Bcrypt processes at most 72 bytes of password input—not necessarily 72 characters. Depending on the library, a longer password may be rejected or silently truncated, so the same long password can behave differently across applications. Check how your implementation handles the limit before setting password rules or changing password-storage code.
What is bcrypt’s maximum password length?
Bcrypt’s maximum effective input is 72 bytes. The Go crypto project documents that GenerateFromPassword rejects passwords longer than 72 bytes, while the Java bcrypt implementation describes 72 bytes as the absolute maximum, equivalent to 18 32-bit words.
The limit applies to the encoded bytes passed to bcrypt. It is not a universal 72-character limit.
Why 72 bytes is not always 72 characters
Text is encoded into bytes before it is processed. In UTF-8, some visible characters use multiple bytes, so a password containing non-ASCII characters can reach 72 bytes with fewer than 72 characters. Measure the byte sequence produced by the same character encoding used by the verifier, rather than counting displayed characters or Unicode code points.
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Also establish whether the application normalizes text before encoding it. Registration, login, reset, import, and migration must apply the same encoding and normalization rules or credentials can stop matching.
What happens when a password exceeds the limit?
Behavior depends on the bcrypt implementation. Go’s GenerateFromPassword returns ErrPasswordTooLong for inputs over 72 bytes. Flask-Bcrypt and Passlib document behavior that ignores or truncates bytes after the limit. Consult the documentation for the exact library and version deployed; do not assume every implementation rejects long input.
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With silent truncation, two passwords that share their first 72 bytes but differ afterward are treated as the same bcrypt input. Passlib also documents truncation at the first NULL byte, so embedded-NUL behavior needs explicit compatibility testing.
How to handle the limit in an application
- Measure after encoding. Determine the byte length using the verifier’s actual character encoding and normalization policy.
- Choose a deliberate over-limit rule. Reject passwords over 72 bytes, or adopt a documented preprocessing scheme. Avoid silently relying on a library’s truncation behavior without accounting for its consequences.
- Apply one rule everywhere. Registration, login, password reset, account import, and migration must use compatible handling.
- Test boundary and compatibility cases. Include inputs of 71, 72, and 73 bytes; multibyte UTF-8 text near the boundary; embedded NUL bytes; and pairs that share the first 72 bytes but differ afterward.
- Record the implementation details. Track the deployed library and version, over-limit behavior, encoding and normalization rules, NUL handling, bcrypt prefix support such as
2aor2b, configurable work factor, and any pre-hash compatibility mode.
Re-test these cases when upgrading the bcrypt library: implementations and versions may not handle long inputs identically.
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Can you pre-hash a long password before bcrypt?
Pre-hashing changes the password-storage scheme; it is not a transparent fix for existing bcrypt hashes. Flask-Bcrypt documents a SHA-256 preprocessing workaround, but switching an existing system to that approach can make current passwords fail verification unless the application distinguishes the old and new schemes and handles migration deliberately.
If adopting preprocessing, specify the exact transformation and apply it consistently at enrollment and verification. Preserve a path for checking existing hashes, then migrate credentials in a controlled way rather than changing the transformation in place.
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What should you use for new password storage?
OWASP recommends Argon2id when available. For legacy systems that continue to use bcrypt, its Password Storage Cheat Sheet advises a work factor of 10 or more and a 72-byte password limit. The work factor and input limit address different concerns: the first controls bcrypt’s computational cost, while the second constrains its input.
Sources: Go bcrypt documentation; Go error documentation; patrickfav/bcrypt documentation; Flask-Bcrypt documentation; Passlib bcrypt documentation; OWASP Password Storage Cheat Sheet.
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