FAQ

note 22100 added to function.md5

Jimw
Feb 8, 2003 at 7:49 pm
wil...@...com, saying there are no decryption functions for md5 in php is a bit of a misnomer. md5 is a hash, not an encryption algorithm. The difference between the two is that encryption is meant to be undone, so any algorithm must both preserve all of the input data and also use functions which are reversible. Hashes are not meant to be undone, so they purposely throw out data or use transforms which cannot be undone. A good example of a trivial hash is modular arithmetic, where 100 mod 7 = 2, given 2 it is impossible to calculate 100 without guessing. PHP does not include an UN-md5 because such a thing cannot exist for long strings, and hopefully never will for short ones (less than 16 or so characters).
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