diff --git a/src/scitokens/scitokens.py b/src/scitokens/scitokens.py
index 7fa6923..2959731 100644
--- a/src/scitokens/scitokens.py
+++ b/src/scitokens/scitokens.py
@@ -21,6 +21,7 @@ from .utils import keycache as KeyCache
 from .utils import config
 from .utils.errors import MissingIssuerException, InvalidTokenFormat, MissingKeyException, UnsupportedKeyException
 from cryptography.hazmat.primitives.serialization import load_pem_public_key
+from cryptography.hazmat.primitives.asymmetric import rsa, ec
 
 class SciToken(object):
     """
@@ -41,10 +42,28 @@ class SciToken(object):
             raise NotImplementedError()
 
         self._key = key
+        derived_alg = None
+        if key:
+            derived_alg = self._derive_algorithm(key)
 
         # Make sure we support the key algorithm
+        if key and not algorithm and not derived_alg:
+            # We don't know the key algorithm
+            raise UnsupportedKeyException("Key was given for SciToken, but algorithm was not "
+                                          "passed to SciToken creation and it cannot be derived "
+                                          "from the provided key")
+        elif derived_alg and not algorithm:
+            self._key_alg = derived_alg
+        elif derived_alg and algorithm and derived_alg != algorithm:
+            error_str = ("Key provided reports algorithm type: {0}, ".format(derived_alg) +
+                         "while scitoken creation argument was {0}".format(algorithm))
+            raise UnsupportedKeyException(error_str)
+        elif key and algorithm:
+            self._key_alg = algorithm
+        else:
+            # If key is not specified, and neither is algorithm
+            self._key_alg = algorithm if algorithm is not None else config.get('default_alg')
 
-        self._key_alg = algorithm if algorithm is not None else config.get('default_alg')
         if self._key_alg not in ["RS256", "ES256"]:
             raise UnsupportedKeyException()
         self._key_id = key_id
@@ -54,6 +73,24 @@ class SciToken(object):
         self.insecure = False
         self._serialized_token = None
 
+    @staticmethod
+    def _derive_algorithm(key):
+        """
+        Derive the algorithm type from the PEM contents of the key
+
+        returns: Key algorithm if known, otherwise None
+        """
+
+        if isinstance(key, rsa.RSAPrivateKey):
+            return "RS256"
+        elif isinstance(key, ec.EllipticCurvePrivateKey):
+            if key.curve.name == "secp256r1":
+                return "ES256"
+
+        # If it gets here, we don't know what type of key
+        return None
+
+
     def claims(self):
         """
         Return an iterator of (key, value) pairs of claims, starting

diff --git a/tests/test_create_scitoken.py b/tests/test_create_scitoken.py
index 89157e7..43defcf 100644
--- a/tests/test_create_scitoken.py
+++ b/tests/test_create_scitoken.py
@@ -262,6 +262,37 @@ class TestCreation(unittest.TestCase):
         with self.assertRaises(UnsupportedKeyException):
             scitokens.SciToken(key = self._private_key, algorithm="doesnotexist")
 
+    def test_autodetect_keytype(self):
+        """
+        Test the autodetection of the key type
+        """
+        private_key = generate_private_key(
+            public_exponent=65537,
+            key_size=2048,
+            backend=default_backend()
+        )
+
+        ec_private_key = ec.generate_private_key(
+            ec.SECP256R1(), default_backend()
+        )
+
+        # Test when we give it the wrong algorithm type
+        with self.assertRaises(scitokens.scitokens.UnsupportedKeyException):
+            token = scitokens.SciToken(key = private_key, algorithm="ES256")
+
+        # Test when we give it the wrong algorithm type
+        with self.assertRaises(scitokens.scitokens.UnsupportedKeyException):
+            token = scitokens.SciToken(key = ec_private_key, algorithm="RS256")
+
+        # Test when we give an unsupported algorithm
+        unsupported_private_key = ec.generate_private_key(
+            ec.SECP192R1(), default_backend()
+        )
+        with self.assertRaises(scitokens.scitokens.UnsupportedKeyException):
+            token = scitokens.SciToken(key = unsupported_private_key)
+
+        token = scitokens.SciToken(key = ec_private_key, algorithm="ES256")
+        token.serialize(issuer="local")
 
 if __name__ == '__main__':
     unittest.main()

Auto-detect signing algorithm
We have two potential signing algorithms - RS256 and ES256 - for SciTokens.  However, the private key for these two are not interchangeable - a RSA key cannot be used to sign ES256.

Since we can probe the key type, we should be able to correctly select the algorithm.
tests/test_create_scitoken.py::TestCreation::test_create_to_validate tests/test_create_scitoken.py::TestCreation::test_opt tests/test_create_scitoken.py::TestCreation::test_ec_public_key tests/test_create_scitoken.py::TestCreation::test_autodetect_keytype tests/test_create_scitoken.py::TestCreation::test_create tests/test_create_scitoken.py::TestCreation::test_multiple_scopes tests/test_create_scitoken.py::TestCreation::test_ver tests/test_create_scitoken.py::TestCreation::test_unsupported_key tests/test_create_scitoken.py::TestCreation::test_ec_create tests/test_create_scitoken.py::TestCreation::test_serialize tests/test_create_scitoken.py::TestCreation::test_aud tests/test_create_scitoken.py::TestCreation::test_contains tests/test_create_scitoken.py::TestCreation::test_no_kid tests/test_create_scitoken.py::TestCreation::test_public_key