Add ECDH module which works by hashing the output of ecmult_const

.travis.yml

@@ -8,12 +8,14 @@ compiler:
   - gcc
 env:
   global:
-    - FIELD=auto  BIGNUM=auto  SCALAR=auto  ENDOMORPHISM=no  STATICPRECOMPUTATION=no ASM=no  BUILD=check  EXTRAFLAGS= HOST=
+    - FIELD=auto  BIGNUM=auto  SCALAR=auto  ENDOMORPHISM=no  STATICPRECOMPUTATION=no ASM=no  BUILD=check  EXTRAFLAGS= HOST= ECDH=no
   matrix:
     - SCALAR=32bit
+    - SCALAR=32bit    FIELD=32bit       ECDH=yes
     - SCALAR=64bit
     - FIELD=64bit
     - FIELD=64bit     ENDOMORPHISM=yes
+    - FIELD=64bit     ENDOMORPHISM=yes  ECDH=yes
     - FIELD=64bit                       ASM=x86_64
     - FIELD=64bit     ENDOMORPHISM=yes  ASM=x86_64
     - FIELD=32bit
@@ -56,5 +58,5 @@ before_script: ./autogen.sh
 script:
  - if [ -n "$HOST" ]; then export USE_HOST="--host=$HOST"; fi
  - if [ "x$HOST" = "xi686-linux-gnu" ]; then export CC="$CC -m32"; fi
- - ./configure --enable-endomorphism=$ENDOMORPHISM --with-field=$FIELD --with-bignum=$BIGNUM --with-scalar=$SCALAR --enable-ecmult-static-precomputation=$STATICPRECOMPUTATION $EXTRAFLAGS $USE_HOST && make -j2 $BUILD
+ - ./configure --enable-endomorphism=$ENDOMORPHISM --with-field=$FIELD --with-bignum=$BIGNUM --with-scalar=$SCALAR --enable-ecmult-static-precomputation=$STATICPRECOMPUTATION --enable-module-ecdh=$ECDH $EXTRAFLAGS $USE_HOST && make -j2 $BUILD
 os: linux

Makefile.am

@@ -97,3 +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])
 
@@ -315,6 +320,10 @@ 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])
@@ -322,6 +331,7 @@ 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_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/modules/ecdh/Makefile.am.include

@@ -0,0 +1,3 @@
+include_HEADERS += include/secp256k1_ecdh.h
+noinst_HEADERS += src/modules/ecdh/main_impl.h
+noinst_HEADERS += src/modules/ecdh/tests_impl.h

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 +1,75 @@
+/**********************************************************************
+ * 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 test_bad_scalar(void) {
+    unsigned char s_zero[32] = { 0 };
+    unsigned char s_overflow[32] = {
+        0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+        0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xfe,
+        0xba, 0xae, 0xdc, 0xe6, 0xaf, 0x48, 0xa0, 0x3b,
+        0xbf, 0xd2, 0x5e, 0x8c, 0xd0, 0x36, 0x41, 0x41
+    };
+    unsigned char s_rand[32] = { 0 };
+    unsigned char output[32];
+    secp256k1_scalar_t rand;
+    secp256k1_pubkey_t point;
+
+    /* Create random point */
+    random_scalar_order(&rand);
+    secp256k1_scalar_get_b32(s_rand, &rand);
+    CHECK(secp256k1_ec_pubkey_create(ctx, &point, s_rand) == 1);
+
+    /* Try to multiply it by bad values */
+    CHECK(secp256k1_ecdh(ctx, output, &point, s_zero) == 0);
+    CHECK(secp256k1_ecdh(ctx, output, &point, s_overflow) == 0);
+    /* ...and a good one */
+    s_overflow[31] -= 1;
+    CHECK(secp256k1_ecdh(ctx, output, &point, s_overflow) == 1);
+}
+
+void run_ecdh_tests(void) {
+    test_ecdh_generator_basepoint();
+    test_bad_scalar();
+}
+
+#endif

src/secp256k1.c

@@ -526,3 +526,7 @@ int secp256k1_context_randomize(secp256k1_context_t* ctx, const unsigned char *s
     secp256k1_ecmult_gen_blind(&ctx->ecmult_gen_ctx, seed32);
     return 1;
 }
+
+#ifdef ENABLE_MODULE_ECDH
+# include "modules/ecdh/main_impl.h"
+#endif

src/tests.c

@@ -2356,6 +2356,10 @@ void run_ecdsa_openssl(void) {
 }
 #endif
 
+#ifdef ENABLE_MODULE_ECDH
+# include "modules/ecdh/tests_impl.h"
+#endif
+
 int main(int argc, char **argv) {
     unsigned char seed16[16] = {0};
     unsigned char run32[32] = {0};
@@ -2444,6 +2448,11 @@ int main(int argc, char **argv) {
     run_endomorphism_tests();
 #endif
 
+#ifdef ENABLE_MODULE_ECDH
+    /* ecdh tests */
+    run_ecdh_tests();
+#endif
+
     /* ecdsa tests */
     run_random_pubkeys();
     run_ecdsa_sign_verify();