gnu.crypto.cipher
Class Cast5

java.lang.Object
  gnu.crypto.cipher.BaseCipher
      gnu.crypto.cipher.Cast5

All Implemented Interfaces:

java.lang.Cloneable, IBlockCipher, IBlockCipherSpi

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:

1. For key sizes up to and including 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;
2. For key sizes greater than 80 bits, the algorithm uses the full 16 rounds;
3. For key sizes less than 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:

1. The CAST-128 Encryption Algorithm.
   Carlisle Adams.

Version:

$Revision: 1.2 $

Field Summary

Fields inherited from class gnu.crypto.cipher.BaseCipher

currentBlockSize, currentKey, defaultBlockSize, defaultKeySize, lock, name

Fields inherited from interface gnu.crypto.cipher.IBlockCipher

CIPHER_BLOCK_SIZE, KEY_MATERIAL

Constructor Summary

Cast5()
Trivial 0-arguments constructor.

Method Summary

java.util.Iterator	blockSizes() Returns an Iterator over the supported block sizes.
java.lang.Object	clone() Returns a clone of this instance.
void	decrypt(byte[] in, int i, byte[] out, int j, java.lang.Object k, int bs) Decrypts exactly one block of ciphertext.
void	encrypt(byte[] in, int i, byte[] out, int j, java.lang.Object k, int bs) The full encryption algorithm is given in the following four steps.
java.util.Iterator	keySizes() Returns an Iterator over the supported key sizes.
java.lang.Object	makeKey(byte[] uk, int bs) Expands a user-supplied key material into a session key for a designated block size.
boolean	selfTest() A correctness test that consists of basic symmetric encryption / decryption test(s) for all supported block and key sizes, as well as one (1) variable key Known Answer Test (KAT).

Methods inherited from class gnu.crypto.cipher.BaseCipher

currentBlockSize, decryptBlock, defaultBlockSize, defaultKeySize, encryptBlock, init, name, reset, testKat, testKat

Methods inherited from class java.lang.Object

equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Cast5

public Cast5()

Trivial 0-arguments constructor.

Method Detail

clone

public java.lang.Object clone()

Description copied from interface: IBlockCipher

Returns a clone of this instance.

Specified by:

clone in interface IBlockCipher

Specified by:

clone in class BaseCipher

blockSizes

public java.util.Iterator blockSizes()

Description copied from interface: IBlockCipher

Returns an Iterator over the supported block sizes. Each element returned by this object is an Integer.

Returns:

an Iterator over the supported block sizes.

keySizes

public java.util.Iterator keySizes()

Description copied from interface: IBlockCipher

Returns an Iterator over the supported key sizes. Each element returned by this object is an Integer.

Returns:

an Iterator over the supported key sizes.

makeKey

public java.lang.Object makeKey(byte[] uk,
                                int bs)
                         throws java.security.InvalidKeyException

Description copied from interface: IBlockCipherSpi

Expands a user-supplied key material into a session key for a designated block size.

Parameters:

uk - the user-supplied key material.

bs - the desired block size in bytes.

Returns:

an Object encapsulating the session key.

Throws:

java.security.InvalidKeyException - if the key data is invalid.

encrypt

public void encrypt(byte[] in,
                    int i,
                    byte[] out,
                    int j,
                    java.lang.Object k,
                    int bs)

The full encryption algorithm is given in the following four steps.

    INPUT:  plaintext m1...m64; key K = k1...k128.
    OUTPUT: ciphertext c1...c64.
 

1. (key schedule) Compute 16 pairs of subkeys {Kmi, Kri} from a user key (see makeKey() method).
2. (L0,R0) <-- (m1...m64). (Split the plaintext into left and right 32-bit halves L0 = m1...m32 and R0 = m33...m64.).
3. (16 rounds) for i from 1 to 16, compute Li and Ri as follows:
   • Li = Ri-1;
   • Ri = Li-1 ^ F(Ri-1,Kmi,Kri), where F is defined in method F() -- f is of Type 1, Type 2, or Type 3, depending on i, and ^ being the bitwise XOR function.
4. c1...c64 <-- (R16,L16). (Exchange final blocks L16, R16 and concatenate to form the ciphertext.)

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.

decrypt

public void decrypt(byte[] in,
                    int i,
                    byte[] out,
                    int j,
                    java.lang.Object k,
                    int bs)

Description copied from interface: IBlockCipherSpi

Decrypts exactly one block of ciphertext.

Parameters:

in - the ciphertext.

i - index of in from which to start considering data.

out - the plaintext.

j - index of out from which to store the result.

k - the session key to use.

bs - the block size to use.

selfTest

public boolean selfTest()

Description copied from interface: IBlockCipher

A correctness test that consists of basic symmetric encryption / decryption test(s) for all supported block and key sizes, as well as one (1) variable key Known Answer Test (KAT).

Specified by:

selfTest in interface IBlockCipher

Overrides:

selfTest in class BaseCipher