Generate ecmult_static_pre_g.h

This header contains a static array that replaces the ecmult_context pre_g and pre_g_128 tables.
The gen_ecmult_static_pre_g program generates this header file.

Author: roconnor-blockstream

.gitattributes

@@ -0,0 +1 @@
+src/ecmult_static_pre_g.h linguist-generated

.gitignore

@@ -9,6 +9,7 @@ bench_internal
 tests
 exhaustive_tests
 gen_context
+gen_ecmult_static_pre_g
 valgrind_ctime_test
 *.exe
 *.so

Makefile.am

@@ -127,12 +127,19 @@ exhaustive_tests_LDFLAGS = -static
 TESTS += exhaustive_tests
 endif
 
+EXTRA_PROGRAMS = gen_ecmult_static_pre_g
+gen_ecmult_static_pre_g_SOURCES = src/gen_ecmult_static_pre_g.c
+# See Automake manual, Section "Errors with distclean"
+src/ecmult_static_pre_g.h:
+	$(MAKE) $(AM_MAKEFLAGS) gen_ecmult_static_pre_g$(EXEEXT)
+	./gen_ecmult_static_pre_g$(EXEEXT)
+
 if USE_ECMULT_STATIC_PRECOMPUTATION
 CPPFLAGS_FOR_BUILD +=-I$(top_srcdir) -I$(builddir)/src
 
 gen_context_OBJECTS = gen_context.o
 gen_context_BIN = gen_context$(BUILD_EXEEXT)
-gen_%.o: src/gen_%.c src/libsecp256k1-config.h
+$(gen_context_OBJECTS): src/gen_context.c src/libsecp256k1-config.h
 	$(CC_FOR_BUILD) $(DEFS) $(CPPFLAGS_FOR_BUILD) $(SECP_CFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
 
 $(gen_context_BIN): $(gen_context_OBJECTS)
@@ -149,7 +156,7 @@ src/ecmult_static_context.h: $(gen_context_BIN)
 CLEANFILES = $(gen_context_BIN) src/ecmult_static_context.h
 endif
 
-EXTRA_DIST = autogen.sh src/gen_context.c src/basic-config.h
+EXTRA_DIST = autogen.sh src/gen_context.c src/ecmult_static_pre_g.h src/basic-config.h
 
 if ENABLE_MODULE_ECDH
 include src/modules/ecdh/Makefile.am.include

configure.ac

@@ -177,6 +177,8 @@ AC_ARG_WITH([ecmult-window], [AS_HELP_STRING([--with-ecmult-window=SIZE|auto],
 [window size for ecmult precomputation for verification, specified as integer in range [2..24].]
 [Larger values result in possibly better performance at the cost of an exponentially larger precomputed table.]
 [The table will store 2^(SIZE-1) * 64 bytes of data but can be larger in memory due to platform-specific padding and alignment.]
+[A window size larger than 15 will require you delete the prebuilt ecmult_static_pre_g.h file so that it can be rebuilt.]
+[For very large window sizes, use "make -j 1" to reduce memory use during compilation.]
 ["auto" is a reasonable setting for desktop machines (currently 15). [default=auto]]
 )],
 [req_ecmult_window=$withval], [req_ecmult_window=auto])

src/ecmult.h

@@ -11,6 +11,24 @@
 #include "scalar.h"
 #include "scratch.h"
 
+/* 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
+
+/** The number of entries a table with precomputed multiples needs to have. */
+#define ECMULT_TABLE_SIZE(w) (1L << ((w)-2))
+
 typedef struct {
     /* For accelerating the computation of a*P + b*G: */
     secp256k1_ge_storage (*pre_g)[];    /* odd multiples of the generator */

src/ecmult_impl.h

@@ -44,28 +44,14 @@
 #  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].
+#if ECMULT_WINDOW_SIZE < WINDOW_G
+#  error ECMULT_WINDOW_SIZE too small for WINDOW_G.
 #endif
 
 #define WNAF_BITS 128
 #define WNAF_SIZE_BITS(bits, w) (((bits) + (w) - 1) / (w))
 #define WNAF_SIZE(w) WNAF_SIZE_BITS(WNAF_BITS, w)
 
-/** The number of entries a table with precomputed multiples needs to have. */
-#define ECMULT_TABLE_SIZE(w) (1 << ((w)-2))
-
 /* The number of objects allocated on the scratch space for ecmult_multi algorithms */
 #define PIPPENGER_SCRATCH_OBJECTS 6
 #define STRAUSS_SCRATCH_OBJECTS 6

src/ecmult_static_pre_g.h

@@ -0,0 +1,137 @@
+/*****************************************************************************************************
+ * Copyright (c) 2013, 2014, 2017, 2021 Pieter Wuille, Andrew Poelstra, Jonas Nick, Russell O'Connor *
+ * Distributed under the MIT software license, see the accompanying                                  *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.                              *
+ *****************************************************************************************************/
+
+#include <inttypes.h>
+#include <stdio.h>
+
+/* Autotools creates libsecp256k1-config.h, of which ECMULT_WINDOW_SIZE is needed.
+   ifndef guard so downstream users can define their own if they do not use autotools. */
+#if !defined(ECMULT_WINDOW_SIZE)
+#include "libsecp256k1-config.h"
+#endif
+
+/* In principle we could use ASM, but this yields only a minor speedup in
+   build time and it's very complicated. In particular when cross-compiling, we'd
+   need to build the ASM for the build and the host machine. */
+#undef USE_EXTERNAL_ASM
+#undef USE_ASM_X86_64
+
+#include "../include/secp256k1.h"
+#include "assumptions.h"
+#include "util.h"
+#include "field_impl.h"
+#include "group_impl.h"
+#include "ecmult.h"
+
+void print_table(FILE *fp, const char *name, int window_g, const secp256k1_gej *gen, int with_conditionals) {
+    static secp256k1_gej gj;
+    static secp256k1_ge ge, dgen;
+    static secp256k1_ge_storage ges;
+    int j;
+    int i;
+
+    gj = *gen;
+    secp256k1_ge_set_gej_var(&ge, &gj);
+    secp256k1_ge_to_storage(&ges, &ge);
+
+    fprintf(fp, "static const secp256k1_ge_storage %s[ECMULT_TABLE_SIZE(WINDOW_G)] = {\n", name);
+    fprintf(fp, " S(%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32
+                  ",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32")\n",
+                SECP256K1_GE_STORAGE_CONST_GET(ges));
+
+    secp256k1_gej_double_var(&gj, gen, NULL);
+    secp256k1_ge_set_gej_var(&dgen, &gj);
+
+    j = 1;
+    for(i = 3; i <= window_g; ++i) {
+        if (with_conditionals) {
+            fprintf(fp, "#if ECMULT_TABLE_SIZE(WINDOW_G) > %ld\n", ECMULT_TABLE_SIZE(i-1));
+        }
+        for(;j < ECMULT_TABLE_SIZE(i); ++j) {
+            secp256k1_gej_set_ge(&gj, &ge);
+            secp256k1_gej_add_ge_var(&gj, &gj, &dgen, NULL);
+            secp256k1_ge_set_gej_var(&ge, &gj);
+            secp256k1_ge_to_storage(&ges, &ge);
+
+            fprintf(fp, ",S(%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32
+                          ",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32",%"PRIx32")\n",
+                        SECP256K1_GE_STORAGE_CONST_GET(ges));
+        }
+        if (with_conditionals) {
+            fprintf(fp, "#endif\n");
+        }
+    }
+    fprintf(fp, "};\n");
+}
+
+void print_two_tables(FILE *fp, int window_g, const secp256k1_ge *g, int with_conditionals) {
+    secp256k1_gej gj;
+    int i;
+
+    secp256k1_gej_set_ge(&gj, g);
+    print_table(fp, "secp256k1_pre_g", window_g, &gj, with_conditionals);
+    for (i = 0; i < 128; ++i) {
+        secp256k1_gej_double_var(&gj, &gj, NULL);
+    }
+    print_table(fp, "secp256k1_pre_g_128", window_g, &gj, with_conditionals);
+}
+
+int main(void) {
+    const secp256k1_ge g = SECP256K1_G;
+    const secp256k1_ge g_13 = SECP256K1_G_ORDER_13;
+    const secp256k1_ge g_199 = SECP256K1_G_ORDER_199;
+    const int window_g_13 = 4;
+    const int window_g_199 = 8;
+    FILE* fp;
+
+    fp = fopen("src/ecmult_static_pre_g.h","w");
+    if (fp == NULL) {
+        fprintf(stderr, "Could not open src/ecmult_static_pre_g.h for writing!\n");
+        return -1;
+    }
+
+    fprintf(fp, "/* This file was automatically generated by gen_ecmult_static_pre_g. */\n");
+    fprintf(fp, "/* This file contains an array secp256k1_pre_g with odd multiples of the base point G and\n");
+    fprintf(fp, " * an array secp256k1_pre_g_128 with odd multiples of 2^128*G for accelerating the computation of a*P + b*G.\n");
+    fprintf(fp, " */\n");
+    fprintf(fp, "#ifndef SECP256K1_ECMULT_STATIC_PRE_G_H\n");
+    fprintf(fp, "#define SECP256K1_ECMULT_STATIC_PRE_G_H\n");
+    fprintf(fp, "#include \"group.h\"\n");
+    fprintf(fp, "#ifdef S\n");
+    fprintf(fp, "   #error macro identifier S already in use.\n");
+    fprintf(fp, "#endif\n");
+    fprintf(fp, "#define S(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p) "
+                "SECP256K1_GE_STORAGE_CONST(0x##a##u,0x##b##u,0x##c##u,0x##d##u,0x##e##u,0x##f##u,0x##g##u,"
+                "0x##h##u,0x##i##u,0x##j##u,0x##k##u,0x##l##u,0x##m##u,0x##n##u,0x##o##u,0x##p##u)\n");
+    fprintf(fp, "#if ECMULT_TABLE_SIZE(ECMULT_WINDOW_SIZE) > %ld\n", ECMULT_TABLE_SIZE(ECMULT_WINDOW_SIZE));
+    fprintf(fp, "   #error configuration mismatch, invalid ECMULT_WINDOW_SIZE. Try deleting ecmult_static_pre_g.h before the build.\n");
+    fprintf(fp, "#endif\n");
+    fprintf(fp, "#if defined(EXHAUSTIVE_TEST_ORDER)\n");
+    fprintf(fp, "#if EXHAUSTIVE_TEST_ORDER == 13\n");
+    fprintf(fp, "#define WINDOW_G %d\n", window_g_13);
+
+    print_two_tables(fp, window_g_13, &g_13, 0);
+
+    fprintf(fp, "#elif EXHAUSTIVE_TEST_ORDER == 199\n");
+    fprintf(fp, "#define WINDOW_G %d\n", window_g_199);
+
+    print_two_tables(fp, window_g_199, &g_199, 0);
+
+    fprintf(fp, "#else\n");
+    fprintf(fp, "   #error No known generator for the specified exhaustive test group order.\n");
+    fprintf(fp, "#endif\n");
+    fprintf(fp, "#else /* !defined(EXHAUSTIVE_TEST_ORDER) */\n");
+    fprintf(fp, "#define WINDOW_G ECMULT_WINDOW_SIZE\n");
+
+    print_two_tables(fp, ECMULT_WINDOW_SIZE, &g, 1);
+
+    fprintf(fp, "#endif\n");
+    fprintf(fp, "#undef S\n");
+    fprintf(fp, "#endif\n");
+    fclose(fp);
+
+    return 0;
+}

src/gen_ecmult_static_pre_g.c

@@ -10,6 +10,27 @@
 #include "field.h"
 #include "group.h"
 
+#define SECP256K1_G_ORDER_13 SECP256K1_GE_CONST(\
+    0xc3459c3d, 0x35326167, 0xcd86cce8, 0x07a2417f,\
+    0x5b8bd567, 0xde8538ee, 0x0d507b0c, 0xd128f5bb,\
+    0x8e467fec, 0xcd30000a, 0x6cc1184e, 0x25d382c2,\
+    0xa2f4494e, 0x2fbe9abc, 0x8b64abac, 0xd005fb24\
+)
+#define SECP256K1_G_ORDER_199 SECP256K1_GE_CONST(\
+    0x226e653f, 0xc8df7744, 0x9bacbf12, 0x7d1dcbf9,\
+    0x87f05b2a, 0xe7edbd28, 0x1f564575, 0xc48dcf18,\
+    0xa13872c2, 0xe933bb17, 0x5d9ffd5b, 0xb5b6e10c,\
+    0x57fe3c00, 0xbaaaa15a, 0xe003ec3e, 0x9c269bae\
+)
+/** Generator for secp256k1, value 'g' defined in
+ *  "Standards for Efficient Cryptography" (SEC2) 2.7.1.
+ */
+#define SECP256K1_G SECP256K1_GE_CONST(\
+    0x79BE667EUL, 0xF9DCBBACUL, 0x55A06295UL, 0xCE870B07UL,\
+    0x029BFCDBUL, 0x2DCE28D9UL, 0x59F2815BUL, 0x16F81798UL,\
+    0x483ADA77UL, 0x26A3C465UL, 0x5DA4FBFCUL, 0x0E1108A8UL,\
+    0xFD17B448UL, 0xA6855419UL, 0x9C47D08FUL, 0xFB10D4B8UL\
+)
 /* These exhaustive group test orders and generators are chosen such that:
  * - The field size is equal to that of secp256k1, so field code is the same.
  * - The curve equation is of the form y^2=x^3+B for some constant B.
@@ -21,23 +42,15 @@
  */
 #if defined(EXHAUSTIVE_TEST_ORDER)
 #  if EXHAUSTIVE_TEST_ORDER == 13
-static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_GE_CONST(
-    0xc3459c3d, 0x35326167, 0xcd86cce8, 0x07a2417f,
-    0x5b8bd567, 0xde8538ee, 0x0d507b0c, 0xd128f5bb,
-    0x8e467fec, 0xcd30000a, 0x6cc1184e, 0x25d382c2,
-    0xa2f4494e, 0x2fbe9abc, 0x8b64abac, 0xd005fb24
-);
+static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_G_ORDER_13;
+
 static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(
     0x3d3486b2, 0x159a9ca5, 0xc75638be, 0xb23a69bc,
     0x946a45ab, 0x24801247, 0xb4ed2b8e, 0x26b6a417
 );
 #  elif EXHAUSTIVE_TEST_ORDER == 199
-static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_GE_CONST(
-    0x226e653f, 0xc8df7744, 0x9bacbf12, 0x7d1dcbf9,
-    0x87f05b2a, 0xe7edbd28, 0x1f564575, 0xc48dcf18,
-    0xa13872c2, 0xe933bb17, 0x5d9ffd5b, 0xb5b6e10c,
-    0x57fe3c00, 0xbaaaa15a, 0xe003ec3e, 0x9c269bae
-);
+static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_G_ORDER_199;
+
 static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(
     0x2cca28fa, 0xfc614b80, 0x2a3db42b, 0x00ba00b1,
     0xbea8d943, 0xdace9ab2, 0x9536daea, 0x0074defb
@@ -46,15 +59,7 @@ static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(
 #    error No known generator for the specified exhaustive test group order.
 #  endif
 #else
-/** Generator for secp256k1, value 'g' defined in
- *  "Standards for Efficient Cryptography" (SEC2) 2.7.1.
- */
-static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_GE_CONST(
-    0x79BE667EUL, 0xF9DCBBACUL, 0x55A06295UL, 0xCE870B07UL,
-    0x029BFCDBUL, 0x2DCE28D9UL, 0x59F2815BUL, 0x16F81798UL,
-    0x483ADA77UL, 0x26A3C465UL, 0x5DA4FBFCUL, 0x0E1108A8UL,
-    0xFD17B448UL, 0xA6855419UL, 0x9C47D08FUL, 0xFB10D4B8UL
-);
+static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_G;
 
 static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 7);
 #endif