Prefer signing_algorithm and digest_algorithm over sign_alg and digest_alg

Continuing the deprecation of saml2 frontend sign_alg and digest_alg
configuration options (see, 04850ee).

The values of the new options should be preferred when set. Otherwise,
we fall back to the deprecate options.

Notice that the new configuration options expect the algothim
identifier, not an internal symbol.

Signed-off-by: Ivan Kanakarakis <ivan.kanak@gmail.com>

Author: c00kiemon5ter

src/satosa/frontends/saml2.py

@@ -362,18 +362,21 @@ def _handle_authn_response(self, context, internal_response, idp):
         logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
         logger.debug(logline)
 
-        policies = self.idp_config.get(
-            'service', {}).get('idp', {}).get('policy', {})
+        idp_conf = self.idp_config.get('service', {}).get('idp', {})
+        policies = idp_conf.get('policy', {})
         sp_policy = policies.get('default', {})
         sp_policy.update(policies.get(sp_entity_id, {}))
 
         sign_assertion = sp_policy.get('sign_assertion', False)
         sign_response = sp_policy.get('sign_response', True)
-        sign_alg = sp_policy.get('sign_alg', 'SIG_RSA_SHA256')
-        digest_alg = sp_policy.get('digest_alg', 'DIGEST_SHA256')
         encrypt_assertion = sp_policy.get('encrypt_assertion', False)
         encrypted_advice_attributes = sp_policy.get('encrypted_advice_attributes', False)
 
+        signing_algorithm = idp_conf.get('signing_algorithm')
+        digest_algorithm = idp_conf.get('digest_algorithm')
+        sign_alg_attr = sp_policy.get('sign_alg', 'SIG_RSA_SHA256')
+        digest_alg_attr = sp_policy.get('digest_alg', 'DIGEST_SHA256')
+
         # Construct arguments for method create_authn_response
         # on IdP Server instance
         args = {
@@ -389,31 +392,35 @@ def _handle_authn_response(self, context, internal_response, idp):
             'encrypted_advice_attributes': encrypted_advice_attributes,
         }
 
-        try:
-            args['sign_alg'] = getattr(xmldsig, sign_alg)
-        except AttributeError as e:
-            msg = "Unsupported sign algorithm {}".format(sign_alg)
-            logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
-            logger.error(logline)
-            raise Exception(msg) from e
-        else:
-            msg = "signing with algorithm {}".format(args['sign_alg'])
-            logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
-            logger.debug(logline)
+        args['sign_alg'] = signing_algorithm
+        if not args['sign_alg']:
+            try:
+                args['sign_alg'] = getattr(xmldsig, sign_alg_attr)
+            except AttributeError as e:
+                msg = "Unsupported sign algorithm {}".format(sign_alg)
+                logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
+                logger.error(logline)
+                raise Exception(msg) from e
+
+        msg = "signing with algorithm {}".format(args['sign_alg'])
+        logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
+        logger.debug(logline)
 
-        try:
-            args['digest_alg'] = getattr(xmldsig, digest_alg)
-        except AttributeError as e:
-            msg = "Unsupported digest algorithm {}".format(digest_alg)
-            logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
-            logger.error(logline)
-            raise Exception(msg) from e
-        else:
-            msg = "using digest algorithm {}".format(args['digest_alg'])
-            logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
-            logger.debug(logline)
+        args['digest_alg'] = digest_algorithm
+        if not args['digest_alg']:
+            try:
+                args['digest_alg'] = getattr(xmldsig, digest_alg_attr)
+            except AttributeError as e:
+                msg = "Unsupported digest algorithm {}".format(digest_alg)
+                logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
+                logger.error(logline)
+                raise Exception(msg) from e
+
+        msg = "using digest algorithm {}".format(args['digest_alg'])
+        logline = lu.LOG_FMT.format(id=lu.get_session_id(context.state), message=msg)
+        logger.debug(logline)
 
-        if 'sign_alg' in args or 'digest_alg' in args:
+        if sign_alg_attr or digest_alg_attr:
             msg = (
                 "sign_alg and digest_alg are deprecated; "
                 "instead, use signing_algorithm and digest_algorithm "