wallet: derive the key pool from a seed, so a phrase could bring it back (#93)

Adds BIP32 derivation behind the wallet key pool, with no user-facing way to create a seed yet. wallet.dat gains hdmaster/hdchaincode/hdnext; the mnemonic is never stored. TopUpKeyPool derives m/n' when a seed exists and falls back to a random key, with a log line, if derivation fails. New -selftest=wallet-hd, wired into make tests on both platforms.

1 files changed, 27 insertions, 0 deletions

diff --git a/src/db.h b/src/db.h
index f51047e..01b8583 100644
--- a/src/db.h
+++ b/src/db.h
@@ -386,6 +386,33 @@ public:
         return Write(string("defaultkey"), vchPubKey);
     }
 
+    // --- Deterministic (BIP32) seed ---------------------------------------
+    //
+    // Stored as the master key and chain code rather than the mnemonic: the
+    // twelve words are the user's to write down, and keeping a copy of them in
+    // the file they are meant to protect defeats the point of having them.
+    //
+    // "hdnext" is the next child index to derive, so the sequence a restore
+    // reproduces is the same one this wallet handed out.
+    bool ReadHDMaster(vector<unsigned char>& vchMasterRet, vector<unsigned char>& vchChainCodeRet)
+    {
+        vchMasterRet.clear();
+        vchChainCodeRet.clear();
+        return Read(string("hdmaster"), vchMasterRet)
+            && Read(string("hdchaincode"), vchChainCodeRet);
+    }
+
+    bool WriteHDMaster(const vector<unsigned char>& vchMaster, const vector<unsigned char>& vchChainCode)
+    {
+        return Write(string("hdmaster"), vchMaster)
+            && Write(string("hdchaincode"), vchChainCode);
+    }
+
+    bool WriteHDNext(unsigned int nNext)
+    {
+        return Write(string("hdnext"), nNext);
+    }
+
     // Keys generated ahead of time and not yet handed out. The private key of
     // each is already stored under its own "key" record by AddKey; a "pool"
     // record only says that this one is still unspoken for.