Methods to create password verifiers for PostgreSQL

encrypt_password.py

# Copyright 2019-2022 Jonathan S. Katz
# 
# MIT License
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
"""
Generate the password hashes / verifiers for use in PostgreSQL

How to use this:

pw = EncryptPassword(
    user="username",
    password="securepassword",
    algorithm="scram-sha-256",
)
print(pw.encrypt())

The output of the ``encrypt`` function can be stored in PostgreSQL in the
password clause, e.g.

    ALTER ROLE username PASSWORD {pw.encrypt()};

where you safely interpolate it in with a quoted literal, of course :)
"""
import base64
import hashlib
import hmac
import secrets
import stringprep
import unicodedata

class EncryptPassword:
    ALGORITHMS = {
        'md5': {
            'encryptor': '_encrypt_md5',
            'digest': hashlib.md5,
            'defaults': {},
        },
        'scram-sha-256': {
            'encryptor': '_encrypt_scram_sha_256',
            'digest': hashlib.sha256,
            'defaults': {
                'salt_length': 16,
                'iterations': 4096,
            },
        }
    }
    # List of characters that are prohibited to be used per PostgreSQL-SASLprep
    SASLPREP_STEP3 = (
        stringprep.in_table_a1, # PostgreSQL treats this as prohibited
        stringprep.in_table_c12,
        stringprep.in_table_c21_c22,
        stringprep.in_table_c3,
        stringprep.in_table_c4,
        stringprep.in_table_c5,
        stringprep.in_table_c6,
        stringprep.in_table_c7,
        stringprep.in_table_c8,
        stringprep.in_table_c9,
    )


    def __init__(self, user, password, algorithm='scram-sha-256', **kwargs):
        self.user = user
        self.password = password
        self.algorithm = algorithm
        self.salt = None
        self.encrypted_password = None
        self.kwargs = kwargs

    def encrypt(self):
        try:
            algorithm = self.ALGORITHMS[self.algorithm]
        except KeyError:
            raise Exception('algorithm "{}" not supported'.format(self.algorithm))
        kwargs = algorithm['defaults'].copy()
        kwargs.update(self.kwargs)
        return getattr(self, algorithm['encryptor'])(algorithm['digest'], **kwargs)

    def _bytes_xor(self, a, b):
        """XOR two bytestrings together"""
        return bytes(a_i ^ b_i for a_i, b_i in zip(a, b))

    def _encrypt_md5(self, digest, **kwargs):
        self.encrypted_password = b"md5" + digest(
            self.password.encode('utf-8') + self.user.encode('utf-8')).hexdigest().encode('utf-8')
        return self.encrypted_password

    def _encrypt_scram_sha_256(self, digest, **kwargs):
        # requires SASL prep
        # password = SASLprep
        iterations = kwargs['iterations']
        salt_length = kwargs['salt_length']
        salted_password = self._scram_sha_256_generate_salted_password(self.password, salt_length, iterations, digest)
        client_key = hmac.HMAC(salted_password, b"Client Key", digest)
        stored_key = digest(client_key.digest()).digest()
        server_key = hmac.HMAC(salted_password, b"Server Key", digest)
        self.encrypted_password = self.algorithm.upper().encode("utf-8") + b"$" + \
            ("{}".format(iterations)).encode("utf-8") + b":" + \
            base64.b64encode(self.salt) + b"$" + \
            base64.b64encode(stored_key) + b":" + base64.b64encode(server_key.digest())
        return self.encrypted_password

    def _normalize_password(self, password):
        """Normalize the password using PostgreSQL-flavored SASLprep. For reference:

        https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/common/saslprep.c
        using the `pg_saslprep` function

        Implementation borrowed from asyncpg implementation:
        https://github.com/MagicStack/asyncpg/blob/master/asyncpg/protocol/scram.pyx#L263
        """
        normalized_password = password

        # if the password is an ASCII string or fails to encode as an UTF8
        # string, we can return
        try:
            normalized_password.encode("ascii")
        except UnicodeEncodeError:
            pass
        else:
            return normalized_password

        # Step 1 of SASLPrep: Map. Per the algorithm, we map non-ascii space
        # characters to ASCII spaces (\x20 or \u0020, but we will use ' ') and
        # commonly mapped to nothing characters are removed
        # Table C.1.2 -- non-ASCII spaces
        # Table B.1 -- "Commonly mapped to nothing"
        normalized_password = u"".join(
            [' ' if stringprep.in_table_c12(c) else c
            for c in normalized_password if not stringprep.in_table_b1(c)])

        # If at this point the password is empty, PostgreSQL uses the original
        # password
        if not normalized_password:
            return password

        # Step 2 of SASLPrep: Normalize. Normalize the password using the
        # Unicode normalization algorithm to NFKC form
        normalized_password = unicodedata.normalize('NFKC', normalized_password)

        # If the password is not empty, PostgreSQL uses the original password
        if not normalized_password:
            return password

        # Step 3 of SASLPrep: Prohobited characters. If PostgreSQL detects any
        # of the prohibited characters in SASLPrep, it will use the original
        # password
        # We also include "unassigned code points" in the prohibited character
        # category as PostgreSQL does the same
        for c in normalized_password:
            if any([in_prohibited_table(c) for in_prohibited_table in
                    self.SASLPREP_STEP3]):
                return password

        # Step 4 of SASLPrep: Bi-directional characters. PostgreSQL follows the
        # rules for bi-directional characters laid on in RFC3454 Sec. 6 which
        # are:
        # 1. Characters in RFC 3454 Sec 5.8 are prohibited (C.8)
        # 2. If a string contains a RandALCat character, it cannot containy any
        #    LCat character
        # 3. If the string contains any RandALCat character, an RandALCat
        #    character must be the first and last character of the string
        # RandALCat characters are found in table D.1, whereas LCat are in D.2
        if any([stringprep.in_table_d1(c) for c in normalized_password]):
            # if the first character or the last character are not in D.1,
            # return the original password
            if not (stringprep.in_table_d1(normalized_password[0]) and
                    stringprep.in_table_d1(normalized_password[-1])):
                return password

            # if any characters are in D.2, use the original password
            if any([stringprep.in_table_d2(c) for c in normalized_password]):
                return password

        # return the normalized password
        return normalized_password


    def _scram_sha_256_generate_salted_password(self, password, salt_length, iterations, digest):
        """This follows the "Hi" algorithm specified in RFC5802"""
        # first, need to normalize the password using PostgreSQL-flavored SASLprep
        normalized_password = self._normalize_password(password)
        # convert the password to a binary string - UTF8 is safe for SASL (though there are SASLPrep rules)
        p = normalized_password.encode("utf8")
        # generate a salt
        self.salt = secrets.token_bytes(salt_length)
        # the initial signature is the salt with a terminator of a 32-bit string ending in 1
        ui = hmac.new(p, self.salt + b'\x00\x00\x00\x01', digest)
        # grab the initial digest
        u = ui.digest()
        # for X number of iterations, recompute the HMAC signature against the password
        # and the latest iteration of the hash, and XOR it with the previous version
        for x in range(iterations - 1):
            ui = hmac.new(p, ui.digest(), hashlib.sha256)
            # this is a fancy way of XORing two byte strings together
            u = self._bytes_xor(u, ui.digest())
        return u

We have adapted your code for one function in Python. This made it possible to create it in the database. The pgsql shell provides password challenge and verification. This is great. Thanks.
Chek function
It is irresponsible to give access to pg_authid to the user being checked. Therefore, the SECURITY DEFINER call mode is used. On behalf of a user who has access but does not have the login right.