The “gummed kraft sealing tape” the agency uses is three inches wide, and the indications from the response to my FOIA request suggest that the agency buys it in 450-foot rolls. The CIA even sent me a photocopy of the catalogue they order from—Miller’s Supplies at Work, Government Sales Division. The product number is UFS-44HD007.
After September 11, 2001, the US government instituted high-tech monitoring of snail mail sent through the US Postal Service. Basically the front of every letter can be scanned and catalogued with the flip of a switch. In fact, there’s some evidence that the USPS might photograph every piece of mail that makes its way through their systems.
But if you’re concerned about someone (the government or otherwise) tampering with the inside of your mail, you might do well to get a roll of this stuff. If it’s good enough for the CIA, it’s probably your best bet.
Image and Source: http://paleofuture.gizmodo.com/how-to-securely-send-snail-mail-the-cia-way-1791501535