descriptor.cpp

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// Copyright (c) 2018 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 <script/descriptor.h>

#include <chainparams.h> // For Params()
#include <config.h>
#include <key_io.h>
#include <pubkey.h>
#include <script/standard.h>
#include <span.h>
#include <util/bip32.h>
#include <util/spanparsing.h>
#include <util/strencodings.h>
#include <util/system.h>
#include <util/vector.h>

#include <memory>
#include <string>

namespace {

// Checksum                                                               //

// This section implements a checksum algorithm for descriptors with the
// following properties:
// * Mistakes in a descriptor string are measured in "symbol errors". The higher
//   the number of symbol errors, the harder it is to detect:
//   * An error substituting a character from 0123456789()[],'/*[email protected]:$%{}
//   for
//     another in that set always counts as 1 symbol error.
//     * Note that hex encoded keys are covered by these characters. Xprvs and
//       xpubs use other characters too, but already have their own checksum
//       mechanism.
//     * Function names like "multi()" use other characters, but mistakes in
//       these would generally result in an unparsable descriptor.
//   * A case error always counts as 1 symbol error.
//   * Any other 1 character substitution error counts as 1 or 2 symbol errors.
// * Any 1 symbol error is always detected.
// * Any 2 or 3 symbol error in a descriptor of up to 49154 characters is always
// detected.
// * Any 4 symbol error in a descriptor of up to 507 characters is always
// detected.
// * Any 5 symbol error in a descriptor of up to 77 characters is always
// detected.
// * Is optimized to minimize the chance a 5 symbol error in a descriptor up to
// 387 characters is undetected
// * Random errors have a chance of 1 in 2**40 of being undetected.
//
// These properties are achieved by expanding every group of 3 (non checksum)
// characters into 4 GF(32) symbols, over which a cyclic code is defined.

/*
 * Interprets c as 8 groups of 5 bits which are the coefficients of a degree 8
 * polynomial over GF(32), multiplies that polynomial by x, computes its
 * remainder modulo a generator, and adds the constant term val.
 *
 * This generator is G(x) = x^8 + {30}x^7 + {23}x^6 + {15}x^5 + {14}x^4 +
 * {10}x^3 + {6}x^2 + {12}x + {9}. It is chosen to define an cyclic error
 * detecting code which is selected by:
 * - Starting from all BCH codes over GF(32) of degree 8 and below, which by
 * construction guarantee detecting 3 errors in windows up to 19000 symbols.
 * - Taking all those generators, and for degree 7 ones, extend them to degree 8
 * by adding all degree-1 factors.
 * - Selecting just the set of generators that guarantee detecting 4 errors in a
 * window of length 512.
 * - Selecting one of those with best worst-case behavior for 5 errors in
 * windows of length up to 512.
 *
 * The generator and the constants to implement it can be verified using this
 * Sage code: B = GF(2) # Binary field BP.<b> = B[] # Polynomials over the
 * binary field F_mod = b**5 + b**3 + 1 F.<f> = GF(32, modulus=F_mod,
 * repr='int') # GF(32) definition FP.<x> = F[] # Polynomials over GF(32) E_mod
 * = x**3 + x + F.fetch_int(8) E.<e> = F.extension(E_mod) # Extension field
 * definition alpha = e**2743 # Choice of an element in extension field for p in
 * divisors(E.order() - 1): # Verify alpha has order 32767. assert((alpha**p ==
 * 1) == (p % 32767 == 0)) G = lcm([(alpha**i).minpoly() for i in
 * [1056,1057,1058]] + [x + 1]) print(G) # Print out the generator for i in
 * [1,2,4,8,16]: # Print out {1,2,4,8,16}*(G mod x^8), packed in hex integers.
 *       v = 0
 *       for coef in reversed((F.fetch_int(i)*(G %
 * x**8)).coefficients(sparse=True)): v = v*32 + coef.integer_representation()
 *       print("0x%x" % v)
 */
uint64_t PolyMod(uint64_t c, int val) {
    uint8_t c0 = c >> 35;
    c = ((c & 0x7ffffffff) << 5) ^ val;
    if (c0 & 1) {
        c ^= 0xf5dee51989;
    }
    if (c0 & 2) {
        c ^= 0xa9fdca3312;
    }
    if (c0 & 4) {
        c ^= 0x1bab10e32d;
    }
    if (c0 & 8) {
        c ^= 0x3706b1677a;
    }
    if (c0 & 16) {
        c ^= 0x644d626ffd;
    }
    return c;
}

std::string DescriptorChecksum(const Span<const char> &span) {
    static std::string INPUT_CHARSET = "0123456789()[],'/*[email protected]:$%{}"
                                       "IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~"
                                       "ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";

    static std::string CHECKSUM_CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";

    uint64_t c = 1;
    int cls = 0;
    int clscount = 0;
    for (auto ch : span) {
        auto pos = INPUT_CHARSET.find(ch);
        if (pos == std::string::npos) {
            return "";
        }
        // Emit a symbol for the position inside the group, for every character.
        c = PolyMod(c, pos & 31);
        // Accumulate the group numbers
        cls = cls * 3 + (pos >> 5);
        if (++clscount == 3) {
            // Emit an extra symbol representing the group numbers, for every 3
            // characters.
            c = PolyMod(c, cls);
            cls = 0;
            clscount = 0;
        }
    }
    if (clscount > 0) {
        c = PolyMod(c, cls);
    }
    for (int j = 0; j < 8; ++j) {
        // Shift further to determine the checksum.
        c = PolyMod(c, 0);
    }
    // Prevent appending zeroes from not affecting the checksum.
    c ^= 1;

    std::string ret(8, ' ');
    for (int j = 0; j < 8; ++j) {
        ret[j] = CHECKSUM_CHARSET[(c >> (5 * (7 - j))) & 31];
    }
    return ret;
}

std::string AddChecksum(const std::string &str) {
    return str + "#" + DescriptorChecksum(MakeSpan(str));
}

// Internal representation                                                //

typedef std::vector<uint32_t> KeyPath;

struct PubkeyProvider {
protected:
    uint32_t m_expr_index;

public:
    PubkeyProvider(uint32_t exp_index) : m_expr_index(exp_index) {}

    virtual ~PubkeyProvider() = default;

    virtual bool GetPubKey(int pos, const SigningProvider &arg, CPubKey &key,
                           KeyOriginInfo &info,
                           const DescriptorCache *read_cache = nullptr,
                           DescriptorCache *write_cache = nullptr) = 0;

    virtual bool IsRange() const = 0;

    virtual size_t GetSize() const = 0;

    virtual std::string ToString() const = 0;

    virtual bool ToPrivateString(const SigningProvider &arg,
                                 std::string &out) const = 0;

    virtual bool GetPrivKey(int pos, const SigningProvider &arg,
                            CKey &key) const = 0;
};

class OriginPubkeyProvider final : public PubkeyProvider {
    KeyOriginInfo m_origin;
    std::unique_ptr<PubkeyProvider> m_provider;

    std::string OriginString() const {
        return HexStr(std::begin(m_origin.fingerprint),
                      std::end(m_origin.fingerprint)) +
               FormatHDKeypath(m_origin.path);
    }

public:
    OriginPubkeyProvider(uint32_t exp_index, KeyOriginInfo info,
                         std::unique_ptr<PubkeyProvider> provider)
        : PubkeyProvider(exp_index), m_origin(std::move(info)),
          m_provider(std::move(provider)) {}
    bool GetPubKey(int pos, const SigningProvider &arg, CPubKey &key,
                   KeyOriginInfo &info,
                   const DescriptorCache *read_cache = nullptr,
                   DescriptorCache *write_cache = nullptr) override {
        if (!m_provider->GetPubKey(pos, arg, key, info, read_cache,
                                   write_cache)) {
            return false;
        }
        std::copy(std::begin(m_origin.fingerprint),
                  std::end(m_origin.fingerprint), info.fingerprint);
        info.path.insert(info.path.begin(), m_origin.path.begin(),
                         m_origin.path.end());
        return true;
    }
    bool IsRange() const override { return m_provider->IsRange(); }
    size_t GetSize() const override { return m_provider->GetSize(); }
    std::string ToString() const override {
        return "[" + OriginString() + "]" + m_provider->ToString();
    }
    bool ToPrivateString(const SigningProvider &arg,
                         std::string &ret) const override {
        std::string sub;
        if (!m_provider->ToPrivateString(arg, sub)) {
            return false;
        }
        ret = "[" + OriginString() + "]" + std::move(sub);
        return true;
    }
    bool GetPrivKey(int pos, const SigningProvider &arg,
                    CKey &key) const override {
        return m_provider->GetPrivKey(pos, arg, key);
    }
};

class ConstPubkeyProvider final : public PubkeyProvider {
    CPubKey m_pubkey;

public:
    ConstPubkeyProvider(uint32_t exp_index, const CPubKey &pubkey)
        : PubkeyProvider(exp_index), m_pubkey(pubkey) {}
    bool GetPubKey(int pos, const SigningProvider &arg, CPubKey &key,
                   KeyOriginInfo &info,
                   const DescriptorCache *read_cache = nullptr,
                   DescriptorCache *write_cache = nullptr) override {
        key = m_pubkey;
        info.path.clear();
        CKeyID keyid = m_pubkey.GetID();
        std::copy(keyid.begin(), keyid.begin() + sizeof(info.fingerprint),
                  info.fingerprint);
        return true;
    }
    bool IsRange() const override { return false; }
    size_t GetSize() const override { return m_pubkey.size(); }
    std::string ToString() const override {
        return HexStr(m_pubkey.begin(), m_pubkey.end());
    }
    bool ToPrivateString(const SigningProvider &arg,
                         std::string &ret) const override {
        CKey key;
        if (!arg.GetKey(m_pubkey.GetID(), key)) {
            return false;
        }
        ret = EncodeSecret(key);
        return true;
    }
    bool GetPrivKey(int pos, const SigningProvider &arg,
                    CKey &key) const override {
        return arg.GetKey(m_pubkey.GetID(), key);
    }
};

enum class DeriveType {
    NO,
    UNHARDENED,
    HARDENED,
};

class BIP32PubkeyProvider final : public PubkeyProvider {
    // Root xpub, path, and final derivation step type being used, if any
    CExtPubKey m_root_extkey;
    KeyPath m_path;
    DeriveType m_derive;
    // Cache of the parent of the final derived pubkeys.
    // Primarily useful for situations when no read_cache is provided
    CExtPubKey m_cached_xpub;

    bool GetExtKey(const SigningProvider &arg, CExtKey &ret) const {
        CKey key;
        if (!arg.GetKey(m_root_extkey.pubkey.GetID(), key)) {
            return false;
        }
        ret.nDepth = m_root_extkey.nDepth;
        std::copy(m_root_extkey.vchFingerprint,
                  m_root_extkey.vchFingerprint + sizeof(ret.vchFingerprint),
                  ret.vchFingerprint);
        ret.nChild = m_root_extkey.nChild;
        ret.chaincode = m_root_extkey.chaincode;
        ret.key = key;
        return true;
    }

    // Derives the last xprv
    bool GetDerivedExtKey(const SigningProvider &arg, CExtKey &xprv) const {
        if (!GetExtKey(arg, xprv)) {
            return false;
        }
        for (auto entry : m_path) {
            xprv.Derive(xprv, entry);
        }
        return true;
    }

    bool IsHardened() const {
        if (m_derive == DeriveType::HARDENED) {
            return true;
        }
        for (auto entry : m_path) {
            if (entry >> 31) {
                return true;
            }
        }
        return false;
    }

public:
    BIP32PubkeyProvider(uint32_t exp_index, const CExtPubKey &extkey,
                        KeyPath path, DeriveType derive)
        : PubkeyProvider(exp_index), m_root_extkey(extkey),
          m_path(std::move(path)), m_derive(derive) {}
    bool IsRange() const override { return m_derive != DeriveType::NO; }
    size_t GetSize() const override { return 33; }
    bool GetPubKey(int pos, const SigningProvider &arg, CPubKey &key_out,
                   KeyOriginInfo &final_info_out,
                   const DescriptorCache *read_cache = nullptr,
                   DescriptorCache *write_cache = nullptr) override {
        // Info of parent of the to be derived pubkey
        KeyOriginInfo parent_info;
        CKeyID keyid = m_root_extkey.pubkey.GetID();
        std::copy(keyid.begin(),
                  keyid.begin() + sizeof(parent_info.fingerprint),
                  parent_info.fingerprint);
        parent_info.path = m_path;

        // Info of the derived key itself which is copied out upon successful
        // completion
        KeyOriginInfo final_info_out_tmp = parent_info;
        if (m_derive == DeriveType::UNHARDENED) {
            final_info_out_tmp.path.push_back((uint32_t)pos);
        }
        if (m_derive == DeriveType::HARDENED) {
            final_info_out_tmp.path.push_back(((uint32_t)pos) | 0x80000000L);
        }

        // Derive keys or fetch them from cache
        CExtPubKey final_extkey = m_root_extkey;
        CExtPubKey parent_extkey = m_root_extkey;
        bool der = true;
        if (read_cache) {
            if (!read_cache->GetCachedDerivedExtPubKey(m_expr_index, pos,
                                                       final_extkey)) {
                if (m_derive == DeriveType::HARDENED) {
                    return false;
                }
                // Try to get the derivation parent
                if (!read_cache->GetCachedParentExtPubKey(m_expr_index,
                                                          parent_extkey)) {
                    return false;
                }
                final_extkey = parent_extkey;
                if (m_derive == DeriveType::UNHARDENED) {
                    der = parent_extkey.Derive(final_extkey, pos);
                }
            }
        } else if (m_cached_xpub.pubkey.IsValid() &&
                   m_derive != DeriveType::HARDENED) {
            parent_extkey = final_extkey = m_cached_xpub;
            if (m_derive == DeriveType::UNHARDENED) {
                der = parent_extkey.Derive(final_extkey, pos);
            }
        } else if (IsHardened()) {
            CExtKey xprv;
            if (!GetDerivedExtKey(arg, xprv)) {
                return false;
            }
            parent_extkey = xprv.Neuter();
            if (m_derive == DeriveType::UNHARDENED) {
                der = xprv.Derive(xprv, pos);
            }
            if (m_derive == DeriveType::HARDENED) {
                der = xprv.Derive(xprv, pos | 0x80000000UL);
            }
            final_extkey = xprv.Neuter();
        } else {
            for (auto entry : m_path) {
                der = parent_extkey.Derive(parent_extkey, entry);
                assert(der);
            }
            final_extkey = parent_extkey;
            if (m_derive == DeriveType::UNHARDENED) {
                der = parent_extkey.Derive(final_extkey, pos);
            }
            assert(m_derive != DeriveType::HARDENED);
        }
        assert(der);

        final_info_out = final_info_out_tmp;
        key_out = final_extkey.pubkey;

        // We rely on the consumer to check that m_derive isn't HARDENED as
        // above But we can't have already cached something in case we read
        // something from the cache and parent_extkey isn't actually the parent.
        if (!m_cached_xpub.pubkey.IsValid()) {
            m_cached_xpub = parent_extkey;
        }

        if (write_cache) {
            // Only cache parent if there is any unhardened derivation
            if (m_derive != DeriveType::HARDENED) {
                write_cache->CacheParentExtPubKey(m_expr_index, parent_extkey);
            } else if (final_info_out.path.size() > 0) {
                write_cache->CacheDerivedExtPubKey(m_expr_index, pos,
                                                   final_extkey);
            }
        }

        return true;
    }
    std::string ToString() const override {
        std::string ret =
            EncodeExtPubKey(m_root_extkey) + FormatHDKeypath(m_path);
        if (IsRange()) {
            ret += "/*";
            if (m_derive == DeriveType::HARDENED) {
                ret += '\'';
            }
        }
        return ret;
    }
    bool ToPrivateString(const SigningProvider &arg,
                         std::string &out) const override {
        CExtKey key;
        if (!GetExtKey(arg, key)) {
            return false;
        }
        out = EncodeExtKey(key) + FormatHDKeypath(m_path);
        if (IsRange()) {
            out += "/*";
            if (m_derive == DeriveType::HARDENED) {
                out += '\'';
            }
        }
        return true;
    }
    bool GetPrivKey(int pos, const SigningProvider &arg,
                    CKey &key) const override {
        CExtKey extkey;
        if (!GetDerivedExtKey(arg, extkey)) {
            return false;
        }
        if (m_derive == DeriveType::UNHARDENED) {
            extkey.Derive(extkey, pos);
        }
        if (m_derive == DeriveType::HARDENED) {
            extkey.Derive(extkey, pos | 0x80000000UL);
        }
        key = extkey.key;
        return true;
    }
};

class DescriptorImpl : public Descriptor {
    const std::vector<std::unique_ptr<PubkeyProvider>> m_pubkey_args;
    const std::string m_name;

protected:
    const std::unique_ptr<DescriptorImpl> m_subdescriptor_arg;

    virtual std::string ToStringExtra() const { return ""; }

    virtual std::vector<CScript>
    MakeScripts(const std::vector<CPubKey> &pubkeys, const CScript *script,
                FlatSigningProvider &out) const = 0;

public:
    DescriptorImpl(std::vector<std::unique_ptr<PubkeyProvider>> pubkeys,
                   std::unique_ptr<DescriptorImpl> script,
                   const std::string &name)
        : m_pubkey_args(std::move(pubkeys)), m_name(name),
          m_subdescriptor_arg(std::move(script)) {}

    bool IsSolvable() const override {
        if (m_subdescriptor_arg) {
            if (!m_subdescriptor_arg->IsSolvable()) {
                return false;
            }
        }
        return true;
    }

    bool IsRange() const final {
        for (const auto &pubkey : m_pubkey_args) {
            if (pubkey->IsRange()) {
                return true;
            }
        }
        if (m_subdescriptor_arg) {
            if (m_subdescriptor_arg->IsRange()) {
                return true;
            }
        }
        return false;
    }

    bool ToStringHelper(const SigningProvider *arg, std::string &out,
                        bool priv) const {
        std::string extra = ToStringExtra();
        size_t pos = extra.size() > 0 ? 1 : 0;
        std::string ret = m_name + "(" + extra;
        for (const auto &pubkey : m_pubkey_args) {
            if (pos++) {
                ret += ",";
            }
            std::string tmp;
            if (priv) {
                if (!pubkey->ToPrivateString(*arg, tmp)) {
                    return false;
                }
            } else {
                tmp = pubkey->ToString();
            }
            ret += std::move(tmp);
        }
        if (m_subdescriptor_arg) {
            if (pos++) {
                ret += ",";
            }
            std::string tmp;
            if (!m_subdescriptor_arg->ToStringHelper(arg, tmp, priv)) {
                return false;
            }
            ret += std::move(tmp);
        }
        out = std::move(ret) + ")";
        return true;
    }

    std::string ToString() const final {
        std::string ret;
        ToStringHelper(nullptr, ret, false);
        return AddChecksum(ret);
    }

    bool ToPrivateString(const SigningProvider &arg,
                         std::string &out) const override final {
        bool ret = ToStringHelper(&arg, out, true);
        out = AddChecksum(out);
        return ret;
    }

    bool ExpandHelper(int pos, const SigningProvider &arg,
                      const DescriptorCache *read_cache,
                      std::vector<CScript> &output_scripts,
                      FlatSigningProvider &out,
                      DescriptorCache *write_cache) const {
        std::vector<std::pair<CPubKey, KeyOriginInfo>> entries;
        entries.reserve(m_pubkey_args.size());

        // Construct temporary data in `entries` and `subscripts`, to avoid
        // producing output in case of failure.
        for (const auto &p : m_pubkey_args) {
            entries.emplace_back();
            if (!p->GetPubKey(pos, arg, entries.back().first,
                              entries.back().second, read_cache, write_cache)) {
                return false;
            }
        }
        std::vector<CScript> subscripts;
        if (m_subdescriptor_arg) {
            FlatSigningProvider subprovider;
            if (!m_subdescriptor_arg->ExpandHelper(pos, arg, read_cache,
                                                   subscripts, subprovider,
                                                   write_cache)) {
                return false;
            }
            out = Merge(out, subprovider);
        }

        std::vector<CPubKey> pubkeys;
        pubkeys.reserve(entries.size());
        for (auto &entry : entries) {
            pubkeys.push_back(entry.first);
            out.origins.emplace(
                entry.first.GetID(),
                std::make_pair<CPubKey, KeyOriginInfo>(
                    CPubKey(entry.first), std::move(entry.second)));
        }
        if (m_subdescriptor_arg) {
            for (const auto &subscript : subscripts) {
                out.scripts.emplace(CScriptID(subscript), subscript);
                std::vector<CScript> addscripts =
                    MakeScripts(pubkeys, &subscript, out);
                for (auto &addscript : addscripts) {
                    output_scripts.push_back(std::move(addscript));
                }
            }
        } else {
            output_scripts = MakeScripts(pubkeys, nullptr, out);
        }
        return true;
    }

    bool Expand(int pos, const SigningProvider &provider,
                std::vector<CScript> &output_scripts, FlatSigningProvider &out,
                DescriptorCache *write_cache = nullptr) const final {
        return ExpandHelper(pos, provider, nullptr, output_scripts, out,
                            write_cache);
    }

    bool ExpandFromCache(int pos, const DescriptorCache &read_cache,
                         std::vector<CScript> &output_scripts,
                         FlatSigningProvider &out) const final {
        return ExpandHelper(pos, DUMMY_SIGNING_PROVIDER, &read_cache,
                            output_scripts, out, nullptr);
    }

    void ExpandPrivate(int pos, const SigningProvider &provider,
                       FlatSigningProvider &out) const final {
        for (const auto &p : m_pubkey_args) {
            CKey key;
            if (!p->GetPrivKey(pos, provider, key)) {
                continue;
            }
            out.keys.emplace(key.GetPubKey().GetID(), key);
        }
        if (m_subdescriptor_arg) {
            FlatSigningProvider subprovider;
            m_subdescriptor_arg->ExpandPrivate(pos, provider, subprovider);
            out = Merge(out, subprovider);
        }
    }

    std::optional<OutputType> GetOutputType() const override {
        return std::nullopt;
    }
};

class AddressDescriptor final : public DescriptorImpl {
    const CTxDestination m_destination;

protected:
    std::string ToStringExtra() const override {
        return EncodeDestination(m_destination, GetConfig());
    }
    std::vector<CScript> MakeScripts(const std::vector<CPubKey> &,
                                     const CScript *,
                                     FlatSigningProvider &) const override {
        return Vector(GetScriptForDestination(m_destination));
    }

public:
    AddressDescriptor(CTxDestination destination)
        : DescriptorImpl({}, {}, "addr"),
          m_destination(std::move(destination)) {}
    bool IsSolvable() const final { return false; }

    std::optional<OutputType> GetOutputType() const override {
        switch (m_destination.which()) {
            case 1 /* PKHash */:
            case 2 /* ScriptHash */:
                return OutputType::LEGACY;
            case 0 /* CNoDestination */:
            default:
                return std::nullopt;
        }
    }
    bool IsSingleType() const final { return true; }
};

class RawDescriptor final : public DescriptorImpl {
    const CScript m_script;

protected:
    std::string ToStringExtra() const override {
        return HexStr(m_script.begin(), m_script.end());
    }
    std::vector<CScript> MakeScripts(const std::vector<CPubKey> &,
                                     const CScript *,
                                     FlatSigningProvider &) const override {
        return Vector(m_script);
    }

public:
    RawDescriptor(CScript script)
        : DescriptorImpl({}, {}, "raw"), m_script(std::move(script)) {}
    bool IsSolvable() const final { return false; }

    std::optional<OutputType> GetOutputType() const override {
        CTxDestination dest;
        ExtractDestination(m_script, dest);
        switch (dest.which()) {
            case 1 /* PKHash */:
            case 2 /* ScriptHash */:
                return OutputType::LEGACY;
            case 0 /* CNoDestination */:
            default:
                return std::nullopt;
        }
    }
    bool IsSingleType() const final { return true; }
};

class PKDescriptor final : public DescriptorImpl {
protected:
    std::vector<CScript> MakeScripts(const std::vector<CPubKey> &keys,
                                     const CScript *,
                                     FlatSigningProvider &) const override {
        return Vector(GetScriptForRawPubKey(keys[0]));
    }

public:
    PKDescriptor(std::unique_ptr<PubkeyProvider> prov)
        : DescriptorImpl(Vector(std::move(prov)), {}, "pk") {}
    bool IsSingleType() const final { return true; }
};

class PKHDescriptor final : public DescriptorImpl {
protected:
    std::vector<CScript> MakeScripts(const std::vector<CPubKey> &keys,
                                     const CScript *,
                                     FlatSigningProvider &out) const override {
        CKeyID id = keys[0].GetID();
        out.pubkeys.emplace(id, keys[0]);
        return Vector(GetScriptForDestination(PKHash(id)));
    }

public:
    PKHDescriptor(std::unique_ptr<PubkeyProvider> prov)
        : DescriptorImpl(Vector(std::move(prov)), {}, "pkh") {}
    std::optional<OutputType> GetOutputType() const override {
        return OutputType::LEGACY;
    }
    bool IsSingleType() const final { return true; }
};

class ComboDescriptor final : public DescriptorImpl {
protected:
    std::vector<CScript> MakeScripts(const std::vector<CPubKey> &keys,
                                     const CScript *,
                                     FlatSigningProvider &out) const override {
        std::vector<CScript> ret;
        CKeyID id = keys[0].GetID();
        out.pubkeys.emplace(id, keys[0]);
        // P2PK
        ret.emplace_back(GetScriptForRawPubKey(keys[0]));
        // P2PKH
        ret.emplace_back(GetScriptForDestination(PKHash(id)));
        return ret;
    }

public:
    ComboDescriptor(std::unique_ptr<PubkeyProvider> prov)
        : DescriptorImpl(Vector(std::move(prov)), {}, "combo") {}
    bool IsSingleType() const final { return false; }
};

class MultisigDescriptor final : public DescriptorImpl {
    const int m_threshold;
    const bool m_sorted;

protected:
    std::string ToStringExtra() const override {
        return strprintf("%i", m_threshold);
    }
    std::vector<CScript> MakeScripts(const std::vector<CPubKey> &keys,
                                     const CScript *,
                                     FlatSigningProvider &) const override {
        if (m_sorted) {
            std::vector<CPubKey> sorted_keys(keys);
            std::sort(sorted_keys.begin(), sorted_keys.end());
            return Vector(GetScriptForMultisig(m_threshold, sorted_keys));
        }
        return Vector(GetScriptForMultisig(m_threshold, keys));
    }

public:
    MultisigDescriptor(int threshold,
                       std::vector<std::unique_ptr<PubkeyProvider>> providers,
                       bool sorted = false)
        : DescriptorImpl(std::move(providers), {},
                         sorted ? "sortedmulti" : "multi"),
          m_threshold(threshold), m_sorted(sorted) {}
    bool IsSingleType() const final { return true; }
};

class SHDescriptor final : public DescriptorImpl {
protected:
    std::vector<CScript> MakeScripts(const std::vector<CPubKey> &,
                                     const CScript *script,
                                     FlatSigningProvider &) const override {
        return Vector(GetScriptForDestination(ScriptHash(*script)));
    }

public:
    SHDescriptor(std::unique_ptr<DescriptorImpl> desc)
        : DescriptorImpl({}, std::move(desc), "sh") {}
    std::optional<OutputType> GetOutputType() const override {
        return OutputType::LEGACY;
    }
    bool IsSingleType() const final { return true; }
};

// Parser                                                                 //

enum class ParseScriptContext {
    TOP,
    P2SH,
};

NODISCARD bool ParseKeyPath(const std::vector<Span<const char>> &split,
                            KeyPath &out, std::string &error) {
    for (size_t i = 1; i < split.size(); ++i) {
        Span<const char> elem = split[i];
        bool hardened = false;
        if (elem.size() > 0 &&
            (elem[elem.size() - 1] == '\'' || elem[elem.size() - 1] == 'h')) {
            elem = elem.first(elem.size() - 1);
            hardened = true;
        }
        uint32_t p;
        if (!ParseUInt32(std::string(elem.begin(), elem.end()), &p)) {
            error = strprintf("Key path value '%s' is not a valid uint32",
                              std::string(elem.begin(), elem.end()).c_str());
            return false;
        } else if (p > 0x7FFFFFFFUL) {
            error = strprintf("Key path value %u is out of range", p);
            return false;
        }
        out.push_back(p | (uint32_t(hardened) << 31));
    }
    return true;
}

std::unique_ptr<PubkeyProvider> ParsePubkeyInner(uint32_t key_exp_index,
                                                 const Span<const char> &sp,
                                                 FlatSigningProvider &out,
                                                 std::string &error) {
    using namespace spanparsing;

    auto split = Split(sp, '/');
    std::string str(split[0].begin(), split[0].end());
    if (str.size() == 0) {
        error = "No key provided";
        return nullptr;
    }
    if (split.size() == 1) {
        if (IsHex(str)) {
            std::vector<uint8_t> data = ParseHex(str);
            CPubKey pubkey(data);
            if (pubkey.IsFullyValid()) {
                return std::make_unique<ConstPubkeyProvider>(key_exp_index,
                                                             pubkey);
            }
            error = strprintf("Pubkey '%s' is invalid", str);
            return nullptr;
        }
        CKey key = DecodeSecret(str);
        if (key.IsValid()) {
            CPubKey pubkey = key.GetPubKey();
            out.keys.emplace(pubkey.GetID(), key);
            return std::make_unique<ConstPubkeyProvider>(key_exp_index, pubkey);
        }
    }
    CExtKey extkey = DecodeExtKey(str);
    CExtPubKey extpubkey = DecodeExtPubKey(str);
    if (!extkey.key.IsValid() && !extpubkey.pubkey.IsValid()) {
        error = strprintf("key '%s' is not valid", str);
        return nullptr;
    }
    KeyPath path;
    DeriveType type = DeriveType::NO;
    if (split.back() == MakeSpan("*").first(1)) {
        split.pop_back();
        type = DeriveType::UNHARDENED;
    } else if (split.back() == MakeSpan("*'").first(2) ||
               split.back() == MakeSpan("*h").first(2)) {
        split.pop_back();
        type = DeriveType::HARDENED;
    }
    if (!ParseKeyPath(split, path, error)) {
        return nullptr;
    }
    if (extkey.key.IsValid()) {
        extpubkey = extkey.Neuter();
        out.keys.emplace(extpubkey.pubkey.GetID(), extkey.key);
    }
    return std::make_unique<BIP32PubkeyProvider>(key_exp_index, extpubkey,
                                                 std::move(path), type);
}

std::unique_ptr<PubkeyProvider> ParsePubkey(uint32_t key_exp_index,
                                            const Span<const char> &sp,
                                            FlatSigningProvider &out,
                                            std::string &error) {
    using namespace spanparsing;

    auto origin_split = Split(sp, ']');
    if (origin_split.size() > 2) {
        error = "Multiple ']' characters found for a single pubkey";
        return nullptr;
    }
    if (origin_split.size() == 1) {
        return ParsePubkeyInner(key_exp_index, origin_split[0], out, error);
    }
    if (origin_split[0].size() < 1 || origin_split[0][0] != '[') {
        error = strprintf("Key origin start '[ character expected but not "
                          "found, got '%c' instead",
                          origin_split[0][0]);
        return nullptr;
    }
    auto slash_split = Split(origin_split[0].subspan(1), '/');
    if (slash_split[0].size() != 8) {
        error = strprintf("Fingerprint is not 4 bytes (%u characters instead "
                          "of 8 characters)",
                          slash_split[0].size());
        return nullptr;
    }
    std::string fpr_hex =
        std::string(slash_split[0].begin(), slash_split[0].end());
    if (!IsHex(fpr_hex)) {
        error = strprintf("Fingerprint '%s' is not hex", fpr_hex);
        return nullptr;
    }
    auto fpr_bytes = ParseHex(fpr_hex);
    KeyOriginInfo info;
    static_assert(sizeof(info.fingerprint) == 4, "Fingerprint must be 4 bytes");
    assert(fpr_bytes.size() == 4);
    std::copy(fpr_bytes.begin(), fpr_bytes.end(), info.fingerprint);
    if (!ParseKeyPath(slash_split, info.path, error)) {
        return nullptr;
    }
    auto provider =
        ParsePubkeyInner(key_exp_index, origin_split[1], out, error);
    if (!provider) {
        return nullptr;
    }
    return std::make_unique<OriginPubkeyProvider>(
        key_exp_index, std::move(info), std::move(provider));
}

std::unique_ptr<DescriptorImpl> ParseScript(uint32_t key_exp_index,
                                            Span<const char> &sp,
                                            ParseScriptContext ctx,
                                            FlatSigningProvider &out,
                                            std::string &error) {
    using namespace spanparsing;

    auto expr = Expr(sp);
    bool sorted_multi = false;
    if (Func("pk", expr)) {
        auto pubkey = ParsePubkey(key_exp_index, expr, out, error);
        if (!pubkey) {
            return nullptr;
        }
        return std::make_unique<PKDescriptor>(std::move(pubkey));
    }
    if (Func("pkh", expr)) {
        auto pubkey = ParsePubkey(key_exp_index, expr, out, error);
        if (!pubkey) {
            return nullptr;
        }
        return std::make_unique<PKHDescriptor>(std::move(pubkey));
    }
    if (ctx == ParseScriptContext::TOP && Func("combo", expr)) {
        auto pubkey = ParsePubkey(key_exp_index, expr, out, error);
        if (!pubkey) {
            return nullptr;
        }
        return std::make_unique<ComboDescriptor>(std::move(pubkey));
    } else if (ctx != ParseScriptContext::TOP && Func("combo", expr)) {
        error = "Cannot have combo in non-top level";
        return nullptr;
    }
    if ((sorted_multi = Func("sortedmulti", expr)) || Func("multi", expr)) {
        auto threshold = Expr(expr);
        uint32_t thres;
        std::vector<std::unique_ptr<PubkeyProvider>> providers;
        if (!ParseUInt32(std::string(threshold.begin(), threshold.end()),
                         &thres)) {
            error = strprintf(
                "Multi threshold '%s' is not valid",
                std::string(threshold.begin(), threshold.end()).c_str());
            return nullptr;
        }
        size_t script_size = 0;
        while (expr.size()) {
            if (!Const(",", expr)) {
                error = strprintf("Multi: expected ',', got '%c'", expr[0]);
                return nullptr;
            }
            auto arg = Expr(expr);
            auto pk = ParsePubkey(key_exp_index, arg, out, error);
            if (!pk) {
                return nullptr;
            }
            script_size += pk->GetSize() + 1;
            providers.emplace_back(std::move(pk));
            key_exp_index++;
        }
        if (providers.size() < 1 || providers.size() > 16) {
            error = strprintf("Cannot have %u keys in multisig; must have "
                              "between 1 and 16 keys, inclusive",
                              providers.size());
            return nullptr;
        } else if (thres < 1) {
            error = strprintf(
                "Multisig threshold cannot be %d, must be at least 1", thres);
            return nullptr;
        } else if (thres > providers.size()) {
            error =
                strprintf("Multisig threshold cannot be larger than the number "
                          "of keys; threshold is %d but only %u keys specified",
                          thres, providers.size());
            return nullptr;
        }
        if (ctx == ParseScriptContext::TOP) {
            if (providers.size() > 3) {
                error = strprintf("Cannot have %u pubkeys in bare multisig; "
                                  "only at most 3 pubkeys",
                                  providers.size());
                return nullptr;
            }
        }
        if (ctx == ParseScriptContext::P2SH) {
            if (script_size + 3 > MAX_SCRIPT_ELEMENT_SIZE) {
                error = strprintf("P2SH script is too large, %d bytes is "
                                  "larger than %d bytes",
                                  script_size + 3, MAX_SCRIPT_ELEMENT_SIZE);
                return nullptr;
            }
        }
        return std::make_unique<MultisigDescriptor>(thres, std::move(providers),
                                                    sorted_multi);
    }
    if (ctx == ParseScriptContext::TOP && Func("sh", expr)) {
        auto desc = ParseScript(key_exp_index, expr, ParseScriptContext::P2SH,
                                out, error);
        if (!desc || expr.size()) {
            return nullptr;
        }
        return std::make_unique<SHDescriptor>(std::move(desc));
    } else if (ctx != ParseScriptContext::TOP && Func("sh", expr)) {
        error = "Cannot have sh in non-top level";
        return nullptr;
    }
    if (ctx == ParseScriptContext::TOP && Func("addr", expr)) {
        CTxDestination dest =
            DecodeDestination(std::string(expr.begin(), expr.end()), Params());
        if (!IsValidDestination(dest)) {
            error = "Address is not valid";
            return nullptr;
        }
        return std::make_unique<AddressDescriptor>(std::move(dest));
    }
    if (ctx == ParseScriptContext::TOP && Func("raw", expr)) {
        std::string str(expr.begin(), expr.end());
        if (!IsHex(str)) {
            error = "Raw script is not hex";
            return nullptr;
        }
        auto bytes = ParseHex(str);
        return std::make_unique<RawDescriptor>(
            CScript(bytes.begin(), bytes.end()));
    }
    if (ctx == ParseScriptContext::P2SH) {
        error = "A function is needed within P2SH";
        return nullptr;
    }
    error = strprintf("%s is not a valid descriptor function",
                      std::string(expr.begin(), expr.end()));
    return nullptr;
}

std::unique_ptr<PubkeyProvider> InferPubkey(const CPubKey &pubkey,
                                            ParseScriptContext,
                                            const SigningProvider &provider) {
    std::unique_ptr<PubkeyProvider> key_provider =
        std::make_unique<ConstPubkeyProvider>(0, pubkey);
    KeyOriginInfo info;
    if (provider.GetKeyOrigin(pubkey.GetID(), info)) {
        return std::make_unique<OriginPubkeyProvider>(0, std::move(info),
                                                      std::move(key_provider));
    }
    return key_provider;
}

std::unique_ptr<DescriptorImpl> InferScript(const CScript &script,
                                            ParseScriptContext ctx,
                                            const SigningProvider &provider) {
    std::vector<std::vector<uint8_t>> data;
    txnouttype txntype = Solver(script, data);

    if (txntype == TX_PUBKEY) {
        CPubKey pubkey(data[0].begin(), data[0].end());
        if (pubkey.IsValid()) {
            return std::make_unique<PKDescriptor>(
                InferPubkey(pubkey, ctx, provider));
        }
    }
    if (txntype == TX_PUBKEYHASH) {
        uint160 hash(data[0]);
        CKeyID keyid(hash);
        CPubKey pubkey;
        if (provider.GetPubKey(keyid, pubkey)) {
            return std::make_unique<PKHDescriptor>(
                InferPubkey(pubkey, ctx, provider));
        }
    }
    if (txntype == TX_MULTISIG) {
        std::vector<std::unique_ptr<PubkeyProvider>> providers;
        for (size_t i = 1; i + 1 < data.size(); ++i) {
            CPubKey pubkey(data[i].begin(), data[i].end());
            providers.push_back(InferPubkey(pubkey, ctx, provider));
        }
        return std::make_unique<MultisigDescriptor>((int)data[0][0],
                                                    std::move(providers));
    }
    if (txntype == TX_SCRIPTHASH && ctx == ParseScriptContext::TOP) {
        uint160 hash(data[0]);
        CScriptID scriptid(hash);
        CScript subscript;
        if (provider.GetCScript(scriptid, subscript)) {
            auto sub =
                InferScript(subscript, ParseScriptContext::P2SH, provider);
            if (sub) {
                return std::make_unique<SHDescriptor>(std::move(sub));
            }
        }
    }

    CTxDestination dest;
    if (ExtractDestination(script, dest)) {
        if (GetScriptForDestination(dest) == script) {
            return std::make_unique<AddressDescriptor>(std::move(dest));
        }
    }

    return std::make_unique<RawDescriptor>(script);
}

} // namespace

bool CheckChecksum(Span<const char> &sp, bool require_checksum,
                   std::string &error, std::string *out_checksum = nullptr) {
    using namespace spanparsing;

    auto check_split = Split(sp, '#');
    if (check_split.size() > 2) {
        error = "Multiple '#' symbols";
        return false;
    }
    if (check_split.size() == 1 && require_checksum) {
        error = "Missing checksum";
        return false;
    }
    if (check_split.size() == 2) {
        if (check_split[1].size() != 8) {
            error =
                strprintf("Expected 8 character checksum, not %u characters",
                          check_split[1].size());
            return false;
        }
    }
    auto checksum = DescriptorChecksum(check_split[0]);
    if (checksum.empty()) {
        error = "Invalid characters in payload";
        return false;
    }
    if (check_split.size() == 2) {
        if (!std::equal(checksum.begin(), checksum.end(),
                        check_split[1].begin())) {
            error = strprintf(
                "Provided checksum '%s' does not match computed checksum '%s'",
                std::string(check_split[1].begin(), check_split[1].end()),
                checksum);
            return false;
        }
    }
    if (out_checksum) {
        *out_checksum = std::move(checksum);
    }
    sp = check_split[0];
    return true;
}

std::unique_ptr<Descriptor> Parse(const std::string &descriptor,
                                  FlatSigningProvider &out, std::string &error,
                                  bool require_checksum) {
    Span<const char> sp(descriptor.data(), descriptor.size());
    if (!CheckChecksum(sp, require_checksum, error)) {
        return nullptr;
    }
    auto ret = ParseScript(0, sp, ParseScriptContext::TOP, out, error);
    if (sp.size() == 0 && ret) {
        return std::unique_ptr<Descriptor>(std::move(ret));
    }
    return nullptr;
}

std::string GetDescriptorChecksum(const std::string &descriptor) {
    std::string ret;
    std::string error;
    Span<const char> sp(descriptor.data(), descriptor.size());
    if (!CheckChecksum(sp, false, error, &ret)) {
        return "";
    }
    return ret;
}

std::unique_ptr<Descriptor> InferDescriptor(const CScript &script,
                                            const SigningProvider &provider) {
    return InferScript(script, ParseScriptContext::TOP, provider);
}

void DescriptorCache::CacheParentExtPubKey(uint32_t key_exp_pos,
                                           const CExtPubKey &xpub) {
    m_parent_xpubs[key_exp_pos] = xpub;
}

void DescriptorCache::CacheDerivedExtPubKey(uint32_t key_exp_pos,
                                            uint32_t der_index,
                                            const CExtPubKey &xpub) {
    auto &xpubs = m_derived_xpubs[key_exp_pos];
    xpubs[der_index] = xpub;
}

bool DescriptorCache::GetCachedParentExtPubKey(uint32_t key_exp_pos,
                                               CExtPubKey &xpub) const {
    const auto &it = m_parent_xpubs.find(key_exp_pos);
    if (it == m_parent_xpubs.end()) {
        return false;
    }
    xpub = it->second;
    return true;
}

bool DescriptorCache::GetCachedDerivedExtPubKey(uint32_t key_exp_pos,
                                                uint32_t der_index,
                                                CExtPubKey &xpub) const {
    const auto &key_exp_it = m_derived_xpubs.find(key_exp_pos);
    if (key_exp_it == m_derived_xpubs.end()) {
        return false;
    }
    const auto &der_it = key_exp_it->second.find(der_index);
    if (der_it == key_exp_it->second.end()) {
        return false;
    }
    xpub = der_it->second;
    return true;
}

const ExtPubKeyMap DescriptorCache::GetCachedParentExtPubKeys() const {
    return m_parent_xpubs;
}

const std::unordered_map<uint32_t, ExtPubKeyMap>
DescriptorCache::GetCachedDerivedExtPubKeys() const {
    return m_derived_xpubs;
}