Lecture_3

Stream Ciphers
Encrypts a digital data stream one bit or one byte at a time
One time pad is example; but practical limitations
Typical approach for stream cipher:
Key (K) used as input to bit-stream generator algorithm
Algorithm generates cryptographic bit stream (ki ) used to encrypt plaintext
Users share a key; use it to generate keystream
Block Ciphers
Encrypt a block of plaintext as a whole to produce same sized ciphertext
Typical block sizes are 64 or 128 bits
Modes of operation used to apply block ciphers to larger plaintexts
Ideal Block Cipher
n-bit input maps to 2n possible input states
Substitution used to produce 2n output states
Output states map to n-bit output
Ideal block cipher allows maximum number of possible encryption mappings from plaintext block
Problems with ideal block cipher:
Small block size: equivalent to classical substitution cipher; cryptanalysis based on statistical characteristics feasible
Large block size: key must be very large; performance/implementation problems