Generate bcrypt password hashes with configurable cost factor and verify passwords against existing hashes.
Bcrypt is a one-way password hash that is deliberately slow: its whole design goal is to make guessing a password from a stolen hash database expensive. This tool runs bcrypt in the browser, so the password never leaves the page: it takes a password and a cost factor, draws a random 16-byte salt, and returns the 60-character hash string you can store. A verify mode checks a candidate password against a hash you already have. Two things surprise most users: hashing the same password twice gives two different hashes, and passwords over 72 bytes are silently cut to their first 72 bytes.
A concrete input and expected output from the current implementation.
Input
Password: rasmuslerdorf Hash: $2y$10$.vGA1O9wmRjrwAVXD98HNOgsNpDczlqm3Jq7KnEd1rVAGv3Fykk1a (entered in the verify fields)
Expected output
Match: the password 'rasmuslerdorf' verifies against this hash (version $2y$, cost factor 10).
This hash is the canonical test vector from the PHP manual's password_verify example: bcrypt of 'rasmuslerdorf' at cost 10. Verification does not need the original salt - it extracts the salt and cost embedded in the hash and recomputes the digest. Generation cannot have a fixed example, because every run draws a fresh random salt and yields a different hash.
A valid bcrypt hash is one 60-character string: a version marker ($2a$, $2b$ or $2y$), a two-digit cost, then 53 characters of bcrypt's base64 encoding holding a 16-byte random salt and a 23-byte digest. The digest comes from expanding the password and salt through 2^cost rounds of the Blowfish key schedule. The verify mode reads the version, cost and salt out of the pasted hash and recomputes the digest.
The cost factor sets the work: cost 10 means 2^10 = 1,024 rounds, cost 12 means 4,096, and each increment doubles the time. Cost 10 runs in tens of milliseconds, cost 12 in a few hundred, and you pay this on every login check, not just at generation. The format permits costs 4 to 31, but past about 14 a hash takes seconds, so practical tools stop near there.
No. Bcrypt is one-way: the digest cannot be reversed, and the salt embedded in the hash only helps verification, not recovery. The only route back to the password is guessing candidates and checking each one - exactly the attack the cost factor exists to slow down. If the plaintext is lost, it is gone; generate a fresh hash and reset the password.
Start with the library default - usually 10 or 12 - and measure on your own hardware. The right value is the highest cost that keeps every login check tolerable, typically 100-250 ms. Raising the cost later is safe: verification reads the cost from the stored hash, so old hashes keep verifying while new ones get the stronger setting.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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