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