Merge bitcoin/bitcoin#28419: fuzz: introduce and use ConsumePrivateKey helper

583af18 fuzz: introduce and use `ConsumePrivateKey` helper (Sebastian Falbesoner)

Pull request description:

  In the course of reviewing BIP324 related PRs I noticed a frequent pattern of creating private keys (`CKey` instances) with data consumed from the fuzz data provider:
  ```
      auto key_data = provider.ConsumeBytes<unsigned char>(32);
      key_data.resize(32);
      CKey key;
      key.Set(key_data.begin(), key_data.end(), /*fCompressedIn=*/true);
  ```
  This PR introduces a corresponding helper `ConsumePrivateKey` in order to deduplicate code. The compressed flag can either be set to a fixed value, or, if `std::nullopt` is passed (=default), is also consumed from the fuzz data provider via `.ConsumeBool()`.

  Note that this is not a pure refactor, as some of the replaced call-sites previously consumed a random length (`ConsumeRandomLengthByteVector`) instead of a fixed size of 32 bytes for key data. As far as I can see, there is not much value in using a random size, as in all those cases we can only proceed or do something useful with a valid private key, and key data with sizes other than 32 bytes always lead to invalid keys.

ACKs for top commit:
  sipa:
    utACK 583af18
  brunoerg:
    crACK 583af18

Tree-SHA512: 58a178432ba1eb0a2f7597b6700e96477e8b10f429ef642445a52db12e74d81aec307888315b772bfda9610f90df3e1d556cf024c2bef1d520303b12584feaaa

Author: fanquake

src/test/fuzz/bip324.cpp

@@ -30,19 +30,13 @@ FUZZ_TARGET(bip324_cipher_roundtrip, .init=Initialize)
     // Load keys from fuzzer.
     FuzzedDataProvider provider(buffer.data(), buffer.size());
     // Initiator key
-    auto init_key_data = provider.ConsumeBytes<unsigned char>(32);
-    init_key_data.resize(32);
-    CKey init_key;
-    init_key.Set(init_key_data.begin(), init_key_data.end(), true);
+    CKey init_key = ConsumePrivateKey(provider, /*compressed=*/true);
     if (!init_key.IsValid()) return;
     // Initiator entropy
     auto init_ent = provider.ConsumeBytes<std::byte>(32);
     init_ent.resize(32);
     // Responder key
-    auto resp_key_data = provider.ConsumeBytes<unsigned char>(32);
-    resp_key_data.resize(32);
-    CKey resp_key;
-    resp_key.Set(resp_key_data.begin(), resp_key_data.end(), true);
+    CKey resp_key = ConsumePrivateKey(provider, /*compressed=*/true);
     if (!resp_key.IsValid()) return;
     // Responder entropy
     auto resp_ent = provider.ConsumeBytes<std::byte>(32);

src/test/fuzz/key.cpp

@@ -17,6 +17,7 @@
 #include <streams.h>
 #include <test/fuzz/FuzzedDataProvider.h>
 #include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
 #include <util/chaintype.h>
 #include <util/strencodings.h>
 
@@ -312,10 +313,7 @@ FUZZ_TARGET(ellswift_roundtrip, .init = initialize_key)
 {
     FuzzedDataProvider fdp{buffer.data(), buffer.size()};
 
-    auto key_bytes = fdp.ConsumeBytes<uint8_t>(32);
-    key_bytes.resize(32);
-    CKey key;
-    key.Set(key_bytes.begin(), key_bytes.end(), true);
+    CKey key = ConsumePrivateKey(fdp, /*compressed=*/true);
     if (!key.IsValid()) return;
 
     auto ent32 = fdp.ConsumeBytes<std::byte>(32);
@@ -332,17 +330,11 @@ FUZZ_TARGET(bip324_ecdh, .init = initialize_key)
     FuzzedDataProvider fdp{buffer.data(), buffer.size()};
 
     // We generate private key, k1.
-    auto rnd32 = fdp.ConsumeBytes<uint8_t>(32);
-    rnd32.resize(32);
-    CKey k1;
-    k1.Set(rnd32.begin(), rnd32.end(), true);
+    CKey k1 = ConsumePrivateKey(fdp, /*compressed=*/true);
     if (!k1.IsValid()) return;
 
     // They generate private key, k2.
-    rnd32 = fdp.ConsumeBytes<uint8_t>(32);
-    rnd32.resize(32);
-    CKey k2;
-    k2.Set(rnd32.begin(), rnd32.end(), true);
+    CKey k2 = ConsumePrivateKey(fdp, /*compressed=*/true);
     if (!k2.IsValid()) return;
 
     // We construct an ellswift encoding for our key, k1_ellswift.

src/test/fuzz/message.cpp

@@ -28,9 +28,7 @@ FUZZ_TARGET(message, .init = initialize_message)
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
     const std::string random_message = fuzzed_data_provider.ConsumeRandomLengthString(1024);
     {
-        const std::vector<uint8_t> random_bytes = ConsumeRandomLengthByteVector(fuzzed_data_provider);
-        CKey private_key;
-        private_key.Set(random_bytes.begin(), random_bytes.end(), fuzzed_data_provider.ConsumeBool());
+        CKey private_key = ConsumePrivateKey(fuzzed_data_provider);
         std::string signature;
         const bool message_signed = MessageSign(private_key, random_message, signature);
         if (private_key.IsValid()) {

src/test/fuzz/rpc.cpp

@@ -285,19 +285,15 @@ std::string ConsumeScalarRPCArgument(FuzzedDataProvider& fuzzed_data_provider)
         },
         [&] {
             // base58 encoded key
-            const std::vector<uint8_t> random_bytes = fuzzed_data_provider.ConsumeBytes<uint8_t>(32);
-            CKey key;
-            key.Set(random_bytes.begin(), random_bytes.end(), fuzzed_data_provider.ConsumeBool());
+            CKey key = ConsumePrivateKey(fuzzed_data_provider);
             if (!key.IsValid()) {
                 return;
             }
             r = EncodeSecret(key);
         },
         [&] {
             // hex encoded pubkey
-            const std::vector<uint8_t> random_bytes = fuzzed_data_provider.ConsumeBytes<uint8_t>(32);
-            CKey key;
-            key.Set(random_bytes.begin(), random_bytes.end(), fuzzed_data_provider.ConsumeBool());
+            CKey key = ConsumePrivateKey(fuzzed_data_provider);
             if (!key.IsValid()) {
                 return;
             }

src/test/fuzz/script_sign.cpp

@@ -79,9 +79,7 @@ FUZZ_TARGET(script_sign, .init = initialize_script_sign)
     }
 
     FillableSigningProvider provider;
-    CKey k;
-    const std::vector<uint8_t> key_data = ConsumeRandomLengthByteVector(fuzzed_data_provider);
-    k.Set(key_data.begin(), key_data.end(), fuzzed_data_provider.ConsumeBool());
+    CKey k = ConsumePrivateKey(fuzzed_data_provider);
     if (k.IsValid()) {
         provider.AddKey(k);
     }

src/test/fuzz/util.cpp

@@ -193,6 +193,16 @@ CTxDestination ConsumeTxDestination(FuzzedDataProvider& fuzzed_data_provider) no
     return tx_destination;
 }
 
+CKey ConsumePrivateKey(FuzzedDataProvider& fuzzed_data_provider, std::optional<bool> compressed) noexcept
+{
+    auto key_data = fuzzed_data_provider.ConsumeBytes<uint8_t>(32);
+    key_data.resize(32);
+    CKey key;
+    bool compressed_value = compressed ? *compressed : fuzzed_data_provider.ConsumeBool();
+    key.Set(key_data.begin(), key_data.end(), compressed_value);
+    return key;
+}
+
 bool ContainsSpentInput(const CTransaction& tx, const CCoinsViewCache& inputs) noexcept
 {
     for (const CTxIn& tx_in : tx.vin) {

src/test/fuzz/util.h

@@ -11,6 +11,7 @@
 #include <compat/compat.h>
 #include <consensus/amount.h>
 #include <consensus/consensus.h>
+#include <key.h>
 #include <merkleblock.h>
 #include <primitives/transaction.h>
 #include <script/script.h>
@@ -165,6 +166,8 @@ template <typename WeakEnumType, size_t size>
 
 [[nodiscard]] CTxDestination ConsumeTxDestination(FuzzedDataProvider& fuzzed_data_provider) noexcept;
 
+[[nodiscard]] CKey ConsumePrivateKey(FuzzedDataProvider& fuzzed_data_provider, std::optional<bool> compressed = std::nullopt) noexcept;
+
 template <typename T>
 [[nodiscard]] bool MultiplicationOverflow(const T i, const T j) noexcept
 {