MD5
acronymcryptography
Stands for: Message Digest 5
An old 128-bit hash function, cryptographically broken and unsafe for security use.
Message Digest 5 (RFC 1321) was once ubiquitous but is trivially collidable today. It survives only as a non-security checksum - for example in some legacy fingerprinting - and must never be used where collision resistance matters.
MD5 is a cryptographically broken hash function that remains in wide use, and holding both halves of that sentence at once is the whole point. Practical collisions have been demonstrable for two decades, so any use depending on two different inputs being unable to share a digest is unsafe, full stop.
That rules out the uses that matter for security. Digital signatures, certificate fingerprints, and integrity verification against a determined adversary are all defeated, because an attacker can construct two files with the same digest and get the benign one signed. Chosen-prefix attacks made this practical rather than theoretical, and real malware has been signed using exactly this technique.
Where it is not wrong is the distinction worth teaching. Using MD5 as a fast checksum against accidental corruption, or as a hash key in a data structure, is fine, because neither has an adversary choosing the input. The failure is not the algorithm appearing anywhere; it is the algorithm appearing where an attacker gets to pick what is hashed. Password storage is the third case, and it was always wrong for a different reason: speed.
Also known as: md5