The key words you're looking for a are "flaps and seals." That's the tradecraft term for the work involved in surreptitiously opening things, gaining knowledge of their contents and resealing so that the intrusion is not detected.
How difficult it is to detect tampering depends of course on the sophistication of your adversaries' surveillance. In your film clip example, the next step up in detection would be sending an undeveloped strip of film with a latent image already exposed on it. On arrival, the strip is developed. If it is fogged, or the expected image is not there, then an intrusion has taken place. As an alternative, a chemical that does not fog the emulsion, but is not normally present there, would make substitution detectable. Etc. etc. But consider the more difficult problem of detection intrustion WITHOUT letting the badguys know you suspect. Very important in spy work because the very fact that a target is using countersurveillance technique may tell the watchers more than the contents of the mail. Best, Marc de Piolenc Thomas Shaddack wrote: > > > Also, in the US, the police can request a "mail cover" > > (which means recording who all your snail mail is from) > > with much less legal formality than a search warrant, > > and if they get a warrant to open all your incoming mail, > > I don't think they're required to notify you. > > Is there a way to RELIABLY find the mail was opened? > > Reason: If the mail sent is eg. a CD with a set of OTP keys, then the > adversary gains next to nothing by intercepting it IF the interception is > detected (the keys just get discarded and new set is sent to another > address). > One of my ideas was to put a small piece of film or photographic paper, > detect that it was exposed to light, but then the adversary can put in a > new piece of the light-sensitive material and reseal the package. The same > problem goes with the various kinds of seals.
