More About DES

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

2. Block CipherModes 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)

3. 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

4. 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

5. 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

6. 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

7. DES Properties • Reference: Handbook of Applied Cryptography by Menezes, van Oorschot and Vanstone (pp. 256-259) • Desirable characteristics • Dependence of a ciphertext bit on all key and plaintext bits • Bit changes in plaintext, key, or ciphertext have unpredictable consequences

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