MD5 (Message-Digest algorithm 5) is a widely-used cryptographic hash function with a 128-bit hash value. MD5 was designed by Ronald Rivest in 1991.

What it does, to quote the executive summary of rfc1321, is:

"[The MD5 algorithm] takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. It is conjectured that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest. The MD5 algorithm is intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA."

In essence, MD5 is a way to verify data integrity, and is much more reliable than checksum and many other commonly used methods.

Additional information about the algorithm can be found in the Wikipeia.

Because of the long time it takes today to reverse enigneer a MD5 hash (with brute force methods), the idea for md5(); was born in january 2005. This website is connected to a database of md5 hashes and their original counterpart. This makes the search for the counterpart of a given md5 hash extremly fast. Additionaly everytime when you enter a non-md5 hash string into the search field, the md5 result for that search strings gets stored in our database for future use.

Implementations

Apple IIgs

• MD5 for the Apple IIgs. Link provided by Andrew Roughan.

ASP

• MD5 in ASP 2.0. Reference by Andrea Giammarchi.

C

• Professor Rivest's original implementation is included in the appendix of rfc1321 and is also available in a more easily extracted (tarred, gzipped) form.
• L. Peter Deutsch's implementation, with a different license that he calls BSD-like: MD5 in C. The author says: "This implementation was written directly from the text of RFC 1321 (exclusive of the Appendices), does not incorporate any part of the RSA documentation or software, and therefore does not require any acknowledgement to RSA."
• Colin Plumb, Branko Lankester, Ian Jackson, and Galen Hazelwood's md5sum: md5sum. Note that this package appears to assume that you use Debian Linux.
• Christophe Devine's MD5. Reference by author.

C#/.Net

• Apparently, MD5 is a built-in for .net/C#. Thanks, Richard Maw, for letting me know.

Coldfusion

• Coldfusion includes MD5 natively per Steve R. Seaquist.

command-line

• Another md5sum. Reference by Tom Lynn.

Delphi

• Sarah Dean includes MD5 support in a collection of Delphi components.

Java

• Jari Juslin reports that as of version 1.4.0, Java supports MD5 natively. You can read more about it at Sun's documentation of MD5 for java.
• MD5 for Java. Reference by benjamin pritchard.

Javascript

• Paul Johnston has created a javascript MD5 implementation of MD5 under the BSD license.
• MD5 for Javascript. Reference by Dominik Deobald.

Lua

• MD5 for Lua, by Roberto Ierusalimschy. Reference by author.

Lotus Script

• MD5 in Lotus Script, by Martin Wendt. Reference by author.

Macromedia Director

• MD5 for Macromedia Director by Chuck Neal. Reference by Chuck Neal.

masm32

• MD5 in ASM. Reference by "Mike"/MB.

Miva

• Petr Kutil and Ivo Truxa have created a Mivo implementation of MD5.

MySQL

• Apparently, MD5 is a builtin for MySQL. More power to them! Thanks to Jared Kuolt for the reference.

Perl5

• MD5 support is now built-in to recent perls, and called Digest::MD5.
• Neil Winton has created an MD5 Perl Module. This is a Perl Extension, requiring additional compilation.

PHP

• Looks like MD5 is a builtin for PHP. More power to them!
• An MD5 file parser class for PHP. Reference from Kirill Zinov.

PostgreSQL

• Looks like MD5 is a builtin for PostgreSQL. More power to them! (Reference provided by Ben Wahlstrand.)

Python

• Several people have written in to say that MD5 is a builtin for Python. More power to them!

REBOL

• Rebol has built-in support for MD5 per Gregg Irwin.

REXX

• MD5 in REXX for OS/2. Reference by Frank Ellermann.
• Another MD5 in REXX. Reference by Frank Ellermann.

Scheme

• MD5 for Scheme. (reference by Jens Axel Søgaard, the author)

SQL Server

• For SQL Server 7/2000 by Joseph Gama, reference by author. NOTE: link appears to be down. Send me email if you want a copy.
• For SQL Server 2000, by Vic Mackey, reference by Mark Kincaid.

TCL

• tcllib includes MD5 support as per the tcllib MD5 docs. (reference by Andrew Ho.)

Visual Basic

• Francisco Carlos Piragibe de Almeida has created a DLL with a BAS wrapper module for use in VB projects (19K download)
  and an ActiveX in-process server wrapper around the original DLL (includes the DLL) (1394K download). (appears to be broken -- 2005-11-14)
• MD5 for VB. Reference by benjamin pritchard.
• MD5 for VB and VBScript. Reference by Dominik Deobald.

Windows

• Note: assorted language implementations also apply.
• MD5 command-line for Windows. Reference by Stacy Curl.
• Irnis apparently produces a commercial GUI for MD5 and other hash algorithms. Reference by Irnis I.Haliullin.
• Fastsum. Reference by Kirill Zinov, the author.
• MD5Check; includes 32-bit and 64-bit implementations. Free for non-commercial use. Reference by Mark Gillespie, the author.

Witango

• Apparently, the Witango product supports MD5 per Jennifer M. Humphreys.

Collision generators

• Patrick Stach wrote an MD5 collision generator and an MD4 collision generator.