Like other algorithm-based classes in Java Security, * MessageDigest has two major components: * *
engineReset method is
* called by the reset method.
* The SPI methods are abstract; providers must supply a
* concrete implementation.
*
* A MessageDigest object starts out initialized. The data is * processed through it using the update * methods. At any point reset can be called * to reset the digest. Once all the data to be updated has been * updated, one of the digest methods should * be called to complete the hash computation. * *
The digest method can be called once for a given number
* of updates. After digest has been called, the MessageDigest
* object is reset to its initialized state.
*
*
Implementations are free to implement the Cloneable interface,
* and doing so will let client applications test cloneability
* using instanceof Cloneable before cloning:
* *
* MessageDigest md = MessageDigest.getInstance("SHA");
*
* if (md instanceof Cloneable) {
* md.update(toChapter1);
* MessageDigest tc1 = md.clone();
* byte[] toChapter1Digest = tc1.digest;
* md.update(toChapter2);
* ...etc.
* } else {
* throw new DigestException("couldn't make digest of partial content");
* }
*
*
* Note that if a given implementation is not cloneable, it is
* still possible to compute intermediate digests by instantiating
* several instances, if the number of digests is known in advance.
*
* @see DigestInputStream
* @see DigestOutputStream
*
* @version 1.31 97/02/03
* @author Benjamin Renaud
*/
public abstract class MessageDigest {
/* Are we in debugging mode? */
private static boolean debug = false;
/* The digest bits, if any. */
private byte[] digestBits;
private String algorithm;
/**
* Creates a message digest with the specified algorithm name.
*
* @param algorithm the standard name of the digest algorithm.
* See Appendix A in the
* Java Cryptography Architecture API Specification & Reference
* for information about standard algorithm names.
*/
protected MessageDigest(String algorithm) {
this.algorithm = algorithm;
}
/**
* Generates a MessageDigest object that implements the specified digest
* algorithm. If the default provider package contains a MessageDigest
* subclass implementing the algorithm, an instance of that subclass
* is returned. If the algorithm is not available in the default
* package, other packages are searched.
*
* @param algorithm the name of the algorithm requested.
* See Appendix A in the
* Java Cryptography Architecture API Specification & Reference
* for information about standard algorithm names.
*
* @return a Message Digest object implementing the specified
* algorithm.
*
* @exception NoSuchAlgorithmException if the algorithm is
* not available in the caller's environment.
*/
public static MessageDigest getInstance(String algorithm)
throws NoSuchAlgorithmException {
try {
return (MessageDigest)Security.getImpl(algorithm,
"MessageDigest", null);
} catch(NoSuchProviderException e) {
throw new InternalError("please send a bug report via " +
System.getProperty("java.vendor.url.bug"));
}
}
/**
* Generates a MessageDigest object implementing the specified
* algorithm, as supplied from the specified provider, if such an
* algorithm is available from the provider.
*
* @param algorithm the name of the algorithm requested.
* See Appendix A in the
* Java Cryptography Architecture API Specification & Reference
* for information about standard algorithm names.
*
* @param provider the name of the provider.
*
* @return a Message Digest object implementing the specified
* algorithm.
*
* @exception NoSuchAlgorithmException if the algorithm is
* not available in the package supplied by the requested
* provider.
*
* @exception NoSuchProviderException if the provider is not
* available in the environment.
*
* @see Provider
*/
public static MessageDigest getInstance(String algorithm, String provider)
throws NoSuchAlgorithmException, NoSuchProviderException {
return (MessageDigest)Security.getImpl(algorithm,
"MessageDigest", provider);
}
/**
* Updates the digest using the specified byte.
*
* @param input the byte with which to update the digest.
*/
public void update(byte input) {
engineUpdate(input);
}
/**
* Updates the digest using the specified array of bytes, starting
* at the specified offset.
*
* @param input the array of bytes.
*
* @param offset the offset to start from in the array of bytes.
*
* @param len the number of bytes to use, starting at
* offset.
*/
public void update(byte[] input, int offset, int len) {
engineUpdate(input, offset, len);
}
/**
* Updates the digest using the specified array of bytes.
*
* @param input the array of bytes.
*/
public void update(byte[] input) {
engineUpdate(input, 0, input.length);
}
/**
* Completes the hash computation by performing final operations
* such as padding. The digest is reset after this call is made.
*
* @return the array of bytes for the resulting hash value.
*/
public byte[] digest() {
/* Resetting is the responsibility of implementors. */
digestBits = engineDigest();
return digestBits;
}
/**
* Performs a final update on the digest using the specified array
* of bytes, then completes the digest computation. That is, this
* method first calls update on the
* array, then calls digest().
*
* @param input the input to be updated before the digest is
* completed.
*
* @return the array of bytes for the resulting hash value.
*/
public byte[] digest(byte[] input) {
update(input);
return digest();
}
/**
* Helper function that prints unsigned two character hex digits.
*/
private static void hexDigit(PrintStream p, byte x) {
char c;
c = (char) ((x >> 4) & 0xf);
if (c > 9) {
c = (char) ((c - 10) + 'a');
} else {
c = (char) (c + '0');
}
p.write(c);
c = (char) (x & 0xf);
if (c > 9) {
c = (char)((c - 10) + 'a');
} else {
c = (char)(c + '0');
}
p.write(c);
}
/**
* Returns a string representation of this message digest object.
*/
public String toString() {
ByteArrayOutputStream ou = new ByteArrayOutputStream();
PrintStream p = new PrintStream(ou);
p.print(this.getClass().getName()+" Message Digest ");
if (digestBits != null) {
p.print("<");
for(int i = 0; i < digestBits.length; i++)
hexDigit(p, digestBits[i]);
p.print(">");
} else {
p.print("offset.
*/
protected abstract void engineUpdate(byte[] input, int offset, int len);
/**
* SPI: Completes the hash computation by performing final
* operations such as padding. Once engineDigest has
* been called, the engine should be reset (see reset). Resetting is the responsibility of the
* engine implementor.
*
* @return the array of bytes for the resulting hash value.
*/
protected abstract byte[] engineDigest();
/**
* SPI: Resets the digest for further use.
*/
protected abstract void engineReset();
/**
* Returns a clone if the implementation is cloneable.
*
* @return a clone if the implementation is cloneable.
*
* @exception CloneNotSupportedException if this is called on an
* implementation that does not support Cloneable.
*/
public Object clone() throws CloneNotSupportedException {
if (this instanceof Cloneable) {
return super.clone();
} else {
throw new CloneNotSupportedException();
}
}
private void debug(String statement) {
if (debug) {
System.err.println(statement);
}
}
}