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Make WINDOW_G configurable

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This makes WINDOW_G a configurable value in the range of [2..24].
The upper limit of 24 is a defensive choice. The code is probably
correct for values up to 27 but those larger values yield in huge
tables (>= 256MiB), which are i) unlikely to be really beneficial
in practice and ii) increasingly difficult to test.
This commit is contained in:
Tim Ruffing
2019-03-06 13:12:33 +01:00
parent 1a02d6ce51
commit 2842dc523e
3 changed files with 70 additions and 13 deletions
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50
src/ecmult_impl.h
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@@ -30,16 +30,32 @@
# endif
#else
/* optimal for 128-bit and 256-bit exponents. */
#define WINDOW_A 5
/** larger numbers may result in slightly better performance, at the cost of
exponentially larger precomputed tables. */
#ifdef USE_ENDOMORPHISM
/** Two tables for window size 15: 1.375 MiB. */
#define WINDOW_G 15
#else
/** One table for window size 16: 1.375 MiB. */
#define WINDOW_G 16
# define WINDOW_A 5
/** Larger values for ECMULT_WINDOW_SIZE result in possibly better
* performance at the cost of an exponentially larger precomputed
* table. The exact table size is
* (1 << (WINDOW_G - 2)) * sizeof(secp256k1_ge_storage) bytes,
* where sizeof(secp256k1_ge_storage) is typically 64 bytes but can
* be larger due to platform-specific padding and alignment.
* If the endomorphism optimization is enabled (USE_ENDOMORMPHSIM)
* two tables of this size are used instead of only one.
*/
# define WINDOW_G ECMULT_WINDOW_SIZE
#endif
/* Noone will ever need more than a window size of 24. The code might
* be correct for larger values of ECMULT_WINDOW_SIZE but this is not
* not tested.
*
* The following limitations are known, and there are probably more:
* If WINDOW_G > 27 and size_t has 32 bits, then the code is incorrect
* because the size of the memory object that we allocate (in bytes)
* will not fit in a size_t.
* If WINDOW_G > 31 and int has 32 bits, then the code is incorrect
* because certain expressions will overflow.
*/
#if ECMULT_WINDOW_SIZE < 2 || ECMULT_WINDOW_SIZE > 24
# error Set ECMULT_WINDOW_SIZE to an integer in range [2..24].
#endif
#ifdef USE_ENDOMORPHISM
@@ -311,7 +327,12 @@ static void secp256k1_ecmult_context_build(secp256k1_ecmult_context *ctx, const
/* get the generator */
secp256k1_gej_set_ge(&gj, &secp256k1_ge_const_g);
ctx->pre_g = (secp256k1_ge_storage (*)[])checked_malloc(cb, sizeof((*ctx->pre_g)[0]) * ECMULT_TABLE_SIZE(WINDOW_G));
{
size_t size = sizeof((*ctx->pre_g)[0]) * ((size_t)ECMULT_TABLE_SIZE(WINDOW_G));
/* check for overflow */
VERIFY_CHECK(size / sizeof((*ctx->pre_g)[0]) == ((size_t)ECMULT_TABLE_SIZE(WINDOW_G)));
ctx->pre_g = (secp256k1_ge_storage (*)[])checked_malloc(cb, size);
}
/* precompute the tables with odd multiples */
secp256k1_ecmult_odd_multiples_table_storage_var(ECMULT_TABLE_SIZE(WINDOW_G), *ctx->pre_g, &gj);
@@ -321,7 +342,10 @@ static void secp256k1_ecmult_context_build(secp256k1_ecmult_context *ctx, const
secp256k1_gej g_128j;
int i;
ctx->pre_g_128 = (secp256k1_ge_storage (*)[])checked_malloc(cb, sizeof((*ctx->pre_g_128)[0]) * ECMULT_TABLE_SIZE(WINDOW_G));
size_t size = sizeof((*ctx->pre_g_128)[0]) * ((size_t) ECMULT_TABLE_SIZE(WINDOW_G));
/* check for overflow */
VERIFY_CHECK(size / sizeof((*ctx->pre_g_128)[0]) == ((size_t)ECMULT_TABLE_SIZE(WINDOW_G)));
ctx->pre_g_128 = (secp256k1_ge_storage (*)[])checked_malloc(cb, size);
/* calculate 2^128*generator */
g_128j = gj;
@@ -338,7 +362,7 @@ static void secp256k1_ecmult_context_clone(secp256k1_ecmult_context *dst,
if (src->pre_g == NULL) {
dst->pre_g = NULL;
} else {
size_t size = sizeof((*dst->pre_g)[0]) * ECMULT_TABLE_SIZE(WINDOW_G);
size_t size = sizeof((*dst->pre_g)[0]) * ((size_t)ECMULT_TABLE_SIZE(WINDOW_G));
dst->pre_g = (secp256k1_ge_storage (*)[])checked_malloc(cb, size);
memcpy(dst->pre_g, src->pre_g, size);
}
@@ -346,7 +370,7 @@ static void secp256k1_ecmult_context_clone(secp256k1_ecmult_context *dst,
if (src->pre_g_128 == NULL) {
dst->pre_g_128 = NULL;
} else {
size_t size = sizeof((*dst->pre_g_128)[0]) * ECMULT_TABLE_SIZE(WINDOW_G);
size_t size = sizeof((*dst->pre_g_128)[0]) * ((size_t)ECMULT_TABLE_SIZE(WINDOW_G));
dst->pre_g_128 = (secp256k1_ge_storage (*)[])checked_malloc(cb, size);
memcpy(dst->pre_g_128, src->pre_g_128, size);
}