•A hash – also known as a message digest. A hash is the output of a one way function that attempts to ensure the integrity of the message (i.e., that the message has not been altered). It is usually combined with authentication information to ensure that the message originator can be authenticated and that the integrity of the message has not been disrupted. You can think of a hash like an advanced checksum or an advanced cyclic redundancy check (CRC).
Let’s cover hashes and digital signatures first. We’ll assume that Jack wants to send John a message. Jack wants to make sure that John knows the message came from him and that the message was not altered in transit. However, Jack doesn’t care about confidentiality – in other words, the actual message can be sent “in the clear” – but does care about authentication and integrity. We can accomplish this through hashes and digital signatures as shown in Figure 17.
Figure 17 - Digital Signature
In Figure 17, Jack has sent John a message with a digital signature. Let’s see how John would validate this message to make sure it came from Jack and was not altered. Refer to Figure 18.
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