gnu.crypto.cipher
public class Cast5 extends BaseCipher
An implmenetation of the CAST5
(a.k.a. CAST-128) algorithm,
as per RFC-2144, dated May 1997.
In this RFC, Carlisle Adams (the CA in CAST, ST stands for Stafford Tavares) describes CAST5 as:
"...a DES-like Substitution-Permutation Network (SPN) cryptosystem which appears to have good resistance to differential cryptanalysis, linear cryptanalysis, and related-key cryptanalysis. This cipher also possesses a number of other desirable cryptographic properties, including avalanche, Strict Avalanche Criterion (SAC), Bit Independence Criterion (BIC), no complementation property, and an absence of weak and semi-weak keys."
CAST5
is a symmetric block cipher with a block-size of 8
bytes and a variable key-size of up to 128 bits. Its authors, and their
employer (Entrust Technologies, a Nortel majority-owned company), made it
available worldwide on a royalty-free basis for commercial and non-commercial
uses.
The CAST5
encryption algorithm has been designed to allow a
key size that can vary from 40
bits to 128
bits,
in 8-bit increments (that is, the allowable key sizes are 40, 48, 56,
64, ..., 112, 120,
and 128
bits. For variable keysize
operation, the specification is as follows:
80
bits (i.e.,
40, 48, 56, 64, 72,
and 80
bits), the algorithm
is exactly as specified but uses 12
rounds instead of
16
;80
bits, the algorithm uses
the full 16
rounds;128
bits, the key is padded with
zero bytes (in the rightmost, or least significant, positions) out to
128
bits (since the CAST5
key schedule assumes
an input key of 128
bits).References:
Version: $Revision: 1.3 $
Constructor Summary | |
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Cast5() Trivial 0-arguments constructor. |
Method Summary | |
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Iterator | blockSizes() |
Object | clone() |
void | decrypt(byte[] in, int i, byte[] out, int j, Object k, int bs) |
void | encrypt(byte[] in, int i, byte[] out, int j, Object k, int bs) The full encryption algorithm is given in the following four steps. INPUT: plaintext m1...m64; key K = k1...k128. |
Iterator | keySizes() |
Object | makeKey(byte[] uk, int bs) |
boolean | selfTest() |
The full encryption algorithm is given in the following four steps.
INPUT: plaintext m1...m64; key K = k1...k128. OUTPUT: ciphertext c1...c64.
Decryption is identical to the encryption algorithm given above, except that the rounds (and therefore the subkey pairs) are used in reverse order to compute (L0,R0) from (R16,L16).
Looking at the iterations/rounds in pairs we have:
(1a) Li = Ri-1; (1b) Ri = Li-1 ^ Fi(Ri-1); (2a) Li+1 = Ri; (2b) Ri+1 = Li ^ Fi+1(Ri);which by substituting (2a) in (2b) becomes
(2c) Ri+1 = Li ^ Fi+1(Li+1);by substituting (1b) in (2a) and (1a) in (2c), we get:
(3a) Li+1 = Li-1 ^ Fi(Ri-1); (3b) Ri+1 = Ri-1 ^ Fi+1(Li+1);Using only one couple of variables L and R, initialised to L0 and R0 respectively, the assignments for each pair of rounds become:
(4a) L ^= Fi(R); (4b) R ^= Fi+1(L);
Parameters: in contains the plain-text 64-bit block. i start index within input where data is considered. out will contain the cipher-text block. j index in out where cipher-text starts. k the session key object. bs the desired block size.