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Anyone who encrypts their emails or uses secure instant message services runs the risk of having their communications stored by the US National Security Agency, according to the latest leaks from former NSA sysadmin Edward Snowden.

The Guardian has published two more explosive documents which set out what sort of information the NSA is allowed to harvest from foreign targets, as well American citizens.

Both were issued by the secret Foreign Intelligence Surveillance Court and were signed by US Attorney General Eric Holder in 2009.

The leaked documents show that data about a US citizen collected "inadvertently" can also be stored for up to five years, giving agents significant breathing space when gathering intelligence. They also show the methods agents use to establish whether targets are based in the US and what they are allowed to do in order to spy on "non-US persons".

The documents clearly state that surveillance should cease the minute a target is on US soil or is deemed to be an American – but there are exceptions to this which allow spooks to store communications from American citizens.

If someone's location can not be clearly established, then they "will not be treated as a United States person" unless other evidence becomes apparent. This would mean that anyone using anonymity software like Tor, which deliberately masks their location, is liable to have their communications stored.

Spies are also told they can retain "all communications that are enciphered or reasonably believed to contain secret meaning" for up to five years, giving them another way to keep American citizens' communications data.

These must be kept to help create a "technical data base", which is spook slang for any data which is useful for cryptanalysis - the breaking of codes - or analysis of internet traffic. In practice, this means that using encrypted messages for security purposes may have the ironic effect of drawing the secret message to the attention of spies.

This data should really be destroyed within five years, unless they are "communications that are enciphered or reasonably believed to contain secret meaning, and sufficient duration may consist of any period of time during which encrypted material is subject to, or of use in, cryptanalysis".

Exchanges between attorneys and clients can also be kept, as long as they contain foreign intelligence information.

The documents follow in the wake of other revelations from Snowden, including an NSA programme called PRISM which trawled data on individuals from popular US cloud and social services. ®

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