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Message Digests
A criticism of previous signature methods (such as that given on
the previous slide) is that they combine both authentication and
secrecy. If authentication only is desired, a message
digest is a one-way hash function which has the following
characteristics:
- Given
P, it is easy to calculate
MD(P).
- Given
MD(P), it is effectively impossible
to find P.
- No one can generate two messages that have the same
MD(P).
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.
It is also possible to use public key cryptography to implement
the message digest function. In this case, a sender first computes
a message digest as above, then encrypts it using her
private key, then finally appends this to the
message as sent. A recipient can read the message, and can be
confident that it originated from the sender.
We shall see more applications of Digital Signatures, and Public
Key Crypto in general, when we look at technical aspects of
E-Commerce a little later in the subject.
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