403 lines
15 KiB
Python
403 lines
15 KiB
Python
#
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# Signature/DSS.py : DSS.py
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#
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# ===================================================================
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#
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# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
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# All rights reserved.
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#
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# Redistribution and use in source and binary forms, with or without
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# modification, are permitted provided that the following conditions
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# are met:
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#
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# 1. Redistributions of source code must retain the above copyright
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# notice, this list of conditions and the following disclaimer.
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# 2. Redistributions in binary form must reproduce the above copyright
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# notice, this list of conditions and the following disclaimer in
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# the documentation and/or other materials provided with the
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# distribution.
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#
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# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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# COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
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# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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# POSSIBILITY OF SUCH DAMAGE.
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# ===================================================================
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from Cryptodome.Util.asn1 import DerSequence
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from Cryptodome.Util.number import long_to_bytes
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from Cryptodome.Math.Numbers import Integer
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from Cryptodome.Hash import HMAC
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from Cryptodome.PublicKey.ECC import EccKey
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from Cryptodome.PublicKey.DSA import DsaKey
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__all__ = ['DssSigScheme', 'new']
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class DssSigScheme(object):
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"""A (EC)DSA signature object.
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Do not instantiate directly.
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Use :func:`Cryptodome.Signature.DSS.new`.
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"""
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def __init__(self, key, encoding, order):
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"""Create a new Digital Signature Standard (DSS) object.
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Do not instantiate this object directly,
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use `Cryptodome.Signature.DSS.new` instead.
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"""
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self._key = key
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self._encoding = encoding
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self._order = order
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self._order_bits = self._order.size_in_bits()
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self._order_bytes = (self._order_bits - 1) // 8 + 1
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def can_sign(self):
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"""Return ``True`` if this signature object can be used
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for signing messages."""
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return self._key.has_private()
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def _compute_nonce(self, msg_hash):
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raise NotImplementedError("To be provided by subclasses")
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def _valid_hash(self, msg_hash):
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raise NotImplementedError("To be provided by subclasses")
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def sign(self, msg_hash):
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"""Compute the DSA/ECDSA signature of a message.
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Args:
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msg_hash (hash object):
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The hash that was carried out over the message.
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The object belongs to the :mod:`Cryptodome.Hash` package.
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Under mode ``'fips-186-3'``, the hash must be a FIPS
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approved secure hash (SHA-2 or SHA-3).
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:return: The signature as ``bytes``
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:raise ValueError: if the hash algorithm is incompatible to the (EC)DSA key
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:raise TypeError: if the (EC)DSA key has no private half
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"""
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if not self._key.has_private():
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raise TypeError("Private key is needed to sign")
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if not self._valid_hash(msg_hash):
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raise ValueError("Hash is not sufficiently strong")
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# Generate the nonce k (critical!)
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nonce = self._compute_nonce(msg_hash)
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# Perform signature using the raw API
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z = Integer.from_bytes(msg_hash.digest()[:self._order_bytes])
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sig_pair = self._key._sign(z, nonce)
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# Encode the signature into a single byte string
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if self._encoding == 'binary':
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output = b"".join([long_to_bytes(x, self._order_bytes)
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for x in sig_pair])
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else:
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# Dss-sig ::= SEQUENCE {
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# r INTEGER,
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# s INTEGER
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# }
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# Ecdsa-Sig-Value ::= SEQUENCE {
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# r INTEGER,
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# s INTEGER
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# }
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output = DerSequence(sig_pair).encode()
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return output
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def verify(self, msg_hash, signature):
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"""Check if a certain (EC)DSA signature is authentic.
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Args:
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msg_hash (hash object):
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The hash that was carried out over the message.
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This is an object belonging to the :mod:`Cryptodome.Hash` module.
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Under mode ``'fips-186-3'``, the hash must be a FIPS
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approved secure hash (SHA-2 or SHA-3).
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signature (``bytes``):
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The signature that needs to be validated.
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:raise ValueError: if the signature is not authentic
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"""
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if not self._valid_hash(msg_hash):
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raise ValueError("Hash is not sufficiently strong")
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if self._encoding == 'binary':
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if len(signature) != (2 * self._order_bytes):
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raise ValueError("The signature is not authentic (length)")
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r_prime, s_prime = [Integer.from_bytes(x)
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for x in (signature[:self._order_bytes],
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signature[self._order_bytes:])]
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else:
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try:
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der_seq = DerSequence().decode(signature, strict=True)
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except (ValueError, IndexError):
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raise ValueError("The signature is not authentic (DER)")
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if len(der_seq) != 2 or not der_seq.hasOnlyInts():
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raise ValueError("The signature is not authentic (DER content)")
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r_prime, s_prime = Integer(der_seq[0]), Integer(der_seq[1])
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if not (0 < r_prime < self._order) or not (0 < s_prime < self._order):
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raise ValueError("The signature is not authentic (d)")
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z = Integer.from_bytes(msg_hash.digest()[:self._order_bytes])
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result = self._key._verify(z, (r_prime, s_prime))
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if not result:
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raise ValueError("The signature is not authentic")
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# Make PyCryptodome code to fail
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return False
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class DeterministicDsaSigScheme(DssSigScheme):
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# Also applicable to ECDSA
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def __init__(self, key, encoding, order, private_key):
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super(DeterministicDsaSigScheme, self).__init__(key, encoding, order)
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self._private_key = private_key
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def _bits2int(self, bstr):
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"""See 2.3.2 in RFC6979"""
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result = Integer.from_bytes(bstr)
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q_len = self._order.size_in_bits()
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b_len = len(bstr) * 8
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if b_len > q_len:
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# Only keep leftmost q_len bits
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result >>= (b_len - q_len)
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return result
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def _int2octets(self, int_mod_q):
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"""See 2.3.3 in RFC6979"""
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assert 0 < int_mod_q < self._order
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return long_to_bytes(int_mod_q, self._order_bytes)
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def _bits2octets(self, bstr):
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"""See 2.3.4 in RFC6979"""
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z1 = self._bits2int(bstr)
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if z1 < self._order:
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z2 = z1
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else:
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z2 = z1 - self._order
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return self._int2octets(z2)
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def _compute_nonce(self, mhash):
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"""Generate k in a deterministic way"""
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# See section 3.2 in RFC6979.txt
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# Step a
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h1 = mhash.digest()
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# Step b
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mask_v = b'\x01' * mhash.digest_size
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# Step c
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nonce_k = b'\x00' * mhash.digest_size
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for int_oct in (b'\x00', b'\x01'):
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# Step d/f
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nonce_k = HMAC.new(nonce_k,
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mask_v + int_oct +
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self._int2octets(self._private_key) +
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self._bits2octets(h1), mhash).digest()
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# Step e/g
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mask_v = HMAC.new(nonce_k, mask_v, mhash).digest()
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nonce = -1
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while not (0 < nonce < self._order):
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# Step h.C (second part)
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if nonce != -1:
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nonce_k = HMAC.new(nonce_k, mask_v + b'\x00',
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mhash).digest()
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mask_v = HMAC.new(nonce_k, mask_v, mhash).digest()
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# Step h.A
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mask_t = b""
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# Step h.B
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while len(mask_t) < self._order_bytes:
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mask_v = HMAC.new(nonce_k, mask_v, mhash).digest()
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mask_t += mask_v
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# Step h.C (first part)
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nonce = self._bits2int(mask_t)
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return nonce
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def _valid_hash(self, msg_hash):
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return True
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class FipsDsaSigScheme(DssSigScheme):
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#: List of L (bit length of p) and N (bit length of q) combinations
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#: that are allowed by FIPS 186-3. The security level is provided in
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#: Table 2 of FIPS 800-57 (rev3).
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_fips_186_3_L_N = (
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(1024, 160), # 80 bits (SHA-1 or stronger)
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(2048, 224), # 112 bits (SHA-224 or stronger)
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(2048, 256), # 128 bits (SHA-256 or stronger)
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(3072, 256) # 256 bits (SHA-512)
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)
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def __init__(self, key, encoding, order, randfunc):
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super(FipsDsaSigScheme, self).__init__(key, encoding, order)
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self._randfunc = randfunc
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L = Integer(key.p).size_in_bits()
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if (L, self._order_bits) not in self._fips_186_3_L_N:
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error = ("L/N (%d, %d) is not compliant to FIPS 186-3"
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% (L, self._order_bits))
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raise ValueError(error)
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def _compute_nonce(self, msg_hash):
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# hash is not used
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return Integer.random_range(min_inclusive=1,
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max_exclusive=self._order,
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randfunc=self._randfunc)
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def _valid_hash(self, msg_hash):
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"""Verify that SHA-1, SHA-2 or SHA-3 are used"""
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return (msg_hash.oid == "1.3.14.3.2.26" or
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msg_hash.oid.startswith("2.16.840.1.101.3.4.2."))
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class FipsEcDsaSigScheme(DssSigScheme):
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def __init__(self, key, encoding, order, randfunc):
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super(FipsEcDsaSigScheme, self).__init__(key, encoding, order)
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self._randfunc = randfunc
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def _compute_nonce(self, msg_hash):
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return Integer.random_range(min_inclusive=1,
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max_exclusive=self._key._curve.order,
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randfunc=self._randfunc)
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def _valid_hash(self, msg_hash):
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"""Verify that the strength of the hash matches or exceeds
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the strength of the EC. We fail if the hash is too weak."""
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modulus_bits = self._key.pointQ.size_in_bits()
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# SHS: SHA-2, SHA-3, truncated SHA-512
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sha224 = ("2.16.840.1.101.3.4.2.4", "2.16.840.1.101.3.4.2.7", "2.16.840.1.101.3.4.2.5")
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sha256 = ("2.16.840.1.101.3.4.2.1", "2.16.840.1.101.3.4.2.8", "2.16.840.1.101.3.4.2.6")
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sha384 = ("2.16.840.1.101.3.4.2.2", "2.16.840.1.101.3.4.2.9")
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sha512 = ("2.16.840.1.101.3.4.2.3", "2.16.840.1.101.3.4.2.10")
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shs = sha224 + sha256 + sha384 + sha512
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try:
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result = msg_hash.oid in shs
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except AttributeError:
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result = False
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return result
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def new(key, mode, encoding='binary', randfunc=None):
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"""Create a signature object :class:`DssSigScheme` that
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can perform (EC)DSA signature or verification.
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.. note::
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Refer to `NIST SP 800 Part 1 Rev 4`_ (or newer release) for an
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overview of the recommended key lengths.
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Args:
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key (:class:`Cryptodome.PublicKey.DSA` or :class:`Cryptodome.PublicKey.ECC`):
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The key to use for computing the signature (*private* keys only)
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or for verifying one.
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For DSA keys, let ``L`` and ``N`` be the bit lengths of the modulus ``p``
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and of ``q``: the pair ``(L,N)`` must appear in the following list,
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in compliance to section 4.2 of `FIPS 186-4`_:
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- (1024, 160) *legacy only; do not create new signatures with this*
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- (2048, 224) *deprecated; do not create new signatures with this*
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- (2048, 256)
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- (3072, 256)
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For ECC, only keys over P-224, P-256, P-384, and P-521 are accepted.
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mode (string):
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The parameter can take these values:
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- ``'fips-186-3'``. The signature generation is randomized and carried out
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according to `FIPS 186-3`_: the nonce ``k`` is taken from the RNG.
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- ``'deterministic-rfc6979'``. The signature generation is not
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randomized. See RFC6979_.
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encoding (string):
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How the signature is encoded. This value determines the output of
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:meth:`sign` and the input to :meth:`verify`.
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The following values are accepted:
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- ``'binary'`` (default), the signature is the raw concatenation
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of ``r`` and ``s``. It is defined in the IEEE P.1363 standard.
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For DSA, the size in bytes of the signature is ``N/4`` bytes
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(e.g. 64 for ``N=256``).
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For ECDSA, the signature is always twice the length of a point
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coordinate (e.g. 64 bytes for P-256).
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- ``'der'``, the signature is a ASN.1 DER SEQUENCE
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with two INTEGERs (``r`` and ``s``). It is defined in RFC3279_.
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The size of the signature is variable.
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randfunc (callable):
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A function that returns random ``bytes``, of a given length.
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If omitted, the internal RNG is used.
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Only applicable for the *'fips-186-3'* mode.
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.. _FIPS 186-3: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf
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.. _FIPS 186-4: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
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.. _NIST SP 800 Part 1 Rev 4: http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57pt1r4.pdf
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.. _RFC6979: http://tools.ietf.org/html/rfc6979
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.. _RFC3279: https://tools.ietf.org/html/rfc3279#section-2.2.2
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"""
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# The goal of the 'mode' parameter is to avoid to
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# have the current version of the standard as default.
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#
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# Over time, such version will be superseded by (for instance)
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# FIPS 186-4 and it will be odd to have -3 as default.
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if encoding not in ('binary', 'der'):
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raise ValueError("Unknown encoding '%s'" % encoding)
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if isinstance(key, EccKey):
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order = key._curve.order
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private_key_attr = 'd'
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if key._curve.name == "ed25519":
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raise ValueError("ECC key is not on a NIST P curve")
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elif isinstance(key, DsaKey):
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order = Integer(key.q)
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private_key_attr = 'x'
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else:
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raise ValueError("Unsupported key type " + str(type(key)))
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if key.has_private():
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private_key = getattr(key, private_key_attr)
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else:
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private_key = None
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if mode == 'deterministic-rfc6979':
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return DeterministicDsaSigScheme(key, encoding, order, private_key)
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elif mode == 'fips-186-3':
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if isinstance(key, EccKey):
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return FipsEcDsaSigScheme(key, encoding, order, randfunc)
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else:
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return FipsDsaSigScheme(key, encoding, order, randfunc)
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else:
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raise ValueError("Unknown DSS mode '%s'" % mode)
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