# # Signature/DSS.py : DSS.py # # =================================================================== # # Copyright (c) 2014, Legrandin # All rights reserved. # # Redistribution and use in source and binary forms, with or without # 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; # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER # CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE # POSSIBILITY OF SUCH DAMAGE. # =================================================================== """ Digital Signature Standard (DSS), as specified in `FIPS PUB 186-3`__. A sender signs a message in the following way: >>> from Crypto.Hash import SHA256 >>> from Crypto.PublicKey import ECC >>> from Crypto.Signature import DSS >>> >>> message = b'I give my permission to order #4355' >>> key = ECC.import_key(open('privkey.der').read()) >>> h = SHA256.new(message) >>> signer = DSS.new(key, 'fips-186-3') >>> signature = signer.sign(h) The receiver can verify authenticity of the message: >>> key = ECC.import_key(open('pubkey.der').read()) >>> h = SHA256.new(received_message) >>> verifier = DSS.new(key, 'fips-186-3') >>> try: >>> verifier.verify(h, signature): >>> print "The message is authentic." >>> except ValueError: >>> print "The message is not authentic." .. __: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf """ __all__ = ['new', 'DssSigScheme'] from Crypto.Util.py3compat import bchr, b 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 _curve, EccKey class DssSigScheme(object): """This signature scheme can perform DSS signature or verification. :undocumented: __init__ """ 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): """Produce the DSS signature of a message. :Parameters: msg_hash : hash object The hash that was carried out over the message. The object belongs to the `Crypto.Hash` package. Under mode *'fips-186-3'*, the hash must be a FIPS approved secure hash (SHA-1 or a member of the SHA-2 family), of cryptographic strength appropriate for the DSA key. For instance, a 3072/256 DSA key can only be used in combination with SHA-512. :Return: The signature encoded as a byte string. :Raise ValueError: If the hash algorithm is incompatible to the DSA key. :Raise TypeError: If the DSA key has no private half. """ 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 OCTET STRING, # s OCTET STRING # } output = DerSequence(sig_pair).encode() return output def verify(self, msg_hash, signature): """Verify that a certain DSS signature is authentic. This function checks if the party holding the private half of the key really signed the message. :Parameters: msg_hash : hash object The hash that was carried out over the message. This is an object belonging to the `Crypto.Hash` module. Under mode *'fips-186-3'*, the hash must be a FIPS approved secure hash (SHA-1 or a member of the SHA-2 family), of cryptographic strength appropriate for the DSA key. For instance, a 3072/256 DSA key can only be used in combination with SHA-512. signature : byte string 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 does not belong to SHS") 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) 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 = der_seq[0], 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: 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 = bchr(1) * mhash.digest_size # Step c nonce_k = bchr(0) * mhash.digest_size for int_oct in 0, 1: # Step d/f nonce_k = HMAC.new(nonce_k, mask_v + bchr(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 + bchr(0), 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=_curve.order, randfunc=self._randfunc) def _valid_hash(self, msg_hash): """Verify that SHA-[23] (256|384|512) bits are used to match the 128-bit security of P-256""" approved = ("2.16.840.1.101.3.4.2.1", "2.16.840.1.101.3.4.2.2", "2.16.840.1.101.3.4.2.3", "2.16.840.1.101.3.4.2.8", "2.16.840.1.101.3.4.2.9", "2.16.840.1.101.3.4.2.10") return msg_hash.oid in approved def new(key, mode, encoding='binary', randfunc=None): """Return a signature scheme object `DSS_SigScheme` that can be used to perform DSS signature or verification. :Parameters: key : a `Crypto.PublicKey.DSA` or `Crypto.PublicKey.ECC` key object If the key has got its private half, both signature and verification are possible. If it only has the public half, verification is possible but not signature generation. For DSA keys, let *L* and *N* be the bit lengths of the modules *p* and *q*: the combination *(L,N)* must appear in the following list, in compliance to section 4.2 of `FIPS-186`__: - (1024, 160) - (2048, 224) - (2048, 256) - (3072, 256) mode : string The parameter can take these values: - *'fips-186-3'*. The signature generation is carried out according to `FIPS-186`__: the nonce *k* is taken from the RNG. - *'deterministic-rfc6979'*. The signature generation process does not rely on a random generator. See RFC6979_. encoding : string How the signature is encoded. This value determines the output of ``sign`` and the input of ``verify``. The following values are accepted: - *'binary'* (default), the signature is the raw concatenation of *r* and *s*. The size in bytes of the signature is always two times the size of *q*. - *'der'*, the signature is a DER encoded SEQUENCE with two INTEGERs, *r* and *s*. The size of the signature is variable. randfunc : callable The source of randomness. If ``None``, the internal RNG is used. Only used for the *'fips-186-3'* mode. .. __: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf .. __: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf .. _RFC6979: http://tools.ietf.org/html/rfc6979 """ # 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 = _curve.order private_key_attr = 'd' else: order = Integer(key.q) private_key_attr = 'x' 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)