# Class MessageDigest

java.lang.Object
java.security.MessageDigestSpi
java.security.MessageDigest

public abstract class MessageDigest extends MessageDigestSpi
This `MessageDigest` class provides applications the functionality of a message digest algorithm, such as SHA-1 or SHA-256. Message digests are secure one-way hash functions that take arbitrary-sized data and output a fixed-length hash value.

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. Client applications can test cloneability by attempting cloning and catching the `CloneNotSupportedException`:

    MessageDigest md = MessageDigest.getInstance("SHA-256");

    try {
        md.update(toChapter1);
        MessageDigest tc1 = md.clone();
        byte[] toChapter1Digest = tc1.digest();
        md.update(toChapter2);
        ...etc.
    } catch (CloneNotSupportedException cnse) {
        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.

Note that this class is abstract and extends from `MessageDigestSpi` for historical reasons. Application developers should only take notice of the methods defined in this `MessageDigest` class; all the methods in the superclass are intended for cryptographic service providers who wish to supply their own implementations of message digest algorithms.

Every implementation of the Java platform is required to support the following standard `MessageDigest` algorithms:

- `SHA-1`
- `SHA-256`
- `SHA-384`

These algorithms are described in the MessageDigest section of the Java Security Standard Algorithm Names Specification. Consult the release documentation for your implementation to see if any other algorithms are supported.

Since:
  1.1
External Specifications
  - Java Security Standard Algorithm Names
See Also:
  - `DigestInputStream`
  - `DigestOutputStream`

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `MessageDigest(String algorithm)`
  Creates a message digest with the specified algorithm name.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `Object`
  `clone()`
  Returns a clone if the implementation is cloneable.
  `byte[]`
  `digest()`
  Completes the hash computation by performing final operations such as padding.
  `byte[]`
  `digest(byte[] input)`
  Performs a final update on the digest using the specified array of bytes, then completes the digest computation.
  `int`
  `digest(byte[] buf, int offset, int len)`
  Completes the hash computation by performing final operations such as padding.
  `final String`
  `getAlgorithm()`
  Returns a string that identifies the algorithm, independent of implementation details.
  `final int`
  `getDigestLength()`
  Returns the length of the digest in bytes, or 0 if this operation is not supported by the provider and the implementation is not cloneable.
  `static MessageDigest`
  `getInstance(String algorithm)`
  Returns a `MessageDigest` object that implements the specified digest algorithm.
  `static MessageDigest`
  `getInstance(String algorithm, String provider)`
  Returns a `MessageDigest` object that implements the specified digest algorithm.
  `static MessageDigest`
  `getInstance(String algorithm, Provider provider)`
  Returns a `MessageDigest` object that implements the specified digest algorithm.
  `final Provider`
  `getProvider()`
  Returns the provider of this message digest object.
  `static boolean`
  `isEqual(byte[] digesta, byte[] digestb)`
  Compares two digests for equality.
  `void`
  `reset()`
  Resets the digest for further use.
  `String`
  `toString()`
  Returns a string representation of this message digest object.
  `void`
  `update(byte input)`
  Updates the digest using the specified byte.
  `void`
  `update(byte[] input)`
  Updates the digest using the specified array of bytes.
  `void`
  `update(byte[] input, int offset, int len)`
  Updates the digest using the specified array of bytes, starting at the specified offset.
  `final void`
  `update(ByteBuffer input)`
  Update the digest using the specified ByteBuffer.

  ### Methods declared in class MessageDigestSpi

  `engineDigest, engineDigest, engineGetDigestLength, engineReset, engineUpdate, engineUpdate, engineUpdate`

  ### Methods declared in class Object

  `equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Constructor Details

  - ### MessageDigest

    protected MessageDigest(String algorithm)
    Creates a message digest with the specified algorithm name.
    Parameters:
      `algorithm` - the standard name of the digest algorithm. See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
    External Specifications
      - Java Security Standard Algorithm Names

- ## Method Details

  - ### getInstance

    public static MessageDigest getInstance(String algorithm) throws NoSuchAlgorithmException
    Returns a `MessageDigest` object that implements the specified digest algorithm.

    This method traverses the list of registered security Providers, starting with the most preferred Provider. A new `MessageDigest` object encapsulating the `MessageDigestSpi` implementation from the first provider that supports the specified algorithm is returned.

    Note that the list of registered providers may be retrieved via the `Security.getProviders()` method.

    Implementation Note:
      The JDK Reference Implementation additionally uses the `jdk.security.provider.preferred` `Security` property to determine the preferred provider order for the specified algorithm. This may be different from the order of providers returned by `Security.getProviders()`.
    Parameters:
      `algorithm` - the name of the algorithm requested. See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
    Returns:
      a `MessageDigest` object that implements the specified algorithm
    Throws:
      `NoSuchAlgorithmException` - if no `Provider` supports a `MessageDigestSpi` implementation for the specified algorithm
      `NullPointerException` - if `algorithm` is `null`
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getInstance

    public static MessageDigest getInstance(String algorithm, String provider) throws NoSuchAlgorithmException, NoSuchProviderException
    Returns a `MessageDigest` object that implements the specified digest algorithm.

    A new `MessageDigest` object encapsulating the `MessageDigestSpi` implementation from the specified provider is returned. The specified provider must be registered in the security provider list.

    Note that the list of registered providers may be retrieved via the `Security.getProviders()` method.

    Parameters:
      `algorithm` - the name of the algorithm requested. See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
      `provider` - the name of the provider.
    Returns:
      a `MessageDigest` object that implements the specified algorithm
    Throws:
      `IllegalArgumentException` - if the provider name is `null` or empty
      `NoSuchAlgorithmException` - if a `MessageDigestSpi` implementation for the specified algorithm is not available from the specified provider
      `NoSuchProviderException` - if the specified provider is not registered in the security provider list
      `NullPointerException` - if `algorithm` is `null`
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getInstance

    public static MessageDigest getInstance(String algorithm, Provider provider) throws NoSuchAlgorithmException
    Returns a `MessageDigest` object that implements the specified digest algorithm.

    A new `MessageDigest` object encapsulating the `MessageDigestSpi` implementation from the specified provider is returned. Note that the specified provider does not have to be registered in the provider list.

    Parameters:
      `algorithm` - the name of the algorithm requested. See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
      `provider` - the provider.
    Returns:
      a `MessageDigest` object that implements the specified algorithm
    Throws:
      `IllegalArgumentException` - if the specified provider is `null`
      `NoSuchAlgorithmException` - if a `MessageDigestSpi` implementation for the specified algorithm is not available from the specified `Provider` object
      `NullPointerException` - if `algorithm` is `null`
    Since:
      1.4
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `Provider`

  - ### getProvider

    public final Provider getProvider()
    Returns the provider of this message digest object.
    Returns:
      the provider of this message digest object

  - ### update

    public void update(byte input)
    Updates the digest using the specified byte.
    Parameters:
      `input` - the byte with which to update the digest.

  - ### update

    public void update(byte[] input, int offset, int len)
    Updates the digest using the specified array of bytes, starting at the specified offset.
    Parameters:
      `input` - the array of bytes.
      `offset` - the offset to start from in the array of bytes.
      `len` - the number of bytes to use, starting at `offset`.

  - ### update

    public void update(byte[] input)
    Updates the digest using the specified array of bytes.
    Parameters:
      `input` - the array of bytes.

  - ### update

    public final void update(ByteBuffer input)
    Update the digest using the specified ByteBuffer. The digest is updated using the `input.remaining()` bytes starting at `input.position()`. Upon return, the buffer's position will be equal to its limit; its limit will not have changed.
    Parameters:
      `input` - the ByteBuffer
    Since:
      1.5

  - ### digest

    public byte[] digest()
    Completes the hash computation by performing final operations such as padding. The digest is reset after this call is made.
    Returns:
      the array of bytes for the resulting hash value.

  - ### digest

    public int digest(byte[] buf, int offset, int len) throws DigestException
    Completes the hash computation by performing final operations such as padding. The digest is reset after this call is made.
    Parameters:
      `buf` - output buffer for the computed digest
      `offset` - offset into the output buffer to begin storing the digest
      `len` - number of bytes within buf allotted for the digest
    Returns:
      the number of bytes placed into `buf`
    Throws:
      `DigestException` - if an error occurs.

  - ### digest

    public byte[] digest(byte[] input)
    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(input)`, passing the input array to the `update` method, then calls `digest()`.
    Parameters:
      `input` - the input to be updated before the digest is completed.
    Returns:
      the array of bytes for the resulting hash value.

  - ### toString

    public String toString()
    Returns a string representation of this message digest object.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string representation of the object

  - ### isEqual

    public static boolean isEqual(byte[] digesta, byte[] digestb)
    Compares two digests for equality. Two digests are equal if they have the same length and all bytes at corresponding positions are equal.
    Implementation Note:
      All bytes in `digesta` are examined to determine equality, unless `digestb` is `null` or has a length of zero bytes. If `digestb` is not `null` and does not have a length of zero bytes, then the calculation time depends only on the length of `digesta`. It does not depend on the length of `digestb` or the contents of `digesta` and `digestb`.
    Parameters:
      `digesta` - one of the digests to compare.
      `digestb` - the other digest to compare.
    Returns:
      `true` if the digests are equal, `false` otherwise.

  - ### reset

    public void reset()
    Resets the digest for further use.

  - ### getAlgorithm

    public final String getAlgorithm()
    Returns a string that identifies the algorithm, independent of implementation details. The name should be a standard Java Security name (such as "SHA-256"). See the MessageDigest section in the Java Security Standard Algorithm Names Specification for information about standard algorithm names.
    Returns:
      the name of the algorithm
    External Specifications
      - Java Security Standard Algorithm Names

  - ### getDigestLength

    public final int getDigestLength()
    Returns the length of the digest in bytes, or 0 if this operation is not supported by the provider and the implementation is not cloneable.
    Returns:
      the digest length in bytes, or 0 if this operation is not supported by the provider and the implementation is not cloneable.
    Since:
      1.2

  - ### clone

    public Object clone() throws CloneNotSupportedException
    Returns a clone if the implementation is cloneable.
    Overrides:
      `clone` in class `MessageDigestSpi`
    Returns:
      a clone if the implementation is cloneable.
    Throws:
      `CloneNotSupportedException` - if this is called on an implementation that does not support `Cloneable`.
    See Also:
      - `Cloneable`