Merge bitcoin/bitcoin#25642: Don't wrap around when deriving an extended key at a too large depth

fb9faffae3a26b8aed8b671864ba679747163019 extended keys: fail to derive too large depth instead of wrapping around (Antoine Poinsot)
8dc6670ce159c2b080e9f735c6603a601d40b6ac descriptor: don't assert success of extended key derivation (Antoine Poinsot)
50cfc9e7613d6cf6b534df6e551238b80678c70d (pubk)key: mark Derive() as nodiscard (Antoine Poinsot)
0ca258a5ace798c4e54308aa8a09b1ab3302cd7e descriptor: never ignore the return value when deriving an extended key (Antoine Poinsot)
d3599c22bd4c6b3cfaaadd675e95ebe3b3cb1749 spkman: don't ignore the return value when deriving an extended key (Antoine Poinsot)

Pull request description:

  We would previously  silently wrap the derived child's depth back to `0`. Instead, explicitly fail when trying to derive an impossible depth, and handle the error in callers.

  An extended fuzzing corpus of `descriptor_parse` triggered this behaviour, which was reported by MarcoFalke.

  Fixes #25751.

ACKs for top commit:
  achow101:
    re-ACK fb9faffae3a26b8aed8b671864ba679747163019
  instagibbs:
    utACK  https://github.com/bitcoin/bitcoin/pull/25642/commits/fb9faffae3a26b8aed8b671864ba679747163019

Tree-SHA512: 9f75c23572ce847239bd15e5497df2960b6bd63c61ea72347959d968b5c4c9a4bfeee284e76bdcd7bacbf9eeb70feee85ffd3e316f353ca6eca30e93aafad343

1 files changed, 5 insertions, 6 deletions

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index db386c9ab8..9bcbe1ceef 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -328,7 +328,7 @@ class BIP32PubkeyProvider final : public PubkeyProvider
     {
         if (!GetExtKey(arg, xprv)) return false;
         for (auto entry : m_path) {
-            xprv.Derive(xprv, entry);
+            if (!xprv.Derive(xprv, entry)) return false;
             if (entry >> 31) {
                 last_hardened = xprv;
             }
@@ -388,14 +388,13 @@ public:
             }
         } else {
             for (auto entry : m_path) {
-                der = parent_extkey.Derive(parent_extkey, entry);
-                assert(der);
+                if (!parent_extkey.Derive(parent_extkey, entry)) return false;
             }
             final_extkey = parent_extkey;
             if (m_derive == DeriveType::UNHARDENED) der = parent_extkey.Derive(final_extkey, pos);
             assert(m_derive != DeriveType::HARDENED);
         }
-        assert(der);
+        if (!der) return false;
 
         final_info_out = final_info_out_tmp;
         key_out = final_extkey.pubkey;
@@ -498,8 +497,8 @@ public:
         CExtKey extkey;
         CExtKey dummy;
         if (!GetDerivedExtKey(arg, extkey, dummy)) return false;
-        if (m_derive == DeriveType::UNHARDENED) extkey.Derive(extkey, pos);
-        if (m_derive == DeriveType::HARDENED) extkey.Derive(extkey, pos | 0x80000000UL);
+        if (m_derive == DeriveType::UNHARDENED && !extkey.Derive(extkey, pos)) return false;
+        if (m_derive == DeriveType::HARDENED && !extkey.Derive(extkey, pos | 0x80000000UL)) return false;
         key = extkey.key;
         return true;
     }