Python bcrypt: how to check a long encrypted by sha256 password?

Question

I have a long password I want to encode. I use the bcrypt main tutorial:

>>> password = b"an incredibly long password" * 10
>>> hashed = bcrypt.hashpw(
...     base64.b64encode(hashlib.sha256(password).digest()),
...     bcrypt.gensalt()
... )

However, when I check using the tutorial as follows, it does not match:

input_password = "..." # some password
bcrypt.checkpw(input_password.encode("utf8"), hashed)

I think I have to decode it as well. Should base64.b64encode(hashlib.sha256(input_password).digest() work?

Answer 1

To anyone reading this in the future: OP is right. From the BCrypt GitHub :

The bcrypt algorithm only handles passwords up to 72 characters, any characters beyond that are ignored. To work around this, a common approach is to hash a password with a cryptographic hash (such as sha256) and then base64 encode it to prevent NULL byte problems before hashing the result with bcrypt:

>>> password = b"an incredibly long password" * 10
>>> hashed = bcrypt.hashpw(
...     base64.b64encode(hashlib.sha256(password).digest()),
...     bcrypt.gensalt()
... )

To now answer the question:
Yes, to check the password, you do the same steps witht he password as you did before, so

>>> new_password = base64.b64encode(hashlib.sha256(raw_password).digest())
>>> bcrypt.checkpw(new_password, saved_password)

Don't forget to encode the passwords first, so for Python 3 something like

>>> raw_password = password_string.encode('utf_8')

It's likely a good a idea to encode to the same format.