190 lines
6.9 KiB
Python
190 lines
6.9 KiB
Python
# -*- coding: utf-8 -*-
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#
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# Cipher/PKCS1-v1_5.py : PKCS#1 v1.5
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#
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# ===================================================================
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# The contents of this file are dedicated to the public domain. To
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# the extent that dedication to the public domain is not available,
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# everyone is granted a worldwide, perpetual, royalty-free,
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# non-exclusive license to exercise all rights associated with the
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# contents of this file for any purpose whatsoever.
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# No rights are reserved.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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# SOFTWARE.
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# ===================================================================
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__all__ = ['new', 'PKCS115_Cipher']
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from Cryptodome import Random
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from Cryptodome.Util.number import bytes_to_long, long_to_bytes
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from Cryptodome.Util.py3compat import bord, is_bytes, _copy_bytes
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from ._pkcs1_oaep_decode import pkcs1_decode
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class PKCS115_Cipher:
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"""This cipher can perform PKCS#1 v1.5 RSA encryption or decryption.
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Do not instantiate directly. Use :func:`Cryptodome.Cipher.PKCS1_v1_5.new` instead."""
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def __init__(self, key, randfunc):
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"""Initialize this PKCS#1 v1.5 cipher object.
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:Parameters:
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key : an RSA key object
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If a private half is given, both encryption and decryption are possible.
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If a public half is given, only encryption is possible.
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randfunc : callable
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Function that returns random bytes.
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"""
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self._key = key
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self._randfunc = randfunc
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def can_encrypt(self):
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"""Return True if this cipher object can be used for encryption."""
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return self._key.can_encrypt()
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def can_decrypt(self):
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"""Return True if this cipher object can be used for decryption."""
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return self._key.can_decrypt()
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def encrypt(self, message):
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"""Produce the PKCS#1 v1.5 encryption of a message.
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This function is named ``RSAES-PKCS1-V1_5-ENCRYPT``, and it is specified in
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`section 7.2.1 of RFC8017
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<https://tools.ietf.org/html/rfc8017#page-28>`_.
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:param message:
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The message to encrypt, also known as plaintext. It can be of
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variable length, but not longer than the RSA modulus (in bytes) minus 11.
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:type message: bytes/bytearray/memoryview
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:Returns: A byte string, the ciphertext in which the message is encrypted.
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It is as long as the RSA modulus (in bytes).
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:Raises ValueError:
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If the RSA key length is not sufficiently long to deal with the given
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message.
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"""
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# See 7.2.1 in RFC8017
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k = self._key.size_in_bytes()
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mLen = len(message)
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# Step 1
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if mLen > k - 11:
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raise ValueError("Plaintext is too long.")
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# Step 2a
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ps = []
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while len(ps) != k - mLen - 3:
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new_byte = self._randfunc(1)
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if bord(new_byte[0]) == 0x00:
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continue
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ps.append(new_byte)
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ps = b"".join(ps)
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# Step 2b
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em = b'\x00\x02' + ps + b'\x00' + _copy_bytes(None, None, message)
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# Step 3a (OS2IP)
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em_int = bytes_to_long(em)
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# Step 3b (RSAEP)
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m_int = self._key._encrypt(em_int)
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# Step 3c (I2OSP)
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c = long_to_bytes(m_int, k)
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return c
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def decrypt(self, ciphertext, sentinel, expected_pt_len=0):
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r"""Decrypt a PKCS#1 v1.5 ciphertext.
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This is the function ``RSAES-PKCS1-V1_5-DECRYPT`` specified in
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`section 7.2.2 of RFC8017
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<https://tools.ietf.org/html/rfc8017#page-29>`_.
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Args:
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ciphertext (bytes/bytearray/memoryview):
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The ciphertext that contains the message to recover.
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sentinel (any type):
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The object to return whenever an error is detected.
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expected_pt_len (integer):
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The length the plaintext is known to have, or 0 if unknown.
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Returns (byte string):
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It is either the original message or the ``sentinel`` (in case of an error).
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.. warning::
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PKCS#1 v1.5 decryption is intrinsically vulnerable to timing
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attacks (see `Bleichenbacher's`__ attack).
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**Use PKCS#1 OAEP instead**.
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This implementation attempts to mitigate the risk
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with some constant-time constructs.
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However, they are not sufficient by themselves: the type of protocol you
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implement and the way you handle errors make a big difference.
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Specifically, you should make it very hard for the (malicious)
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party that submitted the ciphertext to quickly understand if decryption
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succeeded or not.
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To this end, it is recommended that your protocol only encrypts
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plaintexts of fixed length (``expected_pt_len``),
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that ``sentinel`` is a random byte string of the same length,
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and that processing continues for as long
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as possible even if ``sentinel`` is returned (i.e. in case of
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incorrect decryption).
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.. __: https://dx.doi.org/10.1007/BFb0055716
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"""
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# See 7.2.2 in RFC8017
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k = self._key.size_in_bytes()
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# Step 1
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if len(ciphertext) != k:
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raise ValueError("Ciphertext with incorrect length (not %d bytes)" % k)
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# Step 2a (O2SIP)
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ct_int = bytes_to_long(ciphertext)
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# Step 2b (RSADP) and Step 2c (I2OSP)
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em = self._key._decrypt_to_bytes(ct_int)
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# Step 3 (not constant time when the sentinel is not a byte string)
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output = bytes(bytearray(k))
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if not is_bytes(sentinel) or len(sentinel) > k:
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size = pkcs1_decode(em, b'', expected_pt_len, output)
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if size < 0:
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return sentinel
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else:
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return output[size:]
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# Step 3 (somewhat constant time)
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size = pkcs1_decode(em, sentinel, expected_pt_len, output)
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return output[size:]
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def new(key, randfunc=None):
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"""Create a cipher for performing PKCS#1 v1.5 encryption or decryption.
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:param key:
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The key to use to encrypt or decrypt the message. This is a `Cryptodome.PublicKey.RSA` object.
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Decryption is only possible if *key* is a private RSA key.
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:type key: RSA key object
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:param randfunc:
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Function that return random bytes.
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The default is :func:`Cryptodome.Random.get_random_bytes`.
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:type randfunc: callable
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:returns: A cipher object `PKCS115_Cipher`.
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"""
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if randfunc is None:
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randfunc = Random.get_random_bytes
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return PKCS115_Cipher(key, randfunc)
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