Plaintext

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In cryptography, plaintext is information used as input to an encryption algorithm; the output is termed "ciphertext". The plaintext could be, for example, a diplomatic message, a bank transaction, an email, a diary and so forth — any information that someone might want to prevent others from reading.

Secure handling of plaintext

In a cryptosystem, weaknesses can be introduced through insecure handling of the plaintext — an attacker might attempt to bypass the cryptography altogether. Plaintext is vulnerable in storage, whether in electronic or paper format. The area of physical security deals with how media can be secured from non-electronic attacks. An attack might attempt to open locked desk drawers or safes, and buildings are not always well-secured. An attacker can also engage in dumpster diving, and may be able to reconstruct shredded sheets. One countermeasure is to burn or thoroughly crosscut shred discarded printed plaintexts. If plaintext is kept in a computer file, the disk (or perhaps the entire computer) and its components must be secure. In the case of securing a computer, that security must be physical (e.g., against burglary, brazen removal under cover of a repair, etc.) as well as virtual (e.g., bugs, illicit network access, Trojan programs, ...). A removable disk (or extractable disk drive) is an obvious possibility, in which case physical security of the removed disk is probably most important.

Laptop computers are a special problem. The US State Department, the British Secret Service, and the US Department of Defense have all had laptops containing secret information, presumably in readable text form, 'vanish' in recent years. Discarded computers (and disks and disk drives) are also a potential source of plaintexts. Unerased files (including any plaintexts which may have been present) will still be readable; several enterprising projects have demonstrated this recently. Perhaps the most famous was a MIT student project which found a wide variety of personal/proprietary/confidential information on discarded, and on recycled, computer equipment.

Erased files may be accessible as well. Most operating systems do not actually erase anything — they simply mark the disk space occupied by the 'erased' file as 'available for use', thus making the file a former file. The information in a file 'erased' in this way remains fully present until overwritten at some later time when the operating system reuses the disk space. On large disks, this 'later time' may be months, or even never. Even overwriting that part of a disk occupied by a file before erasing it is insufficient in many cases. Peter Gutmann of the University of Auckland wrote a celebrated paper some years ago on the recovery of overwritten information from magnetic disks. Some government agencies (e.g., NSA) require that all disk drives be physically pulverized when they are discarded, and in some cases, chemically treated with corrosives before or after.

Certain types of cryptanalysis are classified depending on whether they require access to known plaintext or even chosen plaintext.

Padding is often used, and it is common to run all messages being sent through a data compression algorithm prior to submitting the result (the actual plaintext for encryption to a cryptosystem).

Russian copulation has also been used to obscure headings and introductions though, in modern contexts, with message material which may not be readily 'decopulated' on simple inspection, this has become less useful in practice.

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