CasperSecurity
# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.
import typing
from cryptography.utils import int_to_bytes
# This module contains a lightweight DER encoder and decoder. See X.690 for the
# specification. This module intentionally does not implement the more complex
# BER encoding, only DER.
#
# Note this implementation treats an element's constructed bit as part of the
# tag. This is fine for DER, where the bit is always computable from the type.
CONSTRUCTED = 0x20
CONTEXT_SPECIFIC = 0x80
INTEGER = 0x02
BIT_STRING = 0x03
OCTET_STRING = 0x04
NULL = 0x05
OBJECT_IDENTIFIER = 0x06
SEQUENCE = 0x10 | CONSTRUCTED
SET = 0x11 | CONSTRUCTED
PRINTABLE_STRING = 0x13
UTC_TIME = 0x17
GENERALIZED_TIME = 0x18
class DERReader(object):
def __init__(self, data):
self.data = memoryview(data)
def __enter__(self):
return self
def __exit__(self, exc_type, exc_value, tb):
if exc_value is None:
self.check_empty()
def is_empty(self):
return len(self.data) == 0
def check_empty(self):
if not self.is_empty():
raise ValueError("Invalid DER input: trailing data")
def read_byte(self) -> int:
if len(self.data) < 1:
raise ValueError("Invalid DER input: insufficient data")
ret = self.data[0]
self.data = self.data[1:]
return ret
def read_bytes(self, n) -> memoryview:
if len(self.data) < n:
raise ValueError("Invalid DER input: insufficient data")
ret = self.data[:n]
self.data = self.data[n:]
return ret
def read_any_element(self) -> typing.Tuple[int, "DERReader"]:
tag = self.read_byte()
# Tag numbers 31 or higher are stored in multiple bytes. No supported
# ASN.1 types use such tags, so reject these.
if tag & 0x1F == 0x1F:
raise ValueError("Invalid DER input: unexpected high tag number")
length_byte = self.read_byte()
if length_byte & 0x80 == 0:
# If the high bit is clear, the first length byte is the length.
length = length_byte
else:
# If the high bit is set, the first length byte encodes the length
# of the length.
length_byte &= 0x7F
if length_byte == 0:
raise ValueError(
"Invalid DER input: indefinite length form is not allowed "
"in DER"
)
length = 0
for i in range(length_byte):
length <<= 8
length |= self.read_byte()
if length == 0:
raise ValueError(
"Invalid DER input: length was not minimally-encoded"
)
if length < 0x80:
# If the length could have been encoded in short form, it must
# not use long form.
raise ValueError(
"Invalid DER input: length was not minimally-encoded"
)
body = self.read_bytes(length)
return tag, DERReader(body)
def read_element(self, expected_tag: int) -> "DERReader":
tag, body = self.read_any_element()
if tag != expected_tag:
raise ValueError("Invalid DER input: unexpected tag")
return body
def read_single_element(self, expected_tag: int) -> "DERReader":
with self:
return self.read_element(expected_tag)
def read_optional_element(
self, expected_tag: int
) -> typing.Optional["DERReader"]:
if len(self.data) > 0 and self.data[0] == expected_tag:
return self.read_element(expected_tag)
return None
def as_integer(self) -> int:
if len(self.data) == 0:
raise ValueError("Invalid DER input: empty integer contents")
first = self.data[0]
if first & 0x80 == 0x80:
raise ValueError("Negative DER integers are not supported")
# The first 9 bits must not all be zero or all be ones. Otherwise, the
# encoding should have been one byte shorter.
if len(self.data) > 1:
second = self.data[1]
if first == 0 and second & 0x80 == 0:
raise ValueError(
"Invalid DER input: integer not minimally-encoded"
)
return int.from_bytes(self.data, "big")
def encode_der_integer(x: int) -> bytes:
if not isinstance(x, int):
raise ValueError("Value must be an integer")
if x < 0:
raise ValueError("Negative integers are not supported")
n = x.bit_length() // 8 + 1
return int_to_bytes(x, n)
def encode_der(tag: int, *children: bytes) -> bytes:
length = 0
for child in children:
length += len(child)
chunks = [bytes([tag])]
if length < 0x80:
chunks.append(bytes([length]))
else:
length_bytes = int_to_bytes(length)
chunks.append(bytes([0x80 | len(length_bytes)]))
chunks.append(length_bytes)
chunks.extend(children)
return b"".join(chunks)