tests: Add fuzzing harness for ec_seckey_import_der(...) and ec_seckey_export_der(...)

src/Makefile.test.include

@@ -132,6 +132,7 @@ FUZZ_TARGETS = \
   test/fuzz/script_sigcache \
   test/fuzz/script_sign \
   test/fuzz/scriptnum_ops \
+  test/fuzz/secp256k1_ec_seckey_import_export_der \
   test/fuzz/secp256k1_ecdsa_signature_parse_der_lax \
   test/fuzz/service_deserialize \
   test/fuzz/signature_checker \
@@ -1095,6 +1096,12 @@ test_fuzz_scriptnum_ops_LDADD = $(FUZZ_SUITE_LD_COMMON)
 test_fuzz_scriptnum_ops_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS) $(LIBTOOL_APP_LDFLAGS)
 test_fuzz_scriptnum_ops_SOURCES = test/fuzz/scriptnum_ops.cpp
 
+test_fuzz_secp256k1_ec_seckey_import_export_der_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES)
+test_fuzz_secp256k1_ec_seckey_import_export_der_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
+test_fuzz_secp256k1_ec_seckey_import_export_der_LDADD = $(FUZZ_SUITE_LD_COMMON)
+test_fuzz_secp256k1_ec_seckey_import_export_der_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS) $(LIBTOOL_APP_LDFLAGS)
+test_fuzz_secp256k1_ec_seckey_import_export_der_SOURCES = test/fuzz/secp256k1_ec_seckey_import_export_der.cpp
+
 test_fuzz_secp256k1_ecdsa_signature_parse_der_lax_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES)
 test_fuzz_secp256k1_ecdsa_signature_parse_der_lax_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 test_fuzz_secp256k1_ecdsa_signature_parse_der_lax_LDADD = $(FUZZ_SUITE_LD_COMMON)

src/key.cpp

@@ -31,7 +31,7 @@ static secp256k1_context* secp256k1_context_sign = nullptr;
  *
  * out32 must point to an output buffer of length at least 32 bytes.
  */
-static int ec_seckey_import_der(const secp256k1_context* ctx, unsigned char *out32, const unsigned char *seckey, size_t seckeylen) {
+int ec_seckey_import_der(const secp256k1_context* ctx, unsigned char *out32, const unsigned char *seckey, size_t seckeylen) {
     const unsigned char *end = seckey + seckeylen;
     memset(out32, 0, 32);
     /* sequence header */
@@ -88,7 +88,7 @@ static int ec_seckey_import_der(const secp256k1_context* ctx, unsigned char *out
  * will be set to the number of bytes used in the buffer.
  * key32 must point to a 32-byte raw private key.
  */
-static int ec_seckey_export_der(const secp256k1_context *ctx, unsigned char *seckey, size_t *seckeylen, const unsigned char *key32, bool compressed) {
+int ec_seckey_export_der(const secp256k1_context *ctx, unsigned char *seckey, size_t *seckeylen, const unsigned char *key32, bool compressed) {
     assert(*seckeylen >= CKey::SIZE);
     secp256k1_pubkey pubkey;
     size_t pubkeylen = 0;

src/test/fuzz/secp256k1_ec_seckey_import_export_der.cpp

@@ -0,0 +1,38 @@
+// Copyright (c) 2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <key.h>
+#include <secp256k1.h>
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+
+#include <cstdint>
+#include <vector>
+
+int ec_seckey_import_der(const secp256k1_context* ctx, unsigned char* out32, const unsigned char* seckey, size_t seckeylen);
+int ec_seckey_export_der(const secp256k1_context* ctx, unsigned char* seckey, size_t* seckeylen, const unsigned char* key32, bool compressed);
+
+void test_one_input(const std::vector<uint8_t>& buffer)
+{
+    FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
+    secp256k1_context* secp256k1_context_sign = secp256k1_context_create(SECP256K1_CONTEXT_SIGN);
+    {
+        std::vector<uint8_t> out32(32);
+        (void)ec_seckey_import_der(secp256k1_context_sign, out32.data(), ConsumeFixedLengthByteVector(fuzzed_data_provider, CKey::SIZE).data(), CKey::SIZE);
+    }
+    {
+        std::vector<uint8_t> seckey(CKey::SIZE);
+        const std::vector<uint8_t> key32 = ConsumeFixedLengthByteVector(fuzzed_data_provider, 32);
+        size_t seckeylen = CKey::SIZE;
+        const bool compressed = fuzzed_data_provider.ConsumeBool();
+        const bool exported = ec_seckey_export_der(secp256k1_context_sign, seckey.data(), &seckeylen, key32.data(), compressed);
+        if (exported) {
+            std::vector<uint8_t> out32(32);
+            const bool imported = ec_seckey_import_der(secp256k1_context_sign, out32.data(), seckey.data(), seckey.size()) == 1;
+            assert(imported && key32 == out32);
+        }
+    }
+    secp256k1_context_destroy(secp256k1_context_sign);
+}