✨ Use pwdlib with Argon2 by default, adding logic (and tests) to autoupdate old passwords using Bcrypt (#2104)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

backend/tests/crud/test_user.py

@@ -1,4 +1,5 @@
 from fastapi.encoders import jsonable_encoder
+from pwdlib.hashers.bcrypt import BcryptHasher
 from sqlmodel import Session
 
 from app import crud
@@ -88,4 +89,42 @@ def test_update_user(db: Session) -> None:
     user_2 = db.get(User, user.id)
     assert user_2
     assert user.email == user_2.email
-    assert verify_password(new_password, user_2.hashed_password)
+    verified, _ = verify_password(new_password, user_2.hashed_password)
+    assert verified
+
+
+def test_authenticate_user_with_bcrypt_upgrades_to_argon2(db: Session) -> None:
+    """Test that a user with bcrypt password hash gets upgraded to argon2 on login."""
+    email = random_email()
+    password = random_lower_string()
+
+    # Create a bcrypt hash directly (simulating legacy password)
+    bcrypt_hasher = BcryptHasher()
+    bcrypt_hash = bcrypt_hasher.hash(password)
+    assert bcrypt_hash.startswith("$2")  # bcrypt hashes start with $2
+
+    # Create user with bcrypt hash directly in the database
+    user = User(email=email, hashed_password=bcrypt_hash)
+    db.add(user)
+    db.commit()
+    db.refresh(user)
+
+    # Verify the hash is bcrypt before authentication
+    assert user.hashed_password.startswith("$2")
+
+    # Authenticate - this should upgrade the hash to argon2
+    authenticated_user = crud.authenticate(session=db, email=email, password=password)
+    assert authenticated_user
+    assert authenticated_user.email == email
+
+    db.refresh(authenticated_user)
+
+    # Verify the hash was upgraded to argon2
+    assert authenticated_user.hashed_password.startswith("$argon2")
+
+    verified, updated_hash = verify_password(
+        password, authenticated_user.hashed_password
+    )
+    assert verified
+    # Should not need another update since it's already argon2
+    assert updated_hash is None