key: add DeriveExtKey() helper

Co-authored-by: w0xlt <94266259+w0xlt@users.noreply.github.com>

src/key.cpp

@@ -16,6 +16,8 @@
 #include <secp256k1_recovery.h>
 #include <secp256k1_schnorrsig.h>
 
+#include <algorithm>
+
 static secp256k1_context* secp256k1_context_sign = nullptr;
 
 /** These functions are taken from the libsecp256k1 distribution and are very ugly. */
@@ -364,6 +366,18 @@ bool CExtKey::Derive(CExtKey &out, unsigned int _nChild) const {
     return key.Derive(out.key, out.chaincode, _nChild, chaincode);
 }
 
+std::optional<std::pair<CExtKey, KeyOriginInfo>> DeriveExtKey(const CExtKey& ext_key, const std::vector<uint32_t>& path)
+{
+    CExtKey descendant = ext_key;
+    KeyOriginInfo origin;
+    origin.fingerprint = ext_key.id_key_fingerprint();
+    origin.path = path;
+    for (uint32_t i : path) {
+        if (!descendant.Derive(descendant, i)) return std::nullopt;
+    }
+    return std::make_pair(descendant, origin);
+}
+
 void CExtKey::SetSeed(std::span<const std::byte> seed)
 {
     Assert(16 <= seed.size() && seed.size() <= 64);