Merge bitcoin#31917: fuzz: provide more realistic values to the base58(check) decoders

d5537c1 fuzz: make sure DecodeBase58(Check) is called with valid values more often (Lőrinc)
bad1433 fuzz: Always restrict base conversion input lengths (Lőrinc)

Pull request description:

  This is a follow-up to bitcoin#30746, expanding coverage by:
  * restricting every input for the base58 conversions, capping max sizes to `100` instead of `1000` or all available input (suggested by marcofleon in bitcoin#30746 (comment)) since most actual usage has lengths of e.g. `21`, `34`, `78`.
  * providing more valid values to the decoder (suggested by maflcko in bitcoin#30746 (comment)) by randomly providing a random input or a valid encoded one; this also enables unifying the roundtrip tests to a single roundtrip per fuzz.

ACKs for top commit:
  mzumsande:
    Code Review / lightly tested ACK d5537c1
  maflcko:
    review ACK d5537c1 🚛

Tree-SHA512: 50365654cdac8a38708a7475eaa43396642b7337e2ee8999374c3faafff4f05457abc1a54c701211e0ed24d36c12af77bcad17b49695699be42664f2be660659

Author: fanquake

src/test/fuzz/base_encode_decode.cpp

@@ -6,6 +6,7 @@
 
 #include <base58.h>
 #include <psbt.h>
+#include <span.h>
 #include <test/fuzz/FuzzedDataProvider.h>
 #include <util/strencodings.h>
 #include <util/string.h>
@@ -19,42 +20,40 @@ using util::TrimStringView;
 
 FUZZ_TARGET(base58_encode_decode)
 {
-    FuzzedDataProvider provider(buffer.data(), buffer.size());
-    const std::string random_string{provider.ConsumeRandomLengthString(1000)};
-    const int max_ret_len{provider.ConsumeIntegralInRange<int>(-1, 1000)};
+    FuzzedDataProvider provider{buffer.data(), buffer.size()};
+    const auto random_string{provider.ConsumeRandomLengthString(100)};
 
-    // Decode/Encode roundtrip
-    std::vector<unsigned char> decoded;
-    if (DecodeBase58(random_string, decoded, max_ret_len)) {
+    const auto encoded{EncodeBase58(MakeUCharSpan(random_string))};
+    const auto decode_input{provider.ConsumeBool() ? random_string : encoded};
+    const int max_ret_len{provider.ConsumeIntegralInRange<int>(-1, decode_input.size() + 1)};
+    if (std::vector<unsigned char> decoded; DecodeBase58(decode_input, decoded, max_ret_len)) {
         const auto encoded_string{EncodeBase58(decoded)};
-        assert(encoded_string == TrimStringView(random_string));
-        assert(encoded_string.empty() || !DecodeBase58(encoded_string, decoded, provider.ConsumeIntegralInRange<int>(0, decoded.size() - 1)));
+        assert(encoded_string == TrimStringView(decode_input));
+        if (decoded.size() > 0) {
+            assert(max_ret_len > 0);
+            assert(decoded.size() <= static_cast<size_t>(max_ret_len));
+            assert(!DecodeBase58(encoded_string, decoded, provider.ConsumeIntegralInRange<int>(0, decoded.size() - 1)));
+        }
     }
-    // Encode/Decode roundtrip
-    const auto encoded{EncodeBase58(buffer)};
-    std::vector<unsigned char> roundtrip_decoded;
-    assert(DecodeBase58(encoded, roundtrip_decoded, buffer.size())
-        && std::ranges::equal(roundtrip_decoded, buffer));
 }
 
 FUZZ_TARGET(base58check_encode_decode)
 {
-    FuzzedDataProvider provider(buffer.data(), buffer.size());
-    const std::string random_string{provider.ConsumeRandomLengthString(1000)};
-    const int max_ret_len{provider.ConsumeIntegralInRange<int>(-1, 1000)};
+    FuzzedDataProvider provider{buffer.data(), buffer.size()};
+    const auto random_string{provider.ConsumeRandomLengthString(100)};
 
-    // Decode/Encode roundtrip
-    std::vector<unsigned char> decoded;
-    if (DecodeBase58Check(random_string, decoded, max_ret_len)) {
+    const auto encoded{EncodeBase58Check(MakeUCharSpan(random_string))};
+    const auto decode_input{provider.ConsumeBool() ? random_string : encoded};
+    const int max_ret_len{provider.ConsumeIntegralInRange<int>(-1, decode_input.size() + 1)};
+    if (std::vector<unsigned char> decoded; DecodeBase58Check(decode_input, decoded, max_ret_len)) {
         const auto encoded_string{EncodeBase58Check(decoded)};
-        assert(encoded_string == TrimStringView(random_string));
-        assert(encoded_string.empty() || !DecodeBase58Check(encoded_string, decoded, provider.ConsumeIntegralInRange<int>(0, decoded.size() - 1)));
+        assert(encoded_string == TrimStringView(decode_input));
+        if (decoded.size() > 0) {
+            assert(max_ret_len > 0);
+            assert(decoded.size() <= static_cast<size_t>(max_ret_len));
+            assert(!DecodeBase58Check(encoded_string, decoded, provider.ConsumeIntegralInRange<int>(0, decoded.size() - 1)));
+        }
     }
-    // Encode/Decode roundtrip
-    const auto encoded{EncodeBase58Check(buffer)};
-    std::vector<unsigned char> roundtrip_decoded;
-    assert(DecodeBase58Check(encoded, roundtrip_decoded, buffer.size())
-        && std::ranges::equal(roundtrip_decoded, buffer));
 }
 
 FUZZ_TARGET(base32_encode_decode)