add SubjectKeyIdentifier.create_from_public_key

1 files changed, 36 insertions, 1 deletions

diff --git a/src/cryptography/x509.py b/src/cryptography/x509.py
index 978eb560..b58166f6 100644
--- a/src/cryptography/x509.py
+++ b/src/cryptography/x509.py
@@ -5,22 +5,34 @@
 from __future__ import absolute_import, division, print_function
 
 import abc
+import binascii
 import datetime
+import hashlib
 import ipaddress
 from email.utils import parseaddr
 from enum import Enum
 
 import idna
 
+from pyasn1.codec.der import decoder
+from pyasn1.type import namedtype, univ
+
 import six
 
 from six.moves import urllib_parse
 
 from cryptography import utils
-from cryptography.hazmat.primitives import hashes
+from cryptography.hazmat.primitives import hashes, serialization
 from cryptography.hazmat.primitives.asymmetric import dsa, ec, rsa
 
 
+class _SubjectPublicKeyInfo(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType('algorithm', univ.Sequence()),
+        namedtype.NamedType('subjectPublicKey', univ.BitString())
+    )
+
+
 _OID_NAMES = {
     "2.5.4.3": "commonName",
     "2.5.4.6": "countryName",
@@ -669,6 +681,29 @@ class SubjectKeyIdentifier(object):
     def __init__(self, digest):
         self._digest = digest
 
+    @classmethod
+    def create_from_public_key(cls, public_key):
+        # This is a very slow way to do this.
+        serialized = public_key.public_bytes(
+            serialization.Encoding.DER,
+            serialization.PublicFormat.SubjectPublicKeyInfo
+        )
+        spki, remaining = decoder.decode(
+            serialized, asn1Spec=_SubjectPublicKeyInfo()
+        )
+        # the univ.BitString object is a tuple of bits. We need bytes and
+        # pyasn1 really doesn't want to give them to us. To get it we'll
+        # build an integer, hex it, then decode the hex.
+        bits = 0
+        for bit in spki.getComponentByName("subjectPublicKey"):
+            bits = bits << 1 | bit
+
+        # convert the integer to bytes
+        hex_string = '%x' % bits
+        n = len(hex_string)
+        data = binascii.unhexlify(hex_string.zfill(n + (n & 1)))
+        return cls(hashlib.sha1(data).digest())
+
     digest = utils.read_only_property("_digest")
 
     def __repr__(self):