Importance

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Abstract

A method and apparatus whereby the senders and receivers of messages sent over a transmission system including a Host CPU may guarantee the integrity of the data content of the message and also the absolute identity of the sender. Each user of the system as well as the Host CPU contains an identical key-controlled block-cipher cryptographic device with data chaining for encrypting and decrypting messages as required, wherein each user has knowledge of only his own cryptographic key and wherein the Host CPU has access to the unique cryptographic keys of all users of the system stored in a high security storage area available only to said CPU. Stated very generally, the originator of a message A sends a message to a receiver B which includes a transaction or message portion X and a unique digital signature portion Y which is a function both of the message and the senders unique cryptographic key K.sub.A. The receiver then communicates with the CPU for verification of the signature Y. The CPU accesses the sender's key K.sub.A from a secure memory and computes the digital signature Y utilizing the message portion X received from B and the key K.sub.A. Upon a successful verification of the signatures by the CPU, the CPU notifies B via an additional message that the signature of A is valid based on the data content of the message and the key K.sub.A. Based on the information received from the CPU, B may be certain that the signature and message originated with A and A may not later deny having sent the message as it would be virtually impossible for the signature to be forged since it is a complex function of the message content itself. A may also be assured that B cannot alter the message as the signature would no longer be valid. According to other aspects of the invention the interrupting of communications between A and B by an eavesdropper and the subsequent sending of stale messages is prevented. As a still further feature of the invention, an eavesdropper is prevented from sending the 'forged' approval from the CPU to the receiver B.

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First Claim