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# -*- 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 Cryptodome import Random
from Cryptodome.Util.number import bytes_to_long, long_to_bytes
from Cryptodome.Util.py3compat import bord, is_bytes, _copy_bytes
from ._pkcs1_oaep_decode import pkcs1_decode
class PKCS115_Cipher:
"""This cipher can perform PKCS#1 v1.5 RSA encryption or decryption.
Do not instantiate directly. Use :func:`Cryptodome.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
k = self._key.size_in_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)
# 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, expected_pt_len=0):
r"""Decrypt a PKCS#1 v1.5 ciphertext.
This is the function ``RSAES-PKCS1-V1_5-DECRYPT`` specified in
`section 7.2.2 of RFC8017
<https://tools.ietf.org/html/rfc8017#page-29>`_.
Args:
ciphertext (bytes/bytearray/memoryview):
The ciphertext that contains the message to recover.
sentinel (any type):
The object to return whenever an error is detected.
expected_pt_len (integer):
The length the plaintext is known to have, or 0 if unknown.
Returns (byte string):
It is either the original message or the ``sentinel`` (in case of an error).
.. warning::
PKCS#1 v1.5 decryption is intrinsically vulnerable to timing
attacks (see `Bleichenbacher's`__ attack).
**Use PKCS#1 OAEP instead**.
This implementation attempts to mitigate the risk
with some constant-time constructs.
However, they are not sufficient by themselves: the type of protocol you
implement and the way you handle errors make a big difference.
Specifically, you should make it very hard for the (malicious)
party that submitted the ciphertext to quickly understand if decryption
succeeded or not.
To this end, it is recommended that your protocol only encrypts
plaintexts of fixed length (``expected_pt_len``),
that ``sentinel`` is a random byte string of the same length,
and that processing continues for as long
as possible even if ``sentinel`` is returned (i.e. in case of
incorrect decryption).
.. __: https://dx.doi.org/10.1007/BFb0055716
"""
# See 7.2.2 in RFC8017
k = self._key.size_in_bytes()
# Step 1
if len(ciphertext) != k:
raise ValueError("Ciphertext with incorrect length (not %d bytes)" % k)
# Step 2a (O2SIP)
ct_int = bytes_to_long(ciphertext)
# Step 2b (RSADP) and Step 2c (I2OSP)
em = self._key._decrypt_to_bytes(ct_int)
# Step 3 (not constant time when the sentinel is not a byte string)
output = bytes(bytearray(k))
if not is_bytes(sentinel) or len(sentinel) > k:
size = pkcs1_decode(em, b'', expected_pt_len, output)
if size < 0:
return sentinel
else:
return output[size:]
# Step 3 (somewhat constant time)
size = pkcs1_decode(em, sentinel, expected_pt_len, output)
return output[size:]
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 `Cryptodome.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:`Cryptodome.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)
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