Add internal BP++ commit API

Author: sanket1729

src/modules/bulletproofs/bulletproofs_pp_norm_product_impl.h

@@ -77,4 +77,66 @@ static void secp256k1_bulletproofs_powers_of_r(secp256k1_scalar *powers, const s
         secp256k1_scalar_sqr(&powers[i], &powers[i - 1]);
     }
 }
+
+typedef struct ec_mult_bp_commit_cb_data {
+    const secp256k1_scalar *n;
+    const secp256k1_ge *g;
+    const secp256k1_scalar *l;
+    size_t g_len;
+} ec_mult_bp_commit_cb_data;
+
+static int ec_mult_bp_commit_cb(secp256k1_scalar *sc, secp256k1_ge *pt, size_t idx, void *cbdata) {
+    ec_mult_bp_commit_cb_data *data = (ec_mult_bp_commit_cb_data*) cbdata;
+    *pt = data->g[idx];
+    if (idx < data->g_len) {
+        *sc = data->n[idx];
+    } else {
+        *sc = data->l[idx - data->g_len];
+    }
+    return 1;
+}
+
+/* Create a commitment C = vG + [n_vec*G_vec]_q + l_vec*H_vec */
+static int secp256k1_bulletproofs_commit(
+    const secp256k1_context* ctx,
+    secp256k1_scratch_space* scratch,
+    secp256k1_ge* commit,
+    const secp256k1_bulletproofs_generators* g_vec,
+    const secp256k1_scalar* n_vec,
+    size_t n_vec_len,
+    const secp256k1_scalar* l_vec,
+    size_t l_vec_len,
+    const secp256k1_scalar* c_vec,
+    size_t c_vec_len,
+    const secp256k1_scalar* q
+) {
+    secp256k1_scalar v, l_c;
+    /* First n_vec_len generators are Gs, rest are Hs*/
+    if (g_vec->n != (n_vec_len + l_vec_len) || l_vec_len != c_vec_len) {
+        return 0;
+    }
+
+    if (!secp256k1_check_power_of_two(n_vec_len) ||  !secp256k1_check_power_of_two(c_vec_len)) {
+        return 0;
+    }
+    /* Compute v = n_vec*n_vec*q + l_vec*c_vec */
+    secp256k1_weighted_scalar_inner_product(&v, n_vec, 0 /*a offset */, n_vec, 0 /*b offset*/, 1 /*step*/, n_vec_len, q);
+    secp256k1_scalar_inner_product(&l_c, l_vec, 0 /*a offset */, c_vec, 0 /*b offset*/, 1 /*step*/, l_vec_len);
+    secp256k1_scalar_add(&v, &v, &l_c);
+
+    {
+        ec_mult_bp_commit_cb_data data;
+        secp256k1_gej commitj;
+        data.g = g_vec->gens;
+        data.n = n_vec;
+        data.l = l_vec;
+        data.g_len = n_vec_len;
+
+        if (!secp256k1_ecmult_multi_var(&ctx->error_callback, scratch, &commitj, &v, ec_mult_bp_commit_cb, (void*) &data, n_vec_len + l_vec_len)) {
+            return 0;
+        }
+        secp256k1_ge_set_gej_var(commit, &commitj);
+    }
+    return 1;
+}
 #endif