Output descriptors module

src/Makefile.am

@@ -158,6 +158,7 @@ BITCOIN_CORE_H = \
   rpc/register.h \
   rpc/util.h \
   scheduler.h \
+  script/descriptor.h \
   script/ismine.h \
   script/sigcache.h \
   script/sign.h \
@@ -387,6 +388,7 @@ libbitcoin_common_a_SOURCES = \
   policy/feerate.cpp \
   protocol.cpp \
   scheduler.cpp \
+  script/descriptor.cpp \
   script/ismine.cpp \
   script/sign.cpp \
   script/standard.cpp \

src/script/descriptor.cpp

@@ -0,0 +1,566 @@
+// 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 <key_io.h>
+#include <pubkey.h>
+#include <script/script.h>
+#include <script/standard.h>
+
+#include <span.h>
+#include <util.h>
+#include <utilstrencodings.h>
+
+#include <memory>
+#include <string>
+#include <vector>
+
+namespace {
+
+////////////////////////////////////////////////////////////////////////////
+// Internal representation                                                //
+////////////////////////////////////////////////////////////////////////////
+
+typedef std::vector<uint32_t> KeyPath;
+
+std::string FormatKeyPath(const KeyPath& path)
+{
+    std::string ret;
+    for (auto i : path) {
+        ret += strprintf("/%i", (i << 1) >> 1);
+        if (i >> 31) ret += '\'';
+    }
+    return ret;
+}
+
+/** Interface for public key objects in descriptors. */
+struct PubkeyProvider
+{
+    virtual ~PubkeyProvider() = default;
+
+    /** Derive a public key. */
+    virtual bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& out) const = 0;
+
+    /** Whether this represent multiple public keys at different positions. */
+    virtual bool IsRange() const = 0;
+
+    /** Get the size of the generated public key(s) in bytes (33 or 65). */
+    virtual size_t GetSize() const = 0;
+
+    /** Get the descriptor string form. */
+    virtual std::string ToString() const = 0;
+
+    /** Get the descriptor string form including private data (if available in arg). */
+    virtual bool ToPrivateString(const SigningProvider& arg, std::string& out) const = 0;
+};
+
+/** An object representing a parsed constant public key in a descriptor. */
+class ConstPubkeyProvider final : public PubkeyProvider
+{
+    CPubKey m_pubkey;
+
+public:
+    ConstPubkeyProvider(const CPubKey& pubkey) : m_pubkey(pubkey) {}
+    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& out) const override
+    {
+        out = m_pubkey;
+        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;
+    }
+};
+
+enum class DeriveType {
+    NO,
+    UNHARDENED,
+    HARDENED,
+};
+
+/** An object representing a parsed extended public key in a descriptor. */
+class BIP32PubkeyProvider final : public PubkeyProvider
+{
+    CExtPubKey m_extkey;
+    KeyPath m_path;
+    DeriveType m_derive;
+
+    bool GetExtKey(const SigningProvider& arg, CExtKey& ret) const
+    {
+        CKey key;
+        if (!arg.GetKey(m_extkey.pubkey.GetID(), key)) return false;
+        ret.nDepth = m_extkey.nDepth;
+        std::copy(m_extkey.vchFingerprint, m_extkey.vchFingerprint + 4, ret.vchFingerprint);
+        ret.nChild = m_extkey.nChild;
+        ret.chaincode = m_extkey.chaincode;
+        ret.key = key;
+        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(const CExtPubKey& extkey, KeyPath path, DeriveType derive) : m_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& out) const override
+    {
+        if (IsHardened()) {
+            CExtKey key;
+            if (!GetExtKey(arg, key)) return false;
+            for (auto entry : m_path) {
+                key.Derive(key, entry);
+            }
+            if (m_derive == DeriveType::UNHARDENED) key.Derive(key, pos);
+            if (m_derive == DeriveType::HARDENED) key.Derive(key, pos | 0x80000000UL);
+            out = key.Neuter().pubkey;
+        } else {
+            // TODO: optimize by caching
+            CExtPubKey key = m_extkey;
+            for (auto entry : m_path) {
+                key.Derive(key, entry);
+            }
+            if (m_derive == DeriveType::UNHARDENED) key.Derive(key, pos);
+            assert(m_derive != DeriveType::HARDENED);
+            out = key.pubkey;
+        }
+        return true;
+    }
+    std::string ToString() const override
+    {
+        std::string ret = EncodeExtPubKey(m_extkey) + FormatKeyPath(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) + FormatKeyPath(m_path);
+        if (IsRange()) {
+            out += "/*";
+            if (m_derive == DeriveType::HARDENED) out += '\'';
+        }
+        return true;
+    }
+};
+
+/** A parsed addr(A) descriptor. */
+class AddressDescriptor final : public Descriptor
+{
+    CTxDestination m_destination;
+
+public:
+    AddressDescriptor(CTxDestination destination) : m_destination(std::move(destination)) {}
+
+    bool IsRange() const override { return false; }
+    std::string ToString() const override { return "addr(" + EncodeDestination(m_destination) + ")"; }
+    bool ToPrivateString(const SigningProvider& arg, std::string& out) const override { out = ToString(); return true; }
+    bool Expand(int pos, const SigningProvider& arg, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override
+    {
+        output_scripts = std::vector<CScript>{GetScriptForDestination(m_destination)};
+        return true;
+    }
+};
+
+/** A parsed raw(H) descriptor. */
+class RawDescriptor final : public Descriptor
+{
+    CScript m_script;
+
+public:
+    RawDescriptor(CScript script) : m_script(std::move(script)) {}
+
+    bool IsRange() const override { return false; }
+    std::string ToString() const override { return "raw(" + HexStr(m_script.begin(), m_script.end()) + ")"; }
+    bool ToPrivateString(const SigningProvider& arg, std::string& out) const override { out = ToString(); return true; }
+    bool Expand(int pos, const SigningProvider& arg, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override
+    {
+        output_scripts = std::vector<CScript>{m_script};
+        return true;
+    }
+};
+
+/** A parsed pk(P), pkh(P), or wpkh(P) descriptor. */
+class SingleKeyDescriptor final : public Descriptor
+{
+    const std::function<CScript(const CPubKey&)> m_script_fn;
+    const std::string m_fn_name;
+    std::unique_ptr<PubkeyProvider> m_provider;
+
+public:
+    SingleKeyDescriptor(std::unique_ptr<PubkeyProvider> prov, const std::function<CScript(const CPubKey&)>& fn, const std::string& name) : m_script_fn(fn), m_fn_name(name), m_provider(std::move(prov)) {}
+
+    bool IsRange() const override { return m_provider->IsRange(); }
+    std::string ToString() const override { return m_fn_name + "(" + m_provider->ToString() + ")"; }
+    bool ToPrivateString(const SigningProvider& arg, std::string& out) const override
+    {
+        std::string ret;
+        if (!m_provider->ToPrivateString(arg, ret)) return false;
+        out = m_fn_name + "(" + std::move(ret) + ")";
+        return true;
+    }
+    bool Expand(int pos, const SigningProvider& arg, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override
+    {
+        CPubKey key;
+        if (!m_provider->GetPubKey(pos, arg, key)) return false;
+        output_scripts = std::vector<CScript>{m_script_fn(key)};
+        out.pubkeys.emplace(key.GetID(), std::move(key));
+        return true;
+    }
+};
+
+CScript P2PKHGetScript(const CPubKey& pubkey) { return GetScriptForDestination(pubkey.GetID()); }
+CScript P2PKGetScript(const CPubKey& pubkey) { return GetScriptForRawPubKey(pubkey); }
+CScript P2WPKHGetScript(const CPubKey& pubkey) { return GetScriptForDestination(WitnessV0KeyHash(pubkey.GetID())); }
+
+/** A parsed multi(...) descriptor. */
+class MultisigDescriptor : public Descriptor
+{
+    int m_threshold;
+    std::vector<std::unique_ptr<PubkeyProvider>> m_providers;
+
+public:
+    MultisigDescriptor(int threshold, std::vector<std::unique_ptr<PubkeyProvider>> providers) : m_threshold(threshold), m_providers(std::move(providers)) {}
+
+    bool IsRange() const override
+    {
+        for (const auto& p : m_providers) {
+            if (p->IsRange()) return true;
+        }
+        return false;
+    }
+
+    std::string ToString() const override
+    {
+        std::string ret = strprintf("multi(%i", m_threshold);
+        for (const auto& p : m_providers) {
+            ret += "," + p->ToString();
+        }
+        return std::move(ret) + ")";
+    }
+
+    bool ToPrivateString(const SigningProvider& arg, std::string& out) const override
+    {
+        std::string ret = strprintf("multi(%i", m_threshold);
+        for (const auto& p : m_providers) {
+            std::string sub;
+            if (!p->ToPrivateString(arg, sub)) return false;
+            ret += "," + std::move(sub);
+        }
+        out = std::move(ret) + ")";
+        return true;
+    }
+
+    bool Expand(int pos, const SigningProvider& arg, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override
+    {
+        std::vector<CPubKey> pubkeys;
+        pubkeys.reserve(m_providers.size());
+        for (const auto& p : m_providers) {
+            CPubKey key;
+            if (!p->GetPubKey(pos, arg, key)) return false;
+            pubkeys.push_back(key);
+        }
+        for (const CPubKey& key : pubkeys) {
+            out.pubkeys.emplace(key.GetID(), std::move(key));
+        }
+        output_scripts = std::vector<CScript>{GetScriptForMultisig(m_threshold, pubkeys)};
+        return true;
+    }
+};
+
+/** A parsed sh(S) or wsh(S) descriptor. */
+class ConvertorDescriptor : public Descriptor
+{
+    const std::function<CScript(const CScript&)> m_convert_fn;
+    const std::string m_fn_name;
+    std::unique_ptr<Descriptor> m_descriptor;
+
+public:
+    ConvertorDescriptor(std::unique_ptr<Descriptor> descriptor, const std::function<CScript(const CScript&)>& fn, const std::string& name) : m_convert_fn(fn), m_fn_name(name), m_descriptor(std::move(descriptor)) {}
+
+    bool IsRange() const override { return m_descriptor->IsRange(); }
+    std::string ToString() const override { return m_fn_name + "(" + m_descriptor->ToString() + ")"; }
+    bool ToPrivateString(const SigningProvider& arg, std::string& out) const override
+    {
+        std::string ret;
+        if (!m_descriptor->ToPrivateString(arg, ret)) return false;
+        out = m_fn_name + "(" + std::move(ret) + ")";
+        return true;
+    }
+    bool Expand(int pos, const SigningProvider& arg, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override
+    {
+        std::vector<CScript> sub;
+        if (!m_descriptor->Expand(pos, arg, sub, out)) return false;
+        output_scripts.clear();
+        for (const auto& script : sub) {
+            CScriptID id(script);
+            out.scripts.emplace(CScriptID(script), script);
+            output_scripts.push_back(m_convert_fn(script));
+        }
+        return true;
+    }
+};
+
+CScript ConvertP2SH(const CScript& script) { return GetScriptForDestination(CScriptID(script)); }
+CScript ConvertP2WSH(const CScript& script) { return GetScriptForDestination(WitnessV0ScriptHash(script)); }
+
+/** A parsed combo(P) descriptor. */
+class ComboDescriptor final : public Descriptor
+{
+    std::unique_ptr<PubkeyProvider> m_provider;
+
+public:
+    ComboDescriptor(std::unique_ptr<PubkeyProvider> provider) : m_provider(std::move(provider)) {}
+
+    bool IsRange() const override { return m_provider->IsRange(); }
+    std::string ToString() const override { return "combo(" + m_provider->ToString() + ")"; }
+    bool ToPrivateString(const SigningProvider& arg, std::string& out) const override
+    {
+        std::string ret;
+        if (!m_provider->ToPrivateString(arg, ret)) return false;
+        out = "combo(" + std::move(ret) + ")";
+        return true;
+    }
+    bool Expand(int pos, const SigningProvider& arg, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override
+    {
+        CPubKey key;
+        if (!m_provider->GetPubKey(pos, arg, key)) return false;
+        CKeyID keyid = key.GetID();
+        {
+            CScript p2pk = GetScriptForRawPubKey(key);
+            CScript p2pkh = GetScriptForDestination(keyid);
+            output_scripts = std::vector<CScript>{std::move(p2pk), std::move(p2pkh)};
+            out.pubkeys.emplace(keyid, key);
+        }
+        if (key.IsCompressed()) {
+            CScript p2wpkh = GetScriptForDestination(WitnessV0KeyHash(keyid));
+            CScriptID p2wpkh_id(p2wpkh);
+            CScript p2sh_p2wpkh = GetScriptForDestination(p2wpkh_id);
+            out.scripts.emplace(p2wpkh_id, p2wpkh);
+            output_scripts.push_back(std::move(p2wpkh));
+            output_scripts.push_back(std::move(p2sh_p2wpkh));
+        }
+        return true;
+    }
+};
+
+////////////////////////////////////////////////////////////////////////////
+// Parser                                                                 //
+////////////////////////////////////////////////////////////////////////////
+
+enum class ParseScriptContext {
+    TOP,
+    P2SH,
+    P2WSH,
+};
+
+/** Parse a constant. If succesful, sp is updated to skip the constant and return true. */
+bool Const(const std::string& str, Span<const char>& sp)
+{
+    if ((size_t)sp.size() >= str.size() && std::equal(str.begin(), str.end(), sp.begin())) {
+        sp = sp.subspan(str.size());
+        return true;
+    }
+    return false;
+}
+
+/** Parse a function call. If succesful, sp is updated to be the function's argument(s). */
+bool Func(const std::string& str, Span<const char>& sp)
+{
+    if ((size_t)sp.size() >= str.size() + 2 && sp[str.size()] == '(' && sp[sp.size() - 1] == ')' && std::equal(str.begin(), str.end(), sp.begin())) {
+        sp = sp.subspan(str.size() + 1, sp.size() - str.size() - 2);
+        return true;
+    }
+    return false;
+}
+
+/** Return the expression that sp begins with, and update sp to skip it. */
+Span<const char> Expr(Span<const char>& sp)
+{
+    int level = 0;
+    auto it = sp.begin();
+    while (it != sp.end()) {
+        if (*it == '(') {
+            ++level;
+        } else if (level && *it == ')') {
+            --level;
+        } else if (level == 0 && (*it == ')' || *it == ',')) {
+            break;
+        }
+        ++it;
+    }
+    Span<const char> ret = sp.first(it - sp.begin());
+    sp = sp.subspan(it - sp.begin());
+    return ret;
+}
+
+/** Split a string on every instance of sep, returning a vector. */
+std::vector<Span<const char>> Split(const Span<const char>& sp, char sep)
+{
+    std::vector<Span<const char>> ret;
+    auto it = sp.begin();
+    auto start = it;
+    while (it != sp.end()) {
+        if (*it == sep) {
+            ret.emplace_back(start, it);
+            start = it + 1;
+        }
+        ++it;
+    }
+    ret.emplace_back(start, it);
+    return ret;
+}
+
+/** Parse a key path, being passed a split list of elements (the first element is ignored). */
+bool ParseKeyPath(const std::vector<Span<const char>>& split, KeyPath& out)
+{
+    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.first(elem.size() - 1);
+            hardened = true;
+        }
+        uint32_t p;
+        if (!ParseUInt32(std::string(elem.begin(), elem.end()), &p) || p > 0x7FFFFFFFUL) return false;
+        out.push_back(p | (((uint32_t)hardened) << 31));
+    }
+    return true;
+}
+
+std::unique_ptr<PubkeyProvider> ParsePubkey(const Span<const char>& sp, bool permit_uncompressed, FlatSigningProvider& out)
+{
+    auto split = Split(sp, '/');
+    std::string str(split[0].begin(), split[0].end());
+    if (split.size() == 1) {
+        if (IsHex(str)) {
+            std::vector<unsigned char> data = ParseHex(str);
+            CPubKey pubkey(data);
+            if (pubkey.IsFullyValid() && (permit_uncompressed || pubkey.IsCompressed())) return MakeUnique<ConstPubkeyProvider>(pubkey);
+        }
+        CKey key = DecodeSecret(str);
+        if (key.IsValid() && (permit_uncompressed || key.IsCompressed())) {
+            CPubKey pubkey = key.GetPubKey();
+            out.keys.emplace(pubkey.GetID(), key);
+            return MakeUnique<ConstPubkeyProvider>(pubkey);
+        }
+    }
+    CExtKey extkey = DecodeExtKey(str);
+    CExtPubKey extpubkey = DecodeExtPubKey(str);
+    if (!extkey.key.IsValid() && !extpubkey.pubkey.IsValid()) 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.pop_back();
+        type = DeriveType::HARDENED;
+    }
+    if (!ParseKeyPath(split, path)) return nullptr;
+    if (extkey.key.IsValid()) {
+        extpubkey = extkey.Neuter();
+        out.keys.emplace(extpubkey.pubkey.GetID(), extkey.key);
+    }
+    return MakeUnique<BIP32PubkeyProvider>(extpubkey, std::move(path), type);
+}
+
+/** Parse a script in a particular context. */
+std::unique_ptr<Descriptor> ParseScript(Span<const char>& sp, ParseScriptContext ctx, FlatSigningProvider& out)
+{
+    auto expr = Expr(sp);
+    if (Func("pk", expr)) {
+        auto pubkey = ParsePubkey(expr, ctx != ParseScriptContext::P2WSH, out);
+        if (!pubkey) return nullptr;
+        return MakeUnique<SingleKeyDescriptor>(std::move(pubkey), P2PKGetScript, "pk");
+    }
+    if (Func("pkh", expr)) {
+        auto pubkey = ParsePubkey(expr, ctx != ParseScriptContext::P2WSH, out);
+        if (!pubkey) return nullptr;
+        return MakeUnique<SingleKeyDescriptor>(std::move(pubkey), P2PKHGetScript, "pkh");
+    }
+    if (ctx == ParseScriptContext::TOP && Func("combo", expr)) {
+        auto pubkey = ParsePubkey(expr, true, out);
+        if (!pubkey) return nullptr;
+        return MakeUnique<ComboDescriptor>(std::move(pubkey));
+    }
+    if (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)) return nullptr;
+        size_t script_size = 0;
+        while (expr.size()) {
+            if (!Const(",", expr)) return nullptr;
+            auto arg = Expr(expr);
+            auto pk = ParsePubkey(arg, ctx != ParseScriptContext::P2WSH, out);
+            if (!pk) return nullptr;
+            script_size += pk->GetSize() + 1;
+            providers.emplace_back(std::move(pk));
+        }
+        if (providers.size() < 1 || providers.size() > 16 || thres < 1 || thres > providers.size()) return nullptr;
+        if (ctx == ParseScriptContext::TOP) {
+            if (providers.size() > 3) return nullptr; // Not more than 3 pubkeys for raw multisig
+        }
+        if (ctx == ParseScriptContext::P2SH) {
+            if (script_size + 3 > 520) return nullptr; // Enforce P2SH script size limit
+        }
+        return MakeUnique<MultisigDescriptor>(thres, std::move(providers));
+    }
+    if (ctx != ParseScriptContext::P2WSH && Func("wpkh", expr)) {
+        auto pubkey = ParsePubkey(expr, false, out);
+        if (!pubkey) return nullptr;
+        return MakeUnique<SingleKeyDescriptor>(std::move(pubkey), P2WPKHGetScript, "wpkh");
+    }
+    if (ctx == ParseScriptContext::TOP && Func("sh", expr)) {
+        auto desc = ParseScript(expr, ParseScriptContext::P2SH, out);
+        if (!desc || expr.size()) return nullptr;
+        return MakeUnique<ConvertorDescriptor>(std::move(desc), ConvertP2SH, "sh");
+    }
+    if (ctx != ParseScriptContext::P2WSH && Func("wsh", expr)) {
+        auto desc = ParseScript(expr, ParseScriptContext::P2WSH, out);
+        if (!desc || expr.size()) return nullptr;
+        return MakeUnique<ConvertorDescriptor>(std::move(desc), ConvertP2WSH, "wsh");
+    }
+    if (ctx == ParseScriptContext::TOP && Func("addr", expr)) {
+        CTxDestination dest = DecodeDestination(std::string(expr.begin(), expr.end()));
+        if (!IsValidDestination(dest)) return nullptr;
+        return MakeUnique<AddressDescriptor>(std::move(dest));
+    }
+    if (ctx == ParseScriptContext::TOP && Func("raw", expr)) {
+        std::string str(expr.begin(), expr.end());
+        if (!IsHex(str)) return nullptr;
+        auto bytes = ParseHex(str);
+        return MakeUnique<RawDescriptor>(CScript(bytes.begin(), bytes.end()));
+    }
+    return nullptr;
+}
+
+} // namespace
+
+std::unique_ptr<Descriptor> Parse(const std::string& descriptor, FlatSigningProvider& out)
+{
+    Span<const char> sp(descriptor.data(), descriptor.size());
+    auto ret = ParseScript(sp, ParseScriptContext::TOP, out);
+    if (sp.size() == 0 && ret) return ret;
+    return nullptr;
+}

src/script/descriptor.h

@@ -0,0 +1,101 @@
+// 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.
+
+#ifndef BITCOIN_SCRIPT_DESCRIPTOR_H
+#define BITCOIN_SCRIPT_DESCRIPTOR_H
+
+#include <script/script.h>
+#include <script/sign.h>
+
+#include <vector>
+
+// Descriptors are strings that describe a set of scriptPubKeys, together with
+// all information necessary to solve them. By combining all information into
+// one, they avoid the need to separately import keys and scripts.
+//
+// Descriptors may be ranged, which occurs when the public keys inside are
+// specified in the form of HD chains (xpubs).
+//
+// Descriptors always represent public information - public keys and scripts -
+// but in cases where private keys need to be conveyed along with a descriptor,
+// they can be included inside by changing public keys to private keys (WIF
+// format), and changing xpubs by xprvs.
+//
+// 1. Examples
+//
+// A P2PK descriptor with a fixed public key:
+// - pk(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798)
+//
+// A P2SH-P2WSH-P2PKH descriptor with a fixed public key:
+// - sh(wsh(pkh(02e493dbf1c10d80f3581e4904930b1404cc6c13900ee0758474fa94abe8c4cd13)))
+//
+// A bare 1-of-2 multisig descriptor:
+// - multi(1,022f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,025cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc)
+//
+// A chain of P2PKH outputs (this needs the corresponding private key to derive):
+// - pkh(xpub68Gmy5EdvgibQVfPdqkBBCHxA5htiqg55crXYuXoQRKfDBFA1WEjWgP6LHhwBZeNK1VTsfTFUHCdrfp1bgwQ9xv5ski8PX9rL2dZXvgGDnw/1'/2/*)
+//
+// 2. Grammar description:
+//
+// X: xpub or xprv encoded extended key
+// I: decimal encoded integer
+// H: Hex encoded byte array
+// A: Address in P2PKH, P2SH, or Bech32 encoding
+//
+// S (Scripts):
+// * pk(P): Pay-to-pubkey (P2PK) output for public key P.
+// * pkh(P): Pay-to-pubkey-hash (P2PKH) output for public key P.
+// * wpkh(P): Pay-to-witness-pubkey-hash (P2WPKH) output for public key P.
+// * sh(S): Pay-to-script-hash (P2SH) output for script S
+// * wsh(S): Pay-to-witness-script-hash (P2WSH) output for script S
+// * combo(P): combination of P2PK, P2PKH, P2WPKH, and P2SH-P2WPKH for public key P.
+// * multi(I,L): k-of-n multisig for given public keys
+// * addr(A): Output to address
+// * raw(H): scriptPubKey with raw bytes
+//
+// P (Public keys):
+// * H: fixed public key (or WIF-encoded private key)
+// * E: extended public key
+// * E/*: (ranged) all unhardened direct children of an extended public key
+// * E/*': (ranged) all hardened direct children of an extended public key
+//
+// L (Comma-separated lists of public keys):
+// * P
+// * L,P
+//
+// E (Extended public keys):
+// * X
+// * E/I: unhardened child
+// * E/I': hardened child
+//
+// The top level is S.
+
+/** Interface for parsed descriptor objects. */
+struct Descriptor {
+    virtual ~Descriptor() = default;
+
+    /** Whether the expansion of this descriptor depends on the position. */
+    virtual bool IsRange() const = 0;
+
+    /** Convert the descriptor back to a string, undoing parsing. */
+    virtual std::string ToString() const = 0;
+
+    /** Convert the descriptor to a private string. This fails if the provided provider does not have the relevant private keys. */
+    virtual bool ToPrivateString(const SigningProvider& provider, std::string& out) const = 0;
+
+    /** Expand a descriptor at a specified position.
+     *
+     * pos: the position at which to expand the descriptor. If IsRange() is false, this is ignored.
+     * provider: the provider to query for private keys in case of hardened derivation.
+     * output_script: the expanded scriptPubKeys will be put here.
+     * out: scripts and public keys necessary for solving the expanded scriptPubKeys will be put here (may be equal to provider).
+     */
+    virtual bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const = 0;
+};
+
+/** Parse a descriptor string. Included private keys are put in out. Returns nullptr if parsing fails. */
+std::unique_ptr<Descriptor> Parse(const std::string& descriptor, FlatSigningProvider& out);
+
+#endif // BITCOIN_SCRIPT_DESCRIPTOR_H
+