Methods to create password verifiers for PostgreSQL

encrypt_password.py

"""
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