200 lines
7.8 KiB
Python
200 lines
7.8 KiB
Python
# -*- coding: utf-8 -*-
|
|
#
|
|
# Cipher/PKCS1-v1_5.py : PKCS#1 v1.5
|
|
#
|
|
# ===================================================================
|
|
# The contents of this file are dedicated to the public domain. To
|
|
# the extent that dedication to the public domain is not available,
|
|
# everyone is granted a worldwide, perpetual, royalty-free,
|
|
# non-exclusive license to exercise all rights associated with the
|
|
# contents of this file for any purpose whatsoever.
|
|
# No rights are reserved.
|
|
#
|
|
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
|
|
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
|
|
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
|
|
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
|
|
# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
|
|
# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
|
|
# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
|
|
# SOFTWARE.
|
|
# ===================================================================
|
|
|
|
__all__ = [ 'new', 'PKCS115_Cipher' ]
|
|
|
|
from Crypto.Util.number import ceil_div, bytes_to_long, long_to_bytes
|
|
from Crypto.Util.py3compat import bord, _copy_bytes
|
|
import Crypto.Util.number
|
|
from Crypto import Random
|
|
|
|
class PKCS115_Cipher:
|
|
"""This cipher can perform PKCS#1 v1.5 RSA encryption or decryption.
|
|
Do not instantiate directly. Use :func:`Crypto.Cipher.PKCS1_v1_5.new` instead."""
|
|
|
|
def __init__(self, key, randfunc):
|
|
"""Initialize this PKCS#1 v1.5 cipher object.
|
|
|
|
:Parameters:
|
|
key : an RSA key object
|
|
If a private half is given, both encryption and decryption are possible.
|
|
If a public half is given, only encryption is possible.
|
|
randfunc : callable
|
|
Function that returns random bytes.
|
|
"""
|
|
|
|
self._key = key
|
|
self._randfunc = randfunc
|
|
|
|
def can_encrypt(self):
|
|
"""Return True if this cipher object can be used for encryption."""
|
|
return self._key.can_encrypt()
|
|
|
|
def can_decrypt(self):
|
|
"""Return True if this cipher object can be used for decryption."""
|
|
return self._key.can_decrypt()
|
|
|
|
def encrypt(self, message):
|
|
"""Produce the PKCS#1 v1.5 encryption of a message.
|
|
|
|
This function is named ``RSAES-PKCS1-V1_5-ENCRYPT``, and it is specified in
|
|
`section 7.2.1 of RFC8017
|
|
<https://tools.ietf.org/html/rfc8017#page-28>`_.
|
|
|
|
:param message:
|
|
The message to encrypt, also known as plaintext. It can be of
|
|
variable length, but not longer than the RSA modulus (in bytes) minus 11.
|
|
:type message: bytes/bytearray/memoryview
|
|
|
|
:Returns: A byte string, the ciphertext in which the message is encrypted.
|
|
It is as long as the RSA modulus (in bytes).
|
|
|
|
:Raises ValueError:
|
|
If the RSA key length is not sufficiently long to deal with the given
|
|
message.
|
|
"""
|
|
|
|
# See 7.2.1 in RFC8017
|
|
modBits = Crypto.Util.number.size(self._key.n)
|
|
k = ceil_div(modBits,8) # Convert from bits to bytes
|
|
mLen = len(message)
|
|
|
|
# Step 1
|
|
if mLen > k - 11:
|
|
raise ValueError("Plaintext is too long.")
|
|
# Step 2a
|
|
ps = []
|
|
while len(ps) != k - mLen - 3:
|
|
new_byte = self._randfunc(1)
|
|
if bord(new_byte[0]) == 0x00:
|
|
continue
|
|
ps.append(new_byte)
|
|
ps = b"".join(ps)
|
|
assert(len(ps) == k - mLen - 3)
|
|
# Step 2b
|
|
em = b'\x00\x02' + ps + b'\x00' + _copy_bytes(None, None, message)
|
|
# Step 3a (OS2IP)
|
|
em_int = bytes_to_long(em)
|
|
# Step 3b (RSAEP)
|
|
m_int = self._key._encrypt(em_int)
|
|
# Step 3c (I2OSP)
|
|
c = long_to_bytes(m_int, k)
|
|
return c
|
|
|
|
def decrypt(self, ciphertext, sentinel):
|
|
r"""Decrypt a PKCS#1 v1.5 ciphertext.
|
|
|
|
This function is named ``RSAES-PKCS1-V1_5-DECRYPT``, and is specified in
|
|
`section 7.2.2 of RFC8017
|
|
<https://tools.ietf.org/html/rfc8017#page-29>`_.
|
|
|
|
:param ciphertext:
|
|
The ciphertext that contains the message to recover.
|
|
:type ciphertext: bytes/bytearray/memoryview
|
|
|
|
:param sentinel:
|
|
The object to return whenever an error is detected.
|
|
:type sentinel: any type
|
|
|
|
:Returns: A byte string. It is either the original message or the ``sentinel`` (in case of an error).
|
|
|
|
:Raises ValueError:
|
|
If the ciphertext length is incorrect
|
|
:Raises TypeError:
|
|
If the RSA key has no private half (i.e. it cannot be used for
|
|
decyption).
|
|
|
|
.. warning::
|
|
You should **never** let the party who submitted the ciphertext know that
|
|
this function returned the ``sentinel`` value.
|
|
Armed with such knowledge (for a fair amount of carefully crafted but invalid ciphertexts),
|
|
an attacker is able to recontruct the plaintext of any other encryption that were carried out
|
|
with the same RSA public key (see `Bleichenbacher's`__ attack).
|
|
|
|
In general, it should not be possible for the other party to distinguish
|
|
whether processing at the server side failed because the value returned
|
|
was a ``sentinel`` as opposed to a random, invalid message.
|
|
|
|
In fact, the second option is not that unlikely: encryption done according to PKCS#1 v1.5
|
|
embeds no good integrity check. There is roughly one chance
|
|
in 2\ :sup:`16` for a random ciphertext to be returned as a valid message
|
|
(although random looking).
|
|
|
|
It is therefore advisabled to:
|
|
|
|
1. Select as ``sentinel`` a value that resembles a plausable random, invalid message.
|
|
2. Not report back an error as soon as you detect a ``sentinel`` value.
|
|
Put differently, you should not explicitly check if the returned value is the ``sentinel`` or not.
|
|
3. Cover all possible errors with a single, generic error indicator.
|
|
4. Embed into the definition of ``message`` (at the protocol level) a digest (e.g. ``SHA-1``).
|
|
It is recommended for it to be the rightmost part ``message``.
|
|
5. Where possible, monitor the number of errors due to ciphertexts originating from the same party,
|
|
and slow down the rate of the requests from such party (or even blacklist it altogether).
|
|
|
|
**If you are designing a new protocol, consider using the more robust PKCS#1 OAEP.**
|
|
|
|
.. __: http://www.bell-labs.com/user/bleichen/papers/pkcs.ps
|
|
|
|
"""
|
|
|
|
# See 7.2.1 in RFC3447
|
|
modBits = Crypto.Util.number.size(self._key.n)
|
|
k = ceil_div(modBits,8) # Convert from bits to bytes
|
|
|
|
# Step 1
|
|
if len(ciphertext) != k:
|
|
raise ValueError("Ciphertext with incorrect length.")
|
|
# Step 2a (O2SIP)
|
|
ct_int = bytes_to_long(ciphertext)
|
|
# Step 2b (RSADP)
|
|
m_int = self._key._decrypt(ct_int)
|
|
# Complete step 2c (I2OSP)
|
|
em = long_to_bytes(m_int, k)
|
|
# Step 3
|
|
sep = em.find(b'\x00', 2)
|
|
if not em.startswith(b'\x00\x02') or sep < 10:
|
|
return sentinel
|
|
# Step 4
|
|
return em[sep + 1:]
|
|
|
|
|
|
def new(key, randfunc=None):
|
|
"""Create a cipher for performing PKCS#1 v1.5 encryption or decryption.
|
|
|
|
:param key:
|
|
The key to use to encrypt or decrypt the message. This is a `Crypto.PublicKey.RSA` object.
|
|
Decryption is only possible if *key* is a private RSA key.
|
|
:type key: RSA key object
|
|
|
|
:param randfunc:
|
|
Function that return random bytes.
|
|
The default is :func:`Crypto.Random.get_random_bytes`.
|
|
:type randfunc: callable
|
|
|
|
:returns: A cipher object `PKCS115_Cipher`.
|
|
"""
|
|
|
|
if randfunc is None:
|
|
randfunc = Random.get_random_bytes
|
|
return PKCS115_Cipher(key, randfunc)
|
|
|