Merge #19379: tests: Add fuzzing harness for SigHasLowR(...) and ecdsa_signature_parse_der_lax(...)

46fcac1e4b tests: Add fuzzing harness for ec_seckey_import_der(...) and ec_seckey_export_der(...) (practicalswift)
b667a90389 tests: Add fuzzing harness for SigHasLowR(...) and ecdsa_signature_parse_der_lax(...) (practicalswift)

Pull request description:

  Add fuzzing harness for `SigHasLowR(...)` and `ecdsa_signature_parse_der_lax(...)`.

  See [`doc/fuzzing.md`](https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md) for information on how to fuzz Bitcoin Core. Don't forget to contribute any coverage increasing inputs you find to the [Bitcoin Core fuzzing corpus repo](https://github.com/bitcoin-core/qa-assets).

  Happy fuzzing :)

ACKs for top commit:
  Crypt-iQ:
    ACK 46fcac1e4b

Tree-SHA512: 11a4856a1efd9a04030a8c8aee2413fd5be1ea248147e649a48a55bacdf732bb48a19ee1ce2761d47d4dd61c9598aec53061b961b319ad824d539dda11a8ccf4

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);
+}

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);
+}