Merge #20257: Update secp256k1 subtree to latest master

6c0259fc2f8bd34ba83ad10a6a11d6d99e8d1fc7 Squashed 'src/secp256k1/' changes from c6b6b8f1bb..3967d96bf1 (Pieter Wuille)

Pull request description:

  Nothing important changed, but this silences this (erroneous) warning in certain GCC 9 versions:

  ```
  In file included from src/secp256k1.c:16:
  src/ecmult_impl.h: In function ‘secp256k1_ecmult’:
  src/ecmult_impl.h:496:48: warning: array subscript [1, 268435456] is outside array bounds of ‘struct secp256k1_strauss_point_state[1]’ [-Warray-bounds]
    496 |             secp256k1_gej tmp = a[state->ps[np].input_pos];
        |                                   ~~~~~~~~~~~~~^~~~~~~~~~
  src/ecmult_impl.h:565:42: note: while referencing ‘ps’
    565 |     struct secp256k1_strauss_point_state ps[1];
        |                                          ^~
  src/ecmult_impl.h:502:139: warning: array subscript [1, 268435456] is outside array bounds of ‘struct secp256k1_strauss_point_state[1]’ [-Warray-bounds]
    502 |             secp256k1_fe_mul(state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), &(a[state->ps[np].input_pos].z));
        |                                                                                                                              ~~~~~~~~~~~~~^~~~~~~~~~
  src/ecmult_impl.h:565:42: note: while referencing ‘ps’
    565 |     struct secp256k1_strauss_point_state ps[1];
        |                                          ^~
  ```

  (see https://github.com/bitcoin-core/secp256k1/issues/834)

ACKs for top commit:
  fanquake:
    ACK 5803f5f5f6030e69b46a46f0511b8173bf89de0d  - performed the update myself and got the same change: [check_20257_subtree](https://github.com/fanquake/bitcoin/tree/check_20257_subtree).
  hebasto:
    ACK 5803f5f5f6030e69b46a46f0511b8173bf89de0d, tested on Linux Mint 20 (x86_64) with `gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0` -- no warnings are emitted.

Tree-SHA512: 386281d23aee93a3b1d1a09fec8319c3a477e46967430c935677eed54abddc62d5a7710f9eeab1ec476ace05adcb194b5b377712e44a6bb95a74ffa35faf77f3

5 files changed, 38 insertions, 16 deletions

diff --git a/src/secp256k1/.travis.yml b/src/secp256k1/.travis.yml
index bcc8c210f5..ce8d6391b2 100644
--- a/src/secp256k1/.travis.yml
+++ b/src/secp256k1/.travis.yml
@@ -33,7 +33,7 @@ env:
     - CFLAGS=-O0 CTIMETEST=no
     - ECMULTGENPRECISION=2
     - ECMULTGENPRECISION=8
-    - RUN_VALGRIND=yes BIGNUM=no ASM=x86_64 EXPERIMENTAL=yes ECDH=yes  RECOVERY=yes EXTRAFLAGS="--disable-openssl-tests" BUILD=
+    - RUN_VALGRIND=yes BIGNUM=no ASM=x86_64 ECDH=yes  RECOVERY=yes EXPERIMENTAL=yes SCHNORRSIG=yes EXTRAFLAGS="--disable-openssl-tests" BUILD=
 matrix:
   fast_finish: true
   include:
@@ -81,7 +81,7 @@ matrix:
             - libc6-dbg:i386
     # S390x build (big endian system)
     - compiler: gcc
-      env: HOST=s390x-unknown-linux-gnu ECDH=yes RECOVERY=yes EXPERIMENTAL=yes CTIMETEST=
+      env: HOST=s390x-unknown-linux-gnu ECDH=yes RECOVERY=yes EXPERIMENTAL=yes SCHNORRSIG=yes CTIMETEST=
       arch: s390x
 
 # We use this to install macOS dependencies instead of the built in `homebrew` plugin,
diff --git a/src/secp256k1/README.md b/src/secp256k1/README.md
index 2602475787..e070937235 100644
--- a/src/secp256k1/README.md
+++ b/src/secp256k1/README.md
@@ -16,7 +16,7 @@ Features:
 * Very efficient implementation.
 * Suitable for embedded systems.
 * Optional module for public key recovery.
-* Optional module for ECDH key exchange (experimental).
+* Optional module for ECDH key exchange.
 
 Experimental features have not received enough scrutiny to satisfy the standard of quality of this library but are made available for testing and review by the community. The APIs of these features should not be considered stable.
 
diff --git a/src/secp256k1/build-aux/m4/bitcoin_secp.m4 b/src/secp256k1/build-aux/m4/bitcoin_secp.m4
index 57595f4499..ece3d655ed 100644
--- a/src/secp256k1/build-aux/m4/bitcoin_secp.m4
+++ b/src/secp256k1/build-aux/m4/bitcoin_secp.m4
@@ -36,16 +36,39 @@ if test x"$has_libcrypto" = x"yes" && test x"$has_openssl_ec" = x; then
   CPPFLAGS_TEMP="$CPPFLAGS"
   CPPFLAGS="$CRYPTO_CPPFLAGS $CPPFLAGS"
   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+    #include <openssl/bn.h>
     #include <openssl/ec.h>
     #include <openssl/ecdsa.h>
     #include <openssl/obj_mac.h>]],[[
-    EC_KEY *eckey = EC_KEY_new_by_curve_name(NID_secp256k1);
-    ECDSA_sign(0, NULL, 0, NULL, NULL, eckey);
+    # if OPENSSL_VERSION_NUMBER < 0x10100000L
+    void ECDSA_SIG_get0(const ECDSA_SIG *sig, const BIGNUM **pr, const BIGNUM **ps) {(void)sig->r; (void)sig->s;}
+    # endif
+
+    unsigned int zero = 0;
+    const unsigned char *zero_ptr = (unsigned char*)&zero;
+    EC_KEY_free(EC_KEY_new_by_curve_name(NID_secp256k1));
+    EC_KEY *eckey = EC_KEY_new();
+    EC_GROUP *group = EC_GROUP_new_by_curve_name(NID_secp256k1);
+    EC_KEY_set_group(eckey, group);
+    ECDSA_sign(0, NULL, 0, NULL, &zero, eckey);
     ECDSA_verify(0, NULL, 0, NULL, 0, eckey);
+    o2i_ECPublicKey(&eckey, &zero_ptr, 0);
+    d2i_ECPrivateKey(&eckey, &zero_ptr, 0);
+    EC_KEY_check_key(eckey);
     EC_KEY_free(eckey);
+    EC_GROUP_free(group);
     ECDSA_SIG *sig_openssl;
     sig_openssl = ECDSA_SIG_new();
+    d2i_ECDSA_SIG(&sig_openssl, &zero_ptr, 0);
+    i2d_ECDSA_SIG(sig_openssl, NULL);
+    ECDSA_SIG_get0(sig_openssl, NULL, NULL);
     ECDSA_SIG_free(sig_openssl);
+    const BIGNUM *bignum = BN_value_one();
+    BN_is_negative(bignum);
+    BN_num_bits(bignum);
+    if (sizeof(zero) >= BN_num_bytes(bignum)) {
+        BN_bn2bin(bignum, (unsigned char*)&zero);
+    }
   ]])],[has_openssl_ec=yes],[has_openssl_ec=no])
   AC_MSG_RESULT([$has_openssl_ec])
   CPPFLAGS="$CPPFLAGS_TEMP"
diff --git a/src/secp256k1/configure.ac b/src/secp256k1/configure.ac
index 5a078e6c81..eb3b449bec 100644
--- a/src/secp256k1/configure.ac
+++ b/src/secp256k1/configure.ac
@@ -122,7 +122,7 @@ AC_ARG_ENABLE(ecmult_static_precomputation,
     [use_ecmult_static_precomputation=auto])
 
 AC_ARG_ENABLE(module_ecdh,
-    AS_HELP_STRING([--enable-module-ecdh],[enable ECDH shared secret computation (experimental)]),
+    AS_HELP_STRING([--enable-module-ecdh],[enable ECDH shared secret computation]),
     [enable_module_ecdh=$enableval],
     [enable_module_ecdh=no])
 
@@ -395,8 +395,8 @@ esac
 
 if test x"$use_tests" = x"yes"; then
   SECP_OPENSSL_CHECK
-  if test x"$has_openssl_ec" = x"yes"; then
-    if test x"$enable_openssl_tests" != x"no"; then
+  if test x"$enable_openssl_tests" != x"no" && test x"$has_openssl_ec" = x"yes"; then
+      enable_openssl_tests=yes
       AC_DEFINE(ENABLE_OPENSSL_TESTS, 1, [Define this symbol if OpenSSL EC functions are available])
       SECP_TEST_INCLUDES="$SSL_CFLAGS $CRYPTO_CFLAGS $CRYPTO_CPPFLAGS"
       SECP_TEST_LIBS="$CRYPTO_LIBS"
@@ -406,16 +406,17 @@ if test x"$use_tests" = x"yes"; then
         SECP_TEST_LIBS="$SECP_TEST_LIBS -lgdi32"
         ;;
       esac
-    fi
   else
     if test x"$enable_openssl_tests" = x"yes"; then
       AC_MSG_ERROR([OpenSSL tests requested but OpenSSL with EC support is not available])
     fi
+    enable_openssl_tests=no
   fi
 else
   if test x"$enable_openssl_tests" = x"yes"; then
     AC_MSG_ERROR([OpenSSL tests requested but tests are not enabled])
   fi
+  enable_openssl_tests=no
 fi
 
 if test x"$set_bignum" = x"gmp"; then
@@ -458,14 +459,10 @@ if test x"$enable_experimental" = x"yes"; then
   AC_MSG_NOTICE([******])
   AC_MSG_NOTICE([WARNING: experimental build])
   AC_MSG_NOTICE([Experimental features do not have stable APIs or properties, and may not be safe for production use.])
-  AC_MSG_NOTICE([Building ECDH module: $enable_module_ecdh])
   AC_MSG_NOTICE([Building extrakeys module: $enable_module_extrakeys])
   AC_MSG_NOTICE([Building schnorrsig module: $enable_module_schnorrsig])
   AC_MSG_NOTICE([******])
 else
-  if test x"$enable_module_ecdh" = x"yes"; then
-    AC_MSG_ERROR([ECDH module is experimental. Use --enable-experimental to allow.])
-  fi
   if test x"$enable_module_extrakeys" = x"yes"; then
     AC_MSG_ERROR([extrakeys module is experimental. Use --enable-experimental to allow.])
   fi
@@ -507,6 +504,8 @@ echo "Build Options:"
 echo "  with ecmult precomp     = $set_precomp"
 echo "  with external callbacks = $use_external_default_callbacks"
 echo "  with benchmarks         = $use_benchmark"
+echo "  with tests              = $use_tests"
+echo "  with openssl tests      = $enable_openssl_tests"
 echo "  with coverage           = $enable_coverage"
 echo "  module ecdh             = $enable_module_ecdh"
 echo "  module recovery         = $enable_module_recovery"
diff --git a/src/secp256k1/src/ecmult_impl.h b/src/secp256k1/src/ecmult_impl.h
index 057a69cf73..a9e8b3c76c 100644
--- a/src/secp256k1/src/ecmult_impl.h
+++ b/src/secp256k1/src/ecmult_impl.h
@@ -443,7 +443,7 @@ struct secp256k1_strauss_state {
     struct secp256k1_strauss_point_state* ps;
 };
 
-static void secp256k1_ecmult_strauss_wnaf(const secp256k1_ecmult_context *ctx, const struct secp256k1_strauss_state *state, secp256k1_gej *r, int num, const secp256k1_gej *a, const secp256k1_scalar *na, const secp256k1_scalar *ng) {
+static void secp256k1_ecmult_strauss_wnaf(const secp256k1_ecmult_context *ctx, const struct secp256k1_strauss_state *state, secp256k1_gej *r, size_t num, const secp256k1_gej *a, const secp256k1_scalar *na, const secp256k1_scalar *ng) {
     secp256k1_ge tmpa;
     secp256k1_fe Z;
     /* Splitted G factors. */
@@ -454,8 +454,8 @@ static void secp256k1_ecmult_strauss_wnaf(const secp256k1_ecmult_context *ctx, c
     int bits_ng_128 = 0;
     int i;
     int bits = 0;
-    int np;
-    int no = 0;
+    size_t np;
+    size_t no = 0;
 
     for (np = 0; np < num; ++np) {
         if (secp256k1_scalar_is_zero(&na[np]) || secp256k1_gej_is_infinity(&a[np])) {