Java I/O

The MessageDigest Class

The java.security.MessageDigest class is an abstract class that represents a hash code and its associated algorithm. Concrete subclasses (actually concrete subclasses of java.security.MessageDigestSPI, though the difference isn’t relevant from a client’s point of view) implement particular, professionally designed, well-known hash code algorithms. Thus, rather than constructing instances of this class directly, you ask the static MessageDigest.getInstance() factory method to provide an implementation of an algorithm with a particular name. Table 10.1 lists the standard names for message digest algorithms. Depending on which service providers are installed, you may or may not have all of these. The JDK 1.1 includes SHA-1 (which is the same as SHA) and MD5 but not MD2. RSA’s paywareCrypto-J cryptography library also supports MD2. (See http://www.rsa.com/rsa/products/jsafe/.)

Table 10-1. Message Digest Algorithms in Java 1.1

Name	Algorithm
SHA-1	The Secure Hash Algorithm, as defined in Secure Hash Standard, NIST FIPS 180-1 (National Institute of Standards and Technology Federal Information Processing Standards Publications 180-1); produces 20-byte digests; see http://www.itl.nist.gov/div897/pubs/fip180-1.htm
SHA	Another name for SHA-1
MD2	RSA-MD2 as defined in RFC 1319 and RFC 1423 (RFC 1423 corrects a mistake in RFC 1319); produces 16-byte digests; suitable for use with digital signatures; see http://www.faqs.org/rfcs/rfc1319.html and http://www.faqs.org/rfcs/rfc1423.html ...