Merge bitcoin-core/secp256k1#1049: Faster fixed-input ecmult tests

070e772 Faster fixed-input ecmult tests (Pieter Wuille)

Pull request description:

  Given how much #920 slowed down the tests with low iteration count, replace it with 3 different similar test:
  * count >= 1: a test with 1024 multiplies that tests any pattern of 6 bits in windows not more than 20 bits wide
  * count >= 3: a test with 2048 multiplies that tests any pattern of 8 consecutive bits
  * count >= 35: the old test (which effectively tests all 2-bit patterns)

ACKs for top commit:
  robot-dreams:
    ACK 070e772, the addition of the `CONDITIONAL_TEST` macro is nice.
  real-or-random:
    ACK 070e772

Tree-SHA512: b4ccca42c71fcd1baa7143f73d1c3ac9d012c296485164a03341dbeee02e4ba9f7c7ad6b441923a5fe0286c97eff60815033adb4e1d30b3ef08bcb79590327ff

src/tests.c

@@ -28,6 +28,8 @@
 #include "modinv64_impl.h"
 #endif
 
+#define CONDITIONAL_TEST(cnt, nam) if (count < (cnt)) { printf("Skipping %s (iteration count too low)\n", nam); } else
+
 static int count = 64;
 static secp256k1_context *ctx = NULL;
 
@@ -4752,8 +4754,8 @@ void test_ecmult_accumulate(secp256k1_sha256* acc, const secp256k1_scalar* x, se
     }
 }
 
-void test_ecmult_constants(void) {
-    /* Test ecmult_gen for:
+void test_ecmult_constants_2bit(void) {
+    /* Using test_ecmult_accumulate, test ecmult for:
      * - For i in 0..36:
      *   - Key i
      *   - Key -i
@@ -4796,8 +4798,81 @@ void test_ecmult_constants(void) {
     secp256k1_scratch_space_destroy(ctx, scratch);
 }
 
+void test_ecmult_constants_sha(uint32_t prefix, size_t iter, const unsigned char* expected32) {
+    /* Using test_ecmult_accumulate, test ecmult for:
+     * - Key 0
+     * - Key 1
+     * - Key -1
+     * - For i in range(iter):
+     *   - Key SHA256(LE32(prefix) || LE16(i))
+     */
+    secp256k1_scalar x;
+    secp256k1_sha256 acc;
+    unsigned char b32[32];
+    unsigned char inp[6];
+    size_t i;
+    secp256k1_scratch_space *scratch = secp256k1_scratch_space_create(ctx, 65536);
+
+    inp[0] = prefix & 0xFF;
+    inp[1] = (prefix >> 8) & 0xFF;
+    inp[2] = (prefix >> 16) & 0xFF;
+    inp[3] = (prefix >> 24) & 0xFF;
+    secp256k1_sha256_initialize(&acc);
+    secp256k1_scalar_set_int(&x, 0);
+    test_ecmult_accumulate(&acc, &x, scratch);
+    secp256k1_scalar_set_int(&x, 1);
+    test_ecmult_accumulate(&acc, &x, scratch);
+    secp256k1_scalar_negate(&x, &x);
+    test_ecmult_accumulate(&acc, &x, scratch);
+
+    for (i = 0; i < iter; ++i) {
+        secp256k1_sha256 gen;
+        inp[4] = i & 0xff;
+        inp[5] = (i >> 8) & 0xff;
+        secp256k1_sha256_initialize(&gen);
+        secp256k1_sha256_write(&gen, inp, sizeof(inp));
+        secp256k1_sha256_finalize(&gen, b32);
+        secp256k1_scalar_set_b32(&x, b32, NULL);
+        test_ecmult_accumulate(&acc, &x, scratch);
+    }
+    secp256k1_sha256_finalize(&acc, b32);
+    CHECK(secp256k1_memcmp_var(b32, expected32, 32) == 0);
+
+    secp256k1_scratch_space_destroy(ctx, scratch);
+}
+
 void run_ecmult_constants(void) {
-    test_ecmult_constants();
+    /* Expected hashes of all points in the tests below. Computed using an
+     * independent implementation. */
+    static const unsigned char expected32_6bit20[32] = {
+        0x68, 0xb6, 0xed, 0x6f, 0x28, 0xca, 0xc9, 0x7f,
+        0x8e, 0x8b, 0xd6, 0xc0, 0x61, 0x79, 0x34, 0x6e,
+        0x5a, 0x8f, 0x2b, 0xbc, 0x3e, 0x1f, 0xc5, 0x2e,
+        0x2a, 0xd0, 0x45, 0x67, 0x7f, 0x95, 0x95, 0x8e
+    };
+    static const unsigned char expected32_8bit8[32] = {
+        0x8b, 0x65, 0x8e, 0xea, 0x86, 0xae, 0x3c, 0x95,
+        0x90, 0xb6, 0x77, 0xa4, 0x8c, 0x76, 0xd9, 0xec,
+        0xf5, 0xab, 0x8a, 0x2f, 0xfd, 0xdb, 0x19, 0x12,
+        0x1a, 0xee, 0xe6, 0xb7, 0x6e, 0x05, 0x3f, 0xc6
+    };
+    /* For every combination of 6 bit positions out of 256, restricted to
+     * 20-bit windows (i.e., the first and last bit position are no more than
+     * 19 bits apart), all 64 bit patterns occur in the input scalars used in
+     * this test. */
+    CONDITIONAL_TEST(1, "test_ecmult_constants_sha 1024") {
+        test_ecmult_constants_sha(4808378u, 1024, expected32_6bit20);
+    }
+
+    /* For every combination of 8 consecutive bit positions, all 256 bit
+     * patterns occur in the input scalars used in this test. */
+    CONDITIONAL_TEST(3, "test_ecmult_constants_sha 2048") {
+        test_ecmult_constants_sha(1607366309u, 2048, expected32_8bit8);
+    }
+
+    CONDITIONAL_TEST(35, "test_ecmult_constants_2bit") {
+        test_ecmult_constants_2bit();
+    }
 }
 
 void test_ecmult_gen_blind(void) {