Cryptography Reference
In-Depth Information
algorithm appears to have settled on 1,024 to 2,048 bits,
or between roughly 310 and 620 alphabetic characters, as
the unit of encryption.
A block cipher breaks the plaintext into blocks of the
same size for encryption using a common key: the block
size for a Playfair cipher is two letters, and for the DES
used in electronic codebook mode it is 64 bits of binary-
encoded plaintext. While a block could consist of a single
symbol, normally it is larger.
A stream cipher also breaks the plaintext into units,
normally of a single character, and then encrypts the
i
th
unit of the plaintext with the i
th
unit of a key stream.
Vernam encryption with a onetime key is an example of
such a system, as are rotor cipher machines and the DES
used in the output feedback mode (in which the cipher-
text from one encryption is fed back in as the plaintext
for the next encryption) to generate a key stream. Stream
ciphers depend on the receiver's using precisely the same
part of the key stream to decrypt the cipher that was
employed to encrypt the plaintext. They thus require that
the transmitter's and receiver's key-stream generators be
synchronized. This means that they must be synchronized
initially and stay in sync thereafter, or else the cipher will
be decrypted into a garbled form until synchrony can be
reestablished. This latter property of self-synchronizing
cipher systems results in what is known as error propaga-
tion, an important parameter in any stream-cipher system.
