What is MD5?
Short answer: MD5 is a cryptographic hash function that produces a 128-bit value (32 hex characters) from any variable-length input.
Detailed explanation
MD5 was designed by Ronald Rivest in 1991. Today it is considered cryptographically broken since 2004 — collisions (two different inputs with the same hash) can be generated in seconds on common hardware. Do NOT use for passwords, digital signatures, or anything requiring security. Still useful for non-critical file integrity checksums and as a deterministic identifier.
Example
MD5('Hello world') = 5eb63bbbe01eeed093cb22bb8f5acdc3Common use cases
- ▸Verifying integrity of downloaded files
- ▸Generating deterministic IDs
- ▸Cache keys (URL hashes)
- ▸Quickly comparing files
- ▸Historical: passwords (DON'T USE TODAY)
Frequently asked questions
Is MD5 still safe to use?
Not for security. Collisions can be produced deliberately, so it must not be used for passwords or digital signatures. It is fine as a quick file checksum.
What should I use instead?
SHA-256 for integrity, and bcrypt or Argon2 for storing passwords. A general-purpose hash is the wrong tool for passwords.
Can an MD5 hash be reversed?
Not mathematically, but common values are already in public lookup tables, so in practice a weak input is trivially recovered.
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