detect pycryptodome, don't use it

fixes #198

backport of e92ba72 from master

.travis.yml

@@ -55,6 +55,10 @@ matrix:
       env: PYCRYPTO=true
     - python: 3.7-dev
       env: PYCRYPTO=true
+    - python: 3.7
+      dist: xenial
+      sudo: true
+      env: PYCRYPTODOME=true
     - python: 2.7
       env: GMPY=true
     - python: 3.4
@@ -99,6 +103,7 @@ install:
   - if [[ $TACKPY == 'true' ]]; then travis_retry pip install tackpy; fi
   - if [[ $M2CRYPTO == 'true' ]]; then travis_retry pip install --pre m2crypto; fi
   - if [[ $PYCRYPTO == 'true' ]]; then travis_retry pip install pycrypto; fi
+  - if [[ $PYCRYPTODOME == 'true' ]]; then travis_retry pip install pycryptodome; fi
   - if [[ $GMPY == 'true' ]]; then travis_retry pip install gmpy; fi
   - travis_retry pip install -r requirements.txt
 

tlslite/utils/cryptomath.py

@@ -46,11 +46,20 @@
     gmpyLoaded = False
 
 #Try to load pycrypto
+# pylint: disable=invalid-name
 try:
     import Crypto.Cipher.AES
-    pycryptoLoaded = True
+    # check if we're not using pycryptodome
+    try:
+        # pycrypto defaults to ECB when just key is provided
+        # pycryptodome requires specifying the mode of operation
+        Crypto.Cipher.AES.AESCipher(b'2' * (128//8))
+        pycryptoLoaded = True
+    except AttributeError:
+        pycryptoLoaded = False
 except ImportError:
     pycryptoLoaded = False
+# pylint: enable=invalid-name
 
 
 # **************************************************************************