Add exhaustive test for ECDH module

diff --git a/src/modules/ecdh/Makefile.am.include b/src/modules/ecdh/Makefile.am.include
index 1866053..81bc627 100644
--- a/src/modules/ecdh/Makefile.am.include
+++ b/src/modules/ecdh/Makefile.am.include
@@ -1,5 +1,6 @@
 include_HEADERS += include/secp256k1_ecdh.h
 noinst_HEADERS += src/modules/ecdh/main_impl.h
 noinst_HEADERS += src/modules/ecdh/tests_impl.h
+noinst_HEADERS += src/modules/ecdh/tests_exhaustive_impl.h
 noinst_HEADERS += src/modules/ecdh/bench_impl.h
 noinst_HEADERS += src/wycheproof/ecdh_secp256k1_test.h
diff --git a/src/modules/ecdh/tests_exhaustive_impl.h b/src/modules/ecdh/tests_exhaustive_impl.h
new file mode 100644
index 0000000..2bcd243
--- /dev/null
+++ b/src/modules/ecdh/tests_exhaustive_impl.h
@@ -0,0 +1,56 @@
+/***********************************************************************
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_MODULE_ECDH_TESTS_EXHAUSTIVE_H
+#define SECP256K1_MODULE_ECDH_TESTS_EXHAUSTIVE_H
+
+#include "../../../include/secp256k1_ecdh.h"
+#include "main_impl.h"
+
+static void test_exhaustive_ecdh(const secp256k1_context *ctx, const secp256k1_ge *group) {
+    int i, j;
+    unsigned char seckeys[EXHAUSTIVE_TEST_ORDER - 1][32];
+    secp256k1_pubkey pubkeys[EXHAUSTIVE_TEST_ORDER - 1];
+
+    /* Construct key pairs (32-byte secret key, public key object) for the entire group. */
+    for (i = 1; i < EXHAUSTIVE_TEST_ORDER; i++) {
+        secp256k1_scalar scalar;
+        secp256k1_scalar_set_int(&scalar, i);
+        secp256k1_scalar_get_b32(seckeys[i - 1], &scalar);
+        CHECK(secp256k1_ec_pubkey_create(ctx, &pubkeys[i - 1], seckeys[i - 1]));
+    }
+
+    /* Loop over key combinations. */
+    for (i = 1; i < EXHAUSTIVE_TEST_ORDER; i++) {
+        for (j = 1; j < EXHAUSTIVE_TEST_ORDER; j++) {
+            unsigned char ecdh_result_ij[32];
+            unsigned char ecdh_result_ji[32];
+
+            /* Calculate ECDH(i*G, j) and ECDH(j*G, i) using API function and verify that the results match. */
+            CHECK(secp256k1_ecdh(ctx, ecdh_result_ij, &pubkeys[i - 1], seckeys[j - 1], NULL, NULL));
+            CHECK(secp256k1_ecdh(ctx, ecdh_result_ji, &pubkeys[j - 1], seckeys[i - 1], NULL, NULL));
+            CHECK(secp256k1_memcmp_var(ecdh_result_ij, ecdh_result_ji, 32) == 0);
+
+            /* Recalculate the expected ECDH result manually by invoking the default ECDH hash
+             * function on the precomputed group element (group[i * j]) coordinates, and verify
+             * that it matches the previously calculated public API results. */
+            {
+                secp256k1_ge ecdh_ge_expected = group[(i * j) % EXHAUSTIVE_TEST_ORDER];
+                unsigned char ecdh_result_expected[32];
+                unsigned char x[32];
+                unsigned char y[32];
+
+                secp256k1_fe_normalize_var(&ecdh_ge_expected.x);
+                secp256k1_fe_normalize_var(&ecdh_ge_expected.y);
+                secp256k1_fe_get_b32(x, &ecdh_ge_expected.x);
+                secp256k1_fe_get_b32(y, &ecdh_ge_expected.y);
+                CHECK(secp256k1_ecdh_hash_function_default(ecdh_result_expected, x, y, NULL));
+                CHECK(secp256k1_memcmp_var(ecdh_result_ij, ecdh_result_expected, 32) == 0);
+            }
+        }
+    }
+}
+
+#endif
diff --git a/src/tests_exhaustive.c b/src/tests_exhaustive.c
index 68d4bec..6d128bc 100644
--- a/src/tests_exhaustive.c
+++ b/src/tests_exhaustive.c
@@ -337,6 +337,10 @@ static void test_exhaustive_sign(const secp256k1_context *ctx, const secp256k1_g
      */
 }
 
+#ifdef ENABLE_MODULE_ECDH
+#include "modules/ecdh/tests_exhaustive_impl.h"
+#endif
+
 #ifdef ENABLE_MODULE_RECOVERY
 #include "modules/recovery/tests_exhaustive_impl.h"
 #endif
@@ -437,6 +441,9 @@ int main(int argc, char** argv) {
         test_exhaustive_sign(ctx, group);
         test_exhaustive_verify(ctx, group);
 
+#ifdef ENABLE_MODULE_ECDH
+        test_exhaustive_ecdh(ctx, group);
+#endif
 #ifdef ENABLE_MODULE_RECOVERY
         test_exhaustive_recovery(ctx, group);
 #endif