What is a hash?
A hash function turns any input into a fixed-length fingerprint. Changing even one character changes the whole hash. Hashes are used to check that downloads aren't corrupted, to store passwords (with salting) and to compare files.
- SHA-256 is the modern standard (used by Bitcoin, TLS certificates and software checksums).
- MD5 and SHA-1 are fast but cryptographically broken, so use them only for non-security checksums.
- Files are hashed in your browser and never uploaded, so this is safe for private documents.
Frequently asked questions
How do I verify a downloaded file?
Click “Open file…” and choose the file, then paste the checksum published by the provider into “Compare with”. A green tick appears when it matches.
Can a hash be reversed?
No. Hashes are one-way. Short or common inputs (such as weak passwords) can still be guessed by trying many inputs, which is why passwords need salted, slow hashes such as bcrypt.