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Digital Signatures
These extend the basic Public Key authentication protocols to
documents or messages. The three
key objectives are:
- The receiver can verify the claimed identity of the sender,
because only the sender's public key will decrypt it.
- The sender cannot later repudiate the contents of the message,
because only the possessor of the specific private key could have
generated it.
- The receiver cannot possibly have concocted the message
himself.
The recipient's public key can (optionally) be
used to encrypt the message, so that only the recipient can read
it. This step is only necessary if both
authentication and secrecy are
needed. We can take a plaintext message P, and encode it thus:
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