batch: Add API usage example

This commit adds an example C program using the batch API.

GNU Autotools will compile this example only if both batch and
schnorrsig modules are enabled.

Author: siv2r

.gitignore

@@ -9,6 +9,7 @@ valgrind_ctime_test
 ecdh_example
 ecdsa_example
 schnorr_example
+batch_example
 *.exe
 *.so
 *.a

Makefile.am

@@ -170,6 +170,17 @@ if BUILD_WINDOWS
 schnorr_example_LDFLAGS += -lbcrypt
 endif
 TESTS += schnorr_example
+if ENABLE_MODULE_BATCH
+noinst_PROGRAMS += batch_example
+batch_example_SOURCES = examples/batch.c
+batch_example_CPPFLAGS = -I$(top_srcdir)/include
+batch_example_LDADD = libsecp256k1.la
+batch_example_LDFLAGS = -static
+if BUILD_WINDOWS
+batch_example_LDFLAGS += -lbcrypt
+endif
+TESTS += batch_example
+endif
 endif
 endif
 

examples/batch.c

@@ -0,0 +1,181 @@
+#include <stdio.h>
+#include <assert.h>
+#include <string.h>
+
+#include <secp256k1.h>
+#include <secp256k1_batch.h>
+#include <secp256k1_schnorrsig_batch.h>
+#include <secp256k1_tweak_check_batch.h>
+
+#include "random.h"
+
+/* key pair data */
+unsigned char sk[32];
+secp256k1_keypair keypair;
+secp256k1_xonly_pubkey pk;
+
+/* schnorrsig verification data */
+#define N_SIGS 10
+unsigned char msg[N_SIGS][32];
+unsigned char sig[N_SIGS][64];
+
+/* xonly pubkey tweak checks data */
+#define N_CHECKS 10
+unsigned char tweaked_pubkey[N_CHECKS][32];
+int tweaked_pk_parity[N_CHECKS];
+unsigned char tweak[N_CHECKS][32];
+
+/* 2*N_SIGS since one schnorrsig creates two scalar-point pairs in batch
+ * whereas one tweak check creates one scalar-point pair in batch */
+#define N_TERMS (N_CHECKS + 2*N_SIGS)
+
+/* generate key pair required for sign and verify */
+int create_keypair(secp256k1_context *ctx) {
+    while(1) {
+        if (!fill_random(sk, sizeof(sk))) {
+            printf("Failed to generate randomness\n");
+            return 1;
+        }
+        if (secp256k1_keypair_create(ctx, &keypair, sk)) {
+            break;
+        }
+    }
+    if (!secp256k1_keypair_xonly_pub(ctx, &pk, NULL, &keypair)) {
+        return 0;
+    }
+
+    return 1;
+}
+
+/* create valid schnorrsigs for N_SIGS random messages */
+int generate_schnorrsigs(secp256k1_context *ctx) {
+    size_t i;
+
+    for (i = 0; i < N_SIGS; i++) {
+        if(!fill_random(msg[i], sizeof(msg[i]))) {
+            printf("Failed to generate randomness\n");
+            return 1;
+        }
+        assert(secp256k1_schnorrsig_sign32(ctx, sig[i], msg[i], &keypair, NULL));
+        assert(secp256k1_schnorrsig_verify(ctx, sig[i], msg[i], sizeof(msg[i]), &pk));
+    }
+
+    return 1;
+}
+
+/* create valid N_CHECKS number of xonly pukey tweak checks */
+int generate_xonlypub_tweak_checks(secp256k1_context *ctx) {
+    secp256k1_pubkey output_pk;
+    secp256k1_xonly_pubkey output_xonly_pk;
+    size_t i;
+
+    for (i = 0; i < N_CHECKS; i++) {
+        if (!fill_random(tweak[i], sizeof(tweak[i]))) {
+            printf("Failed to generate randomness\n");
+            return 1;
+        }
+        assert(secp256k1_xonly_pubkey_tweak_add(ctx, &output_pk, &pk, tweak[i]));
+        assert(secp256k1_xonly_pubkey_from_pubkey(ctx, &output_xonly_pk, &tweaked_pk_parity[i], &output_pk));
+        assert(secp256k1_xonly_pubkey_serialize(ctx, tweaked_pubkey[i], &output_xonly_pk));
+        assert(secp256k1_xonly_pubkey_tweak_add_check(ctx, tweaked_pubkey[i], tweaked_pk_parity[i], &pk, tweak[i]));
+    }
+
+    return 1;
+}
+
+int main(void) {
+    int ret;
+    size_t i;
+    /* batch object uses secp256k1_context only for the error callback function
+     * here, we create secp256k1_context that can sign and verify, only to generate
+     * input data (schnorrsigs, tweak checks) required for the batch */
+    secp256k1_context *ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
+    secp256k1_batch *batch;
+    unsigned char auxiliary_rand[16];
+
+    /* Generate 16 bytes of randomness to use during batch creation. */
+    if (!fill_random(auxiliary_rand, sizeof(auxiliary_rand))) {
+        printf("Failed to generate randomness\n");
+        return 1;
+    }
+
+    batch = secp256k1_batch_create(ctx, N_TERMS, auxiliary_rand);
+
+    assert(ctx != NULL);
+    assert(batch != NULL);
+
+    /* key pair generation */
+    printf("Creating a key pair.........................");
+    if(!create_keypair(ctx)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+
+    /* create schnorrsigs for N_SIGS random messages */
+    printf("Signing messages............................");
+    if(!generate_schnorrsigs(ctx)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+
+    printf("Adding signatures to the batch object.......");
+    for (i = 0; i < N_SIGS; i++) {
+        /* It is recommended to check the validity of the batch before adding a
+         * new input (schnorrsig/tweak check) to it. The `secp256k1_batch_add_` APIs
+         * won't add any new input to invalid batch since the final `secp256k1_batch_verify`
+         * API call will fail even if the new input is valid. */
+        if(secp256k1_batch_usable(ctx, batch)) {
+            ret = secp256k1_batch_add_schnorrsig(ctx, batch, sig[i], msg[i], sizeof(msg[i]), &pk);
+        } else {
+            printf("INVALID BATCH\n");
+            return 1;
+        }
+
+        if(!ret) {
+            printf("FAILED\n");
+            return 1;
+        }
+    }
+    printf("ok\n");
+
+    printf("Generating xonlypub tweak checks............");
+    if(!generate_xonlypub_tweak_checks(ctx)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+
+    printf("Adding tweak checks to the batch object.....");
+    for (i = 0; i < N_CHECKS; i++) {
+        /* It is recommended to check the validity of the batch before adding a
+         * new input (schnorrsig/tweak check) to it. The `secp256k1_batch_add_` APIs
+         * won't add any new input to invalid batch since the final `secp256k1_batch_verify`
+         * API call will fail even if the new input is valid. */
+        if(secp256k1_batch_usable(ctx, batch)) {
+            ret = secp256k1_batch_add_xonlypub_tweak_check(ctx, batch, tweaked_pubkey[i], tweaked_pk_parity[i], &pk, tweak[i]);
+        } else {
+            printf("INVALID BATCH\n");
+            return 1;
+        }
+
+        if(!ret) {
+            printf("FAILED\n");
+            return 1;
+        }
+    }
+    printf("ok\n");
+
+    printf("Verifying the batch object..................");
+    if(!secp256k1_batch_verify(ctx, batch)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+
+    secp256k1_batch_destroy(ctx, batch);
+    secp256k1_context_destroy(ctx);
+
+    return 0;
+}