Whoa. You can do this in core?

Will mean everyone needs to export and reimport their privkeys to the new wallet structure. But I can confirm this works as it is in https://github.com/duality-solutions/dynamic

What that HD stands for?

@SCDeveloper Yep, there will be a need in some migration for old users (optional though). Another option would be to create a completely new wallet and then simply send all funds from an old wallet to a new one. This way ALL keys will be recoverable. And yes, that repo was where I started from first and was able to compile wallet actually after resolving some conflicts. But I wasn't happy with way too many changes all over the place due to embedded refactoring, so had to backport upstream changes selectively instead. Anyway, thanks for the starting point :)

@akijuh Hierarchical Deterministic (HD) Wallets https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki

This should be ready for review and testing now.

Changes:

• top up keypool on hd wallet encryption (fix tests);
• backport, fix and slightly refactor bitcoin core 9294 and 10235 (hd chain split).

NOTE: This breaks compatibility with hd wallets created on previous commits. If you have such wallet, apply this patch locally and run it once:

diff --git a/src/hdchain.h b/src/hdchain.h
index 6cdc7c831..b0fee515b 100644
--- a/src/hdchain.h
+++ b/src/hdchain.h
@@ -29,7 +29,17 @@ public:
         READWRITE(vchSeed);
         READWRITE(id);
         READWRITE(nExternalChainCounter);
-        READWRITE(nInternalChainCounter);
+        if (ser_action.ForRead()) {
+            try {
+                READWRITE(nInternalChainCounter);
+            }
+            catch (std::ios_base::failure&) {
+                nInternalChainCounter = 0;
+            }
+        }
+        else {
+            READWRITE(nInternalChainCounter);
+        }
     }
 
     bool SetNull();

Wallet should now store mnemonic/mnemonicpassphrase.

NOTE: This is incompatible with previous hd wallets once again, run-once patch is should be similar to the one above but it's not(?).

EDIT: err... smth wasn't quite right with that commit, so I had to force push, sorry :/
EDIT2: and again... also patch for wallets on old commit doesn't seem to work for some reason... investigating..
EDIT3: ok, so it's smth with vector serialization I guess. Don't want to dig too much into it, so let's just accept that it's incompatible 😇 dump hdseed using old build and start new wallet using new build and -hdseed if you need one (might also require to run keypoolrefill to increase number of known addresses to the size of the old wallet and rescan).

Rebased to resolve merge conflict after #1473 (around this line https://github.com/dashpay/dash/pull/1405/files#diff-8bdec64a6a2adb83b9696e5ca6d5522bR1282)