Fix for CVE-2017-11424

Add "RSA PUBLIC KEY" to the forbidden key strings in HMAC. Prevents the
use of PKCS1 keys, cited by this CVE as exposing a key-confusion attack.
Also add a test case for it, doing the obvious thing.

Closes #62

Author: sirosen

jose/jwk.py

@@ -105,6 +105,7 @@ def __init__(self, key, algorithm):
 
         invalid_strings = [
             b'-----BEGIN PUBLIC KEY-----',
+            b'-----BEGIN RSA PUBLIC KEY-----',
             b'-----BEGIN CERTIFICATE-----',
             b'ssh-rsa'
         ]

tests/algorithms/test_HMAC.py

@@ -17,6 +17,10 @@ def test_RSA_key(self):
         with pytest.raises(JOSEError):
             HMACKey(key, ALGORITHMS.HS256)
 
+        key = "-----BEGIN RSA PUBLIC KEY-----"
+        with pytest.raises(JOSEError):
+            HMACKey(key, ALGORITHMS.HS256)
+
         key = "-----BEGIN CERTIFICATE-----"
         with pytest.raises(JOSEError):
             HMACKey(key, ALGORITHMS.HS256)