Fixed issue aws#135. Took longer than I expected because was trying to

get the value of the key in the encryption context dictionary to be the
same (as well as name of the key), but then learned this wasn't what
we wanted.  Wrote a test that I confirm works only after the change is
added.

src/aws_encryption_sdk/materials_managers/default.py

@@ -49,13 +49,21 @@ def _generate_signing_key_and_update_encryption_context(self, algorithm, encrypt
         :param dict encryption_context: Encryption context from request
         :returns: Signing key bytes
         :rtype: bytes or None
+        :raises ValueError if a signer key that is already present in the encryption
+            context is added
         """
         _LOGGER.debug("Generating signing key")
         if algorithm.signing_algorithm_info is None:
             return None
 
         signer = Signer(algorithm=algorithm, key=generate_ecc_signing_key(algorithm=algorithm))
-        encryption_context[ENCODED_SIGNER_KEY] = to_str(signer.encoded_public_key())
+        signer_key = signer.encoded_public_key()
+        # raise error if key already present, even if different value
+        # we don't care about the value, but we do care about the key (in the DICT)
+        # remember this is dict(key -> value) = dict(key_name -> key)
+        if ENCODED_SIGNER_KEY in encryption_context:
+            raise ValueError("Tried to add key that was already present in Encryption Context")
+        encryption_context[ENCODED_SIGNER_KEY] = to_str(signer_key)
         return signer.key_bytes()
 
     def get_encryption_materials(self, request):
@@ -68,6 +76,8 @@ def get_encryption_materials(self, request):
         :raises MasterKeyProviderError: if no master keys are available from the underlying master key provider
         :raises MasterKeyProviderError: if the primary master key provided by the underlying master key provider
             is not included in the full set of master keys provided by that provider
+        :raises ValueError if in calling _generate_signing_key_and_update_encryption_context()
+            a key is attempted to be added to the encryption context, when it already has that key.
         """
         algorithm = request.algorithm if request.algorithm is not None else self.algorithm
         encryption_context = request.encryption_context.copy()

test/unit/test_deserialize.py

@@ -27,6 +27,14 @@
 
 pytestmark = [pytest.mark.unit, pytest.mark.local]
 
+def test_deserialize_malformed_encryption_context():
+    """
+    If the client deserialization receives a malformed ciphertext 
+    that defines the AAD length as 0 and then also defines a AAD fields 
+    as 0, the deserialization logic SHOULD raise an error.
+    """
+    bad_ciphertext = b'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'
+    pass
 
 def test_deserialize_non_framed_values():
     iv = b"\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x10\x11"

test/unit/test_material_managers_default.py

@@ -12,6 +12,7 @@
 # language governing permissions and limitations under the License.
 """Test suite for aws_encryption_sdk.materials_managers.default"""
 import pytest
+import os
 from mock import MagicMock, sentinel
 from pytest_mock import mocker  # noqa pylint: disable=unused-import
 
@@ -20,10 +21,14 @@
 from aws_encryption_sdk.identifiers import Algorithm
 from aws_encryption_sdk.internal.defaults import ALGORITHM, ENCODED_SIGNER_KEY
 from aws_encryption_sdk.key_providers.base import MasterKeyProvider
-from aws_encryption_sdk.materials_managers import EncryptionMaterials
+from aws_encryption_sdk.materials_managers import EncryptionMaterials, EncryptionMaterialsRequest
 from aws_encryption_sdk.materials_managers.default import DefaultCryptoMaterialsManager
 from aws_encryption_sdk.structures import DataKey
 
+from aws_encryption_sdk.internal.crypto.authentication import Signer
+from aws_encryption_sdk.internal.crypto.elliptic_curve import generate_ecc_signing_key
+from aws_encryption_sdk.internal.str_ops import to_str
+
 pytestmark = [pytest.mark.unit, pytest.mark.local]
 
 
@@ -234,3 +239,21 @@ def test_decrypt_materials(mocker, patch_for_dcmm_decrypt):
     )
     assert test.data_key is cmm.master_key_provider.decrypt_data_key_from_list.return_value
     assert test.verification_key == patch_for_dcmm_decrypt
+
+# tests that we correctly through an error when we try to add a key 
+# to the encryption context but it is already present
+def test_signer_key_in_encryption_context():
+    cmm = build_cmm()
+    algo = ALGORITHM # default
+
+    # key value does not matter
+    key = to_str(Signer(algorithm=algo, key=generate_ecc_signing_key(algorithm=algo)).encoded_public_key())
+    context = {ENCODED_SIGNER_KEY: key}
+    request = EncryptionMaterialsRequest(
+        algorithm=algo,
+        encryption_context=context,
+        frame_length=4096
+    )
+    with pytest.raises(ValueError) as excinfo:
+        cmm.get_encryption_materials(request)
+    excinfo.match(r"Tried to add key that was already present in Encryption Context")