They could even be implemented as tokens given in limited quantities to each user. 7. The invention in its preferred embodiment is symmetrical--that is, users can both send stamped electronic mail and receive stamped electronic mail. It is possible, however, that a user external to the system may desire to send electronic mail to a user internal to the system. In such a case, a stamping service could be set up that would stamp electronic mail and send it for a fee. 8. The invention in its preferred embodiment appends electronic stamps to the body of the electronic mail. This was done merely as a matter of convenience. An electronic stamp may be appended, prepended, or stored in the header information of the electronic mail (in electronic mail systems that permit user defined headers), among other places. Accordingly, the foregoing description is to be understood as being in every respect illustrative and exemplary, but not restrictive. And that the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.
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