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