Add hazmat usage example

.gitignore

@@ -12,6 +12,7 @@ ecdsa_example
 schnorr_example
 ellswift_example
 musig_example
+hazmat_example
 *.exe
 *.so
 *.a

Makefile.am

@@ -206,6 +206,17 @@ musig_example_LDFLAGS += -lbcrypt
 endif
 TESTS += musig_example
 endif
+if ENABLE_MODULE_HAZMAT
+noinst_PROGRAMS += hazmat_example
+hazmat_example_SOURCES = examples/hazmat.c
+hazmat_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATIC
+hazmat_example_LDADD = libsecp256k1.la
+hazmat_example_LDFLAGS = -static
+if BUILD_WINDOWS
+hazmat_example_LDFLAGS += -lbcrypt
+endif
+TESTS += hazmat_example
+endif
 endif
 
 ### Precomputed tables

examples/CMakeLists.txt

@@ -29,3 +29,7 @@ endif()
 if(SECP256K1_ENABLE_MODULE_MUSIG)
   add_example(musig)
 endif()
+
+if(SECP256K1_ENABLE_MODULE_HAZMAT)
+  add_example(hazmat)
+endif()

examples/hazmat.c

@@ -0,0 +1,196 @@
+/*************************************************************************
+ * To the extent possible under law, the author(s) have dedicated all    *
+ * copyright and related and neighboring rights to the software in this  *
+ * file to the public domain worldwide. This software is distributed     *
+ * without any warranty. For the CC0 Public Domain Dedication, see       *
+ * EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 *
+ *************************************************************************/
+
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <secp256k1.h>
+#include <secp256k1_hazmat.h>
+
+#include "examples_util.h"
+
+int main(void) {
+    secp256k1_context* ctx;
+    unsigned char randomize[32];
+    secp256k1_hazmat_scalar a[3], a_sum;
+    secp256k1_hazmat_point A[3], A_sum;
+    unsigned char lhs_ser[33], rhs_ser[33];
+    int return_val, i;
+
+    /* Create a secp256k1 context
+     * Note that in the hazmat module, the context is only needed for multiplication
+     * with the generator point (function `secp256k1_hazmat_multiply_with_generator`).
+     */
+    ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
+    if (!fill_random(randomize, sizeof(randomize))) {
+        printf("Failed to generate randomness\n");
+        return 1;
+    }
+    /* Randomizing the context is recommended to protect against side-channel
+     * leakage. See `secp256k1_context_randomize` in secp256k1.h for more
+     * information about it. This should never fail.
+     */
+    return_val = secp256k1_context_randomize(ctx, randomize);
+    assert(return_val);
+
+    /* Generate keypairs */
+    for (i = 0; i < 3; i++) {
+        unsigned char scalar_buf[32];
+        unsigned char point_ser[33];
+
+        if (!fill_random(scalar_buf, sizeof(scalar_buf))) {
+            printf("Failed to generate randomness\n");
+            return 1;
+        }
+        if (!secp256k1_hazmat_scalar_parse(&a[i], scalar_buf) || secp256k1_hazmat_scalar_is_zero(&a[i])) {
+            printf("Generated secret key is invalid. This indicates an issue with the random number generator.\n");
+            return 1;
+        }
+        secp256k1_hazmat_multiply_with_generator(ctx, &A[i], &a[i]);
+
+        secp256k1_hazmat_point_serialize(point_ser, &A[i]);
+        printf("scalar a_%d: ", i+1); print_hex(scalar_buf, sizeof(scalar_buf));
+        printf("point  A_%d: ", i+1); print_hex(point_ser, sizeof(point_ser));
+
+        secure_erase(scalar_buf, sizeof(scalar_buf));
+    }
+
+    /* Simple example: verify that (a_1 + a_2 + a_3) * G = A_1 + A_2 + A_3 holds */
+    secp256k1_hazmat_scalar_set_zero(&a_sum);
+    secp256k1_hazmat_point_set_infinity(&A_sum);
+    for (i = 0; i < 3; i++) {
+        secp256k1_hazmat_scalar_add(&a_sum, &a_sum, &a[i]);
+        secp256k1_hazmat_point_add(&A_sum, &A_sum, &A[i]);
+    }
+
+    {
+        secp256k1_hazmat_point A_lhs;
+
+        secp256k1_hazmat_multiply_with_generator(ctx, &A_lhs, &a_sum);
+        secp256k1_hazmat_point_serialize(lhs_ser, &A_lhs);
+        secp256k1_hazmat_point_serialize(rhs_ser, &A_sum);
+
+        printf("\n");
+        printf("(a_1 + a_2 + a_3) * G: ");
+        print_hex(lhs_ser, sizeof(lhs_ser));
+        printf(" A_1 + A_2 + A_3:      ");
+        print_hex(rhs_ser, sizeof(rhs_ser));
+
+        /* Verify equality for both the hazmat points and their serialization */
+        return_val = secp256k1_hazmat_point_equal(&A_lhs, &A_sum);
+        assert(return_val == 1);
+        return_val = memcmp(lhs_ser, rhs_ser, sizeof(lhs_ser));
+        assert(return_val == 0);
+    }
+
+    /* Next example: verify that a_1 * A_2 = A_1 * a_2 (ECDH) */
+    {
+        secp256k1_hazmat_point lhs, rhs;
+
+        secp256k1_hazmat_multiply_with_point(&lhs, &a[0], &A[1]);
+        secp256k1_hazmat_multiply_with_point(&rhs, &a[1], &A[0]);
+        secp256k1_hazmat_point_serialize(lhs_ser, &lhs);
+        secp256k1_hazmat_point_serialize(rhs_ser, &rhs);
+
+        printf("\n");
+        printf(" a_1 * A_2: ");
+        print_hex(lhs_ser, sizeof(lhs_ser));
+        printf(" A_1 * a_2: ");
+        print_hex(rhs_ser, sizeof(rhs_ser));
+
+        return_val = secp256k1_hazmat_point_equal(&lhs, &rhs);
+        assert(return_val == 1);
+        return_val = memcmp(lhs_ser, rhs_ser, sizeof(lhs_ser));
+        assert(return_val == 0);
+    }
+
+    /* Yet another example, to demonstrate also scalar multiplication:
+     * verify that (a_1 * a_2) * A_3 = a_1 * (a_2 * A_3) */
+    {
+        secp256k1_hazmat_point lhs, rhs;
+        secp256k1_hazmat_scalar tmp_scalar;
+        secp256k1_hazmat_point tmp_point;
+
+        secp256k1_hazmat_scalar_mul(&tmp_scalar, &a[0], &a[1]);
+        secp256k1_hazmat_multiply_with_point(&lhs, &tmp_scalar, &A[2]);
+        secp256k1_hazmat_multiply_with_point(&tmp_point, &a[1], &A[2]);
+        secp256k1_hazmat_multiply_with_point(&rhs, &a[0], &tmp_point);
+        secp256k1_hazmat_point_serialize(lhs_ser, &lhs);
+        secp256k1_hazmat_point_serialize(rhs_ser, &rhs);
+
+        printf("\n");
+        printf("(a_1 * a_2) * A_3:  ");
+        print_hex(lhs_ser, sizeof(lhs_ser));
+        printf(" a_1 * (a_2 * A_3): ");
+        print_hex(rhs_ser, sizeof(rhs_ser));
+
+        return_val = secp256k1_hazmat_point_equal(&lhs, &rhs);
+        assert(return_val == 1);
+        return_val = memcmp(lhs_ser, rhs_ser, sizeof(lhs_ser));
+        assert(return_val == 0);
+    }
+
+    /* Show negation and neutral elements for scalars and points:
+     * a_i - a_i = 0
+     * A_i - A_i = point at infinity
+     */
+    for (i = 0; i < 3; i++) {
+        secp256k1_hazmat_scalar a_result, a_negated;
+        secp256k1_hazmat_point A_result, A_negated;
+
+        a_negated = a[i];
+        secp256k1_hazmat_scalar_negate(&a_negated);
+        secp256k1_hazmat_scalar_add(&a_result, &a[i], &a_negated);
+        assert(secp256k1_hazmat_scalar_is_zero(&a_result));
+
+        A_negated = A[i];
+        secp256k1_hazmat_point_negate(&A_negated);
+        secp256k1_hazmat_point_add(&A_result, &A[i], &A_negated);
+        assert(secp256k1_hazmat_point_is_infinity(&A_result));
+    }
+
+    /* To demonstrate parsing points and scalars, verify that the discrete log
+     * of the generator point is the scalar with value 1. */
+    {
+        secp256k1_hazmat_point generator, generator_calculated;
+        secp256k1_hazmat_scalar scalar_one;
+        unsigned char generator_ser[33] =
+            "\x02\x79\xBE\x66\x7E\xF9\xDC\xBB\xAC\x55\xA0\x62\x95\xCE\x87\x0B\x07"
+                "\x02\x9B\xFC\xDB\x2D\xCE\x28\xD9\x59\xF2\x81\x5B\x16\xF8\x17\x98";
+        unsigned char scalar_one_ser[32] =
+            "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
+            "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01";
+        unsigned char generator_calculated_ser[33];
+
+        return_val = secp256k1_hazmat_point_parse(&generator, generator_ser);
+        assert(return_val);
+        return_val = secp256k1_hazmat_scalar_parse(&scalar_one, scalar_one_ser);
+        assert(return_val);
+        secp256k1_hazmat_multiply_with_generator(ctx, &generator_calculated, &scalar_one);
+        secp256k1_hazmat_point_serialize(generator_calculated_ser, &generator_calculated);
+        return_val = secp256k1_hazmat_point_equal(&generator, &generator_calculated);
+        assert(return_val == 1);
+        return_val = memcmp(generator_ser, generator_calculated_ser, sizeof(generator_ser));
+        assert(return_val == 0);
+    }
+
+    /* It's best practice to try to clear secrets from memory after using them.
+     * This is done because some bugs can allow an attacker to leak memory, for
+     * example through "out of bounds" array access (see Heartbleed), or the OS
+     * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
+     *
+     * Here we are preventing these writes from being optimized out, as any good compiler
+     * will remove any writes that aren't used. */
+    for (i = 0; i < 3; i++) {
+        secure_erase(&a[i], sizeof(a[i]));
+    }
+    secure_erase(&a_sum, sizeof(a_sum));
+
+    return 0;
+}