Cryptography
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Cryptography - TOC

RC5


    The RC5 block cipher has a word-oriented architecture for variable word sizes w = 16, 32, or 64 bits. It has an extremely compact description, and is suitable for hardware or software. The number of rounds r and the key byte-length b are also variable. It is successively more completely identified as RC5-w, RC5-w/r, and RC5-w/r/b. RC5-32/12/16 is considered a common choice of parameters; r = 12 rounds are recommended for RC5-32, and r = 16 for RC5-64.


    Algorithm specifies RC5. Plaintext and ciphertext are blocks of bitlength 2w. Each of r rounds updates both w-bit data halves, using 2 subkeys in an input transformation and 2 more for each round. The only operations used, all on w-bit words, are addition mod
2w (), XOR (), and rotations (left and right ). The XOR operation is linear, while the addition may be considered nonlinear depending on the metric for linearity. The data dependent rotations featured in RC5 are the main nonlinear operation used: x y denotes cyclically shifting a w-bit word left y bits; the rotation-count y may be reduced mod w (the low-order lg(w) bits of y suffice). The key schedule expands a key of b bytes into 2r + 2 subkeys Ki of w bits each. Regarding packing/unpacking bytes into words, the byte-order is little-endian: for w = 32, the first plaintext byte goes in the low-order end of A, the fourth in A's high-order end, the fifth in B's low order end, and so on.

RC5 Algorithm

RC5 Key Schedule


    RC5 decryption: Decryption uses the Algorithm subkeys, operating on ciphertext
C = (A,B) as follows (subtraction is mod 2w, denoted ). For i from r down to 1 do:
B ((B K2i+1) A) A, A ((A K2i) B)B. Finally M (A K0;B K1).

RC5 Constants

    RC5-32/12/16 test vectors: For the hexadecimal plaintext M = 65C178B2 84D197CC and key K = 5269F149 D41BA015 2497574D 7F153125, RC5 with w = 32, r = 12, and b = 16 generates ciphertext C = EB44E415 DA319824.


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