1 files changed, 150 insertions, 0 deletions
diff --git a/src/cryptography/hazmat/_der.py b/src/cryptography/hazmat/_der.py
new file mode 100644
index 00000000..3a121a85
--- /dev/null
+++ b/src/cryptography/hazmat/_der.py
@@ -0,0 +1,150 @@
+# 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 six
+
+from cryptography.utils import int_from_bytes, 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 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):
+        if len(self.data) < 1:
+            raise ValueError("Invalid DER input: insufficient data")
+        ret = six.indexbytes(self.data, 0)
+        self.data = self.data[1:]
+        return ret
+
+    def read_bytes(self, n):
+        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):
+        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):
+        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):
+        ret = self.read_element(expected_tag)
+        self.check_empty()
+        return ret
+
+    def read_optional_element(self, expected_tag):
+        if len(self.data) > 0 and six.indexbytes(self.data, 0) == expected_tag:
+            return self.read_element(expected_tag)
+        return None
+
+    def as_integer(self):
+        if len(self.data) == 0:
+            raise ValueError("Invalid DER input: empty integer contents")
+        first = six.indexbytes(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 = six.indexbytes(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):
+    if not isinstance(x, six.integer_types):
+        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, *children):
+    length = 0
+    for child in children:
+        length += len(child)
+    chunks = [six.int2byte(tag)]
+    if length < 0x80:
+        chunks.append(six.int2byte(length))
+    else:
+        length_bytes = int_to_bytes(length)
+        chunks.append(six.int2byte(0x80 | len(length_bytes)))
+        chunks.append(length_bytes)
+    chunks.extend(children)
+    return b"".join(chunks)