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Cipher - DES Algorithm

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This chapter describes:

• What is block cipher.
• The DES (Data Encryption Standard) cipher algorithm.
• The DES key schedule algorithm.
• The DES decryption algorithm

Block Cipher

Block Cipher - An encryption scheme that "the clear text is broken up into blocks of fixed length, and encrypted one block at a time".

Usually, a block cipher encrypts a block of clear text into a block of cipher text of the same length. In this case, a block cipher can be viewed as a simple substitute cipher with character size equal to the block size.

ECB Operation Mode - Blocks of clear text are encrypted independently. ECB stands for Electronic Code Book. Main properties of this mode:

• Identical clear text blocks are encrypted to identical cipher text blocks.
• Re-ordering clear text blocks results in re-ordering cipher text blocks.
• An encryption error affects only the block where it occurs.

CBC Operation Mode - The previous cipher text block is XORed with the clear text block before applying the encryption mapping. Main properties of this mode:

• An encryption error affects only the block where is occurs and one next block.

Product Cipher - An encryption scheme that "uses multiple ciphers in which the cipher text of one cipher is used as the clear text of the next cipher". Usually, substitution ciphers and transposition ciphers are used alternatively to construct a product cipher.

Iterated Block Cipher - A block cipher that "iterates a fixed number of times of another block cipher, called round function, with a different key, called round key, for each iteration".

Feistel Cipher - An iterate block cipher that uses the following algorithm:

Input: 
   T: 2t bits of clear text
   k1, k2, ..., kr: r round keys
   f: a block cipher with bock size of t
   
Output: 
   C: 2t bits of cipher text

Algorithm: 
   (L0, R0) = T, dividing T in two t-bit parts 
   (L1, R1) = (R0, L0 ^ f(R0, k1))
   (L2, R2) = (R1, L1 ^ f(R1, k2))
   ......
   C = (Rr, Lr), swapping the two parts

^ is the XOR operation.

DES Cipher - A 16-round Feistel cipher with block size of 64 bits. DES stands for Data Encryption Standard.

DES (Data Encryption Standard) Cipher Algorithm

DES Cipher - A 16-round Feistel cipher with block size of 64 bits. DES stands for Data Encryption Standard.

DES was developed by IBM in 1974 in response to a federal government public invitation for data encryption algorithms. In 977, DES was published as a federal standard, FIPS PUB 46.

(Continued on next part...)

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