public/unlock-store: idempotent call for passphrases

This allows the user to call it even after the store has been unlock in
order to get the admin token.

secretstore/secret-store.go

@@ -175,9 +175,21 @@ func (s *Store) WriteTo(out io.Writer) (n int64, err error) {
 
 var ErrNoSuchKey = errors.New("no such key")
 
+func (s *Store) HasKey(passphrase []byte) bool {
+	key, hash := s.keyPairFromPassword(passphrase)
+	defer memzero(key[:])
+
+	for _, k := range s.Keys {
+		if k.Hash == hash {
+			return true
+		}
+	}
+
+	return false
+}
+
 func (s *Store) Unlock(passphrase []byte) (ok bool) {
 	key, hash := s.keyPairFromPassword(passphrase)
-	memzero(passphrase)
 	defer memzero(key[:])
 
 	var idx = -1