they were not altered after their collection. Special tamper-evident evidence bags are available, to be used following a strict protocol. Video recordings can be protected to some degree against tampering by recording a timestamp. Security seals are commonly employed on devices such as electronic voting machines in an attempt to detect tampering. However, testing by Argonne National Laboratory and others demonstrates that existing seals can usually be quickly defeated by a trained person using low-tech methods. They offer ideas on countermeasures, and are exploring the promising option of "anti-evidence" seals. To prevent gas and electricity meters from being interfered with to show lower chargeable readings, they may be sealed with a lead or plastic seal with a government marking, typically fixed to a wire that passes through part of the meter housing. The meter cannot be opened without cutting the wire or damaging the seal. Computer systems In cryptographic terminology, cryptographic hash functions and cryptographic signatures are used to add a tamper-evident layer of protection to document, often referred to as an electronic signature. Hardware-encrypted full disk drives utilise tamper-evident cases, so when it is retrieved the owner can be assured that the data has not been compromised, thus preventing costly further actions such as notifying the data owners. The document, email, or file to be protected is used to generate a signed hash, a number generated from the contents of the document. Any change to the document, no matter how trivial, such as changing a single bit from a 1 to a 0, will cause it to have a different hash, which will make the signature invalid. To alter a document while purposely maintaining the same hash, assuming the hash function and the program implementing it are properly designed, is extremely difficult. See Avalanche effect and Hash collision. See also Active packaging 1982 Chicago Tylenol murders Dye pack Hardware-based full disk encryption Ink tag Package pilferage Packaging and labeling Sealing wax Security printing Security seal Tamperproofing References External links FDA Compliance Policy Guides – CPG Sec. 450.500 Tamper-Resistant Packaging Requirements for Certain Over-the-Counter Human Drug Products "Improving Tamper-Evident Packaging: Problems, Tests and Solutions", Jack L. Rosette, 1992 "Tamper Evident Microprocessors", Adam Waksman and Simha Sethumadhavan, 2010 Packaging Security