Cryptography 3 Flashcards
for each key value, we want the block cipher to be a random permutation, and the different permutations for the different key values should be chosen independently.
Ideal Block Cipher
is generic if we can find a similar distinguisher for almost any block cipher. In the above case, the distinguisher is generic because we can construct one just like it for any block cipher. This “attack” would even allow us to distinguish between two ideal block ciphers.
distinguisher
for a given key, extract the permutation by encrypting all possible plaintexts. If the permutation is odd, we know that we have an ideal block cipher, because the real block cipher never generates an odd permutation. If the permutation is even, we claim to have a real block cipher.
Parity Attack
Virtually all block ciphers consist of several repetitions of a weak block cipher, known as a round. Several of these weak rounds in sequence make a strong block cipher.
Round info …
key size of 56 bits and small block size of 64 bits (plaintext). Consists of 16 rounds using each round a 48-bit round key from the 56-bit key and this selection is different each round. The algorithm that derives these round keys from the main block cipher key is called the key schedule. (Feistel Cipher)
DES
one 56-bit key, decrypt w/2nd 56-bit key and encrypt again w/1st key or 3rd 56-bit key.
3DES
consists of 10-14 rounds depending on key size; 10 rds for 128-bit, 12 rds for 192-bit, 14 rds for 256-bit keys.
AES
consists of 32 rounds, 128-bit round key. Slow compared to AES.
Serpent
also uses whitening. At both the start and the end of the cipher, additional key material is added to the data. This makes the cipher harder to attack for most types of attacks, and it costs very little.
Twofish
If two plaintext blocks are the same, then the corresponding ciphertext blocks will be identical, and that is visible to the attacker.
ECB issues ?
