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, 1 insertions, 0 deletions

diff --git a/src/pubkey.cpp b/src/pubkey.cpp
index a4a1be6388..2e37e16690 100644
--- a/src/pubkey.cpp
+++ b/src/pubkey.cpp
@@ -365,6 +365,7 @@ void CExtPubKey::DecodeWithVersion(const unsigned char code[BIP32_EXTKEY_WITH_VE
 }
 
 bool CExtPubKey::Derive(CExtPubKey &out, unsigned int _nChild) const {
+    if (nDepth == std::numeric_limits<unsigned char>::max()) return false;
     out.nDepth = nDepth + 1;
     CKeyID id = pubkey.GetID();
     memcpy(out.vchFingerprint, &id, 4);