All Packages  Class Hierarchy  This Package  Previous  Next  Index

Class cryptix.provider.mac.HMAC

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

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.

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: 1.2 $

Author:
David Hopwood

Constructor Index

 o HMAC(String, int)
Constructs an HMAC object for the given MD algorithm name and block size.

Method Index

 o clone()
Returns a copy of this HMAC object.
 o engineDigest()
Calculates the final MAC.
 o engineGetDigestLength()
SPI: Returns the digest length in bytes.
 o engineGetParameter(String)
 o engineReset()
Resets this object disregarding any temporary data present at the time of the invocation of this call.
 o engineSetParameter(String, Object)
 o engineUpdate(byte)
Continues an HMAC digest using the input byte.
 o engineUpdate(byte[], int, int)
Hashes a byte array from a given offset for a specified length.
 o getParameter(String)
 o setParameter(String, Object)

Constructors

 o 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.

Methods

 o clone
 public Object clone() throws CloneNotSupportedException
Returns a copy of this HMAC object.

Overrides:
clone in class MessageDigest
 o 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 MessageDigest
 o engineUpdate
 protected void engineUpdate(byte input)
Continues an HMAC digest using the input byte.

Overrides:
engineUpdate in class MessageDigest
 o engineUpdate
 protected void engineUpdate(byte input[],
                             int offset,
                             int len)
Hashes a byte array from a given offset for a specified length.

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.
Overrides:
engineUpdate in class MessageDigest
 o 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 MessageDigest
 o engineGetDigestLength
 protected int engineGetDigestLength()
SPI: Returns the digest length in bytes.

 o setParameter
 public void setParameter(String param,
                          Object value) throws InvalidParameterException
 o getParameter
 public Object getParameter(String param) throws InvalidParameterException
 o engineSetParameter
 protected void engineSetParameter(String param,
                                   Object value) throws InvalidParameterException
 o engineGetParameter
 protected Object engineGetParameter(String param) throws InvalidParameterException

All Packages  Class Hierarchy  This Package  Previous  Next  Index