tests: Add fuzzing harness for SigHasLowR(...) and ecdsa_signature_parse_der_lax(...)

Author: practicalswift

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_ecdsa_signature_parse_der_lax \
   test/fuzz/service_deserialize \
   test/fuzz/signature_checker \
   test/fuzz/snapshotmetadata_deserialize \
@@ -1094,6 +1095,12 @@ 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_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)
+test_fuzz_secp256k1_ecdsa_signature_parse_der_lax_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS) $(LIBTOOL_APP_LDFLAGS)
+test_fuzz_secp256k1_ecdsa_signature_parse_der_lax_SOURCES = test/fuzz/secp256k1_ecdsa_signature_parse_der_lax.cpp
+
 test_fuzz_service_deserialize_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) -DSERVICE_DESERIALIZE=1
 test_fuzz_service_deserialize_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 test_fuzz_service_deserialize_LDADD = $(FUZZ_SUITE_LD_COMMON)

src/pubkey.cpp

@@ -24,7 +24,7 @@ secp256k1_context* secp256k1_context_verify = nullptr;
  *  strict DER before being passed to this module, and we know it supports all
  *  violations present in the blockchain before that point.
  */
-static int ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_signature* sig, const unsigned char *input, size_t inputlen) {
+int ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_signature* sig, const unsigned char *input, size_t inputlen) {
     size_t rpos, rlen, spos, slen;
     size_t pos = 0;
     size_t lenbyte;

src/test/fuzz/secp256k1_ecdsa_signature_parse_der_lax.cpp

@@ -0,0 +1,33 @@
+// 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>
+
+bool SigHasLowR(const secp256k1_ecdsa_signature* sig);
+int ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_signature* sig, const unsigned char* input, size_t inputlen);
+
+void test_one_input(const std::vector<uint8_t>& buffer)
+{
+    FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
+    const std::vector<uint8_t> signature_bytes = ConsumeRandomLengthByteVector(fuzzed_data_provider);
+    if (signature_bytes.data() == nullptr) {
+        return;
+    }
+    secp256k1_context* secp256k1_context_verify = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY);
+    secp256k1_ecdsa_signature sig_der_lax;
+    const bool parsed_der_lax = ecdsa_signature_parse_der_lax(secp256k1_context_verify, &sig_der_lax, signature_bytes.data(), signature_bytes.size()) == 1;
+    if (parsed_der_lax) {
+        ECC_Start();
+        (void)SigHasLowR(&sig_der_lax);
+        ECC_Stop();
+    }
+    secp256k1_context_destroy(secp256k1_context_verify);
+}