ssh/agent: remove len check in Unlock

Unlock compares the length of the passphrase with the given one before
calling subtle.ConstantTimeCompare. This is redundant, since
ConstantTimeCompare already perform a lengths check before doing
anything. Remove the check from Unlock.

Updates golang/go#25173

Change-Id: Ib5fec3a94392bddf2996f5c6bf5a414529e86f2f
Reviewed-on: https://go-review.googlesource.com/110068
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/agent/keyring.go

@@ -102,7 +102,7 @@ func (r *keyring) Unlock(passphrase []byte) error {
 	if !r.locked {
 		return errors.New("agent: not locked")
 	}
-	if len(passphrase) != len(r.passphrase) || 1 != subtle.ConstantTimeCompare(passphrase, r.passphrase) {
+	if 1 != subtle.ConstantTimeCompare(passphrase, r.passphrase) {
 		return fmt.Errorf("agent: incorrect passphrase")
 	}