Automatic switch to internal backend on missing PyCA module

RNS/Cryptography/Provider.py

@@ -1,5 +1,38 @@
+import importlib
+
+PROVIDER_NONE     = 0x00
 PROVIDER_INTERNAL = 0x01
 PROVIDER_PYCA     = 0x02
 
-PROVIDER = PROVIDER_PYCA
+PROVIDER = PROVIDER_NONE
-# PROVIDER = PROVIDER_INTERNAL
+
+pyca_v = None
+use_pyca = False
+
+try:
+    if importlib.util.find_spec('cryptography') != None:
+        import cryptography
+        pyca_v = cryptography.__version__
+        v = pyca_v.split(".")
+
+        if int(v[0]) == 2:
+            if int(v[1]) >= 8:
+                use_pyca = True
+        elif int(v[0]) >= 3:
+            use_pyca = True
+
+except Exception as e:
+    pass
+
+if use_pyca:
+    PROVIDER = PROVIDER_PYCA
+else:
+    PROVIDER = PROVIDER_INTERNAL
+
+def backend():
+    if PROVIDER == PROVIDER_NONE:
+        return "none"
+    elif PROVIDER == PROVIDER_INTERNAL:
+        return "internal"
+    elif PROVIDER == PROVIDER_PYCA:
+        return "openssl, PyCA "+str(pyca_v)

RNS/Cryptography/__init__.py

@@ -6,6 +6,7 @@ from .Hashes import sha512
 from .HKDF import hkdf
 from .PKCS7 import PKCS7
 from .Fernet import Fernet
+from .Provider import backend
 
 import RNS.Cryptography.Provider as cp