The MessageDigest Class

The MessageDigest class is an engine class designed to provide the functionality of cryptographically secure message digests such as SHA-1 or MD5. A cryptographically secure message digest takes arbitrary-sized input (a byte array), and generates a fixed-size output, called a digest or hash. A digest has two properties:
  • It should be computationally infeasible to find two messages that hashed to the same value.
  • The digest should not reveal anything about the input that was used to generate it.
Message digests are used to produce unique and reliable identifiers of data. They are sometimes called the digital fingerprints of data.
Creating a MessageDigest Object
The first step for computing a digest is to create a message digest instance. As with all engine classes, the way to get a MessageDigest object for a particular type of message digest algorithm is to call the getInstance static factory method on the MessageDigest class:
static MessageDigest getInstance(String algorithm)
Note: The algorithm name is not case-sensitive. For example, all the following calls are equivalent:
A caller might optionally specify the name of a provider or a Provider instance, which guarantees that the implementation of the algorithm requested is from the specified provider:
static MessageDigest getInstance(String algorithm, String provider)
static MessageDigest getInstance(String algorithm, Provider provider)

A call to getInstance returns an initialized message digest object. It therefore does not need further initialization.

Updating a Message Digest Object
The next step for calculating the digest of some data is to supply the data to the initialized message digest object. This step is done by calling one of the update methods:
void update(byte input)
void update(byte[] input)
void update(byte[] input, int offset, int len)
Computing the Digest
After the data has been supplied by calls to update methods, the digest is computed using a call to one of the digest methods:
byte[] digest()
byte[] digest(byte[] input)
int digest(byte[] buf, int offset, int len)

The first two methods return the computed digest. The latter method stores the computed digest in the provided buffer buf, starting at offset. len is the number of bytes in buf allotted for the digest. The method returns the number of bytes actually stored in buf.

A call to the digest method that takes an input byte array argument is equivalent to making a call to the following method with the specified input, followed by a call to the digest method without any arguments:
void update(byte[] input)

See the Examples section for more details.