Block and Stream CiphersReferenceMatt Bishop, Computer Security,
Addison Wesley, 2003.
Block and Stream Ciphers
Block CiphersBlock ciphers break messages into fixed length
blocks, and encrypt each block using the same key.The Data
Encryption Standard (DES) is an example of a block cipher, where
blocks of 64 bits are encrypted using a 56-bit key.
Block and Stream Ciphers
Formal Definition of a Block CipherLet E be an encipherment
algorithm, and let Ek(b) be the encipherment of the message b with
key k. Let a message m=b1b2...where each bi is of a fixed length.A
block cipher is a cipher for which Ek(m) = Ek(b1)Ek(b2)...
Block and Stream Ciphers
Stream Ciphers Stream ciphers, like block ciphers, break message
into fixed length blocks, but use a sequence of keys to encrypt the
blocks.The Vigenre cipher is an example of a stream cipher.
Block and Stream Ciphers
Formal Definition of a Stream CipherLet E be an encipherment
algorithm, and let Ek(b) be the encipherment of the message b with
key k. Let a message m=b1b2...where each bi is of a fixed length,
and let k = k1k2....A stream cipher is a cipher for which Ek(m) =
Ek1 (b1)Ek2(b2)...
Block and Stream Ciphers
Synchronous Stream CiphersThe one-time pad is also a stream
cipher, and can be implemented by exclusive-oring each bit of the
key with one bit of the message.A synchronous stream cipher
generates the key string from a source other than the message
itself.A source for the key could come from an randomly generated
binary sequence.
Block and Stream Ciphers
Self-Synchronous Stream CiphersSelf-synchronous stream ciphers
obtain a key from the message itself.An autokey cipher is a
self-synchronous cipher that uses the message itself as a key.
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