Stream Ciphers and Block Ciphers

by Todd Sundsted

August 2, 2002 —

A stream cipher is a symmetric (secret key) cipher that operates on
small units of data (as small as a single bit) at a time. Stream ciphers
are a practical attempt to emulate the cryptographically attractive
features of a one-time pad. A stream cipher generates a keystream (a
sequence of bits) using the secret key as a seed. The keystream is
combined with the plaintext, typically via a simple operations such as
an exclusive-or, to generate the ciphertext.

Common stream ciphers with Java implementations include RC4 (one of the
most well known) and SEAL (written by Michael Roe and described in
Applied Cryptography).

A block cipher is a symmetric cipher that operates on a fixed size block
of bytes at one time. Block ciphers transform plaintext messages into
ciphertext messages block by block. The transformation is secured by
secret key. Applying the reverse transformation using the same secret
key performs decryption.

Blocks ciphers are more popular that steam ciphers. Common block ciphers
with Java implementations include DES and AES (both NIST FIPS
standards), Blowfish, IDEA, Skipjack, RC2 and RC5.

ITworld