Separate helper functions for pubkey_create and seckey_tweak_add

Summary:
This is in preparation for allowing code reuse by keypair functions

This is a partial backport of secp256k1 [[bitcoin-core/secp256k1#558 | PR558]] : bitcoin-core/secp256k1@f001034

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7642

src/secp256k1.c

@@ -550,26 +550,33 @@ int secp256k1_ec_seckey_verify(const secp256k1_context* ctx, const unsigned char
     return ret;
 }
 
-int secp256k1_ec_pubkey_create(const secp256k1_context* ctx, secp256k1_pubkey *pubkey, const unsigned char *seckey) {
+static int secp256k1_ec_pubkey_create_helper(const secp256k1_ecmult_gen_context *ecmult_gen_ctx, secp256k1_scalar *seckey_scalar, secp256k1_ge *p, const unsigned char *seckey) {
     secp256k1_gej pj;
+    int ret;
+
+    ret = secp256k1_scalar_set_b32_seckey(seckey_scalar, seckey);
+    secp256k1_scalar_cmov(seckey_scalar, &secp256k1_scalar_one, !ret);
+
+    secp256k1_ecmult_gen(ecmult_gen_ctx, &pj, seckey_scalar);
+    secp256k1_ge_set_gej(p, &pj);
+    return ret;
+}
+
+int secp256k1_ec_pubkey_create(const secp256k1_context* ctx, secp256k1_pubkey *pubkey, const unsigned char *seckey) {
     secp256k1_ge p;
-    secp256k1_scalar sec;
+    secp256k1_scalar seckey_scalar;
     int ret = 0;
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(pubkey != NULL);
     memset(pubkey, 0, sizeof(*pubkey));
     ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
     ARG_CHECK(seckey != NULL);
 
-    ret = secp256k1_scalar_set_b32_seckey(&sec, seckey);
-    secp256k1_scalar_cmov(&sec, &secp256k1_scalar_one, !ret);
-
-    secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &pj, &sec);
-    secp256k1_ge_set_gej(&p, &pj);
+    ret = secp256k1_ec_pubkey_create_helper(&ctx->ecmult_gen_ctx, &seckey_scalar, &p, seckey);
     secp256k1_pubkey_save(pubkey, &p);
     memczero(pubkey, sizeof(*pubkey), !ret);
 
-    secp256k1_scalar_clear(&sec);
+    secp256k1_scalar_clear(&seckey_scalar);
     return ret;
 }
 
@@ -607,24 +614,31 @@ int secp256k1_ec_pubkey_negate(const secp256k1_context* ctx, secp256k1_pubkey *p
     return ret;
 }
 
-int secp256k1_ec_seckey_tweak_add(const secp256k1_context* ctx, unsigned char *seckey, const unsigned char *tweak) {
+
+static int secp256k1_ec_seckey_tweak_add_helper(secp256k1_scalar *sec, const unsigned char *tweak) {
     secp256k1_scalar term;
+    int overflow = 0;
+    int ret = 0;
+
+    secp256k1_scalar_set_b32(&term, tweak, &overflow);
+    ret = (!overflow) & secp256k1_eckey_privkey_tweak_add(sec, &term);
+    secp256k1_scalar_clear(&term);
+    return ret;
+}
+
+int secp256k1_ec_seckey_tweak_add(const secp256k1_context* ctx, unsigned char *seckey, const unsigned char *tweak) {
     secp256k1_scalar sec;
     int ret = 0;
-    int overflow = 0;
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(seckey != NULL);
     ARG_CHECK(tweak != NULL);
 
-    secp256k1_scalar_set_b32(&term, tweak, &overflow);
     ret = secp256k1_scalar_set_b32_seckey(&sec, seckey);
-
-    ret &= (!overflow) & secp256k1_eckey_privkey_tweak_add(&sec, &term);
+    ret &= secp256k1_ec_seckey_tweak_add_helper(&sec, tweak);
     secp256k1_scalar_cmov(&sec, &secp256k1_scalar_zero, !ret);
     secp256k1_scalar_get_b32(seckey, &sec);
 
     secp256k1_scalar_clear(&sec);
-    secp256k1_scalar_clear(&term);
     return ret;
 }