Hashing is the process of running data through a one-way function that produces a fixed-length value called a hash or digest. The same input always yields the same hash, but you cannot feasibly recover the input from it.
Why it matters
Hashes verify integrity: if a downloaded file's hash matches the published one, it was not corrupted or tampered with. They are also how systems store passwords without keeping the passwords themselves. Unlike encryption, hashing has no key and is not meant to be undone.
Example
The SHA-256 hash of the word hello always begins 2cf24dba5fb0a30e.... Changing one letter to Hello produces a completely different digest. A site checks your login by hashing what you typed and comparing it with the stored hash.
Common mistakes
- Calling hashing encryption. There is no key and no decryption.
- Using fast general hashes such as MD5 or plain SHA-256 for passwords instead of bcrypt, scrypt or Argon2.
- Relying on MD5 or SHA-1 for security, since collisions can be created.
Related
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