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Use trivial algorithm in ecmult_multi if scratch space is small #592

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May 25, 2019
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Use trivial algorithm in ecmult_multi if scratch space is small #592

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10 changes: 6 additions & 4 deletions src/ecmult_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1152,16 +1152,18 @@ static int secp256k1_ecmult_multi_var(const secp256k1_ecmult_context *ctx, secp2
return secp256k1_ecmult_multi_simple_var(ctx, r, inp_g_sc, cb, cbdata, n);
}

/* Compute the batch sizes for pippenger given a scratch space. If it's greater than a threshold
* use pippenger. Otherwise use strauss */
/* Compute the batch sizes for Pippenger's algorithm given a scratch space. If it's greater than
* a threshold use Pippenger's algorithm. Otherwise use Strauss' algorithm.
* As a first step check if there's enough space for Pippenger's algo (which requires less space
* than Strauss' algo) and if not, use the simple algorithm. */
if (!secp256k1_ecmult_multi_batch_size_helper(&n_batches, &n_batch_points, secp256k1_pippenger_max_points(scratch), n)) {
return 0;
return secp256k1_ecmult_multi_simple_var(ctx, r, inp_g_sc, cb, cbdata, n);
}
if (n_batch_points >= ECMULT_PIPPENGER_THRESHOLD) {
f = secp256k1_ecmult_pippenger_batch;
} else {
if (!secp256k1_ecmult_multi_batch_size_helper(&n_batches, &n_batch_points, secp256k1_strauss_max_points(scratch), n)) {
return 0;
return secp256k1_ecmult_multi_simple_var(ctx, r, inp_g_sc, cb, cbdata, n);
}
f = secp256k1_ecmult_strauss_batch;
}
Expand Down
42 changes: 31 additions & 11 deletions src/tests.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2572,7 +2572,6 @@ void test_ecmult_multi(secp256k1_scratch *scratch, secp256k1_ecmult_multi_func e
secp256k1_gej r;
secp256k1_gej r2;
ecmult_multi_data data;
secp256k1_scratch *scratch_empty;

data.sc = sc;
data.pt = pt;
Expand Down Expand Up @@ -2607,11 +2606,6 @@ void test_ecmult_multi(secp256k1_scratch *scratch, secp256k1_ecmult_multi_func e
secp256k1_gej_add_var(&r, &r, &r2, NULL);
CHECK(secp256k1_gej_is_infinity(&r));

/* Try to multiply 1 point, but scratch space is empty */
scratch_empty = secp256k1_scratch_create(&ctx->error_callback, 0);
CHECK(!ecmult_multi(&ctx->ecmult_ctx, scratch_empty, &r, &szero, ecmult_multi_callback, &data, 1));
secp256k1_scratch_destroy(scratch_empty);

/* Try to multiply 1 point, but callback returns false */
CHECK(!ecmult_multi(&ctx->ecmult_ctx, scratch, &r, &szero, ecmult_multi_false_callback, &data, 1));

Expand Down Expand Up @@ -2809,6 +2803,24 @@ void test_ecmult_multi(secp256k1_scratch *scratch, secp256k1_ecmult_multi_func e
}
}

void test_ecmult_multi_batch_single(secp256k1_ecmult_multi_func ecmult_multi) {
secp256k1_scalar szero;
secp256k1_scalar sc[32];
secp256k1_ge pt[32];
secp256k1_gej r;
ecmult_multi_data data;
secp256k1_scratch *scratch_empty;

data.sc = sc;
data.pt = pt;
secp256k1_scalar_set_int(&szero, 0);

/* Try to multiply 1 point, but scratch space is empty.*/
scratch_empty = secp256k1_scratch_create(&ctx->error_callback, 0);
CHECK(!ecmult_multi(&ctx->ecmult_ctx, scratch_empty, &r, &szero, ecmult_multi_callback, &data, 1));
secp256k1_scratch_destroy(scratch_empty);
}

void test_secp256k1_pippenger_bucket_window_inv(void) {
int i;

Expand Down Expand Up @@ -2932,19 +2944,25 @@ void test_ecmult_multi_batching(void) {
}
data.sc = sc;
data.pt = pt;
secp256k1_gej_neg(&r2, &r2);

/* Test with empty scratch space */
/* Test with empty scratch space. It should compute the correct result using
* ecmult_mult_simple algorithm which doesn't require a scratch space. */
scratch = secp256k1_scratch_create(&ctx->error_callback, 0);
CHECK(!secp256k1_ecmult_multi_var(&ctx->ecmult_ctx, scratch, &r, &scG, ecmult_multi_callback, &data, 1));
CHECK(secp256k1_ecmult_multi_var(&ctx->ecmult_ctx, scratch, &r, &scG, ecmult_multi_callback, &data, n_points));
secp256k1_gej_add_var(&r, &r, &r2, NULL);
CHECK(secp256k1_gej_is_infinity(&r));
secp256k1_scratch_destroy(scratch);

/* Test with space for 1 point in pippenger. That's not enough because
* ecmult_multi selects strauss which requires more memory. */
* ecmult_multi selects strauss which requires more memory. It should
* therefore select the simple algorithm. */
scratch = secp256k1_scratch_create(&ctx->error_callback, secp256k1_pippenger_scratch_size(1, 1) + PIPPENGER_SCRATCH_OBJECTS*ALIGNMENT);
CHECK(!secp256k1_ecmult_multi_var(&ctx->ecmult_ctx, scratch, &r, &scG, ecmult_multi_callback, &data, 1));
CHECK(secp256k1_ecmult_multi_var(&ctx->ecmult_ctx, scratch, &r, &scG, ecmult_multi_callback, &data, n_points));
secp256k1_gej_add_var(&r, &r, &r2, NULL);
CHECK(secp256k1_gej_is_infinity(&r));
secp256k1_scratch_destroy(scratch);

secp256k1_gej_neg(&r2, &r2);
for(i = 1; i <= n_points; i++) {
if (i > ECMULT_PIPPENGER_THRESHOLD) {
int bucket_window = secp256k1_pippenger_bucket_window(i);
Expand Down Expand Up @@ -2972,7 +2990,9 @@ void run_ecmult_multi_tests(void) {
test_ecmult_multi(scratch, secp256k1_ecmult_multi_var);
test_ecmult_multi(NULL, secp256k1_ecmult_multi_var);
test_ecmult_multi(scratch, secp256k1_ecmult_pippenger_batch_single);
test_ecmult_multi_batch_single(secp256k1_ecmult_pippenger_batch_single);
test_ecmult_multi(scratch, secp256k1_ecmult_strauss_batch_single);
test_ecmult_multi_batch_single(secp256k1_ecmult_strauss_batch_single);
secp256k1_scratch_destroy(scratch);

/* Run test_ecmult_multi with space for exactly one point */
Expand Down