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# Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Functions for PKCS#1 version 2 encryption and signing
This module implements certain functionality from PKCS#1 version 2. Main
documentation is RFC 2437: https://tools.ietf.org/html/rfc2437
"""
from rsa import (
common,
pkcs1,
transform,
)
def mgf1(seed: bytes, length: int, hasher: str = 'SHA-1') -> bytes:
"""
MGF1 is a Mask Generation Function based on a hash function.
A mask generation function takes an octet string of variable length and a
desired output length as input, and outputs an octet string of the desired
length. The plaintext-awareness of RSAES-OAEP relies on the random nature of
the output of the mask generation function, which in turn relies on the
random nature of the underlying hash.
:param bytes seed: seed from which mask is generated, an octet string
:param int length: intended length in octets of the mask, at most 2^32(hLen)
:param str hasher: hash function (hLen denotes the length in octets of the hash
function output)
:return: mask, an octet string of length `length`
:rtype: bytes
:raise OverflowError: when `length` is too large for the specified `hasher`
:raise ValueError: when specified `hasher` is invalid
"""
try:
hash_length = pkcs1.HASH_METHODS[hasher]().digest_size
except KeyError as ex:
raise ValueError(
'Invalid `hasher` specified. Please select one of: {hash_list}'.format(
hash_list=', '.join(sorted(pkcs1.HASH_METHODS.keys()))
)
) from ex
# If l > 2^32(hLen), output "mask too long" and stop.
if length > (2**32 * hash_length):
raise OverflowError(
"Desired length should be at most 2**32 times the hasher's output "
"length ({hash_length} for {hasher} function)".format(
hash_length=hash_length,
hasher=hasher,
)
)
# Looping `counter` from 0 to ceil(l / hLen)-1, build `output` based on the
# hashes formed by (`seed` + C), being `C` an octet string of length 4
# generated by converting `counter` with the primitive I2OSP
output = b''.join(
pkcs1.compute_hash(
seed + transform.int2bytes(counter, fill_size=4),
method_name=hasher,
)
for counter in range(common.ceil_div(length, hash_length) + 1)
)
# Output the leading `length` octets of `output` as the octet string mask.
return output[:length]
__all__ = [
'mgf1',
]
if __name__ == '__main__':
print('Running doctests 1000x or until failure')
import doctest
for count in range(1000):
(failures, tests) = doctest.testmod()
if failures:
break
if count % 100 == 0 and count:
print('%i times' % count)
print('Doctests done')
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