Now that we understand how the public/private key
algorithms work, it??™s easy to see the mechanism behind signatures.
Let??™s assume that Alice wants from Bob a statement of
some sort, like a contract; let??™s also assume that Alice wants the
Alice
Eve
Bob
Bob
Bob
Bob
Bob??™s Public Key
Bob??™s Private Key
Encryption Operation
Bob
E
Bob E
Figure 1-5 Alice uses public key cryptography for sending a secure
message to Bob. Eve can??™t access the data exchanged.
Digital signatures
prevent illicit
modi?¬?cations of the
signed data and can
certify the source of
the signature
The Babel of Cryptography 43
guarantee that Bob will not deny the content of the statement at
a later time (a guarantee known as nonrepudiation). Bob can
reuse the same asymmetric keys infrastructure we introduced
before. The statement, a fragment of data, can be reduced to a
smaller string by a function known as thumbprint. The
thumbprint of a data fragment is a bit like a printout of your
DNA. It still uniquely identi?¬?es you, but it??™s much cheaper to
send via mail than to actually send your entire body via courier.
The thumbprint can then be encrypted with Bob??™s private key.
The result is the signature, and it can be attached to the statement.
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