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Signature
pss.py
# =================================================================== # # Copyright (c) 2014, Legrandin <helderijs@gmail.com> # 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. # =================================================================== from Crypto.Util.py3compat import bchr, bord, iter_range import Crypto.Util.number from Crypto.Util.number import (ceil_div, long_to_bytes, bytes_to_long ) from Crypto.Util.strxor import strxor from Crypto import Random class PSS_SigScheme: """A signature object for ``RSASSA-PSS``. Do not instantiate directly. Use :func:`Crypto.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:`Crypto.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 = Crypto.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:`Crypto.Hash` module. :type parameter: hash object :parameter signature: The signature that needs to be validated. :type signature: bytes :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 = Crypto.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:`Crypto.Hash` :type hash_object: :return: the mask, as a *byte string* """ T = b"" for counter in iter_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): r""" 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 iter_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 iter_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("Incorrect signature") # 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 an object for making or verifying PKCS#1 PSS signatures. :parameter rsa_key: The RSA key to use for signing or verifying the message. This is a :class:`Crypto.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 function that returns the mask (as `bytes`). It must accept two parameters: a seed (as `bytes`) and the length of the data to return. If not specified, it will be the function :func:`MGF1` defined in `RFC8017 <https://tools.ietf.org/html/rfc8017#page-67>`_ and combined with the same hash algorithm applied to the message to sign or verify. If you want to use a different function, for instance still :func:`MGF1` but together with another hash, you can do:: from Crypto.Hash import SHA256 from Crypto.Signature.pss import MGF1 mgf = lambda x, y: MGF1(x, y, SHA256) * *salt_bytes* (``integer``) -- Length of the salt, in bytes. It is a value between 0 and ``emLen - hLen - 2``, where ``emLen`` is the size of the RSA modulus and ``hLen`` is the size of the digest applied to the message to sign or verify. The salt is generated internally, you don't need to provide it. If not specified, the salt length will be ``hLen``. If it is zero, the signature scheme becomes deterministic. Note that in some implementations such as OpenSSL the default salt length is ``emLen - hLen - 2`` (even though it is not more secure than ``hLen``). * *rand_func* (``callable``) -- A function that returns random ``bytes``, of the desired length. The default is :func:`Crypto.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(kwargs.keys())) return PSS_SigScheme(rsa_key, mask_func, salt_len, rand_func)
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