# Copyright (c) 2015, Hubert Kario
#
# See the LICENSE file for legal information regarding use of this file.
"""Implementation of Poly1305 authenticator for RFC 7539"""
from .cryptomath import divceil
[docs]
class Poly1305(object):
"""Poly1305 authenticator"""
P = 0x3fffffffffffffffffffffffffffffffb # 2^130-5
[docs]
@staticmethod
def le_bytes_to_num(data):
"""Convert a number from little endian byte format"""
ret = 0
for i in range(len(data) - 1, -1, -1):
ret <<= 8
ret += data[i]
return ret
[docs]
@staticmethod
def num_to_16_le_bytes(num):
"""Convert number to 16 bytes in little endian format"""
ret = [0]*16
for i, _ in enumerate(ret):
ret[i] = num & 0xff
num >>= 8
return bytearray(ret)
[docs]
def __init__(self, key):
"""Set the authenticator key"""
if len(key) != 32:
raise ValueError("Key must be 256 bit long")
self.acc = 0
self.r = self.le_bytes_to_num(key[0:16])
self.r &= 0x0ffffffc0ffffffc0ffffffc0fffffff
self.s = self.le_bytes_to_num(key[16:32])
[docs]
def create_tag(self, data):
"""Calculate authentication tag for data"""
for i in range(0, divceil(len(data), 16)):
n = self.le_bytes_to_num(data[i*16:(i+1)*16] + b'\x01')
self.acc += n
self.acc = (self.r * self.acc) % self.P
self.acc += self.s
return self.num_to_16_le_bytes(self.acc)