Merge #15024: Allow specific private keys to be derived from descriptor

53b7de629d3d9281dc6f8fa10e32c4cdec59c140 Add test for dumping the private key imported from descriptor (MeshCollider)
2857bc4a64cc8dc7914bc984ac878397ac64292d Extend importmulti descriptor tests (MeshCollider)
81a884bbd0dbee108d11776794d9627ca07504aa Import private keys from descriptor with importmulti if provided (MeshCollider)
a4d1bd1a29be2dcc5e00c63b6b41916b1c466de0 Add private key derivation functions to descriptors (MeshCollider)

Pull request description:

  ~This is based on #14491, review the last 3 commits only.~

  Currently, descriptors have an Expand() function which returns public keys and scripts for a specific index of a ranged descriptor. But the private key for a specific index is not given. This allows private keys for specific indices to be derived. This also allows those keys to be imported through the `importmulti` RPC rather than having to provide them separately.

ACKs for commit 53b7de:
  achow101:
    ACK 53b7de629d3d9281dc6f8fa10e32c4cdec59c140

Tree-SHA512: c060bc01358a1adc76d3d470fefc2bdd39c837027f452e9bc4bd2e726097e1ece4af9d5627efd942a5f8819271e15ba54f010b169b50a9435a1f0f40fd1cebf3

3 files changed, 52 insertions, 10 deletions

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index 9be87fabb0..50119ba184 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -164,6 +164,9 @@ struct PubkeyProvider
 
     /** Get the descriptor string form including private data (if available in arg). */
     virtual bool ToPrivateString(const SigningProvider& arg, std::string& out) const = 0;
+
+    /** Derive a private key, if private data is available in arg. */
+    virtual bool GetPrivKey(int pos, const SigningProvider& arg, CKey& key) const = 0;
 };
 
 class OriginPubkeyProvider final : public PubkeyProvider
@@ -195,6 +198,10 @@ public:
         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);
+    }
 };
 
 /** An object representing a parsed constant public key in a descriptor. */
@@ -222,6 +229,10 @@ public:
         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 {
@@ -266,14 +277,9 @@ public:
     {
         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;
+                CKey priv_key;
+                if (!GetPrivKey(pos, arg, priv_key)) return false;
+                *key = priv_key.GetPubKey();
             } else {
                 // TODO: optimize by caching
                 CExtPubKey extkey = m_extkey;
@@ -312,6 +318,18 @@ public:
         }
         return true;
     }
+    bool GetPrivKey(int pos, const SigningProvider& arg, CKey& key) const override
+    {
+        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.key;
+        return true;
+    }
 };
 
 /** Base class for all Descriptor implementations. */
@@ -462,6 +480,20 @@ public:
         Span<const unsigned char> span = MakeSpan(cache);
         return ExpandHelper(pos, DUMMY_SIGNING_PROVIDER, &span, output_scripts, out, nullptr) && span.size() == 0;
     }
+
+    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_script_arg) {
+            FlatSigningProvider subprovider;
+            m_script_arg->ExpandPrivate(pos, provider, subprovider);
+            out = Merge(out, subprovider);
+        }
+    }
 };
 
 /** Construct a vector with one element, which is moved into it. */
diff --git a/src/script/descriptor.h b/src/script/descriptor.h
index 907a102284..af7ae229ca 100644
--- a/src/script/descriptor.h
+++ b/src/script/descriptor.h
@@ -60,6 +60,14 @@ struct Descriptor {
      * 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;
+
+    /** Expand the private key for a descriptor at a specified position, if possible.
+     *
+     * pos: the position at which to expand the descriptor. If IsRange() is false, this is ignored.
+     * provider: the provider to query for the private keys.
+     * out: any private keys available for the specified pos will be placed here.
+     */
+    virtual void ExpandPrivate(int pos, const SigningProvider& provider, FlatSigningProvider& out) const = 0;
 };
 
 /** Parse a descriptor string. Included private keys are put in out.
diff --git a/src/wallet/rpcdump.cpp b/src/wallet/rpcdump.cpp
index 2f2fe33ba4..3112dca9f5 100644
--- a/src/wallet/rpcdump.cpp
+++ b/src/wallet/rpcdump.cpp
@@ -1165,8 +1165,7 @@ static UniValue ProcessImportDescriptor(ImportData& import_data, std::map<CKeyID
 
     const UniValue& priv_keys = data.exists("keys") ? data["keys"].get_array() : UniValue();
 
-    // Expand all descriptors to get public keys and scripts.
-    // TODO: get private keys from descriptors too
+    // Expand all descriptors to get public keys and scripts, and private keys if available.
     for (int i = range_start; i <= range_end; ++i) {
         FlatSigningProvider out_keys;
         std::vector<CScript> scripts_temp;
@@ -1180,7 +1179,10 @@ static UniValue ProcessImportDescriptor(ImportData& import_data, std::map<CKeyID
             import_data.import_scripts.emplace(x.second);
         }
 
+        parsed_desc->ExpandPrivate(i, keys, out_keys);
+
         std::copy(out_keys.pubkeys.begin(), out_keys.pubkeys.end(), std::inserter(pubkey_map, pubkey_map.end()));
+        std::copy(out_keys.keys.begin(), out_keys.keys.end(), std::inserter(privkey_map, privkey_map.end()));
         import_data.key_origins.insert(out_keys.origins.begin(), out_keys.origins.end());
     }