Merge #14646: Add expansion cache functions to descriptors (unused for now)

26879509f Add comments to descriptor tests (Pieter Wuille)
82df4c64f Add descriptor expansion cache (Pieter Wuille)
1eda33aab [refactor] Combine the ToString and ToPrivateString implementations (Pieter Wuille)
24d3a7b3a [refactor] Use DescriptorImpl internally, permitting access to new methods (Pieter Wuille)
6be0fb4b3 [refactor] Add a base DescriptorImpl with most common logic (Pieter Wuille)

Pull request description:

  This patch modifies the internal `Descriptor` class to optionally construct and use an "expansion cache". Such a cache is a byte array that encodes all information necessary to expand a `Descriptor` a second time without access to private keys, and without the need to perform expensive BIP32 derivations. For all currently defined descriptors, the cache simply contains a concatenation of all public keys used.

  This is motivated by the goal of importing a descriptor into the wallet and using it as a replacement for the keypool, where it would be impossible to expand descriptors if they use hardened derivation.

Tree-SHA512: f531a0a82ec1eecc30b78ba8a31724d1249826b028cc3543ad32372e1aedd537f137ab03dbffc222c5df444d5865ecd5cec754c1ae1d4989b6e9baeaffade32a

4 files changed, 287 insertions, 207 deletions

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index ca80d3451f..a702be5b78 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -40,8 +40,8 @@ struct PubkeyProvider
 {
     virtual ~PubkeyProvider() = default;
 
-    /** Derive a public key. */
-    virtual bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info) const = 0;
+    /** Derive a public key. If key==nullptr, only info is desired. */
+    virtual bool GetPubKey(int pos, const SigningProvider& arg, CPubKey* key, KeyOriginInfo& info) const = 0;
 
     /** Whether this represent multiple public keys at different positions. */
     virtual bool IsRange() const = 0;
@@ -68,7 +68,7 @@ class OriginPubkeyProvider final : public PubkeyProvider
 
 public:
     OriginPubkeyProvider(KeyOriginInfo info, std::unique_ptr<PubkeyProvider> provider) : m_origin(std::move(info)), m_provider(std::move(provider)) {}
-    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info) const override
+    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey* key, KeyOriginInfo& info) const override
     {
         if (!m_provider->GetPubKey(pos, arg, key, info)) return false;
         std::copy(std::begin(m_origin.fingerprint), std::end(m_origin.fingerprint), info.fingerprint);
@@ -94,9 +94,9 @@ class ConstPubkeyProvider final : public PubkeyProvider
 
 public:
     ConstPubkeyProvider(const CPubKey& pubkey) : m_pubkey(pubkey) {}
-    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info) const override
+    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey* key, KeyOriginInfo& info) const override
     {
-        key = m_pubkey;
+        if (key) *key = m_pubkey;
         info.path.clear();
         CKeyID keyid = m_pubkey.GetID();
         std::copy(keyid.begin(), keyid.begin() + sizeof(info.fingerprint), info.fingerprint);
@@ -152,26 +152,28 @@ 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& key, KeyOriginInfo& info) const override
+    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey* key, KeyOriginInfo& info) const override
     {
-        if (IsHardened()) {
-            CExtKey extkey;
-            if (!GetExtKey(arg, extkey)) return false;
-            for (auto entry : m_path) {
-                extkey.Derive(extkey, entry);
+        if (key) {
+            if (IsHardened()) {
+                CExtKey extkey;
+                if (!GetExtKey(arg, extkey)) return false;
+                for (auto entry : m_path) {
+                    extkey.Derive(extkey, entry);
+                }
+                if (m_derive == DeriveType::UNHARDENED) extkey.Derive(extkey, pos);
+                if (m_derive == DeriveType::HARDENED) extkey.Derive(extkey, pos | 0x80000000UL);
+                *key = extkey.Neuter().pubkey;
+            } else {
+                // TODO: optimize by caching
+                CExtPubKey extkey = m_extkey;
+                for (auto entry : m_path) {
+                    extkey.Derive(extkey, entry);
+                }
+                if (m_derive == DeriveType::UNHARDENED) extkey.Derive(extkey, pos);
+                assert(m_derive != DeriveType::HARDENED);
+                *key = extkey.pubkey;
             }
-            if (m_derive == DeriveType::UNHARDENED) extkey.Derive(extkey, pos);
-            if (m_derive == DeriveType::HARDENED) extkey.Derive(extkey, pos | 0x80000000UL);
-            key = extkey.Neuter().pubkey;
-        } else {
-            // TODO: optimize by caching
-            CExtPubKey extkey = m_extkey;
-            for (auto entry : m_path) {
-                extkey.Derive(extkey, entry);
-            }
-            if (m_derive == DeriveType::UNHARDENED) extkey.Derive(extkey, pos);
-            assert(m_derive != DeriveType::HARDENED);
-            key = extkey.pubkey;
         }
         CKeyID keyid = m_extkey.pubkey.GetID();
         std::copy(keyid.begin(), keyid.begin() + sizeof(info.fingerprint), info.fingerprint);
@@ -202,129 +204,119 @@ public:
     }
 };
 
-/** 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; }
-    bool IsSolvable() 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; }
-    bool IsSolvable() 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
+/** Base class for all Descriptor implementations. */
+class DescriptorImpl : public Descriptor
 {
-    const std::function<CScript(const CPubKey&)> m_script_fn;
-    const std::string m_fn_name;
-    std::unique_ptr<PubkeyProvider> m_provider;
+    //! Public key arguments for this descriptor (size 1 for PK, PKH, WPKH; any size of Multisig).
+    const std::vector<std::unique_ptr<PubkeyProvider>> m_pubkey_args;
+    //! The sub-descriptor argument (nullptr for everything but SH and WSH).
+    const std::unique_ptr<DescriptorImpl> m_script_arg;
+    //! The string name of the descriptor function.
+    const std::string m_name;
+
+protected:
+    //! Return a serialization of anything except pubkey and script arguments, to be prepended to those.
+    virtual std::string ToStringExtra() const { return ""; }
+
+    /** A helper function to construct the scripts for this descriptor.
+     *
+     *  This function is invoked once for every CScript produced by evaluating
+     *  m_script_arg, or just once in case m_script_arg is nullptr.
+
+     *  @param pubkeys The evaluations of the m_pubkey_args field.
+     *  @param script The evaluation of m_script_arg (or nullptr when m_script_arg is nullptr).
+     *  @param out A FlatSigningProvider to put scripts or public keys in that are necessary to the solver.
+     *             The script and pubkeys argument to this function are automatically added.
+     *  @return A vector with scriptPubKeys for this descriptor.
+     */
+    virtual std::vector<CScript> MakeScripts(const std::vector<CPubKey>& pubkeys, const CScript* script, FlatSigningProvider& out) const = 0;
 
 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)) {}
+    DescriptorImpl(std::vector<std::unique_ptr<PubkeyProvider>> pubkeys, std::unique_ptr<DescriptorImpl> script, const std::string& name) : m_pubkey_args(std::move(pubkeys)), m_script_arg(std::move(script)), m_name(name) {}
 
-    bool IsRange() const override { return m_provider->IsRange(); }
-    bool IsSolvable() const override { return true; }
-    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
+    bool IsSolvable() const override
     {
-        CPubKey key;
-        KeyOriginInfo info;
-        if (!m_provider->GetPubKey(pos, arg, key, info)) return false;
-        output_scripts = std::vector<CScript>{m_script_fn(key)};
-        out.origins.emplace(key.GetID(), std::move(info));
-        out.pubkeys.emplace(key.GetID(), key);
+        if (m_script_arg) {
+            if (!m_script_arg->IsSolvable()) return false;
+        }
         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
+    bool IsRange() const final
     {
-        for (const auto& p : m_providers) {
-            if (p->IsRange()) return true;
+        for (const auto& pubkey : m_pubkey_args) {
+            if (pubkey->IsRange()) return true;
+        }
+        if (m_script_arg) {
+            if (m_script_arg->IsRange()) return true;
         }
         return false;
     }
 
-    bool IsSolvable() const override { return true; }
-
-    std::string ToString() const override
+    bool ToStringHelper(const SigningProvider* arg, std::string& out, bool priv) const
     {
-        std::string ret = strprintf("multi(%i", m_threshold);
-        for (const auto& p : m_providers) {
-            ret += "," + p->ToString();
+        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);
         }
-        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);
+        if (m_script_arg) {
+            if (pos++) ret += ",";
+            std::string tmp;
+            if (!m_script_arg->ToStringHelper(arg, tmp, priv)) return false;
+            ret += std::move(tmp);
         }
         out = std::move(ret) + ")";
         return true;
     }
 
-    bool Expand(int pos, const SigningProvider& arg, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const override
+    std::string ToString() const final
+    {
+        std::string ret;
+        ToStringHelper(nullptr, ret, false);
+        return ret;
+    }
+
+    bool ToPrivateString(const SigningProvider& arg, std::string& out) const override final { return ToStringHelper(&arg, out, true); }
+
+    bool ExpandHelper(int pos, const SigningProvider& arg, Span<const unsigned char>* cache_read, std::vector<CScript>& output_scripts, FlatSigningProvider& out, std::vector<unsigned char>* cache_write) const
     {
         std::vector<std::pair<CPubKey, KeyOriginInfo>> entries;
-        entries.reserve(m_providers.size());
-        // Construct temporary data in `entries`, to avoid producing output in case of failure.
-        for (const auto& p : m_providers) {
+        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)) return false;
+            if (!p->GetPubKey(pos, arg, cache_read ? nullptr : &entries.back().first, entries.back().second)) return false;
+            if (cache_read) {
+                // Cached expanded public key exists, use it.
+                if (cache_read->size() == 0) return false;
+                bool compressed = ((*cache_read)[0] == 0x02 || (*cache_read)[0] == 0x03) && cache_read->size() >= 33;
+                bool uncompressed = ((*cache_read)[0] == 0x04) && cache_read->size() >= 65;
+                if (!(compressed || uncompressed)) return false;
+                CPubKey pubkey(cache_read->begin(), cache_read->begin() + (compressed ? 33 : 65));
+                entries.back().first = pubkey;
+                *cache_read = cache_read->subspan(compressed ? 33 : 65);
+            }
+            if (cache_write) {
+                cache_write->insert(cache_write->end(), entries.back().first.begin(), entries.back().first.end());
+            }
         }
+        std::vector<CScript> subscripts;
+        if (m_script_arg) {
+            FlatSigningProvider subprovider;
+            if (!m_script_arg->ExpandHelper(pos, arg, cache_read, subscripts, subprovider, cache_write)) return false;
+            out = Merge(out, subprovider);
+        }
+
         std::vector<CPubKey> pubkeys;
         pubkeys.reserve(entries.size());
         for (auto& entry : entries) {
@@ -332,89 +324,141 @@ public:
             out.origins.emplace(entry.first.GetID(), std::move(entry.second));
             out.pubkeys.emplace(entry.first.GetID(), entry.first);
         }
-        output_scripts = std::vector<CScript>{GetScriptForMultisig(m_threshold, pubkeys)};
+        if (m_script_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, std::vector<unsigned char>* cache = nullptr) const final
+    {
+        return ExpandHelper(pos, provider, nullptr, output_scripts, out, cache);
+    }
+
+    bool ExpandFromCache(int pos, const std::vector<unsigned char>& cache, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const final
+    {
+        Span<const unsigned char> span = MakeSpan(cache);
+        return ExpandHelper(pos, DUMMY_SIGNING_PROVIDER, &span, output_scripts, out, nullptr) && span.size() == 0;
+    }
 };
 
-/** A parsed sh(S) or wsh(S) descriptor. */
-class ConvertorDescriptor : public Descriptor
+/** Construct a vector with one element, which is moved into it. */
+template<typename T>
+std::vector<T> Singleton(T elem)
 {
-    const std::function<CScript(const CScript&)> m_convert_fn;
-    const std::string m_fn_name;
-    std::unique_ptr<Descriptor> m_descriptor;
+    std::vector<T> ret;
+    ret.emplace_back(std::move(elem));
+    return ret;
+}
 
+/** A parsed addr(A) descriptor. */
+class AddressDescriptor final : public DescriptorImpl
+{
+    const CTxDestination m_destination;
+protected:
+    std::string ToStringExtra() const override { return EncodeDestination(m_destination); }
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>&, const CScript*, FlatSigningProvider&) const override { return Singleton(GetScriptForDestination(m_destination)); }
 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)) {}
+    AddressDescriptor(CTxDestination destination) : DescriptorImpl({}, {}, "addr"), m_destination(std::move(destination)) {}
+    bool IsSolvable() const final { return false; }
+};
 
-    bool IsRange() const override { return m_descriptor->IsRange(); }
-    bool IsSolvable() const override { return m_descriptor->IsSolvable(); }
-    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;
-    }
+/** A parsed raw(H) descriptor. */
+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 Singleton(m_script); }
+public:
+    RawDescriptor(CScript script) : DescriptorImpl({}, {}, "raw"), m_script(std::move(script)) {}
+    bool IsSolvable() const final { return false; }
 };
 
-CScript ConvertP2SH(const CScript& script) { return GetScriptForDestination(CScriptID(script)); }
-CScript ConvertP2WSH(const CScript& script) { return GetScriptForDestination(WitnessV0ScriptHash(script)); }
+/** A parsed pk(P) descriptor. */
+class PKDescriptor final : public DescriptorImpl
+{
+protected:
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, const CScript*, FlatSigningProvider&) const override { return Singleton(GetScriptForRawPubKey(keys[0])); }
+public:
+    PKDescriptor(std::unique_ptr<PubkeyProvider> prov) : DescriptorImpl(Singleton(std::move(prov)), {}, "pk") {}
+};
 
-/** A parsed combo(P) descriptor. */
-class ComboDescriptor final : public Descriptor
+/** A parsed pkh(P) descriptor. */
+class PKHDescriptor final : public DescriptorImpl
 {
-    std::unique_ptr<PubkeyProvider> m_provider;
+protected:
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, const CScript*, FlatSigningProvider&) const override { return Singleton(GetScriptForDestination(keys[0].GetID())); }
+public:
+    PKHDescriptor(std::unique_ptr<PubkeyProvider> prov) : DescriptorImpl(Singleton(std::move(prov)), {}, "pkh") {}
+};
 
+/** A parsed wpkh(P) descriptor. */
+class WPKHDescriptor final : public DescriptorImpl
+{
+protected:
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, const CScript*, FlatSigningProvider&) const override { return Singleton(GetScriptForDestination(WitnessV0KeyHash(keys[0].GetID()))); }
 public:
-    ComboDescriptor(std::unique_ptr<PubkeyProvider> provider) : m_provider(std::move(provider)) {}
+    WPKHDescriptor(std::unique_ptr<PubkeyProvider> prov) : DescriptorImpl(Singleton(std::move(prov)), {}, "wpkh") {}
+};
 
-    bool IsRange() const override { return m_provider->IsRange(); }
-    bool IsSolvable() const override { return true; }
-    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
+/** A parsed combo(P) descriptor. */
+class ComboDescriptor final : public DescriptorImpl
+{
+protected:
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, const CScript*, FlatSigningProvider& out) const override
     {
-        CPubKey key;
-        KeyOriginInfo info;
-        if (!m_provider->GetPubKey(pos, arg, key, info)) 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);
-            out.origins.emplace(keyid, std::move(info));
-        }
-        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));
+        std::vector<CScript> ret;
+        CKeyID id = keys[0].GetID();
+        ret.emplace_back(GetScriptForRawPubKey(keys[0])); // P2PK
+        ret.emplace_back(GetScriptForDestination(id)); // P2PKH
+        if (keys[0].IsCompressed()) {
+            CScript p2wpkh = GetScriptForDestination(WitnessV0KeyHash(id));
+            out.scripts.emplace(CScriptID(p2wpkh), p2wpkh);
+            ret.emplace_back(p2wpkh);
+            ret.emplace_back(GetScriptForDestination(CScriptID(p2wpkh))); // P2SH-P2WPKH
         }
-        return true;
+        return ret;
     }
+public:
+    ComboDescriptor(std::unique_ptr<PubkeyProvider> prov) : DescriptorImpl(Singleton(std::move(prov)), {}, "combo") {}
+};
+
+/** A parsed multi(...) descriptor. */
+class MultisigDescriptor final : public DescriptorImpl
+{
+    const int m_threshold;
+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 { return Singleton(GetScriptForMultisig(m_threshold, keys)); }
+public:
+    MultisigDescriptor(int threshold, std::vector<std::unique_ptr<PubkeyProvider>> providers) : DescriptorImpl(std::move(providers), {}, "multi"), m_threshold(threshold) {}
+};
+
+/** A parsed sh(...) descriptor. */
+class SHDescriptor final : public DescriptorImpl
+{
+protected:
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>&, const CScript* script, FlatSigningProvider&) const override { return Singleton(GetScriptForDestination(CScriptID(*script))); }
+public:
+    SHDescriptor(std::unique_ptr<DescriptorImpl> desc) : DescriptorImpl({}, std::move(desc), "sh") {}
+};
+
+/** A parsed wsh(...) descriptor. */
+class WSHDescriptor final : public DescriptorImpl
+{
+protected:
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>&, const CScript* script, FlatSigningProvider&) const override { return Singleton(GetScriptForDestination(WitnessV0ScriptHash(*script))); }
+public:
+    WSHDescriptor(std::unique_ptr<DescriptorImpl> desc) : DescriptorImpl({}, std::move(desc), "wsh") {}
 };
 
 ////////////////////////////////////////////////////////////////////////////
@@ -562,18 +606,18 @@ std::unique_ptr<PubkeyProvider> ParsePubkey(const Span<const char>& sp, bool per
 }
 
 /** Parse a script in a particular context. */
-std::unique_ptr<Descriptor> ParseScript(Span<const char>& sp, ParseScriptContext ctx, FlatSigningProvider& out)
+std::unique_ptr<DescriptorImpl> 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");
+        return MakeUnique<PKDescriptor>(std::move(pubkey));
     }
     if (Func("pkh", expr)) {
         auto pubkey = ParsePubkey(expr, ctx != ParseScriptContext::P2WSH, out);
         if (!pubkey) return nullptr;
-        return MakeUnique<SingleKeyDescriptor>(std::move(pubkey), P2PKHGetScript, "pkh");
+        return MakeUnique<PKHDescriptor>(std::move(pubkey));
     }
     if (ctx == ParseScriptContext::TOP && Func("combo", expr)) {
         auto pubkey = ParsePubkey(expr, true, out);
@@ -606,17 +650,17 @@ std::unique_ptr<Descriptor> ParseScript(Span<const char>& sp, ParseScriptContext
     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");
+        return MakeUnique<WPKHDescriptor>(std::move(pubkey));
     }
     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");
+        return MakeUnique<SHDescriptor>(std::move(desc));
     }
     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");
+        return MakeUnique<WSHDescriptor>(std::move(desc));
     }
     if (ctx == ParseScriptContext::TOP && Func("addr", expr)) {
         CTxDestination dest = DecodeDestination(std::string(expr.begin(), expr.end()));
@@ -642,7 +686,7 @@ std::unique_ptr<PubkeyProvider> InferPubkey(const CPubKey& pubkey, ParseScriptCo
     return key_provider;
 }
 
-std::unique_ptr<Descriptor> InferScript(const CScript& script, ParseScriptContext ctx, const SigningProvider& provider)
+std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptContext ctx, const SigningProvider& provider)
 {
     std::vector<std::vector<unsigned char>> data;
     txnouttype txntype = Solver(script, data);
@@ -650,7 +694,7 @@ std::unique_ptr<Descriptor> InferScript(const CScript& script, ParseScriptContex
     if (txntype == TX_PUBKEY) {
         CPubKey pubkey(data[0].begin(), data[0].end());
         if (pubkey.IsValid()) {
-            return MakeUnique<SingleKeyDescriptor>(InferPubkey(pubkey, ctx, provider), P2PKGetScript, "pk");
+            return MakeUnique<PKDescriptor>(InferPubkey(pubkey, ctx, provider));
         }
     }
     if (txntype == TX_PUBKEYHASH) {
@@ -658,7 +702,7 @@ std::unique_ptr<Descriptor> InferScript(const CScript& script, ParseScriptContex
         CKeyID keyid(hash);
         CPubKey pubkey;
         if (provider.GetPubKey(keyid, pubkey)) {
-            return MakeUnique<SingleKeyDescriptor>(InferPubkey(pubkey, ctx, provider), P2PKHGetScript, "pkh");
+            return MakeUnique<PKHDescriptor>(InferPubkey(pubkey, ctx, provider));
         }
     }
     if (txntype == TX_WITNESS_V0_KEYHASH && ctx != ParseScriptContext::P2WSH) {
@@ -666,7 +710,7 @@ std::unique_ptr<Descriptor> InferScript(const CScript& script, ParseScriptContex
         CKeyID keyid(hash);
         CPubKey pubkey;
         if (provider.GetPubKey(keyid, pubkey)) {
-            return MakeUnique<SingleKeyDescriptor>(InferPubkey(pubkey, ctx, provider), P2WPKHGetScript, "wpkh");
+            return MakeUnique<WPKHDescriptor>(InferPubkey(pubkey, ctx, provider));
         }
     }
     if (txntype == TX_MULTISIG) {
@@ -683,7 +727,7 @@ std::unique_ptr<Descriptor> InferScript(const CScript& script, ParseScriptContex
         CScript subscript;
         if (provider.GetCScript(scriptid, subscript)) {
             auto sub = InferScript(subscript, ParseScriptContext::P2SH, provider);
-            if (sub) return MakeUnique<ConvertorDescriptor>(std::move(sub), ConvertP2SH, "sh");
+            if (sub) return MakeUnique<SHDescriptor>(std::move(sub));
         }
     }
     if (txntype == TX_WITNESS_V0_SCRIPTHASH && ctx != ParseScriptContext::P2WSH) {
@@ -692,7 +736,7 @@ std::unique_ptr<Descriptor> InferScript(const CScript& script, ParseScriptContex
         CScript subscript;
         if (provider.GetCScript(scriptid, subscript)) {
             auto sub = InferScript(subscript, ParseScriptContext::P2WSH, provider);
-            if (sub) return MakeUnique<ConvertorDescriptor>(std::move(sub), ConvertP2WSH, "wsh");
+            if (sub) return MakeUnique<WSHDescriptor>(std::move(sub));
         }
     }
 
@@ -712,7 +756,7 @@ std::unique_ptr<Descriptor> Parse(const std::string& descriptor, FlatSigningProv
 {
     Span<const char> sp(descriptor.data(), descriptor.size());
     auto ret = ParseScript(sp, ParseScriptContext::TOP, out);
-    if (sp.size() == 0 && ret) return ret;
+    if (sp.size() == 0 && ret) return std::unique_ptr<Descriptor>(std::move(ret));
     return nullptr;
 }
 
diff --git a/src/script/descriptor.h b/src/script/descriptor.h
index 0111972f85..44f0efca03 100644
--- a/src/script/descriptor.h
+++ b/src/script/descriptor.h
@@ -48,8 +48,18 @@ struct Descriptor {
      * 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).
+     * cache: vector which will be overwritten with cache data necessary to-evaluate the descriptor at this point without access to private keys.
      */
-    virtual bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out) const = 0;
+    virtual bool Expand(int pos, const SigningProvider& provider, std::vector<CScript>& output_scripts, FlatSigningProvider& out, std::vector<unsigned char>* cache = nullptr) const = 0;
+
+    /** Expand a descriptor at a specified position using cached expansion data.
+     *
+     * pos: the position at which to expand the descriptor. If IsRange() is false, this is ignored.
+     * cache: vector from which cached expansion data will be read.
+     * 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 ExpandFromCache(int pos, const std::vector<unsigned char>& cache, 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. */
diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index e5651710f1..635e4fa3d2 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -705,5 +705,7 @@ FlatSigningProvider Merge(const FlatSigningProvider& a, const FlatSigningProvide
     ret.pubkeys.insert(b.pubkeys.begin(), b.pubkeys.end());
     ret.keys = a.keys;
     ret.keys.insert(b.keys.begin(), b.keys.end());
+    ret.origins = a.origins;
+    ret.origins.insert(b.origins.begin(), b.origins.end());
     return ret;
 }
diff --git a/src/test/descriptor_tests.cpp b/src/test/descriptor_tests.cpp
index 0e98f5a826..8da8cfc00c 100644
--- a/src/test/descriptor_tests.cpp
+++ b/src/test/descriptor_tests.cpp
@@ -79,19 +79,42 @@ void Check(const std::string& prv, const std::string& pub, int flags, const std:
     BOOST_CHECK_EQUAL(parse_pub->IsRange(), (flags & RANGE) != 0);
     BOOST_CHECK_EQUAL(parse_priv->IsRange(), (flags & RANGE) != 0);
 
-
-    // Is not ranged descriptor, only a single result is expected.
+    // * For ranged descriptors,  the `scripts` parameter is a list of expected result outputs, for subsequent
+    //   positions to evaluate the descriptors on (so the first element of `scripts` is for evaluating the
+    //   descriptor at 0; the second at 1; and so on). To verify this, we evaluate the descriptors once for
+    //   each element in `scripts`.
+    // * For non-ranged descriptors, we evaluate the descriptors at positions 0, 1, and 2, but expect the
+    //   same result in each case, namely the first element of `scripts`. Because of that, the size of
+    //   `scripts` must be one in that case.
     if (!(flags & RANGE)) assert(scripts.size() == 1);
-
     size_t max = (flags & RANGE) ? scripts.size() : 3;
+
+    // Iterate over the position we'll evaluate the descriptors in.
     for (size_t i = 0; i < max; ++i) {
+        // Call the expected result scripts `ref`.
         const auto& ref = scripts[(flags & RANGE) ? i : 0];
+        // When t=0, evaluate the `prv` descriptor; when t=1, evaluate the `pub` descriptor.
         for (int t = 0; t < 2; ++t) {
+            // When the descriptor is hardened, evaluate with access to the private keys inside.
             const FlatSigningProvider& key_provider = (flags & HARDENED) ? keys_priv : keys_pub;
-            FlatSigningProvider script_provider;
-            std::vector<CScript> spks;
-            BOOST_CHECK((t ? parse_priv : parse_pub)->Expand(i, key_provider, spks, script_provider));
+
+            // Evaluate the descriptor selected by `t` in poisition `i`.
+            FlatSigningProvider script_provider, script_provider_cached;
+            std::vector<CScript> spks, spks_cached;
+            std::vector<unsigned char> cache;
+            BOOST_CHECK((t ? parse_priv : parse_pub)->Expand(i, key_provider, spks, script_provider, &cache));
+
+            // Compare the output with the expected result.
             BOOST_CHECK_EQUAL(spks.size(), ref.size());
+
+            // Try to expand again using cached data, and compare.
+            BOOST_CHECK(parse_pub->ExpandFromCache(i, cache, spks_cached, script_provider_cached));
+            BOOST_CHECK(spks == spks_cached);
+            BOOST_CHECK(script_provider.pubkeys == script_provider_cached.pubkeys);
+            BOOST_CHECK(script_provider.scripts == script_provider_cached.scripts);
+            BOOST_CHECK(script_provider.origins == script_provider_cached.origins);
+
+            // For each of the produced scripts, verify solvability, and when possible, try to sign a transaction spending it.
             for (size_t n = 0; n < spks.size(); ++n) {
                 BOOST_CHECK_EQUAL(ref[n], HexStr(spks[n].begin(), spks[n].end()));
                 BOOST_CHECK_EQUAL(IsSolvable(Merge(key_provider, script_provider), spks[n]), (flags & UNSOLVABLE) == 0);
@@ -123,6 +146,7 @@ void Check(const std::string& prv, const std::string& pub, int flags, const std:
             }
         }
     }
+
     // Verify no expected paths remain that were not observed.
     BOOST_CHECK_MESSAGE(left_paths.empty(), "Not all expected key paths found: " + prv);
 }