accounts: prevent early drops and zero keys in memory when dropping

Private keys would be locked early if SignLocked was called more than
once because the unlockLater was still running. Terminate it properly.

accounts/account_manager.go

@@ -54,15 +54,22 @@ type Account struct {
 
 type AccountManager struct {
 	keyStore   crypto.KeyStore2
-	unlockedKeys map[string]crypto.Key
+	unlocked   map[string]*unlocked
 	unlockTime time.Duration
 	mutex      sync.RWMutex
 }
 
+type unlocked struct {
+	addr  []byte
+	abort chan struct{}
+
+	*crypto.Key
+}
+
 func NewAccountManager(keyStore crypto.KeyStore2, unlockTime time.Duration) *AccountManager {
 	return &AccountManager{
 		keyStore:   keyStore,
-		unlockedKeys: make(map[string]crypto.Key),
+		unlocked:   make(map[string]*unlocked),
 		unlockTime: unlockTime,
 	}
 }
@@ -97,9 +104,9 @@ func (am *AccountManager) DeleteAccount(address []byte, auth string) error {
 
 func (am *AccountManager) Sign(a Account, toSign []byte) (signature []byte, err error) {
 	am.mutex.RLock()
-	unlockedKey := am.unlockedKeys[string(a.Address)]
+	unlockedKey, found := am.unlocked[string(a.Address)]
 	am.mutex.RUnlock()
-	if unlockedKey.Address == nil {
+	if !found {
 		return nil, ErrLocked
 	}
 	signature, err = crypto.Sign(toSign, unlockedKey.PrivateKey)
@@ -111,10 +118,8 @@ func (am *AccountManager) SignLocked(a Account, keyAuth string, toSign []byte) (
 	if err != nil {
 		return nil, err
 	}
-	am.mutex.Lock()
-	am.unlockedKeys[string(a.Address)] = *key
-	am.mutex.Unlock()
-	go unlockLater(am, a.Address)
+	u := am.addUnlocked(a.Address, key)
+	go am.dropLater(u)
 	signature, err = crypto.Sign(toSign, key.PrivateKey)
 	return signature, err
 }
@@ -143,14 +148,40 @@ func (am *AccountManager) Accounts() ([]Account, error) {
 	return accounts, err
 }
 
-func unlockLater(am *AccountManager, addr []byte) {
-	select {
-	case <-time.After(am.unlockTime):
-	}
+func (am *AccountManager) addUnlocked(addr []byte, key *crypto.Key) *unlocked {
+	u := &unlocked{addr: addr, abort: make(chan struct{}), Key: key}
 	am.mutex.Lock()
-	// TODO: how do we know the key is actually gone from memory?
-	delete(am.unlockedKeys, string(addr))
+	prev, found := am.unlocked[string(addr)]
+	if found {
+		// terminate dropLater for this key to avoid unexpected drops.
+		close(prev.abort)
+		zeroKey(prev.PrivateKey)
+	}
+	am.unlocked[string(addr)] = u
 	am.mutex.Unlock()
+	return u
 }
 
+func (am *AccountManager) dropLater(u *unlocked) {
+	t := time.NewTimer(am.unlockTime)
+	defer t.Stop()
+	select {
+	case <-u.abort:
+		// just quit
+	case <-t.C:
+		am.mutex.Lock()
+		if am.unlocked[string(u.addr)] == u {
+			zeroKey(u.PrivateKey)
+			delete(am.unlocked, string(u.addr))
+		}
+		am.mutex.Unlock()
+	}
+}
+
+// zeroKey zeroes a private key in memory.
+func zeroKey(k *ecdsa.PrivateKey) {
+	b := k.D.Bits()
+	for i := range b {
+		b[i] = 0
+	}
 }