/usr/lib/python3/dist-packages/Cryptodome/Signature/pss.py is in python3-pycryptodome 3.4.7-1ubuntu1.
This file is owned by root:root, with mode 0o644.
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#
# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
# All rights reserved.
#
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# modification, are permitted provided that the following conditions
# are met:
#
# 1. Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
# 2. Redistributions in binary form must reproduce the above copyright
# notice, this list of conditions and the following disclaimer in
# the documentation and/or other materials provided with the
# distribution.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
# COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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# ===================================================================
from Cryptodome.Util.py3compat import b, bchr, bord
import Cryptodome.Util.number
from Cryptodome.Util.number import (ceil_div,
long_to_bytes,
bytes_to_long
)
from Cryptodome.Util.strxor import strxor
from Cryptodome import Random
class PSS_SigScheme:
"""A signature object for ``RSASSA-PSS``.
Do not instantiate directly.
Use :func:`Cryptodome.Signature.pss.new`.
"""
def __init__(self, key, mgfunc, saltLen, randfunc):
"""Initialize this PKCS#1 PSS signature scheme object.
:Parameters:
key : an RSA key object
If a private half is given, both signature and
verification are possible.
If a public half is given, only verification is possible.
mgfunc : callable
A mask generation function that accepts two parameters:
a string to use as seed, and the lenth of the mask to
generate, in bytes.
saltLen : integer
Length of the salt, in bytes.
randfunc : callable
A function that returns random bytes.
"""
self._key = key
self._saltLen = saltLen
self._mgfunc = mgfunc
self._randfunc = randfunc
def can_sign(self):
"""Return ``True`` if this object can be used to sign messages."""
return self._key.has_private()
def sign(self, msg_hash):
"""Create the PKCS#1 PSS signature of a message.
This function is also called ``RSASSA-PSS-SIGN`` and
it is specified in
`section 8.1.1 of RFC8017 <https://tools.ietf.org/html/rfc8017#section-8.1.1>`_.
:parameter msg_hash:
This is an object from the :mod:`Cryptodome.Hash` package.
It has been used to digest the message to sign.
:type msg_hash: hash object
:return: the signature encoded as a *byte string*.
:raise ValueError: if the RSA key is not long enough for the given hash algorithm.
:raise TypeError: if the RSA key has no private half.
"""
# Set defaults for salt length and mask generation function
if self._saltLen is None:
sLen = msg_hash.digest_size
else:
sLen = self._saltLen
if self._mgfunc is None:
mgf = lambda x, y: MGF1(x, y, msg_hash)
else:
mgf = self._mgfunc
modBits = Cryptodome.Util.number.size(self._key.n)
# See 8.1.1 in RFC3447
k = ceil_div(modBits, 8) # k is length in bytes of the modulus
# Step 1
em = _EMSA_PSS_ENCODE(msg_hash, modBits-1, self._randfunc, mgf, sLen)
# Step 2a (OS2IP)
em_int = bytes_to_long(em)
# Step 2b (RSASP1)
m_int = self._key._decrypt(em_int)
# Step 2c (I2OSP)
signature = long_to_bytes(m_int, k)
return signature
def verify(self, msg_hash, signature):
"""Check if the PKCS#1 PSS signature over a message is valid.
This function is also called ``RSASSA-PSS-VERIFY`` and
it is specified in
`section 8.1.2 of RFC8037 <https://tools.ietf.org/html/rfc8017#section-8.1.2>`_.
:parameter msg_hash:
The hash that was carried out over the message. This is an object
belonging to the :mod:`Cryptodome.Hash` module.
:type parameter: hash object
:parameter signature:
The signature that needs to be validated.
:type signature: byte string
:raise ValueError: if the signature is not valid.
"""
# Set defaults for salt length and mask generation function
if self._saltLen is None:
sLen = msg_hash.digest_size
else:
sLen = self._saltLen
if self._mgfunc:
mgf = self._mgfunc
else:
mgf = lambda x, y: MGF1(x, y, msg_hash)
modBits = Cryptodome.Util.number.size(self._key.n)
# See 8.1.2 in RFC3447
k = ceil_div(modBits, 8) # Convert from bits to bytes
# Step 1
if len(signature) != k:
raise ValueError("Incorrect signature")
# Step 2a (O2SIP)
signature_int = bytes_to_long(signature)
# Step 2b (RSAVP1)
em_int = self._key._encrypt(signature_int)
# Step 2c (I2OSP)
emLen = ceil_div(modBits - 1, 8)
em = long_to_bytes(em_int, emLen)
# Step 3/4
_EMSA_PSS_VERIFY(msg_hash, em, modBits-1, mgf, sLen)
def MGF1(mgfSeed, maskLen, hash_gen):
"""Mask Generation Function, described in `B.2.1 of RFC8017
<https://tools.ietf.org/html/rfc8017>`_.
:param mfgSeed:
seed from which the mask is generated
:type mfgSeed: byte string
:param maskLen:
intended length in bytes of the mask
:type maskLen: integer
:param hash_gen:
A module or a hash object from :mod:`Cryptodome.Hash`
:type hash_object:
:return: the mask, as a *byte string*
"""
T = b("")
for counter in range(ceil_div(maskLen, hash_gen.digest_size)):
c = long_to_bytes(counter, 4)
hobj = hash_gen.new()
hobj.update(mgfSeed + c)
T = T + hobj.digest()
assert(len(T) >= maskLen)
return T[:maskLen]
def _EMSA_PSS_ENCODE(mhash, emBits, randFunc, mgf, sLen):
"""
Implement the ``EMSA-PSS-ENCODE`` function, as defined
in PKCS#1 v2.1 (RFC3447, 9.1.1).
The original ``EMSA-PSS-ENCODE`` actually accepts the message ``M``
as input, and hash it internally. Here, we expect that the message
has already been hashed instead.
:Parameters:
mhash : hash object
The hash object that holds the digest of the message being signed.
emBits : int
Maximum length of the final encoding, in bits.
randFunc : callable
An RNG function that accepts as only parameter an int, and returns
a string of random bytes, to be used as salt.
mgf : callable
A mask generation function that accepts two parameters: a string to
use as seed, and the lenth of the mask to generate, in bytes.
sLen : int
Length of the salt, in bytes.
:Return: An ``emLen`` byte long string that encodes the hash
(with ``emLen = \ceil(emBits/8)``).
:Raise ValueError:
When digest or salt length are too big.
"""
emLen = ceil_div(emBits, 8)
# Bitmask of digits that fill up
lmask = 0
for i in range(8*emLen-emBits):
lmask = lmask >> 1 | 0x80
# Step 1 and 2 have been already done
# Step 3
if emLen < mhash.digest_size+sLen+2:
raise ValueError("Digest or salt length are too long"
" for given key size.")
# Step 4
salt = randFunc(sLen)
# Step 5
m_prime = bchr(0)*8 + mhash.digest() + salt
# Step 6
h = mhash.new()
h.update(m_prime)
# Step 7
ps = bchr(0)*(emLen-sLen-mhash.digest_size-2)
# Step 8
db = ps + bchr(1) + salt
# Step 9
dbMask = mgf(h.digest(), emLen-mhash.digest_size-1)
# Step 10
maskedDB = strxor(db, dbMask)
# Step 11
maskedDB = bchr(bord(maskedDB[0]) & ~lmask) + maskedDB[1:]
# Step 12
em = maskedDB + h.digest() + bchr(0xBC)
return em
def _EMSA_PSS_VERIFY(mhash, em, emBits, mgf, sLen):
"""
Implement the ``EMSA-PSS-VERIFY`` function, as defined
in PKCS#1 v2.1 (RFC3447, 9.1.2).
``EMSA-PSS-VERIFY`` actually accepts the message ``M`` as input,
and hash it internally. Here, we expect that the message has already
been hashed instead.
:Parameters:
mhash : hash object
The hash object that holds the digest of the message to be verified.
em : string
The signature to verify, therefore proving that the sender really
signed the message that was received.
emBits : int
Length of the final encoding (em), in bits.
mgf : callable
A mask generation function that accepts two parameters: a string to
use as seed, and the lenth of the mask to generate, in bytes.
sLen : int
Length of the salt, in bytes.
:Raise ValueError:
When the encoding is inconsistent, or the digest or salt lengths
are too big.
"""
emLen = ceil_div(emBits, 8)
# Bitmask of digits that fill up
lmask = 0
for i in range(8*emLen-emBits):
lmask = lmask >> 1 | 0x80
# Step 1 and 2 have been already done
# Step 3
if emLen < mhash.digest_size+sLen+2:
return False
# Step 4
if ord(em[-1:]) != 0xBC:
raise ValueError("Incorrect signature")
# Step 5
maskedDB = em[:emLen-mhash.digest_size-1]
h = em[emLen-mhash.digest_size-1:-1]
# Step 6
if lmask & bord(em[0]):
raise ValueError("Incorrect signature")
# Step 7
dbMask = mgf(h, emLen-mhash.digest_size-1)
# Step 8
db = strxor(maskedDB, dbMask)
# Step 9
db = bchr(bord(db[0]) & ~lmask) + db[1:]
# Step 10
if not db.startswith(bchr(0)*(emLen-mhash.digest_size-sLen-2) + bchr(1)):
raise ValueError("Incorrect signature")
# Step 11
if sLen > 0:
salt = db[-sLen:]
else:
salt = b("")
# Step 12
m_prime = bchr(0)*8 + mhash.digest() + salt
# Step 13
hobj = mhash.new()
hobj.update(m_prime)
hp = hobj.digest()
# Step 14
if h != hp:
raise ValueError("Incorrect signature")
def new(rsa_key, **kwargs):
"""Create a signature object for creating
or verifying PKCS#1 PSS signatures.
:parameter rsa_key:
The RSA key to use for signing or verifying the message.
This is a :class:`Cryptodome.PublicKey.RSA` object.
Signing is only possible when ``rsa_key`` is a **private** RSA key.
:type rsa_key: RSA object
:Keyword Arguments:
* *mask_func* (``callable``) --
A mask generation function that accepts two parameters: a string to
use as seed, and the length of the mask in bytes to generate.
If not specified, the standard :func:`MGF1` is used.
* *salt_bytes* (``integer``) --
Length of the salt, in bytes.
If not specified, it matches the output size of the hash function.
If zero, the signature scheme becomes deterministic.
* *rand_func* (``callable``) --
A function that returns random *byte string*, given the desired
length.
The default is :func:`Cryptodome.Random.get_random_bytes`.
:return: a :class:`PSS_SigScheme` signature object
"""
mask_func = kwargs.pop("mask_func", None)
salt_len = kwargs.pop("salt_bytes", None)
rand_func = kwargs.pop("rand_func", None)
if rand_func is None:
rand_func = Random.get_random_bytes
if kwargs:
raise ValueError("Unknown keywords: " + str(list(kwargs.keys())))
return PSS_SigScheme(rsa_key, mask_func, salt_len, rand_func)
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