Removed pyasn1 dependencies

Note that get_ext_by_oid() still returns the raw bytes of the extension.
I'm still undecided on whether keeping this behaviour is the right thing
to do or not.

References Yubico#19

setup.py

@@ -35,7 +35,9 @@
     maintainer='Yubico Open Source Maintainers',
     maintainer_email='ossmaint@yubico.com',
     url='https://github.com/Yubico/python-u2flib-server',
-    install_requires=['cryptography>=1.2', 'pyasn1>=0.1.7', 'pyasn1-modules'],
+    install_requires=[
+        'cryptography>=1.2',
+    ],
     test_suite='test',
     tests_require=[],
     extras_require={

u2flib_server/attestation/matchers.py

@@ -25,15 +25,18 @@
 # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
+from cryptography.x509 import (
+    ExtensionNotFound, ObjectIdentifier
+)
+
+
 __all__ = [
     'DeviceMatcher',
     'FingerprintMatcher',
     'ExtensionMatcher',
     'DEFAULT_MATCHERS'
 ]
 
-from cryptography.hazmat.primitives.serialization import Encoding
-
 
 class DeviceMatcher(object):
     selector_type = None
@@ -50,16 +53,14 @@ def matches(self, certificate, parameters=[]):
         return certificate.get_fingerprint('sha1').lower() in fingerprints
 
 
-# This is needed since older versions of M2Crypto don't have a way of getting
-# extensions by their OID.
 def get_ext_by_oid(cert, oid):
-    from pyasn1.codec.der import decoder
-    from pyasn1_modules import rfc2459
-    cert, _ = decoder.decode(cert.public_bytes(Encoding.DER), asn1Spec=rfc2459.Certificate())
-    for ext in cert['tbsCertificate']['extensions']:
-        if ext['extnID'].prettyPrint() == oid:
-            return decoder.decode(ext['extnValue'])[0].asOctets()
-    return None
+    oid = ObjectIdentifier(oid)
+    try:
+        extension = cert.extensions.get_extension_for_oid(oid)
+    except ExtensionNotFound:
+        return None
+
+    return extension.value.value
 
 
 class ExtensionMatcher(DeviceMatcher):