Merge bitcoin-core/secp256k1#1371: Add exhaustive tests for ellswift (with create+decode roundtrip)

2792119 Add exhaustive test for ellswift (create+decode roundtrip) (Sebastian Falbesoner)

Pull request description:

  This PR adds the basic structure for ellswift exhaustive tests. Right now only a `secp256k1_ellswift_create` + `secp256k1_ellswift_decode` indirect roundtrip (exhaustive loop scalar -> ellswift pubkey -> decoded pubkey -> decoded group element, compared with exhaustive precomputed group element) is included.

  The exhaustive tests passes locally with all currently supported orders (n=13 [default] and n=199). Note that for n=7, the test is skipped, as the used curve in this case is even-ordered and ellswift only supports odd-ordered curves.

ACKs for top commit:
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    utACK 2792119
  real-or-random:
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Tree-SHA512: c51d3d99e9839793b3c15d75b9a29f01080db160ab8819973abd877288f9f0af972ea4264290220ab1cd035fdebcfac7767436aa39154d924ef0bf6a5733a55d

Author: real-or-random

src/modules/ellswift/Makefile.am.include

@@ -2,3 +2,4 @@ include_HEADERS += include/secp256k1_ellswift.h
 noinst_HEADERS += src/modules/ellswift/bench_impl.h
 noinst_HEADERS += src/modules/ellswift/main_impl.h
 noinst_HEADERS += src/modules/ellswift/tests_impl.h
+noinst_HEADERS += src/modules/ellswift/tests_exhaustive_impl.h

src/modules/ellswift/tests_exhaustive_impl.h

@@ -0,0 +1,39 @@
+/***********************************************************************
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_MODULE_ELLSWIFT_TESTS_EXHAUSTIVE_H
+#define SECP256K1_MODULE_ELLSWIFT_TESTS_EXHAUSTIVE_H
+
+#include "../../../include/secp256k1_ellswift.h"
+#include "main_impl.h"
+
+static void test_exhaustive_ellswift(const secp256k1_context *ctx, const secp256k1_ge *group) {
+    int i;
+
+    /* Note that SwiftEC/ElligatorSwift are inherently curve operations, not
+     * group operations, and this test only checks the curve points which are in
+     * a tiny subgroup. In that sense it can't be really seen as exhaustive as
+     * it doesn't (and for computational reasons obviously cannot) test the
+     * entire domain ellswift operates under. */
+    for (i = 1; i < EXHAUSTIVE_TEST_ORDER; i++) {
+        secp256k1_scalar scalar_i;
+        unsigned char sec32[32];
+        unsigned char ell64[64];
+        secp256k1_pubkey pub_decoded;
+        secp256k1_ge ge_decoded;
+
+        /* Construct ellswift pubkey from exhaustive loop scalar i. */
+        secp256k1_scalar_set_int(&scalar_i, i);
+        secp256k1_scalar_get_b32(sec32, &scalar_i);
+        CHECK(secp256k1_ellswift_create(ctx, ell64, sec32, NULL));
+
+        /* Decode ellswift pubkey and check that it matches the precomputed group element. */
+        secp256k1_ellswift_decode(ctx, &pub_decoded, ell64);
+        secp256k1_pubkey_load(ctx, &ge_decoded, &pub_decoded);
+        ge_equals_ge(&ge_decoded, &group[i]);
+    }
+}
+
+#endif

src/tests_exhaustive.c

@@ -13,6 +13,9 @@
 #define EXHAUSTIVE_TEST_ORDER 13
 #endif
 
+/* These values of B are all values in [1, 8] that result in a curve with even order. */
+#define EXHAUSTIVE_TEST_CURVE_HAS_EVEN_ORDER (SECP256K1_B == 1 || SECP256K1_B == 6 || SECP256K1_B == 8)
+
 #ifdef USE_EXTERNAL_DEFAULT_CALLBACKS
     #pragma message("Ignoring USE_EXTERNAL_CALLBACKS in exhaustive_tests.")
     #undef USE_EXTERNAL_DEFAULT_CALLBACKS
@@ -395,6 +398,10 @@ static void test_exhaustive_sign(const secp256k1_context *ctx, const secp256k1_g
 #include "modules/schnorrsig/tests_exhaustive_impl.h"
 #endif
 
+#ifdef ENABLE_MODULE_ELLSWIFT
+#include "modules/ellswift/tests_exhaustive_impl.h"
+#endif
+
 int main(int argc, char** argv) {
     int i;
     secp256k1_gej groupj[EXHAUSTIVE_TEST_ORDER];
@@ -490,6 +497,15 @@ int main(int argc, char** argv) {
 #ifdef ENABLE_MODULE_SCHNORRSIG
         test_exhaustive_schnorrsig(ctx);
 #endif
+#ifdef ENABLE_MODULE_ELLSWIFT
+    /* The ellswift algorithm does have additional edge cases when operating on
+     * curves of even order, which are not included in the code as secp256k1 is
+     * of odd order. Skip the ellswift tests if the used exhaustive tests curve
+     * is even-ordered accordingly. */
+    #if !EXHAUSTIVE_TEST_CURVE_HAS_EVEN_ORDER
+        test_exhaustive_ellswift(ctx, group);
+    #endif
+#endif
 
         secp256k1_context_destroy(ctx);
     }