Cryptography Reference
In-Depth Information
using the public key
d
to recover the digest, which he can
also compute independently from the message. If the two
agree, he must conclude that
A
originated the cipher,
since only
A
knew
e
and hence could have encrypted the
message.
Thus far, all proposed two-key cryptosystems exact a
very high price for the separation of the privacy or secrecy
channel from the authentication or signature channel.
The greatly increased amount of computation involved
in the asymmetric encryption/decryption process sig-
nificantly cuts the channel capacity (bits per second of
message information communicated). As a result, the
main application of two-key cryptography is in hybrid
systems. In such a system a two-key algorithm is used for
authentication and digital signatures or to exchange a ran-
domly generated session key to be used with a single-key
algorithm at high speed for the main communication. At
the end of the session this key is discarded.
block and sTream ciphers
In general, cipher systems transform fixed-size pieces of
plaintext into ciphertext. In older manual systems these
pieces were usually single letters or characters—or occa-
sionally, as in the Playfair cipher, digraphs, since this was as
large a unit as could feasibly be encrypted and decrypted by
hand. Systems that operated on trigrams or larger groups
of letters were proposed and understood to be potentially
more secure, but they were never implemented because
of the difficulty in manual encryption and decryption. In
modern single-key cryptography the units of information
are often as large as 64 bits, or about 13
1
/
2
alphabetic char-
acters, whereas two-key cryptography based on the RSA
