Migrate password hashing from bcrypt to argon2

[exhuma]

Background

Passwords are currently hashed with bcrypt (src/powonline/model.py:10, 488–498). Argon2 (specifically argon2-cffi) is the current recommended algorithm by OWASP and the Password Hashing Competition winner. It is memory-hard, which makes GPU/ASIC brute-force significantly more expensive than bcrypt.

Work required

Dependency change

• Remove bcrypt from pyproject.toml and requirements.txt
• Add argon2-cffi (provides argon2.PasswordHasher)

src/powonline/model.py

Replace the three bcrypt call sites with argon2.PasswordHasher:

Current	Replacement
hashpw(password.encode("utf8"), gensalt())	ph.hash(password)
checkpw(password.encode("utf8"), self.password)	ph.verify(self.password, password)
setpw method — same as above	same

Argon2 hashes are stored as strings (str), so the password column type should be changed from BYTEA to TEXT (requires an Alembic migration).

Alembic migration

A new migration is needed to:

1. ALTER COLUMN password TYPE TEXT USING password::text (or re-hash on first login — see note below)
2. Optionally set a password_algorithm marker column to distinguish legacy bcrypt rows from new argon2 rows, enabling a transparent upgrade path on login (similar to the existing password_is_plaintext flag).

Transparent upgrade path (recommended)

On User.checkpw():

1. If the stored hash starts with $2b$ (bcrypt prefix), verify with bcrypt, then re-hash and store as argon2.
2. Otherwise verify with argon2.

This avoids a forced password reset for existing users.

Tests

Update tests/seed.sql and any fixtures that insert raw bcrypt hashes.

References

• OWASP Password Storage Cheat Sheet
• argon2-cffi docs
• Existing password_is_plaintext upgrade-path pattern in model.py:491–494