Don't fail if decapsulating with wrong keys doesn't return -1

Closes #11

README.md

@@ -42,7 +42,7 @@ _The checking of items on this list is still being developed. Checked items shou
   * [x] `randombytes.c`
 * [ ] API functions return `0` on success, negative on failure
     * [x] 0 on success
-    * [ ] Negative on failure (currently: partially)
+    * [ ] Negative on failure (within restrictions of FO transform).
 * [ ] No dynamic memory allocations
 * [ ] No branching on secret data (dynamically checked using valgrind)
 * [ ] No access to secret memory locations (dynamically checked using valgrind)

test/crypto_kem/functest.c

@@ -137,8 +137,8 @@ static int test_invalid_ciphertext(void) {
         sendb[pos % CRYPTO_CIPHERTEXTBYTES] ^= 23;
 
         // Alice uses Bobs response to get her secret key
-        if ((returncode = crypto_kem_dec(key_a, sendb, sk_a)) > -1) {
-            printf("ERROR crypto_kem_dec should fail (negative returncode) but returned %d\n", returncode);
+        if ((returncode = crypto_kem_dec(key_a, sendb, sk_a)) > 0) {
+            printf("ERROR crypto_kem_dec should either fail (negative returncode) or succeed (return 0) but returned %d\n", returncode);
             return -1;
         }