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IT Security 1 Symmetric Encryption (Knowledge of Attacker (Cipher-text…
IT Security 1
Symmetric Encryption
Knowledge of Attacker
Cipher-text only attack
Known plain-text attack
knows some P-C pairs
Chosen plain-text attack
can obtain ciphertext for any plaintext of choice
Chosen ciphertext attack
can decrypt any ciphertext except target before target is known
Adaptive chosen ciphertext attack
can decrypt any ciphertext chosen adaptively (depending on target and the result of previous ciphertexts)
Chosen key attack
For any chosen P, the attacker can query an oracle returning E{K1}(P), E{K2}(P), ... where he can either
choose the relation between the keys
know the relation between the keys
The attacker then tries to recover the one/more of the key(s)
Attacks against ciphers
Brute force/Exhaustive key search
Ciphertext only attack
Time-memory tradeoff attack
Differential Cryptoanalysis
Chosen plaintext attack
Algebraic Cryptoanalysis
Block Ciphers
DES
weakness: key length of 54
3DES
Two Types
3-key 3DES gives 112 key len
2-key 3DES ie K1 = K2 gives 80 key len
AES
more secure and efficient than 3DES
128, 192, 256 key lengths
Twofish
Mars
Serpent
Qualities of a secure encryption algo
Confusion
Diffusion
Encrypting a large message
no integrity protection
Electronic Code Book (ECB)
BLOCK
Cipher Block Chain (CBC)
BLOCK
Cipher Feedback Mode (CFB)
BLOCK
Output Feedback Mode (OFB)
STREAM
Counter Mode (CTR)
STREAM
