Merge bitcoin#553: add static context object which has no capabilities

40fde61 prevent attempts to modify `secp256k1_context_no_precomp` (Andrew Poelstra)
ed7c084 add static context object which has no capabilities (Andrew Poelstra)

Pull request description:

Tree-SHA512: a843ed7ba00a00a46eec3146ce428d4b49eb440af766f44d731b1f51553d08de8cc9a0af5ed114d0dfdca6f4bf4a2ede4dbd6a37d6bd818b81630089424a0ba5

include/secp256k1.h

@@ -179,6 +179,13 @@ typedef int (*secp256k1_nonce_function)(
 #define SECP256K1_TAG_PUBKEY_HYBRID_EVEN 0x06
 #define SECP256K1_TAG_PUBKEY_HYBRID_ODD 0x07
 
+/** A simple secp256k1 context object with no precomputed tables. These are useful for
+ *  type serialization/parsing functions which require a context object to maintain
+ *  API consistency, but currently do not require expensive precomputations or dynamic
+ *  allocations.
+ */
+SECP256K1_API extern const secp256k1_context *secp256k1_context_no_precomp;
+
 /** Create a secp256k1 context object.
  *
  *  Returns: a newly created context object.

src/secp256k1.c

@@ -56,6 +56,14 @@ struct secp256k1_context_struct {
     secp256k1_callback error_callback;
 };
 
+static const secp256k1_context secp256k1_context_no_precomp_ = {
+    { 0 },
+    { 0 },
+    { default_illegal_callback_fn, 0 },
+    { default_error_callback_fn, 0 }
+};
+const secp256k1_context *secp256k1_context_no_precomp = &secp256k1_context_no_precomp_;
+
 secp256k1_context* secp256k1_context_create(unsigned int flags) {
     secp256k1_context* ret = (secp256k1_context*)checked_malloc(&default_error_callback, sizeof(secp256k1_context));
     ret->illegal_callback = default_illegal_callback;
@@ -91,6 +99,7 @@ secp256k1_context* secp256k1_context_clone(const secp256k1_context* ctx) {
 }
 
 void secp256k1_context_destroy(secp256k1_context* ctx) {
+    CHECK(ctx != secp256k1_context_no_precomp);
     if (ctx != NULL) {
         secp256k1_ecmult_context_clear(&ctx->ecmult_ctx);
         secp256k1_ecmult_gen_context_clear(&ctx->ecmult_gen_ctx);
@@ -100,6 +109,7 @@ void secp256k1_context_destroy(secp256k1_context* ctx) {
 }
 
 void secp256k1_context_set_illegal_callback(secp256k1_context* ctx, void (*fun)(const char* message, void* data), const void* data) {
+    CHECK(ctx != secp256k1_context_no_precomp);
     if (fun == NULL) {
         fun = default_illegal_callback_fn;
     }
@@ -108,6 +118,7 @@ void secp256k1_context_set_illegal_callback(secp256k1_context* ctx, void (*fun)(
 }
 
 void secp256k1_context_set_error_callback(secp256k1_context* ctx, void (*fun)(const char* message, void* data), const void* data) {
+    CHECK(ctx != secp256k1_context_no_precomp);
     if (fun == NULL) {
         fun = default_error_callback_fn;
     }
@@ -559,6 +570,7 @@ int secp256k1_ec_pubkey_tweak_mul(const secp256k1_context* ctx, secp256k1_pubkey
 
 int secp256k1_context_randomize(secp256k1_context* ctx, const unsigned char *seed32) {
     VERIFY_CHECK(ctx != NULL);
+    CHECK(ctx != secp256k1_context_no_precomp);
     ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
     secp256k1_ecmult_gen_blind(&ctx->ecmult_gen_ctx, seed32);
     return 1;

src/tests.c

@@ -3599,6 +3599,7 @@ void run_ec_pubkey_parse_test(void) {
     ecount = 0;
     VG_UNDEF(&pubkey, sizeof(pubkey));
     CHECK(secp256k1_ec_pubkey_parse(ctx, &pubkey, pubkeyc, 65) == 1);
+    CHECK(secp256k1_ec_pubkey_parse(secp256k1_context_no_precomp, &pubkey, pubkeyc, 65) == 1);
     VG_CHECK(&pubkey, sizeof(pubkey));
     CHECK(ecount == 0);
     VG_UNDEF(&ge, sizeof(ge));