Add ability to use a statically generated ecmult context.

This vastly shrinks the size of the context required for signing on devices with
memory-mapped Flash.

Tables are generated by the new gen_context tool into a header.

src/basic-config.h

@@ -0,0 +1,32 @@
+/**********************************************************************
+ * Copyright (c) 2013, 2014 Pieter Wuille                             *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_BASIC_CONFIG_
+#define _SECP256K1_BASIC_CONFIG_
+
+#ifdef USE_BASIC_CONFIG
+
+#undef USE_ASM_X86_64
+#undef USE_ENDOMORPHISM
+#undef USE_FIELD_10X26
+#undef USE_FIELD_5X52
+#undef USE_FIELD_INV_BUILTIN
+#undef USE_FIELD_INV_NUM
+#undef USE_NUM_GMP
+#undef USE_NUM_NONE
+#undef USE_SCALAR_4X64
+#undef USE_SCALAR_8X32
+#undef USE_SCALAR_INV_BUILTIN
+#undef USE_SCALAR_INV_NUM
+
+#define USE_NUM_NONE 1
+#define USE_FIELD_INV_BUILTIN 1
+#define USE_SCALAR_INV_BUILTIN 1
+#define USE_FIELD_10X26 1
+#define USE_SCALAR_8X32 1
+
+#endif // USE_BASIC_CONFIG
+#endif // _SECP256K1_BASIC_CONFIG_

src/ecmult_gen_impl.h

@@ -11,21 +11,25 @@
 #include "group.h"
 #include "ecmult_gen.h"
 #include "hash_impl.h"
-
+#ifdef USE_ECMULT_STATIC_PRECOMPUTATION
+#include "ecmult_static_context.h"
+#endif
 static void secp256k1_ecmult_gen_context_init(secp256k1_ecmult_gen_context_t *ctx) {
     ctx->prec = NULL;
 }
 
 static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context_t *ctx) {
+#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
     secp256k1_ge_t prec[1024];
     secp256k1_gej_t gj;
     secp256k1_gej_t nums_gej;
     int i, j;
+#endif
 
     if (ctx->prec != NULL) {
         return;
     }
-
+#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
     ctx->prec = (secp256k1_ge_storage_t (*)[64][16])checked_malloc(sizeof(*ctx->prec));
 
     /* get the generator */
@@ -75,6 +79,9 @@ static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context_t *c
             secp256k1_ge_to_storage(&(*ctx->prec)[j][i], &prec[j*16 + i]);
         }
     }
+#else
+    ctx->prec = (secp256k1_ge_storage_t (*)[64][16])secp256k1_ecmult_static_context;
+#endif
     secp256k1_ecmult_gen_blind(ctx, NULL);
 }
 
@@ -87,15 +94,21 @@ static void secp256k1_ecmult_gen_context_clone(secp256k1_ecmult_gen_context_t *d
     if (src->prec == NULL) {
         dst->prec = NULL;
     } else {
+#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
         dst->prec = (secp256k1_ge_storage_t (*)[64][16])checked_malloc(sizeof(*dst->prec));
         memcpy(dst->prec, src->prec, sizeof(*dst->prec));
+#else
+        dst->prec = src->prec;
+#endif
         dst->initial = src->initial;
         dst->blind = src->blind;
     }
 }
 
 static void secp256k1_ecmult_gen_context_clear(secp256k1_ecmult_gen_context_t *ctx) {
+#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
     free(ctx->prec);
+#endif
     secp256k1_scalar_clear(&ctx->blind);
     secp256k1_gej_clear(&ctx->initial);
     ctx->prec = NULL;

src/field_10x26.h

@@ -43,5 +43,5 @@ typedef struct {
 } secp256k1_fe_storage_t;
 
 #define SECP256K1_FE_STORAGE_CONST(d7, d6, d5, d4, d3, d2, d1, d0) {{ (d0), (d1), (d2), (d3), (d4), (d5), (d6), (d7) }}
-
+#define SECP256K1_FE_STORAGE_CONST_GET(d) d.n[7], d.n[6], d.n[5], d.n[4],d.n[3], d.n[2], d.n[1], d.n[0]
 #endif

src/gen_context.c

@@ -0,0 +1,63 @@
+/**********************************************************************
+ * Copyright (c) 2013, 2014, 2015 Thomas Daede, Cory Fields           *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#define USE_BASIC_CONFIG 1
+
+#include "basic-config.h"
+#include "include/secp256k1.h"
+#include "field_impl.h"
+#include "scalar_impl.h"
+#include "group_impl.h"
+#include "ecmult_gen_impl.h"
+
+int main(int argc, char **argv) {
+    secp256k1_ecmult_gen_context_t ctx;
+    int inner;
+    int outer;
+    FILE* fp;
+
+    (void)argc;
+    (void)argv;
+
+    fp = fopen("src/ecmult_static_context.h","w");
+    if (fp == NULL) {
+        fprintf(stderr, "Could not open src/ecmult_static_context.h for writing!\n");
+        return -1;
+    }
+    
+    fprintf(fp, "#ifndef _SECP256K1_ECMULT_STATIC_CONTEXT_\n");
+    fprintf(fp, "#define _SECP256K1_ECMULT_STATIC_CONTEXT_\n");
+    fprintf(fp, "#include \"group.h\"\n");
+    fprintf(fp, "#define SC SECP256K1_GE_STORAGE_CONST\n");
+    fprintf(fp, "static const secp256k1_ge_storage_t secp256k1_ecmult_static_context[64][16] = {\n");
+
+    secp256k1_ecmult_gen_context_init(&ctx);
+    secp256k1_ecmult_gen_context_build(&ctx);
+    for(outer = 0; outer != 64; outer++) {
+        fprintf(fp,"{\n");
+        for(inner = 0; inner != 16; inner++) {
+            fprintf(fp,"    SC(%uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu, %uu)", SECP256K1_GE_STORAGE_CONST_GET((*ctx.prec)[outer][inner]));
+            if (inner != 15) {
+                fprintf(fp,",\n");
+            } else {
+                fprintf(fp,"\n");
+            }
+        }
+        if (outer != 63) {
+            fprintf(fp,"},\n");
+        } else {
+            fprintf(fp,"}\n");
+        }
+    }
+    fprintf(fp,"};\n");
+    secp256k1_ecmult_gen_context_clear(&ctx);
+    
+    fprintf(fp, "#undef SC\n");
+    fprintf(fp, "#endif\n");
+    fclose(fp);
+    
+    return 0;
+}

src/group.h

@@ -38,6 +38,8 @@ typedef struct {
 
 #define SECP256K1_GE_STORAGE_CONST(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) {SECP256K1_FE_STORAGE_CONST((a),(b),(c),(d),(e),(f),(g),(h)), SECP256K1_FE_STORAGE_CONST((i),(j),(k),(l),(m),(n),(o),(p))}
 
+#define SECP256K1_GE_STORAGE_CONST_GET(t) SECP256K1_FE_STORAGE_CONST_GET(t.x), SECP256K1_FE_STORAGE_CONST_GET(t.y)
+
 /** Set a group element equal to the point at infinity */
 static void secp256k1_ge_set_infinity(secp256k1_ge_t *r);