CS 161 Introduction to Cryptography Symmetric Cryptography 11 September 2006 2006 Doug Tygar 1 CS 161 11 September 2006 Cryptography History Gallic Wars to WW2 Enigma Purple Ciphers vs codes Cryptology Cryptography making ciphers Cryptanalysis breaking ciphers Traffic analysis watching patterns of communications Need communications can be tapped Building block for cryptographic protocols In the US National Security Agency 2006 Doug Tygar 2 CS 161 11 September 2006 1 Notation Ciphertext Encryption Plaintext encryption Key sometimes we use cleartext instead of plaintext Key Keyspace Keysize log2 Keyspace c E m k or c Ek m or c m k Also Plaintext Decryption Ciphertext decryption Key encyption Key decryption Key symmetric encyption Key decryption Key asymmetric m D c k E 1 c k or c Dk m 2006 Doug Tygar 3 CS 161 11 September 2006 Attacks on cryptography Direct attack example exhaustive search Known plaintext Chosen plaintext Usual assumptions chosen plaintext attack attacker knows E D but not key 2006 Doug Tygar 4 CS 161 11 September 2006 2 Perfect cryptosystem One time pad Share a common key key size message size XOR key with message No information at all is leaked Why What problem does this system have 2006 Doug Tygar 5 CS 161 11 September 2006 DES Origins mid 70s History Lucifer NIST NSA 56 bit key 64 bit block cipher Differential cryptanalysis Exhaustive search AES Rijndael 128 256 bit key 128 bit block cipher 2006 Doug Tygar 6 CS 161 11 September 2006 3 Symmetric crypto Advantages Fast Reasonably well understood Standardized Can be implemented in hardware easily Exhaustive search attack hard with large key size Disadvantages Key distribution Single target Still needs to be implemented in protocols 2006 Doug Tygar 7 CS 161 11 September 2006 4
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