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>Hybrid ciphers</A
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>Public-key ciphers are no panacea.
Many symmetric ciphers are stronger from a security standpoint,
and public-key encryption and decryption are more expensive than the
corresponding operations in symmetric systems.
Public-key ciphers are nevertheless an effective tool for distributing
symmetric cipher keys, and that is how they are used in hybrid cipher
systems.</P
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>A hybrid cipher uses both a symmetric cipher and a public-key cipher.
It works by using a public-key cipher to share a key for the symmetric
cipher.
The actual message being sent is then encrypted using the key and sent
to the recipient.
Since symmetric key sharing is secure, the symmetric key used is different
for each message sent.
Hence it is sometimes called a session key.</P
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>Both PGP and GnuPG use hybrid ciphers.
The session key, encrypted using the public-key cipher, and the message
being sent, encrypted with the symmetric cipher, are automatically
combined in one package.
The recipient uses his private-key to decrypt the session key and the
session key is then used to decrypt the message.</P
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>A hybrid cipher is no stronger than the public-key cipher or symmetric
cipher it uses, whichever is weaker.
In PGP and GnuPG, the public-key cipher is probably the weaker of
the pair.
Fortunately, however, if an attacker could decrypt a session key it
would only be useful for reading the one message encrypted with that
session key.
The attacker would have to start over and decrypt another session
key in order to read any other message.</P
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