Add test to check deterministic signatures

Wrote this to find what turned out to be an external error when
using the interfacing programs, but felt like it might as well be added.

Under the same key and message, the signature is expected to be identical.
However, as the index changes, this case will not happen in real use.

.gitignore

@@ -11,6 +11,7 @@ test/test_xmss_core_XMSS*
 test/test_xmss_core_fast_XMSS*
 test/test_xmssmt_core_XMSSMT*
 test/test_xmssmt_core_fast_XMSSMT*
+test/test_determinism
 test/speed
 test/gen_testvectors
 test/xmss_keypair

Makefile

@@ -15,6 +15,7 @@ TESTS = test/test_wots \
 		test/test_xmssmt_core_fast \
 		test/test_xmssmt_core \
 		test/test_xmssmt \
+		test/test_determinism \
 
 UI = test/xmss_keypair \
 	 test/xmss_sign \

test/test_determinism.c

@@ -0,0 +1,57 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+#include "../params.h"
+#include "../xmss.h"
+#include "../randombytes.h"
+
+#define MLEN 32
+
+int main()
+{
+    xmss_params params;
+    char *oidstr = "XMSS-SHA2_10_256";
+    uint32_t oid = 0x01000001;
+    unsigned int i;
+
+    fprintf(stderr, "Testing if XMSS-SHA2_10_256 signing is deterministic.. ");
+
+    xmss_str_to_oid(&oid, oidstr);
+    xmss_parse_oid(&params, oid);
+
+    unsigned char pk[XMSS_OID_LEN + params.publickey_bytes];
+    unsigned char sk[XMSS_OID_LEN + params.privatekey_bytes];
+    unsigned char sk2[XMSS_OID_LEN + params.privatekey_bytes];
+
+    unsigned char m[MLEN];
+    unsigned char sm[params.bytes + MLEN];
+    unsigned char sm2[params.bytes + MLEN];
+    unsigned long long smlen;
+
+    xmss_keypair(pk, sk, oid);
+
+    /* Duplicate the key, because the original will be modified. */
+    memcpy(sk2, sk, XMSS_OID_LEN + params.privatekey_bytes);
+
+    /* Sign a random message (but twice the same one). */
+    randombytes(m, MLEN);
+
+    xmss_sign(sk, sm, &smlen, m, MLEN);
+    xmss_sign(sk2, sm2, &smlen, m, MLEN);
+
+    /* Compare signature, and, if applicable, print the differences. */
+    if (memcmp(sm, sm2, params.bytes + MLEN)) {
+        fprintf(stderr, "signatures differ!\n");
+        for (i = 0; i < params.bytes + MLEN; i++) {
+            fprintf(stderr, (sm[i] != sm2[i] ? "x" : "."));
+        }
+        fprintf(stderr, "\n");
+        return -1;
+    }
+    else {
+        fprintf(stderr, "signatures are identical.\n");
+    }
+
+    return 0;
+}