gnu.crypto.mac
Class UHash32

java.lang.Object
  |
  +--gnu.crypto.mac.BaseMac
        |
        +--gnu.crypto.mac.UHash32

All Implemented Interfaces:

java.lang.Cloneable, IMac

public class UHash32

extends BaseMac

UHASH is a keyed hash function, which takes as input a string of arbitrary length, and produces as output a string of fixed length (such as 8 bytes). The actual output length depends on the parameter UMAC-OUTPUT-LEN.

UHASH has been shown to be epsilon-ASU ("Almost Strongly Universal"), where epsilon is a small (parameter-dependent) real number. Informally, saying that a keyed hash function is epsilon-ASU means that for any two distinct fixed input strings, the two outputs of the hash function with a random key "look almost like a pair of random strings". The number epsilon measures how non-random the output strings may be.

UHASH has been designed to be fast by exploiting several architectural features of modern commodity processors. It was specifically designed for use in UMAC. But UHASH is useful beyond that domain, and can be easily adopted for other purposes.

UHASH does its work in three layers. First, a hash function called NH is used to compress input messages into strings which are typically many times smaller than the input message. Second, the compressed message is hashed with an optimized polynomial hash function into a fixed-length 16-byte string. Finally, the 16-byte string is hashed using an inner-product hash into a string of length WORD-LEN bytes. These three layers are repeated (with a modified key) until the outputs total UMAC-OUTPUT-LEN bytes.

References:

1. UMAC: Message Authentication Code using Universal Hashing.
   T. Krovetz, J. Black, S. Halevi, A. Hevia, H. Krawczyk, and P. Rogaway.

Field Summary

Fields inherited from class gnu.crypto.mac.BaseMac

name, truncatedSize, underlyingHash

Fields inherited from interface gnu.crypto.mac.IMac

MAC_KEY_MATERIAL, TRUNCATED_SIZE

Constructor Summary

UHash32()
Trivial 0-arguments constructor.

Method Summary

java.lang.Object	clone() Returns a clone copy of this instance.
byte[]	digest() Completes the MAC by performing final operations such as padding and resetting the instance.
void	init(java.util.Map attributes) Initialises the algorithm with designated attributes.
int	macSize() Returns the output length in bytes of this MAC algorithm.
void	reset() Resets the algorithm instance for re-initialisation and use with other characteristics.
boolean	selfTest() A basic test.
void	update(byte b) Continues a MAC operation using the input byte.
void	update(byte[] b, int offset, int len) Continues a MAC operation, by filling the buffer, processing data in the algorithm's MAC_SIZE-bit block(s), updating the context and count, and buffering the remaining bytes in buffer for the next operation.

Methods inherited from class gnu.crypto.mac.BaseMac

name

Methods inherited from class java.lang.Object

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

Constructor Detail

UHash32

public UHash32()

Trivial 0-arguments constructor.

Method Detail

clone

public java.lang.Object clone()

Description copied from interface: IMac

Returns a clone copy of this instance.

Specified by:

clone in interface IMac

Specified by:

clone in class BaseMac

Returns:

a clone copy of this instance.

macSize

public int macSize()

Description copied from interface: IMac

Returns the output length in bytes of this MAC algorithm.

Specified by:

macSize in interface IMac

Overrides:

macSize in class BaseMac

Returns:

the output length in bytes of this MAC algorithm.

init

public void init(java.util.Map attributes)
          throws java.security.InvalidKeyException,
                 java.lang.IllegalStateException

Description copied from interface: IMac

Initialises the algorithm with designated attributes. Permissible names and values are described in the class documentation above.

Specified by:

init in interface IMac

Specified by:

init in class BaseMac

Parameters:

attributes - a set of name-value pairs that describe the desired future instance behaviour.

Throws:

java.lang.IllegalStateException - if the instance is already initialised.

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

See Also:

IMac.MAC_KEY_MATERIAL

update

public void update(byte b)

Description copied from interface: IMac

Continues a MAC operation using the input byte.

Specified by:

update in interface IMac

Overrides:

update in class BaseMac

Parameters:

b - the input byte to digest.

update

public void update(byte[] b,
                   int offset,
                   int len)

Description copied from interface: IMac

Continues a MAC operation, by filling the buffer, processing data in the algorithm's MAC_SIZE-bit block(s), updating the context and count, and buffering the remaining bytes in buffer for the next operation.

Specified by:

update in interface IMac

Overrides:

update in class BaseMac

Parameters:

b - the input block.

offset - start of meaningful bytes in input block.

len - number of bytes, in input block, to consider.

digest

public byte[] digest()

Description copied from interface: IMac

Completes the MAC by performing final operations such as padding and resetting the instance.

Specified by:

digest in interface IMac

Specified by:

digest in class BaseMac

Returns:

the array of bytes representing the MAC value.

reset

public void reset()

Description copied from interface: IMac

Resets the algorithm instance for re-initialisation and use with other characteristics. This method always succeeds.

Specified by:

reset in interface IMac

Overrides:

reset in class BaseMac

selfTest

public boolean selfTest()

Description copied from interface: IMac

A basic test. Ensures that the MAC of a pre-determined message is equal to a known pre-computed value.

Specified by:

selfTest in interface IMac

Specified by:

selfTest in class BaseMac

Returns:

true if the implementation passes a basic self-test. Returns false otherwise.