Introduce a test case for the extension code.

eqc_test/enacl_ext_eqc.erl

@@ -0,0 +1,24 @@
+-module(enacl_ext_eqc).
+
+-include_lib("eqc/include/eqc.hrl").
+-compile(export_all).
+
+public_keypair() ->
+    ?LET(#{ public := PK, secret := SK}, enacl_ext:curve25519_keypair(),
+         {PK, SK}).
+
+prop_public_key() ->
+    ?FORALL({PK, SK}, public_keypair(),
+        begin
+            equals(PK, enacl_ext:curve25519_public_key(SK))
+        end).
+
+prop_shared_secret() ->
+    ?FORALL([{PK1, SK1}, {PK2, SK2}],
+            [public_keypair(), public_keypair()],
+            begin
+                Alice = enacl_ext:curve25519_shared(SK1, PK2),
+                Bob = enacl_ext:curve25519_shared(SK2, PK1),
+                equals(Alice, Bob)
+            end).
+