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Message Digests

The digital signature method of the previous slide creates an encrypted message. It's often preferable to leave the message as plaintext, but append a signature which verifies its integrity. A message digest is a one-way hash function which has the following characteristics:
 

 
The Internet standard for message digests is the MD5 algorithm, invented by Rivest. Software implementations of this algorithm are widely available. MD5 produces a 128 bit (16 byte) message digest which can be appended to the message. Message digests such as MD5 are often referred to as cryptograhic checksums, because they reveal whether the message has been altered.
 
Typical usage of message digests combines public key cryptography with the message digest function. In this case, a sender first computes an MD5 digest as above, then encrypts it using her private key, and finally appends the encrypted digest to the message. A recipient can read the message, and can be confident that it originated from the sender.
 


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