1. A crytographic operation where an entire message is run through some mathematical operations resulting in a fixed-length (e.g. 128-bit) string that is probably unique. This "hash" has two important properties: It is "one-way"; given a hash, somebody cannot figure out what input message generated the output hash. It is unique; there is more chance of an asteroid hitting the earth and wiping out all life than two messages accidentally hashing to the same value. No two messages should produce the same Example: Some common uses of hashes are: Creating an encryption key from a text password. Creating a unique "fingerprint" of a message that is then encrypted with a private key in order to sign a message. Create unique fingerprints of files in order to detect when they have changed. Example: The program "tripwire" detects intrusions by calculating a hash of all programs/binaries. On a regular basis, it recalculates the hash. If a file has changed, then the hash will also have changed. Tripwire then "trips" whenever the latest calculated hash of the file does not match the one stored in its database. Example: Some common hash algorithms are: SHA-1 If you need to choose a hash algorithm, this is probably the best one to choose (unless speed is the paramount concern). Of the most popular hash algorithms, this is currently (year 2001) considered to be the most secure. MD5 In the year 2001, more data is probably being hashed by MD5 than any other algorithm. However, a lot of people recommend moving to SHA-1 because of weaknesses discovered in MD5. RIPEMD MD4 A historically significant hash algorithm, but useless by today's standards. See also: integrity From Hacking-Lexicon