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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from __future__ import absolute_import, division, print_function
import cffi
import six
from cryptography import utils
from cryptography.hazmat.primitives import interfaces
_ffi = cffi.FFI()
_ffi.cdef("""
uint8_t Cryptography_check_pkcs7_padding(const uint8_t *, uint8_t);
""")
_lib = _ffi.verify(
"""
/* Returns the value of the input with the most-significant-bit copied to all
of the bits. */
static uint8_t Cryptography_DUPLICATE_MSB_TO_ALL(uint8_t a) {
return (1 - (a >> (sizeof(uint8_t) * 8 - 1))) - 1;
}
/* This returns 0xFF if a < b else 0x00, but does so in a constant time
fashion */
static uint8_t Cryptography_constant_time_lt(uint8_t a, uint8_t b) {
a -= b;
return Cryptography_DUPLICATE_MSB_TO_ALL(a);
}
uint8_t Cryptography_check_pkcs7_padding(const uint8_t *data,
uint8_t block_len) {
uint8_t i;
uint8_t pad_size = data[block_len - 1];
uint8_t mismatch = 0;
for (i = 0; i < block_len; i++) {
unsigned int mask = Cryptography_constant_time_lt(i, pad_size);
uint8_t b = data[block_len - 1 - i];
mismatch |= (mask & (pad_size ^ b));
}
/* Check to make sure the pad_size was within the valid range. */
mismatch |= ~Cryptography_constant_time_lt(0, pad_size);
mismatch |= Cryptography_constant_time_lt(block_len, pad_size);
/* Make sure any bits set are copied to the lowest bit */
mismatch |= mismatch >> 4;
mismatch |= mismatch >> 2;
mismatch |= mismatch >> 1;
/* Now check the low bit to see if it's set */
return (mismatch & 1) == 0;
}
""",
ext_package="cryptography",
)
class PKCS7(object):
def __init__(self, block_size):
if not (0 <= block_size < 256):
raise ValueError("block_size must be in range(0, 256)")
if block_size % 8 != 0:
raise ValueError("block_size must be a multiple of 8")
self.block_size = block_size
def padder(self):
return _PKCS7PaddingContext(self.block_size)
def unpadder(self):
return _PKCS7UnpaddingContext(self.block_size)
@utils.register_interface(interfaces.PaddingContext)
class _PKCS7PaddingContext(object):
def __init__(self, block_size):
self.block_size = block_size
# TODO: more copies than necessary, we should use zero-buffer (#193)
self._buffer = b""
def update(self, data):
if self._buffer is None:
raise ValueError("Context was already finalized")
if isinstance(data, six.text_type):
raise TypeError("Unicode-objects must be encoded before padding")
self._buffer += data
finished_blocks = len(self._buffer) // (self.block_size // 8)
result = self._buffer[:finished_blocks * (self.block_size // 8)]
self._buffer = self._buffer[finished_blocks * (self.block_size // 8):]
return result
def finalize(self):
if self._buffer is None:
raise ValueError("Context was already finalized")
pad_size = self.block_size // 8 - len(self._buffer)
result = self._buffer + six.int2byte(pad_size) * pad_size
self._buffer = None
return result
@utils.register_interface(interfaces.PaddingContext)
class _PKCS7UnpaddingContext(object):
def __init__(self, block_size):
self.block_size = block_size
# TODO: more copies than necessary, we should use zero-buffer (#193)
self._buffer = b""
def update(self, data):
if self._buffer is None:
raise ValueError("Context was already finalized")
if isinstance(data, six.text_type):
raise TypeError("Unicode-objects must be encoded before unpadding")
self._buffer += data
finished_blocks = max(
len(self._buffer) // (self.block_size // 8) - 1,
0
)
result = self._buffer[:finished_blocks * (self.block_size // 8)]
self._buffer = self._buffer[finished_blocks * (self.block_size // 8):]
return result
def finalize(self):
if self._buffer is None:
raise ValueError("Context was already finalized")
if len(self._buffer) != self.block_size // 8:
raise ValueError("Invalid padding bytes")
valid = _lib.Cryptography_check_pkcs7_padding(
self._buffer, self.block_size // 8
)
if not valid:
raise ValueError("Invalid padding bytes")
pad_size = six.indexbytes(self._buffer, -1)
res = self._buffer[:-pad_size]
self._buffer = None
return res
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