Move ECDH into its own module

Makefile.am

@@ -13,14 +13,14 @@ noinst_HEADERS += src/group.h
 noinst_HEADERS += src/group_impl.h
 noinst_HEADERS += src/num_gmp.h
 noinst_HEADERS += src/num_gmp_impl.h
-noinst_HEADERS += src/ecdh.h
-noinst_HEADERS += src/ecdh_impl.h
 noinst_HEADERS += src/ecdsa.h
 noinst_HEADERS += src/ecdsa_impl.h
 noinst_HEADERS += src/eckey.h
 noinst_HEADERS += src/eckey_impl.h
 noinst_HEADERS += src/ecmult.h
 noinst_HEADERS += src/ecmult_impl.h
+noinst_HEADERS += src/ecmult_const.h
+noinst_HEADERS += src/ecmult_const_impl.h
 noinst_HEADERS += src/ecmult_gen.h
 noinst_HEADERS += src/ecmult_gen_impl.h
 noinst_HEADERS += src/num.h
@@ -51,7 +51,7 @@ libsecp256k1_la_LIBADD = $(SECP_LIBS)
 
 noinst_PROGRAMS =
 if USE_BENCHMARK
-noinst_PROGRAMS += bench_verify bench_recover bench_sign bench_internal bench_ecdh
+noinst_PROGRAMS += bench_verify bench_recover bench_sign bench_internal
 bench_verify_SOURCES = src/bench_verify.c
 bench_verify_LDADD = libsecp256k1.la $(SECP_LIBS)
 bench_verify_LDFLAGS = -static
@@ -65,10 +65,6 @@ bench_internal_SOURCES = src/bench_internal.c
 bench_internal_LDADD = $(SECP_LIBS)
 bench_internal_LDFLAGS = -static
 bench_internal_CPPFLAGS = $(SECP_INCLUDES)
-bench_ecdh_SOURCES = src/bench_ecdh.c
-bench_ecdh_LDADD = libsecp256k1.la $(SECP_LIBS)
-bench_ecdh_LDFLAGS = -static
-bench_ecdh_CPPFLAGS = $(SECP_INCLUDES)
 endif
 
 if USE_TESTS
@@ -101,3 +97,7 @@ CLEANFILES = gen_context src/ecmult_static_context.h
 endif
 
 EXTRA_DIST = autogen.sh src/gen_context.c src/basic-config.h
+
+if ENABLE_MODULE_ECDH
+include src/modules/ecdh/Makefile.am.include
+endif

configure.ac

@@ -102,6 +102,11 @@ AC_ARG_ENABLE(ecmult_static_precomputation,
     [use_ecmult_static_precomputation=$enableval],
     [use_ecmult_static_precomputation=yes])
 
+AC_ARG_ENABLE(module_ecdh,
+    AS_HELP_STRING([--enable-module-ecdh],[enable ECDH shared secret computation (default is no)]),
+    [enable_module_ecdh=$enableval],
+    [enable_module_ecdh=no])
+
 AC_ARG_WITH([field], [AS_HELP_STRING([--with-field=64bit|32bit|auto],
 [Specify Field Implementation. Default is auto])],[req_field=$withval], [req_field=auto])
 
@@ -315,13 +320,18 @@ if test x"$use_ecmult_static_precomputation" = x"yes"; then
   AC_DEFINE(USE_ECMULT_STATIC_PRECOMPUTATION, 1, [Define this symbol to use a statically generated ecmult table])
 fi
 
+if test x"$enable_module_ecdh" = x"yes"; then
+  AC_DEFINE(ENABLE_MODULE_ECDH, 1, [Define this symbol to enable the ECDH module])
+fi
+
 AC_C_BIGENDIAN()
 
 AC_MSG_NOTICE([Using assembly optimizations: $set_asm])
 AC_MSG_NOTICE([Using field implementation: $set_field])
 AC_MSG_NOTICE([Using bignum implementation: $set_bignum])
 AC_MSG_NOTICE([Using scalar implementation: $set_scalar])
 AC_MSG_NOTICE([Using endomorphism optimizations: $use_endomorphism])
+AC_MSG_NOTICE([Building ECDH module: $enable_module_ecdh])
 
 AC_CONFIG_HEADERS([src/libsecp256k1-config.h])
 AC_CONFIG_FILES([Makefile libsecp256k1.pc])
@@ -332,6 +342,7 @@ AC_SUBST(SECP_TEST_INCLUDES)
 AM_CONDITIONAL([USE_TESTS], [test x"$use_tests" != x"no"])
 AM_CONDITIONAL([USE_BENCHMARK], [test x"$use_benchmark" = x"yes"])
 AM_CONDITIONAL([USE_ECMULT_STATIC_PRECOMPUTATION], [test x"$use_ecmult_static_precomputation" = x"yes"])
+AM_CONDITIONAL([ENABLE_MODULE_ECDH], [test x"$enable_module_ecdh" = x"yes"])
 
 dnl make sure nothing new is exported so that we don't break the cache
 PKGCONFIG_PATH_TEMP="$PKG_CONFIG_PATH"

include/secp256k1.h

@@ -339,22 +339,6 @@ SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdsa_recover(
   secp256k1_pubkey_t *pubkey
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
 
-/** Compute an EC Diffie-Hellman secret in constant time
- *  Returns: 1: exponentiation was successful
- *           0: scalar was invalid (zero or overflow)
- *  In:      ctx:      pointer to a context object (cannot be NULL)
- *           point:    pointer to a public point
- *           scalar:   a 32-byte scalar with which to multiply the point
- *  Out:     result:   a 32-byte array which will be populated by an ECDH
- *                     secret computed from the point and scalar
- */
-SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdh(
-  const secp256k1_context_t* ctx,
-  unsigned char *result,
-  const secp256k1_pubkey_t *point,
-  const unsigned char *scalar
-) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
-
 /** Verify an ECDSA secret key.
  *  Returns: 1: secret key is valid
  *           0: secret key is invalid

include/secp256k1_ecdh.h

@@ -0,0 +1,30 @@
+#ifndef _SECP256K1_ECDH_
+# define _SECP256K1_ECDH_
+
+# include "secp256k1.h"
+
+# ifdef __cplusplus
+extern "C" {
+# endif
+
+/** Compute an EC Diffie-Hellman secret in constant time
+ *  Returns: 1: exponentiation was successful
+ *           0: scalar was invalid (zero or overflow)
+ *  In:      ctx:      pointer to a context object (cannot be NULL)
+ *           point:    pointer to a public point
+ *           scalar:   a 32-byte scalar with which to multiply the point
+ *  Out:     result:   a 32-byte array which will be populated by an ECDH
+ *                     secret computed from the point and scalar
+ */
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdh(
+  const secp256k1_context_t* ctx,
+  unsigned char *result,
+  const secp256k1_pubkey_t *point,
+  const unsigned char *scalar
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+# ifdef __cplusplus
+}
+# endif
+
+#endif

src/bench_ecdh.c

@@ -7,6 +7,7 @@
 #include <string.h>
 
 #include "include/secp256k1.h"
+#include "include/secp256k1_ecdh.h"
 #include "util.h"
 #include "bench.h"
 

src/bench_internal.c

@@ -13,7 +13,7 @@
 #include "field_impl.h"
 #include "group_impl.h"
 #include "scalar_impl.h"
-#include "ecdh_impl.h"
+#include "ecmult_const_impl.h"
 #include "ecmult_impl.h"
 #include "bench.h"
 
@@ -241,12 +241,11 @@ void bench_ecdh_wnaf(void* arg) {
     bench_inv_t *data = (bench_inv_t*)arg;
 
     for (i = 0; i < 20000; i++) {
-        secp256k1_ecdh_wnaf(data->wnaf, data->scalar_x, WINDOW_A);
+        secp256k1_wnaf_const(data->wnaf, data->scalar_x, WINDOW_A);
         secp256k1_scalar_add(&data->scalar_x, &data->scalar_x, &data->scalar_y);
     }
 }
 
-
 void bench_sha256(void* arg) {
     int i;
     bench_inv_t *data = (bench_inv_t*)arg;
@@ -321,7 +320,7 @@ int main(int argc, char **argv) {
     if (have_flag(argc, argv, "group") || have_flag(argc, argv, "add")) run_benchmark("group_add_affine", bench_group_add_affine, bench_setup, NULL, &data, 10, 200000);
     if (have_flag(argc, argv, "group") || have_flag(argc, argv, "add")) run_benchmark("group_add_affine_var", bench_group_add_affine_var, bench_setup, NULL, &data, 10, 200000);
 
-    if (have_flag(argc, argv, "ecdh") || have_flag(argc, argv, "wnaf")) run_benchmark("ecdh_wnaf", bench_ecdh_wnaf, bench_setup, NULL, &data, 10, 20000);
+    if (have_flag(argc, argv, "ecmult") || have_flag(argc, argv, "wnaf")) run_benchmark("ecmult_const_wnaf", bench_ecdh_wnaf, bench_setup, NULL, &data, 10, 20000);
     if (have_flag(argc, argv, "ecmult") || have_flag(argc, argv, "wnaf")) run_benchmark("ecmult_wnaf", bench_ecmult_wnaf, bench_setup, NULL, &data, 10, 20000);
 
     if (have_flag(argc, argv, "hash") || have_flag(argc, argv, "sha256")) run_benchmark("hash_sha256", bench_sha256, bench_setup, NULL, &data, 10, 20000);

src/ecdh.h → src/ecmult_const.h

@@ -1,15 +1,15 @@
 /**********************************************************************
- * Copyright (c) 2015 Pieter Wuille, Andrew Poelstra                  *
+ * Copyright (c) 2015 Andrew Poelstra                                 *
  * Distributed under the MIT software license, see the accompanying   *
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_ECDH_
-#define _SECP256K1_ECDH_
+#ifndef _SECP256K1_ECMULT_CONST_
+#define _SECP256K1_ECMULT_CONST_
 
 #include "scalar.h"
 #include "group.h"
 
-static void secp256k1_point_multiply(secp256k1_gej_t *r, const secp256k1_ge_t *a, const secp256k1_scalar_t *q);
+static void secp256k1_ecmult_const(secp256k1_gej_t *r, const secp256k1_ge_t *a, const secp256k1_scalar_t *q);
 
 #endif

src/ecdh_impl.h → src/ecmult_const_impl.h

@@ -4,12 +4,12 @@
  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
  **********************************************************************/
 
-#ifndef _SECP256K1_ECDH_IMPL_
-#define _SECP256K1_ECDH_IMPL_
+#ifndef _SECP256K1_ECMULT_CONST_IMPL_
+#define _SECP256K1_ECMULT_CONST_IMPL_
 
 #include "scalar.h"
 #include "group.h"
-#include "ecdh.h"
+#include "ecmult_const.h"
 #include "ecmult_impl.h"
 
 #ifdef USE_ENDOMORPHISM
@@ -53,7 +53,7 @@
  *
  *  Numbers reference steps of `Algorithm SPA-resistant Width-w NAF with Odd Scalar` on pp. 335
  */
-static int secp256k1_ecdh_wnaf(int *wnaf, secp256k1_scalar_t s, int w) {
+static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar_t s, int w) {
     int global_sign = 1;
     int skew = 0;
     int word = 0;
@@ -119,7 +119,7 @@ static int secp256k1_ecdh_wnaf(int *wnaf, secp256k1_scalar_t s, int w) {
 }
 
 
-static void secp256k1_point_multiply(secp256k1_gej_t *r, const secp256k1_ge_t *a, const secp256k1_scalar_t *scalar) {
+static void secp256k1_ecmult_const(secp256k1_gej_t *r, const secp256k1_ge_t *a, const secp256k1_scalar_t *scalar) {
     secp256k1_ge_t pre_a[ECMULT_TABLE_SIZE(WINDOW_A)];
     secp256k1_ge_t tmpa;
     secp256k1_fe_t Z;
@@ -144,16 +144,16 @@ static void secp256k1_point_multiply(secp256k1_gej_t *r, const secp256k1_ge_t *a
     secp256k1_scalar_split_lambda(&q_1, &q_lam, &sc);
     /* no need for zero correction when using endomorphism since even
      * numbers have one added to them anyway */
-    skew_1   = secp256k1_ecdh_wnaf(wnaf_1,   q_1,   WINDOW_A - 1);
-    skew_lam = secp256k1_ecdh_wnaf(wnaf_lam, q_lam, WINDOW_A - 1);
+    skew_1   = secp256k1_wnaf_const(wnaf_1,   q_1,   WINDOW_A - 1);
+    skew_lam = secp256k1_wnaf_const(wnaf_lam, q_lam, WINDOW_A - 1);
 #else
     int is_zero = secp256k1_scalar_is_zero(scalar);
     /* the wNAF ladder cannot handle zero, so bump this to one .. we will
      * correct the result after the fact */
     sc.d[0] += is_zero;
     VERIFY_CHECK(!secp256k1_scalar_is_zero(&sc));
 
-    secp256k1_ecdh_wnaf(wnaf, sc, WINDOW_A - 1);
+    secp256k1_wnaf_const(wnaf, sc, WINDOW_A - 1);
 #endif
 
     /* Calculate odd multiples of a.

src/modules/ecdh/Makefile.am.include

@@ -0,0 +1,9 @@
+include_HEADERS += include/secp256k1_ecdh.h
+noinst_HEADERS += src/modules/ecdh/main_impl.h
+noinst_HEADERS += src/modules/ecdh/tests_impl.h
+if USE_BENCHMARK
+noinst_PROGRAMS += bench_ecdh
+bench_ecdh_SOURCES = src/bench_ecdh.c
+bench_ecdh_LDADD = libsecp256k1.la $(SECP_LIBS)
+bench_ecdh_LDFLAGS = -static
+endif

src/modules/ecdh/main_impl.h

@@ -0,0 +1,53 @@
+/**********************************************************************
+ * Copyright (c) 2015 Andrew Poelstra                                 *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_MODULE_ECDH_MAIN_
+#define _SECP256K1_MODULE_ECDH_MAIN_
+
+#include "ecmult_const_impl.h"
+
+int secp256k1_ecdh(const secp256k1_context_t* ctx, unsigned char *result, const secp256k1_pubkey_t *point, const unsigned char *scalar) {
+    int ret = 0;
+    int overflow = 0;
+    secp256k1_gej_t res;
+    secp256k1_ge_t pt;
+    secp256k1_scalar_t s;
+    ARG_CHECK(result != NULL);
+    ARG_CHECK(point != NULL);
+    ARG_CHECK(scalar != NULL);
+    (void)ctx;
+
+    secp256k1_pubkey_load(ctx, &pt, point);
+    secp256k1_scalar_set_b32(&s, scalar, &overflow);
+    if (overflow || secp256k1_scalar_is_zero(&s)) {
+        ret = 0;
+    } else {
+        unsigned char x[32];
+        unsigned char y[1];
+        secp256k1_sha256_t sha;
+
+        secp256k1_ecmult_const(&res, &pt, &s);
+        secp256k1_ge_set_gej(&pt, &res);
+        /* Compute a hash of the point in compressed form
+         * Note we cannot use secp256k1_eckey_pubkey_serialize here since it does not
+         * expect its output to be secret and has a timing sidechannel. */
+        secp256k1_fe_normalize(&pt.x);
+        secp256k1_fe_normalize(&pt.y);
+        secp256k1_fe_get_b32(x, &pt.x);
+        y[0] = 0x02 | secp256k1_fe_is_odd(&pt.y);
+
+        secp256k1_sha256_initialize(&sha);
+        secp256k1_sha256_write(&sha, y, sizeof(y));
+        secp256k1_sha256_write(&sha, x, sizeof(x));
+        secp256k1_sha256_finalize(&sha, result);
+        ret = 1;
+    }
+
+    secp256k1_scalar_clear(&s);
+    return ret;
+}
+
+#endif

src/modules/ecdh/tests_impl.h

@@ -0,0 +1,48 @@
+/**********************************************************************
+ * Copyright (c) 2015 Andrew Poelstra                                 *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_MODULE_ECDH_TESTS_
+#define _SECP256K1_MODULE_ECDH_TESTS_
+
+void test_ecdh_generator_basepoint(void) {
+    unsigned char s_one[32] = { 0 };
+    secp256k1_pubkey_t point[2];
+    int i;
+
+    s_one[31] = 1;
+    /* Check against pubkey creation when the basepoint is the generator */
+    for (i = 0; i < 100; ++i) {
+        secp256k1_sha256_t sha;
+        unsigned char s_b32[32];
+        unsigned char output_ecdh[32];
+        unsigned char output_ser[32];
+        unsigned char point_ser[33];
+        int point_ser_len = sizeof(point_ser);
+        secp256k1_scalar_t s;
+
+        random_scalar_order(&s);
+        secp256k1_scalar_get_b32(s_b32, &s);
+
+        /* compute using ECDH function */
+        CHECK(secp256k1_ec_pubkey_create(ctx, &point[0], s_one) == 1);
+        CHECK(secp256k1_ecdh(ctx, output_ecdh, &point[0], s_b32) == 1);
+        /* compute "explicitly" */
+        CHECK(secp256k1_ec_pubkey_create(ctx, &point[1], s_b32) == 1);
+        CHECK(secp256k1_ec_pubkey_serialize(ctx, point_ser, &point_ser_len, &point[1], 1) == 1);
+        CHECK(point_ser_len == sizeof(point_ser));
+        secp256k1_sha256_initialize(&sha);
+        secp256k1_sha256_write(&sha, point_ser, point_ser_len);
+        secp256k1_sha256_finalize(&sha, output_ser);
+        /* compare */
+        CHECK(memcmp(output_ecdh, output_ser, sizeof(output_ser)) == 0);
+    }
+}
+
+void run_ecdh_tests(void) {
+    test_ecdh_generator_basepoint();
+}
+
+#endif