Scrypt.py

import os
import base64
import hashlib

class Scrypt():
    @staticmethod
    def _make_bytes(pw, encoding):
        if isinstance(pw, str):
            try:
                pw = pw.encode(encoding)
            except UnicodeEncodeError as e:
                e.args = ('Invalid string. Hashes must be ASCII and passwords should be UTF-8',)
                raise e
        if not isinstance(pw, bytes):
            raise ValueError(
                'Value must be {} or {}, not {}'\
                .format(repr(bytes), repr(str), type(pw))
            )
        return pw
      
    @staticmethod
    def _const_time_comparison(a, b):
        if len(a) != len(b):
            return False
        result = 0
        for j,k in zip(a,b):
            result ^= j ^ k
        if result == 0:
            return True
        else:
            return False
    
    @staticmethod
    def hash(password, salt=None, n=17, p=1, r=8, dklen=18, encoding='UTF-8'):
        maxmem = 0x7fffffff
        password = Scrypt._make_bytes(password, encoding)
        _n = 2**n
        salt = os.urandom(18) if salt is None else salt
        hash = hashlib.scrypt(password, salt=salt, n=_n, p=p, r=r, maxmem=maxmem, dklen=dklen)
        salt = base64.b64encode(salt).decode()
        hash = base64.b64encode(hash).decode()
        PHC = f'$scrypt$n={n}$p={p}$r={r}${salt}${hash}'
        return PHC
        
    @staticmethod
    def verify(param_string, password):
        params = param_string.split('$')[1:]
        salt = params[-2]
        try:
            params = {k:int(v) for k,v in map(lambda s: s.split('='), params[1:-2])}
        except ValueError:
            raise ValueError(
                'invalid parameter string'
            )
        salt = base64.b64decode(salt)
        return Scrypt._const_time_comparison(
                   Scrypt._make_bytes(Scrypt.hash(password, salt=salt, **params), 'ascii')
                  ,Scrypt._make_bytes(param_string, 'ascii'))