Nothing is foolproof, [ Yseult agrees, but the considering furrow between her brows adds that it's a good question. ]

It could have been useful. But a passphrase can be stolen, so it may give either a false sense of security or very little. Assigning each agent an individual passphrase helps but would require records. [ She wobbles a hand back and forth uncertainly. ]

Perhaps as a first line of defense. Some sort of marking is another option, but less appealing.