Remove asn1crypto dependency (#4941)

* Remove non-test dependencies on asn1crypto.

cryptography.io actually contains two OpenSSL bindings right now, the
expected cffi one, and an optional one hidden in asn1crypto. asn1crypto
contains a lot of things that cryptography.io doesn't use, including a
BER parser and a hand-rolled and not constant-time EC implementation.

Instead, check in a much small DER-only parser in cryptography/hazmat. A
quick benchmark suggests this parser is also faster than asn1crypto:

  from __future__ import absolute_import, division, print_function
  import timeit

  print(timeit.timeit(
      "decode_dss_signature(sig)",
      setup=r"""
  from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
  sig=b"\x30\x2d\x02\x15\x00\xb5\xaf\x30\x78\x67\xfb\x8b\x54\x39\x00\x13\xcc\x67\x02\x0d\xdf\x1f\x2c\x0b\x81\x02\x14\x62\x0d\x3b\x22\xab\x50\x31\x44\x0c\x3e\x35\xea\xb6\xf4\x81\x29\x8f\x9e\x9f\x08"
  """,
      number=10000))

Python 2.7:
  asn1crypto: 0.25
  _der.py: 0.098

Python 3.5:
  asn1crypto: 0.17
  _der.py: 0.10

* Remove test dependencies on asn1crypto.

The remaining use of asn1crypto was some sanity-checking of
Certificates. Add a minimal X.509 parser to extract the relevant fields.

* Add a read_single_element helper function.

The outermost read is a little tedious.

* Address flake8 warnings

* Fix test for long-form vs short-form lengths.

Testing a zero length trips both this check and the non-minimal long
form check. Use a one-byte length to cover the missing branch.

* Remove support for negative integers.

These never come up in valid signatures. Note, however, this does
change public API.

* Update src/cryptography/hazmat/primitives/asymmetric/utils.py

Co-Authored-By: Alex Gaynor <alex.gaynor@gmail.com>

* Review comments

* Avoid hardcoding the serialization of NULL in decode_asn1.py too.

2 files changed, 222 insertions, 4 deletions

diff --git a/tests/hazmat/primitives/test_asym_utils.py b/tests/hazmat/primitives/test_asym_utils.py
index d817c651..b49ca3f2 100644
--- a/tests/hazmat/primitives/test_asym_utils.py
+++ b/tests/hazmat/primitives/test_asym_utils.py
@@ -31,10 +31,6 @@ def test_dss_signature():
     assert sig3 == b"0\x06\x02\x01\x00\x02\x01\x00"
     assert decode_dss_signature(sig3) == (0, 0)
 
-    sig4 = encode_dss_signature(-1, 0)
-    assert sig4 == b"0\x06\x02\x01\xFF\x02\x01\x00"
-    assert decode_dss_signature(sig4) == (-1, 0)
-
 
 def test_encode_dss_non_integer():
     with pytest.raises(ValueError):
@@ -53,6 +49,11 @@ def test_encode_dss_non_integer():
         encode_dss_signature("hello", "world")
 
 
+def test_encode_dss_negative():
+    with pytest.raises(ValueError):
+        encode_dss_signature(-1, 0)
+
+
 def test_decode_dss_trailing_bytes():
     with pytest.raises(ValueError):
         decode_dss_signature(b"0\x06\x02\x01\x01\x02\x01\x01\x00\x00\x00")
diff --git a/tests/hazmat/test_der.py b/tests/hazmat/test_der.py
new file mode 100644
index 00000000..d81c0d3e
--- /dev/null
+++ b/tests/hazmat/test_der.py
@@ -0,0 +1,217 @@
+# 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.
+
+from __future__ import absolute_import, division, print_function
+
+import pytest
+
+from cryptography.hazmat._der import (
+    DERReader, INTEGER, NULL, OCTET_STRING, SEQUENCE, encode_der,
+    encode_der_integer
+)
+
+
+def test_der_reader_basic():
+    reader = DERReader(b"123456789")
+    assert reader.read_byte() == ord(b"1")
+    assert reader.read_bytes(1).tobytes() == b"2"
+    assert reader.read_bytes(4).tobytes() == b"3456"
+
+    with pytest.raises(ValueError):
+        reader.read_bytes(4)
+
+    assert reader.read_bytes(3).tobytes() == b"789"
+
+    # The input is now empty.
+    with pytest.raises(ValueError):
+        reader.read_bytes(1)
+    with pytest.raises(ValueError):
+        reader.read_byte()
+
+
+def test_der():
+    # This input is the following structure, using
+    # https://github.com/google/der-ascii
+    #
+    # SEQUENCE {
+    #   SEQUENCE {
+    #     NULL {}
+    #     INTEGER { 42 }
+    #     OCTET_STRING { "hello" }
+    #   }
+    # }
+    der = b"\x30\x0e\x30\x0c\x05\x00\x02\x01\x2a\x04\x05\x68\x65\x6c\x6c\x6f"
+    reader = DERReader(der)
+    with pytest.raises(ValueError):
+        reader.check_empty()
+
+    # Parse the outer element.
+    outer = reader.read_element(SEQUENCE)
+    reader.check_empty()
+    assert outer.data.tobytes() == der[2:]
+
+    # Parse the outer element with read_any_element.
+    reader = DERReader(der)
+    tag, outer2 = reader.read_any_element()
+    reader.check_empty()
+    assert tag == SEQUENCE
+    assert outer2.data.tobytes() == der[2:]
+
+    # Parse the outer element with read_single_element.
+    outer3 = DERReader(der).read_single_element(SEQUENCE)
+    assert outer3.data.tobytes() == der[2:]
+
+    # read_single_element rejects trailing data.
+    with pytest.raises(ValueError):
+        DERReader(der + der).read_single_element(SEQUENCE)
+
+    # Continue parsing the structure.
+    inner = outer.read_element(SEQUENCE)
+    outer.check_empty()
+
+    # Parsing a missing optional element should work.
+    assert inner.read_optional_element(INTEGER) is None
+
+    null = inner.read_element(NULL)
+    null.check_empty()
+
+    # Parsing a present optional element should work.
+    integer = inner.read_optional_element(INTEGER)
+    assert integer.as_integer() == 42
+
+    octet_string = inner.read_element(OCTET_STRING)
+    assert octet_string.data.tobytes() == b"hello"
+
+    # Parsing a missing optional element should work when the input is empty.
+    inner.check_empty()
+    assert inner.read_optional_element(INTEGER) is None
+
+    # Re-encode the same structure.
+    der2 = encode_der(
+        SEQUENCE,
+        encode_der(
+            SEQUENCE,
+            encode_der(NULL),
+            encode_der(INTEGER, encode_der_integer(42)),
+            encode_der(OCTET_STRING, b"hello"),
+        )
+    )
+    assert der2 == der
+
+
+@pytest.mark.parametrize(
+    "length,header",
+    [
+        # Single-byte lengths.
+        (0, b"\x04\x00"),
+        (1, b"\x04\x01"),
+        (2, b"\x04\x02"),
+        (127, b"\x04\x7f"),
+        # Long-form lengths.
+        (128, b"\x04\x81\x80"),
+        (129, b"\x04\x81\x81"),
+        (255, b"\x04\x81\xff"),
+        (0x100, b"\x04\x82\x01\x00"),
+        (0x101, b"\x04\x82\x01\x01"),
+        (0xffff, b"\x04\x82\xff\xff"),
+        (0x10000, b"\x04\x83\x01\x00\x00"),
+    ]
+)
+def test_der_lengths(length, header):
+    body = length * b"a"
+    der = header + body
+
+    reader = DERReader(der)
+    element = reader.read_element(OCTET_STRING)
+    reader.check_empty()
+    assert element.data.tobytes() == body
+
+    assert encode_der(OCTET_STRING, body) == der
+
+
+@pytest.mark.parametrize(
+    "bad_input",
+    [
+        # The input ended before the tag.
+        b"",
+        # The input ended before the length.
+        b"\x30",
+        # The input ended before the second byte of the length.
+        b"\x30\x81",
+        # The input ended before the body.
+        b"\x30\x01",
+        # The length used long form when it should be short form.
+        b"\x30\x81\x01\x00",
+        # The length was not minimally-encoded.
+        b"\x30\x82\x00\x80" + (0x80 * b"a"),
+        # Indefinite-length encoding is not valid DER.
+        b"\x30\x80\x00\x00"
+        # Tag number (the bottom 5 bits) 31 indicates long form tags, which we
+        # do not support.
+        b"\x1f\x00",
+        b"\x9f\x00",
+        b"\xbf\x00",
+        b"\xff\x00",
+    ]
+)
+def test_der_reader_bad_input(bad_input):
+    reader = DERReader(bad_input)
+    with pytest.raises(ValueError):
+        reader.read_any_element()
+
+
+def test_der_reader_wrong_tag():
+    reader = DERReader(b"\x04\x00")
+    with pytest.raises(ValueError):
+        reader.read_element(SEQUENCE)
+
+
+@pytest.mark.parametrize(
+    "value,der",
+    [
+        (0, b'\x00'),
+        (1, b'\x01'),
+        (2, b'\x02'),
+        (3, b'\x03'),
+        (127, b'\x7f'),
+        (128, b'\x00\x80'),
+        (0x112233445566778899aabbccddeeff,
+         b'\x11\x22\x33\x44\x55\x66\x77\x88\x99\xaa\xbb\xcc\xdd\xee\xff'),
+    ]
+)
+def test_integer(value, der):
+    assert encode_der_integer(value) == der
+    assert DERReader(der).as_integer() == value
+
+
+@pytest.mark.parametrize(
+    "bad_input",
+    [
+        # Zero is encoded as b"\x00", not the empty string.
+        b"",
+        # Too many leading zeros.
+        b"\x00\x00",
+        b"\x00\x7f",
+        # Too many leading ones.
+        b"\xff\xff",
+        b"\xff\x80",
+        # Negative integers are not supported.
+        b"\x80",
+        b"\x81",
+        b"\x80\x00\x00",
+        b"\xff",
+    ]
+)
+def test_invalid_integer(bad_input):
+    reader = DERReader(bad_input)
+    with pytest.raises(ValueError):
+        reader.as_integer()
+
+
+def test_invalid_integer_encode():
+    with pytest.raises(ValueError):
+        encode_der_integer(-1)
+
+    with pytest.raises(ValueError):
+        encode_der_integer("not an integer")