Cryptography Tutorials - Herong's Tutorial Examples - v5.40, by Dr. Herong Yang
What Is Block Cipher?
This section describes what is block cipher - An encryption scheme in which 'the clear text is broken up into blocks of fixed length, and encrypted one block at a time'.
Block Cipher - An encryption scheme in which "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:
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:
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.
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