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gnu.javax.crypto.mac

Interface IMac

Known Subinterfaces:
IAuthenticatedMode
Known Implementing Classes:
BaseMac, EAX, HMac, OMAC, TMMH16, UHash32, UMac32
public interface IMac
The basic visible methods of any MAC (Message Authentication Code) algorithm.

A MAC provides a way to check the integrity of information transmitted over, or stored in, an unreliable medium, based on a secret key. Typically, MACs are used between two parties, that share a common secret key, in order to validate information transmitted between them.

When a MAC algorithm is based on a cryptographic hash function, it is then called to a HMAC (Hashed Message Authentication Code) --see RFC-2104.

Another type of MAC algorithms exist: UMAC or Universal Message Authentication Code, described in draft-krovetz-umac-01.txt.

With UMACs, the sender and receiver share a common secret key (the MAC key) which determines:

References:

  1. RFC 2104HMAC: Keyed-Hashing for Message Authentication.
    H. Krawczyk, M. Bellare, and R. Canetti.
  2. UMAC: Message Authentication Code using Universal Hashing.
    T. Krovetz, J. Black, S. Halevi, A. Hevia, H. Krawczyk, and P. Rogaway.

Field Summary

static String
MAC_KEY_MATERIAL
Property name of the user-supplied key material.
static String
TRUNCATED_SIZE
Property name of the desired truncated output size in bytes.

Method Summary

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(Map<K,V> attributes)
Initialises the algorithm with designated attributes.
int
macSize()
Returns the output length in bytes of this MAC algorithm.
String
name()
Returns the canonical name of this 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[] in, int offset, int length)
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.

Field Details

MAC_KEY_MATERIAL

public static final String MAC_KEY_MATERIAL
Property name of the user-supplied key material. The value associated to this property name is taken to be a byte array.
Field Value:
"gnu.crypto.mac.key.material"

TRUNCATED_SIZE

public static final String TRUNCATED_SIZE
Property name of the desired truncated output size in bytes. The value associated to this property name is taken to be an integer. If no value is specified in the attributes map at initialisation time, then all bytes of the underlying hash algorithm's output are emitted.

This implementation, follows the recommendation of the RFC 2104 authors; specifically: 

     We recommend that the output length t be not less than half the
     length of the hash output (to match the birthday attack bound)
     and not less than 80 bits (a suitable lower bound on the number
     of bits that need to be predicted by an attacker).
Field Value:
"gnu.crypto.mac.truncated.size"

Method Details

clone

public Object clone()
            throws CloneNotSupportedException
Returns a clone copy of this instance.
Overrides:
clone in interface Object
Returns:
a clone copy of this instance.

digest

public byte[] digest()
Completes the MAC by performing final operations such as padding and resetting the instance.
Returns:
the array of bytes representing the MAC value.

init

public void init(Map<K,V> attributes)
            throws InvalidKeyException,
                   IllegalStateException
Initialises the algorithm with designated attributes. Permissible names and values are described in the class documentation above.
Parameters:
attributes - a set of name-value pairs that describe the desired future instance behaviour.
Throws:
InvalidKeyException - if the key data is invalid.
IllegalStateException - if the instance is already initialised.
See Also:
MAC_KEY_MATERIAL

macSize

public int macSize()
Returns the output length in bytes of this MAC algorithm.
Returns:
the output length in bytes of this MAC algorithm.

name

public String name()
Returns the canonical name of this algorithm.
Returns:
the canonical name of this algorithm.

reset

public void reset()
Resets the algorithm instance for re-initialisation and use with other characteristics. This method always succeeds.

selfTest

public boolean selfTest()
A basic test. Ensures that the MAC of a pre-determined message is equal to a known pre-computed value.
Returns:
true if the implementation passes a basic self-test. Returns false otherwise.

update

public void update(byte b)
Continues a MAC operation using the input byte.
Parameters:
b - the input byte to digest.

update

public void update(byte[] in,
                   int offset,
                   int length)
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.
Parameters:
in - the input block.
offset - start of meaningful bytes in input block.
length - number of bytes, in input block, to consider.
IMac.java -- Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc. This file is a part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; if not, write to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.

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