Editing Letter (message) (section)

==Security methods==
[[Cryptography]] (secret writing) sometimes played a role in letters in centuries past, as correspondents would use previously agreed [[code]] to try to shield the [[plaintext]] from the comprehension of prying eyes during the letter's transit. This could be done in business letters to lessen [[industrial espionage|spying by competitors on prices and methods]] and in personal letters to try to evade [[postal censorship]] (either of [[censorship#State secrets and prevention of attention|wartime censors]] or of peacetime [[political censorship|authoritarian censors]]) or the gossip of townsfolk.  It could be in either cryptic form (for example, "AEDFX GHSTR HTFXV") or in deceptively readable form (for example, "the dog will run at sunset unless the rains come"). By the standards of modern digital applied cryptography, the security was often not especially high (that is, the [[cryptanalysis|codebreaking]] was not necessarily difficult), but it was usually high enough to meet the demands of the context (that is, the degree of risk, the likelihood or stakes of any codebreaking efforts, and the state of the codebreaking art in each era). 

Various forms or precursors of [[tamper-evident technology]] were developed over the centuries to enable the sending and receiving of letters whereby it would be evident to the recipient if anyone else had opened the letter before they received it. The principal class of these methods was [[sealing wax]]. Another method was to apply a small thin disc of [[adhesive]] material known as a wafer. A more elaborate class was [[letterlocking]], including a type called [[spiral locking]], which was especially relevant to government ministers, royal courts, judicial courts, and legislators.<!--For references on letterlocking, click through to its article.--> 

[[Envelope]]s are available in plain types as well as types with somewhat higher [[privacy]] protection in which a pattern of ink is printed on the inner side, making it more difficult for anyone trying to [[candling|candle]] a sealed letter (that is, examine it [[transparency and translucency|translucently]] via [[wikt:backlight#Noun|backlight]]).  Such envelopes are usually called privacy envelopes or security envelopes. Another [[word sense|sense]] of the term ''security envelope'' refers to [[security bag]]s. 

Diplomatic mail pouch systems are special, small, closed postal systems run by each country's [[ministry of foreign affairs]] or department of state. A general theme of the diplomatic mail pouch is that outsiders never have physical access to it during the entire chain of custody; it never gets sent off out of sight of authorized persons, which would otherwise be the weak link in the chain where [[intelligence agency|intelligence agencies]] could surreptitiously examine it in non-evident ways. The mail pouch itself in a diplomatic mail pouch system is often a [[security bag]] instead of merely any cloth pouch or sack.