More About DES Cryptography and Network Security Reference: Sec 3.1 of Stallings Text.

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Presentation transcript:

More About DES Cryptography and Network Security Reference: Sec 3.1 of Stallings Text

Block Cipher Modes of Operation Sec 3.7 of text How is DES used in applications? Input to encryption is not always the bits of the plaintext Modes Electronic Codebook (ECB) Cipher Block Chaining (CBC) Cipher Feedback (CFB) Output Feedback (OFB)

Electronic Codebook Break plaintext into 64-bit blocks Encoded each block independently using the same key Ideal for short amounts of data May not be secure for long messages Regularity of certain sequences/blocks may be used to analysis ciphertext

Cipher Block Chaining Mode Input to encryption is the plaintext XOR-ed with the ciphertext of preceding block Initialization Vector (IV) needed for first block (sent using ECB encryption) Identical plaintext blocks do not necessarily produce the same ciphertext block Repeating patterns are not exposed Useful for long messages; authentication

Cipher Feedback Mode Unit of transmission If plaintext is a stream of j-bit characters (e.g., 8 bits for ASCII characters), it is preferable to have output in j-bit pieces Encryption Start with IV (shift register), encrypt and select 8- bits XOR-ed with plaintext character to produce ciphertext character Use ciphertext character for next encryption step by shifting it into the shift register; repeat the process

Output Feedback Mode Similar to CFB but use the 8-bit output of encryption for the shift register Advantage: transmission errors do not propagate to other ciphertext characters

DES Properties Reference: Handbook of Applied Cryptography by Menezes, van Oorschot and Vanstone (pp ) Desirable characteristics Dependence of a ciphertext bit on all key and plaintext bits Bit changes in plaintext, key, or ciphertext have unpredictable consequences

Other Properties of the DES Complementation Weak keys DES is not a group Susceptibility to linear/differential cryptanalysis