A public one-way hash function can be used to speed up a public-key digital signature system. Rather than directly compute the signature of a long message which can take an inordinately long time, you compute the one-way hash of the message, and then digitally sign just the hash. The result can be verified with a public key, but created only with a private key. The receiver can verify the digest and hence be assured the file indeed came from you and that none of its bytes have been modified since you signed it. There are two common digest algorithms SHA-1 and MD5. Even though Jar files for signed Applets have a signed digest, oddly, Sun did not provide access to classes for computing them until Java 1.3 when it introduced the java.security.MessageDigest class.

If you want digests that others can’t backwards guess to the original text, condition your original data with secret1 + data + secret2 where + represents concatenation. For a slightly fancier technique use HMAC described in RFC 2104.

MD-5 and SHA-1 are heavyweight digests. If you want to compute them quickly and are not concerned about cryptographic maliciousness, just random error, you can get away with much simpler digests such as:

• CRC16
• java.util.zip.CRC32
• java.util.zip.Adler32 Very fast.
• java.lang.Object.hashCode() % someLargePrime

For earlier versions of Java, Mitch Gallant has provided an MD5 and SHA-1. Mr. Tines has also provided an MD5 SHA-0 and SHA-1.

In theory, if you wanted a variable length digest, you could cook one up by concatenating a string of random numbers generated by using your value as a seed. However, making such a digest longer does not improve its quality. It actually makes it even easier to crack. It has no more variability than the size of the seed.