note 27344 added to function.md5

Recommended readed: OpenSSL from O'reilly! It has chapters on SSL and PHP!! but it also covers cryptography in more depth (chapters 1 and 2 are highly recommended to all here!). It has lots of good information! Talks in depth about lots of stuff that I cannot begin to explain here. MD5 is a repeatable hashes / digest process. Taking something of unknown size or content and reducing it to a known size but retaining a high degree of unknown content. A good hash / digest is said to alter the output significantly changing ~50% of the bits in the "fixed-in-size" output stream) in the event of changing one bit (at random) from the "unknown-in-size" input stream (or even changing the length of the input stream by one bit*/byte, *=with padding if necessary) MD5 is such a hash / digest. Other than that, it doesn't do much on it's own. MD5 is a cheap way to test a file transfer (like a CRC32). If either the file or the MD5 is downloaded with errors, the chances that the MD5 of the file and the "PUBLIC" copy of the MD5 will match is highly unlikely. Both would have to error in a highly unpredictable way. However relying of MD5s as a way to validate that the file hasn't been tamptered with (tainted) is not good. If you can download the file from one place and a public MD5 from a second place, you at least are using a 3rd party method to attempt to validate the file's contents against tainting. MD5 can ONLY be used to validate the contents against tainting if there is something secret (private) between the two end-points. Lets examine MD5 in a typical and extremely effective email validating process. The two parties via a trusted method exchange a word / phrase / password (something private) that hopefully nobody else knows. The first party publically composes an email with an MD5. But instead of sending that MD5. the MD5 is used against this word / phrase / password (private) in a Message Authentication Code (MAC), or better Hash-MAC (HMAC) (see http://www.rsasecurity.com/rsalabs/faq/2-1-7.html) One way would be to MD5 the word / phrase / password (private) part and the public part (the message body) as two different MD5's. the MD5 the two MD5s together as a single MD5 and send the composite MD5 in the public. The receiver can (using all available parts, the private part, public part and the composite MD5) authenicate (testing against the computed part) the message hasn't been tampered during transit. The message body and the composite MD5 is sent in plain text, yet the contents have been authenicated with a high level of confidence. No encryption was used. MD5 is often used to authenicate parts of encrypted streams and thus is the reason why many confuse MD5 as encryption (or even authenication) rather than what it is. A hash / digest. An alternate to MD5 is SHA1. The output size of SHA1 is a little bigger (I think 164bits). More bits, means a higher degree of complexity. 128 bits is concidered minimim by experts in the field.... For cipher lengths and symmetric key sizes (due to computational power now available for brute force attacks). --Doug -- http://www.php.net/manual/en/function.md5.php http://master.php.net/manage/user-notes.php?action=edit+27344 http://master.php.net/manage/user-notes.php?action=delete+27344 http://master.php.net/manage/user-notes.php?action=reject+27344