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Chapter 3 Cryptography 59
Cryptography Overview
4. Digest the message file.
5. If the digest matches the 16 bytes you obtained from decrypting the original 96-
byte block, the message is verified. That is, you can assume the 96-byte block is
the file’s digest encrypted with the RSA private key associated with the public
key you used. It would have been computationally infeasible to produce that 96-
byte block any other way.
There are other uses for a digital signature. Suppose that Bob wishes to buy
something from Alice over the Internet. He e-mails her a credit card number. Alice
can easily find out from the credit card issuer that the number she received is valid
and indeed belongs to Bob. But how does she know that it was Bob who sent the
number and not someone posing as Bob? She sends the purchaser a randomly
generated message and asks him to digitally sign it with his private key. She then
retrieves his public key from a certification authority and verifies the signature. Only
the person with access to Bob’s private key will be able to generate a digital signature
from the message she generated in such a way that Bob’s public key will verify it
properly. In this way, Alice authenticates Bob’s identity.
Figure 3-11 RSA Digital Signature
Message
Digest
RSA Private
Encryption
Signature
Private Key
Signature Operation
Original
Message
Message
Digest
Signature
Valid
Original
Message
RSA Public
Decryption
Signature
Not Valid
Public Key
Verification Operation
Signature
YES
NO
EQUAL?