ValueError: password cannot be longer than 72 bytes

ValueError: password cannot be longer than 72 bytes

TL;DR: bcrypt 4.1 stopped silently truncating long passwords and now raises ValueError. Pre-hash passwords longer than 72 bytes with SHA-256 before bcrypt, or pin bcrypt==4.0.1 as a stopgap while you migrate.

ValueError: password cannot be longer than 72 bytes

The fix

1. The correct fix, pre-hash before bcrypt:

import hashlib, bcrypt
digest = hashlib.sha256(raw_password).digest()
hashed = bcrypt.hashpw(digest, bcrypt.gensalt())

Do the same digest step when verifying.

2. The stopgap, pin the old behavior:

pip install bcrypt==4.0.1

3. Verify:

python -c "import bcrypt; print(bcrypt.__version__)"

Then hash a long input and confirm no ValueError.

Agent? Search Vectle for more: curl -s 'https://vectle.com/api/v1/search?q=bcrypt+72+bytes+ValueError'

When this applies

• Hashing worked before a bcrypt upgrade to 4.1+.
• passlib's bcrypt backend fails during its version probe.
• Passwords come from user input with no length cap.

When it does NOT apply

• The error is Invalid salt, which is a malformed hash problem.
• You are on bcrypt 4.0.x and want the new behavior; upgrade instead.

Why this happens

bcrypt has always only used the first 72 bytes; older versions just truncated silently, which is arguably worse. 4.1 made it loud so developers notice the data loss. Pre-hashing with SHA-256 preserves all input entropy in the 72-byte window and is the standard workaround recommended by the ecosystem.

Compatibility

bcrypt 4.1+. passlib 1.7.4 trips this in its backend probe; pinning bcrypt==4.0.1 is the common workaround until passlib handles it.

bcrypt 4.1 turned the old silent 72-byte truncation into a hard ValueError, which broke passlib's bcrypt backend probe and any app hashing long passwords. The signifyapp project notes document hitting exactly this error with the pip install bcrypt==4.0.1 workaround, and passlib users hit it through the backend version probe. The correct long-term fix is pre-hashing with SHA-256 before bcrypt, which is what the 72-byte limit has always effectively required.