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aes_drbg.py
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aes_drbg.py
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#MIT License
#Copyright (c) 2019 Samuele Cornell
#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
#LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
#OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
#SOFTWARE.
import pyaes
class AES_DRBG(object):
"""AES based DRBG class compliant with SP-900 80A NIST standard.
This implementation follows closely the above Specification in order to be as clear as possible.
Constructor requires only the specification of the desired AES version (128/192 or 256 Keylen):
Parameters
----------
keylen : keylength in bits for AES block cipher used in the DRBG
Returns
-------
drbg_object
The DRBG has 3 main methods:
1 INSTANTIATE: initialize and instantiate the DRBG
2 RESEED: reseed the DRBG (this must be done every 2**48 calls)
3 GENERATE obtain pseudo-random bits from DRBG
"""
def __init__(self, keylen):
self.keylen = keylen
self.reseed_counter = 0
self.key = False
self.V = False
self.outlen = 16 # same for all
if keylen == 256:
self.seedlen = 48
self.keylen = 32
elif keylen == 192:
self.seedlen = 40
self.keylen = 24
elif keylen == 128:
self.seedlen = 32
self.keylen = 16
else:
raise ValueError("Keylen not supported")
self.reseed_interval = 2 ** 48 # same for all
def instantiate(self, entropy_in, per_string=b''):
'''
Method handling initialization of the DRBG (see Specification)
Parameters
----------
entropy_in : hex byterray (e.g. \xFF\xF1....etc (len = seedlen_bits /8))
full entropy seed for DRBG, it must be seedlen bits
per_string : hex byterray (e.g. \xFF\xF1....etc (len must be less or equal seedlen))
additional input which will be xored with the input entropy for added security (optional)
Returns
-------
'''
if len(per_string) is not 0:
temp = len(per_string) # NB len is in bytes
if temp < self.seedlen:
per_string = per_string + b"\x00" * (self.seedlen - temp) # pad
else:
raise ValueError("Length of personalization string must be equal or less than seedlen")
else:
per_string = b"\x00" * self.seedlen
seed_material = int(entropy_in.hex(), 16) ^ int(per_string.hex(), 16)
seed_material = seed_material.to_bytes(self.seedlen, byteorder='big', signed=False)
self.key = b"\x00" * self.keylen
self.V = b"\x00" * self.outlen
self.aes = pyaes.AESModeOfOperationECB(self.key)
self._update(seed_material)
self.reseed_counter = 1
def _update(self, provided_data):
'''
DRBG internal Update function (see Specification)
Parameters
----------
provided_data : hex byterray (e.g. \xFF\xF1....etc (len = seedlen_bits /8))
input data to the update function (it is ensured in other methods is seedlen bits)
Returns
-------
'''
temp = b""
while (len(temp) < self.seedlen):
# increment V
self.V = (int(self.V.hex(), 16) + 1) % 2 ** (self.outlen * 8)
self.V = self.V.to_bytes(self.outlen, byteorder='big', signed=False)
output_block = self.aes.encrypt(self.V) # generate keystream
temp = temp + output_block # concat keystream
temp = temp[0:self.seedlen]
temp = int(temp.hex(), 16) ^ int(provided_data.hex(), 16) # xor keystream
temp = temp.to_bytes(self.seedlen, byteorder='big', signed=False)
self.key = temp[0:self.keylen]
self.V = temp[-self.outlen:]
self.aes = pyaes.AESModeOfOperationECB(self.key) # update the key
def reseed(self, entropy_in, add_in=b''):
'''
DRBG Reseed function (see Specification)
Similar to instantiate except the previous DRBG state (self.key, self.V, self.reseed_counter) is
preserved and updated with full entropy.
Parameters
----------
entropy_in : hex byterray (e.g. \xFF\xF1....etc (len = seedlen_bits /8))
full entropy seed for DRBG, it must be seedlen bits
add_in : hex byterray (e.g. \xFF\xF1....etc (len must be less or equal seedlen))
additional input which will be xored with the input entropy for added security (optional)
Returns
-------
'''
if len(add_in) is not 0:
temp = len(add_in) # NB len is in bytes
if temp < self.seedlen:
add_in = add_in + b"\x00" * (self.seedlen - temp) # pad
else:
raise ValueError("Length of personalization string must be equal or less than seedlen")
else:
add_in = b"\x00" * self.seedlen
seed_material = int(entropy_in.hex(), 16) ^ int(add_in.hex(), 16)
seed_material = seed_material.to_bytes(self.seedlen, byteorder='big', signed=False)
self._update(seed_material)
self.reseed_counter = 1
def generate(self, req_bytes, add_in=b''):
''' DRBG Generate Funtion (see Specification)
returns req_bytes pseudo-random bits from the DRBG
Parameters
----------
req_bytes : int
number of bytes requested from the DRBG
add_in : hex byterray (e.g. \xFF\xF1....etc (len must be less or equal seedlen))
additional input which will be xored with the output of DRBG (optional)
Returns
-------
returned_bytes: hex byterray (e.g. \xFF\xF1....etc (len is req_bytes))
pseudo-random bytes from DRBG ready to be used in whatever application
'''
if self.reseed_counter > self.reseed_interval:
raise Warning("the DBRG should be reseeded !!!")
if len(add_in) is not 0:
temp = len(add_in)
if temp < self.seedlen:
add_in = add_in + b"\x00" * (temp - self.seedlen)
self._update(add_in)
else:
add_in = b"\x00" * self.seedlen
temp = b''
while (len(temp) < req_bytes):
self.V = (int(self.V.hex(), 16) + 1) % 2 ** (self.outlen * 8)
self.V = self.V.to_bytes(self.outlen, byteorder='big', signed=False)
output_block = self.aes.encrypt(self.V)
temp = temp + output_block
returned_bytes = temp[0:req_bytes]
self._update(add_in)
self.reseed_counter = self.reseed_counter + 1
return returned_bytes