Inside Bitcoin

1// Copyright (c) 2023-present The Bitcoin Core developers
2// Distributed under the MIT software license, see the accompanying
3// file COPYING or http://www.opensource.org/licenses/mit-license.php.
4
5#ifndef BITCOIN_TEST_FUZZ_UTIL_DESCRIPTOR_H
6#define BITCOIN_TEST_FUZZ_UTIL_DESCRIPTOR_H
7
8#include <array>
9#include <cinttypes>
10#include <cstddef>
11#include <limits>
12#include <optional>
13#include <span>
14#include <string>
15#include <string_view>
16
17/**
18 * Converts a mocked descriptor string to a valid one. Every key in a mocked descriptor is
19 * represented by 2 hex characters preceded by the '%' character. We parse the two hex characters
20 * as an index in a list of pre-generated keys. This list contains keys of the various types
21 * accepted in descriptor key expressions.
22 */
23class MockedDescriptorConverter {
24private:
25    //! Types are raw (un)compressed pubkeys, raw xonly pubkeys, raw privkeys (WIF), xpubs, xprvs.
26    static constexpr uint8_t KEY_TYPES_COUNT{6};
27    //! How many keys we'll generate in total.
28    static constexpr size_t TOTAL_KEYS_GENERATED{std::numeric_limits<uint8_t>::max() + 1};
29    //! 256 keys of various types.
30    std::array<std::string, TOTAL_KEYS_GENERATED> keys_str;
31
32public:
33    // We derive the type of key to generate from the 1-byte id parsed from hex.
34    bool IdIsCompPubKey(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 0; }
35    bool IdIsUnCompPubKey(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 1; }
36    bool IdIsXOnlyPubKey(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 2; }
37    bool IdIsConstPrivKey(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 3; }
38    bool IdIsXpub(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 4; }
39    bool IdIsXprv(uint8_t idx) const { return idx % KEY_TYPES_COUNT == 5; }
40
41    //! When initializing the target, populate the list of keys.
42    void Init();
43
44    //! Parse an id in the keys vectors from a 2-characters hex string.
45    std::optional<uint8_t> IdxFromHex(std::string_view hex_characters) const;
46
47    //! Get an actual descriptor string from a descriptor string whose keys were mocked.
48    std::optional<std::string> GetDescriptor(std::string_view mocked_desc) const;
49};
50
51//! Default maximum number of derivation indexes in a single derivation path when limiting its depth.
52inline constexpr int MAX_DEPTH{2};
53
54/**
55 * Whether the buffer, if it represents a valid descriptor, contains a derivation path deeper than
56 * a given maximum depth. Note this may also be hit for deriv paths in origins.
57 */
58bool HasDeepDerivPath(std::span<const uint8_t> buff, int max_depth = MAX_DEPTH);
59
60//! Default maximum number of sub-fragments.
61inline constexpr int MAX_SUBS{1'000};
62//! Maximum number of nested sub-fragments we'll allow in a descriptor.
63inline constexpr size_t MAX_NESTED_SUBS{10'000};
64
65/**
66 * Whether the buffer, if it represents a valid descriptor, contains a fragment with more
67 * sub-fragments than the given maximum.
68 */
69bool HasTooManySubFrag(std::span<const uint8_t> buff, int max_subs = MAX_SUBS,
70                       size_t max_nested_subs = MAX_NESTED_SUBS);
71
72//! Default maximum number of wrappers per fragment.
73inline constexpr int MAX_WRAPPERS{100};
74
75/**
76 * Whether the buffer, if it represents a valid descriptor, contains a fragment with more
77 * wrappers than the given maximum.
78 */
79bool HasTooManyWrappers(std::span<const uint8_t> buff, int max_wrappers = MAX_WRAPPERS);
80
81/// Default maximum leaf size. This should be large enough to cover an extended
82/// key, including paths "/", inside and outside of "[]".
83inline constexpr uint32_t MAX_LEAF_SIZE{200};
84
85/// Whether the expanded buffer (after calling GetDescriptor() in
86/// MockedDescriptorConverter) has a leaf size too large.
87bool HasTooLargeLeafSize(std::span<const uint8_t> buff, uint32_t max_leaf_size = MAX_LEAF_SIZE);
88
89/// Deriving "expensive" descriptors will consume useful fuzz compute. The
90/// compute is better spent on a smaller subset of descriptors, which still
91/// covers all real end-user settings.
92///
93/// Use this function after MockedDescriptorConverter::GetDescriptor()
94inline bool IsTooExpensive(std::span<const uint8_t> buffer)
95{
96    // Key derivation is expensive. Deriving deep derivation paths takes a lot of compute and we'd
97    // rather spend time elsewhere in this target, like on the actual descriptor syntax. So rule
98    // out strings which could correspond to a descriptor containing a too large derivation path.
99    if (HasDeepDerivPath(buffer)) return true;
100
101    // Some fragments can take a virtually unlimited number of sub-fragments (thresh, multi_a) but
102    // may perform quadratic operations on them. Limit the number of sub-fragments per fragment.
103    if (HasTooManySubFrag(buffer)) return true;
104
105    // The script building logic performs quadratic copies in the number of nested wrappers. Limit
106    // the number of nested wrappers per fragment.
107    if (HasTooManyWrappers(buffer)) return true;
108
109    // If any suspected leaf is too large, it will likely not represent a valid
110    // use-case. Also, possible base58 parsing in the leaf is quadratic. So
111    // limit the leaf size.
112    if (HasTooLargeLeafSize(buffer)) return true;
113
114    return false;
115}
116#endif // BITCOIN_TEST_FUZZ_UTIL_DESCRIPTOR_H
117