Checksum

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javacard.security
Class Checksum

java.lang.Object
  extended by javacard.security.Checksum
Direct Known Subclasses:
CRC16, CRC32
public abstract class Checksum
extends Object

The Checksum class is the base class for CRC (cyclic redundancy check) checksum algorithms. Implementations of Checksum algorithms must extend this class and implement all the abstract methods.

A tear or card reset event resets a Checksum object to the initial state (state upon construction).

Even if a transaction is in progress, update of intermediate result state in the implementation instance shall not participate in the transaction.

Field Summary
static byte ALG_ISO3309_CRC16
          ISO/IEC 3309 compliant 16 bit CRC algorithm.
static byte ALG_ISO3309_CRC32
          ISO/IEC 3309 compliant 32 bit CRC algorithm.
 
Constructor Summary
protected Checksum()
          Protected Constructor
 
Method Summary
abstract  short doFinal(byte[] inBuff, short inOffset, short inLength, byte[] outBuff, short outOffset)
          Generates a CRC checksum of all/last input data.
abstract  byte getAlgorithm()
          Gets the Checksum algorithm.
static Checksum getInstance(byte algorithm, boolean externalAccess)
          Creates a Checksum object instance of the selected algorithm.
abstract  void init(byte[] bArray, short bOff, short bLen)
          Resets and initializes the Checksum object with the algorithm specific Note: The ALG_ISO3309_CRC16 algorithm expects 2 bytes of parameter information in bArray representing the initial checksum value.
abstract  void update(byte[] inBuff, short inOffset, short inLength)
          Accumulates a partial checksum of the input data.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

ALG_ISO3309_CRC16

public static final byte ALG_ISO3309_CRC16
ISO/IEC 3309 compliant 16 bit CRC algorithm.

See Also:
Constant Field Values

ALG_ISO3309_CRC32

public static final byte ALG_ISO3309_CRC32
ISO/IEC 3309 compliant 32 bit CRC algorithm.

See Also:
Constant Field Values
Constructor Detail

Checksum

protected Checksum()
Protected Constructor

Method Detail

getInstance

public static final Checksum getInstance(byte algorithm,
                                         boolean externalAccess)
                                  throws CryptoException
Creates a Checksum object instance of the selected algorithm.

Parameters:
algorithm - the desired checksum algorithm.
externalAccess - true indicates that the instance will be shared among multiple applet instances and that the Checksum instance will also be accessed (via a Shareable. interface) when the owner of the Checksum instance is not the currently selected applet. If true the implementation must not allocate CLEAR_ON_DESELECT transient space for internal data.
Returns:
the Checksum object instance of the requested algorithm.
Throws:
CryptoException - with the following reason codes:
  • CryptoException.NO_SUCH_ALGORITHM if the requested algorithm or shared access mode is not supported.

getAlgorithm

public abstract byte getAlgorithm()
Gets the Checksum algorithm. Valid codes listed in ALG_ .. constants above, for example, ALG_ISO3309_CRC16

Returns:
the algorithm code defined above

init

public abstract void init(byte[] bArray,
                          short bOff,
                          short bLen)
                   throws CryptoException
Resets and initializes the Checksum object with the algorithm specific

Note:

  • The ALG_ISO3309_CRC16 algorithm expects 2 bytes of parameter information in bArray representing the initial checksum value.
  • The ALG_ISO3309_CRC32 algorithm expects 4 bytes of parameter information in bArray representing the initial checksum value.

Parameters:
bArray - byte array containing algorithm specific initialization information
bOff - offset within bArray where the algorithm specific data begins
bLen - byte length of algorithm specific parameter data.
Throws:
CryptoException - with the following reason codes:
  • CryptoException.ILLEGAL_VALUE if a byte array parameter option is not supported by the algorithm or if the bLen is an incorrect byte length for the algorithm specific data.

doFinal

public abstract short doFinal(byte[] inBuff,
                              short inOffset,
                              short inLength,
                              byte[] outBuff,
                              short outOffset)
Generates a CRC checksum of all/last input data. The CRC engine processes input data starting with the byte at offset inOffset and continuing on until the byte at (inOffset+inLength-1) of the inBuff array. Within each byte the processing proceeds from the least significant bit to the most.

Completes and returns the checksum computation. The Checksum object is reset to the initial state(state upon construction) when this method completes.

Note:

  • The ALG_ISO3309_CRC16 and ALG_ISO3309_CRC32 algorithms reset the initial checksum value to 0. The initial checksum value can be re-initialized using the init(byte[], short, short) method.

The input and output buffer data may overlap.

Parameters:
inBuff - the input buffer of data to be checksummed
inOffset - the offset into the input buffer at which to begin checksum generation
inLength - the byte length to checksum
outBuff - the output buffer, may be the same as the input buffer
outOffset - the offset into the output buffer where the resulting checksum value begins
Returns:
number of bytes of checksum output in outBuff

update

public abstract void update(byte[] inBuff,
                            short inOffset,
                            short inLength)
Accumulates a partial checksum of the input data. The CRC engine processes input data starting with the byte at offset inOffset and continuing on until the byte at (inOffset+inLength-1) of the inBuff array. Within each byte the processing proceeds from the least significant bit to the most.

This method requires temporary storage of intermediate results. This may result in additional resource consumption and/or slow performance. This method should only be used if all the input data required for the checksum is not available in one byte array. The Checksum.html#doFinal(byte[], short, short, byte[], short) doFinal(byte[], short, short, byte[], short) method is recommended whenever possible.

Note:

  • If inLength is 0 this method does nothing.

Parameters:
inBuff - the input buffer of data to be checksummed
inOffset - the offset into the input buffer at which to begin checksum generation
inLength - the byte length to checksum
See Also:
doFinal(byte[], short, short, byte[], short)
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