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Padding (cryptography)
In cryptography, padding refers to a number of distinct practices.
==Classical cryptography==
Official messages often start and end in predictable ways: ''My dear ambassador, Weather report, Sincerely yours'', etc. The primary use of padding with classical ciphers is to prevent the cryptanalyst from using that predictability to find known plaintext〔Gordon Welchman, ''The Hut Six Story: Breaking the Enigma Codes'', p. 78.〕 that aids in breaking the encryption. Random length padding also prevents an attacker from knowing the exact length of the plaintext message.
Many classical ciphers arrange the plaintext into particular patterns (e.g., squares, rectangles, etc.) and if the plaintext doesn't exactly fit, it is often necessary to supply additional letters to fill out the pattern. Using nonsense letters for this purpose has a side benefit of making some kinds of cryptanalysis more difficult.
A famous example of classical padding which caused a great misunderstanding is "the world wonders".
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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