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## cryptix.provider.mac
Class HMAC

    java.lang.Object
      |
      +--java.security.MessageDigestSpi
            |
            +--java.security.MessageDigest
                  |
                  +--cryptix.provider.mac.HMAC

Direct Known Subclasses:
  HMAC_HAVAL, HMAC_MD2, HMAC_MD4, HMAC_MD5, HMAC_RIPEMD128, HMAC_RIPEMD160, HMAC_SHA0, HMAC_SHA1

public class HMAC
extends MessageDigest
implements Parameterized, Cloneable

A class to implement the HMAC message authentication code, as described in RFC 2104.

The definition of HMAC-X for a given message digest algorithm X depends on that algorithm's internal block size, which is passed into the constructor when this class is used directly. Normally, this class will be obtained indirectly via the JCA API, e.g. `MessageDigest.getInstance("HMAC-SHA-1")` if SHA-1 is to be used as the digest algorithm.

The key can be set as a byte array using the "key" parameter. It is not reset after the MAC has been returned (this enables successive MACs using the same key to be calculated efficiently). For example:

        import java.security.Parameterized;

        MessageDigest hmac = MessageDigest.getInstance("HMAC-SHA-1");
        byte[] key = ..., input1 = ..., input2 = ...;
        ((Parameterized) mac).setParameter("key", key);
        byte[] mac1 = hmac.digest(input1);
        byte[] mac2 = hmac.digest(input2);

Parameters other than "key" are passed through to the MessageDigest object, but this can only be done when the key is not set. An explicit call to `reset()` will 'unset' the key.

This implementation does not support truncation of the output MAC. If truncation is desired, the caller should use only the relevant initial bits of the output array.

Note that although this class implements `Cloneable`, its `clone()` method will throw a `CloneNotSupportedException` if the underlying MessageDigest cannot be cloned. This should not be the case for any of the Cryptix-supported digest algorithms.

HMAC is similar, but not identical to the MAC used by Secure Sockets Layer version 3. The JCA standard algorithm name of the variant used in SSL v3 is "SSL3MAC-X", for a message digest name X.

References:

1. RFC 2104, HMAC: Keyed-Hashing for Message Authentication
2. RFC 2202, Test Cases for HMAC-MD5 and HMAC-SHA-1

Copyright © 1997 Systemics Ltd on behalf of the Cryptix Development Team.
All rights reserved.

$Revision: 1.3 $

Since:
  Cryptix 3.0.3
Author:
  David Hopwood

Constructor Summary
`HMAC(String mdAlgorithm, int mdBlockSize)` Constructs an HMAC object for the given MD algorithm name and block size.

Method Summary
`Object` | `clone()` Returns a copy of this HMAC object.
`protected byte[]` | `engineDigest()` Calculates the final MAC.
`protected int` | `engineGetDigestLength()` SPI: Returns the digest length in bytes.
`protected Object` | `engineGetParameter(String param)`
`protected void` | `engineReset()` Resets this object disregarding any temporary data present at the time of the invocation of this call.
`protected void` | `engineSetParameter(String param, Object value)`
`protected void` | `engineUpdate(byte input)` Continues an HMAC digest using the input byte.
`protected void` | `engineUpdate(byte[] input, int offset, int len)` Hashes a byte array from a given offset for a specified length.
`Object` | `getParameter(String param)` Gets the value of the specified algorithm parameter.
`void` | `setParameter(String param, Object value)` Sets the specified algorithm parameter to the specified value.

Methods inherited from class java.security.MessageDigest
`digest, digest, digest, getAlgorithm, getDigestLength, getInstance, getInstance, getProvider, isEqual, reset, toString, update, update, update`

Methods inherited from class java.security.MessageDigestSpi
`engineDigest`

Methods inherited from class java.lang.Object
`equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Constructor Detail

### HMAC

    public HMAC(String mdAlgorithm,
                int mdBlockSize)

  Constructs an HMAC object for the given MD algorithm name and block size.

  Do not rely on being able to use this constructor to obtain a subclass of MessageDigest; when Cryptix is changed to use the Java 1.2 provider architecture, it will be a subclass of MessageDigestSpi instead.

  Parameters:
    `mdAlgorithm` - the standard JCA algorithm name of the message digest to be used.
    `mdBlockSize` - the internal block size of the digest algorithm.

Method Detail

### clone

    public Object clone()
                 throws CloneNotSupportedException

  Returns a copy of this HMAC object.
  Overrides:
    clone in class MessageDigest
  Throws:
    CloneNotSupportedException - if the underlying MessageDigest is not cloneable.

### engineReset

    protected void engineReset()

  Resets this object disregarding any temporary data present at the time of the invocation of this call.
  Overrides:
    engineReset in class MessageDigestSpi

### engineUpdate

    protected void engineUpdate(byte input)

  Continues an HMAC digest using the input byte.
  Overrides:
    engineUpdate in class MessageDigestSpi

### engineUpdate

    protected void engineUpdate(byte[] input,
                                int offset,
                                int len)

  Hashes a byte array from a given offset for a specified length.
  Overrides:
    engineUpdate in class MessageDigestSpi
  Parameters:
    `input` - byte array from which data is to be hashed.
    `offset` - start index of bytes to hash in input.
    `len` - number of bytes to hash.

### engineDigest

    protected byte[] engineDigest()

  Calculates the final MAC.

  RFC 2104 describes HMAC as follows:

  The definition of HMAC requires a cryptographic hash function, which we denote by H, and a secret key K. We assume H to be a cryptographic hash function where data is hashed by iterating a basic compression function on blocks of data. We denote by B the byte-length of such blocks (B=64 for all the above mentioned examples of hash functions), and by L the byte-length of hash outputs (L=16 for MD5, L=20 for SHA-1). The authentication key K can be of any length up to B, the block length of the hash function. Applications that use keys longer than B bytes will first hash the key using H and then use the resultant L byte string as the actual key to HMAC. In any case the minimal recommended length for K is L bytes (as the hash output length). See section 3 for more information on keys.

  We define two fixed and different strings ipad and opad as follows (the 'i' and 'o' are mnemonics for inner and outer):

  > ipad = the byte 0x36 repeated B times
  > opad = the byte 0x5C repeated B times.

  To compute HMAC over the data 'text' we perform

  > H(K XOR opad, H(K XOR ipad, text))

  Namely,

  1. append zeros to the end of K to create a B byte string (e.g., if K is of length 20 bytes and B=64, then K will be appended with 44 zero bytes 0x00)
  2. XOR (bitwise exclusive-OR) the B byte string computed in step (1) with ipad
  3. append the stream of data 'text' to the B byte string resulting from step (2)
  4. apply H to the stream generated in step (3)
  5. XOR (bitwise exclusive-OR) the B byte string computed in step (1) with opad
  6. append the H result from step (4) to the B byte string resulting from step (5)
  7. apply H to the stream generated in step (6) and output the result
  Overrides:
    engineDigest in class MessageDigestSpi

### engineGetDigestLength

    protected int engineGetDigestLength()

  SPI: Returns the digest length in bytes.
  Overrides:
    engineGetDigestLength in class MessageDigestSpi

### setParameter

    public void setParameter(String param,
                             Object value)
                      throws InvalidParameterException

  Description copied from interface: Parameterized
  Sets the specified algorithm parameter to the specified value.

  This method supplies a general-purpose mechanism through which it is possible to set the various parameters of this object. A uniform algorithm-specific naming scheme for each parameter is desirable but left unspecified at this time.

  Specified by:
    setParameter in interface Parameterized
  Tags copied from interface: Parameterized
  Parameters:
    `param` - the string identifier of the parameter.
    `value` - the parameter value.
  Throws:
    NullPointerException - if param == null
    NoSuchParameterException - if there is no parameter with name param for this cipher implementation.
    InvalidParameterException - if the parameter exists but cannot be set (for example because the object is in the wrong state).
    InvalidParameterTypeException - if value is the wrong type for this parameter.

### getParameter

    public Object getParameter(String param)
                        throws InvalidParameterException

  Description copied from interface: Parameterized
  Gets the value of the specified algorithm parameter.

  This method supplies a general-purpose mechanism through which it is possible to get the various parameters of this object. A uniform algorithm-specific naming scheme for each parameter is desirable but left unspecified at this time.

  Specified by:
    getParameter in interface Parameterized
  Tags copied from interface: Parameterized
  Parameters:
    `param` - the string name of the parameter.
  Returns:
    the object that represents the parameter value.
  Throws:
    NullPointerException - if param == null
    NoSuchParameterException - if there is no parameter with name param for this implementation.
    InvalidParameterException - if the parameter exists but cannot be read.

### engineSetParameter

    protected void engineSetParameter(String param,
                                      Object value)
                               throws InvalidParameterException

### engineGetParameter

    protected Object engineGetParameter(String param)
                                 throws InvalidParameterException

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Copyright (C) 1995-2000 The Cryptix Foundation Ltd. All rights reserved.