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The poem code is a simple, and insecure, cryptographic method which was used by SOE to communicate with their agents in Nazi-occupied Europe. The method works by the sender and receiver pre-arranging a poem to use. The sender chooses a set number of words at random from the poem and gives each letter in the chosen words a number. The numbers are then used as a key for some cipher to conceal the plaintext of the message. The cipher used was often double transposition. To indicate to the receiver which words had been chosen an indicator group is sent at the start of the message. == Description == To encrypt a message, the agent would select words from the poem as the key. Every poem code message commenced with an indicator-group of five letters, which showed which five words of an agent's poem had been used to encrypt the message. The words would be written sequentially, and their letters numbered to create a transposition key to encrypt a message. For example, if the words are YEO THOMAS IS A PAIN IN THE ARSE, then the transposition key is: 25 5 16, 23 8 17 13 1 20, 10 21, 2, 18 3 11 14, 12 15, 24 9 6, 4 19 22 7. These are the locations of the first appearances of A's, B, etc. in the sentence. This defines a permutation which is used for encryption (25->1, 5->2 etc.). First, the plaintext message is arranged in columns. Then the columns are permuted, and then the rows are permuted. For example, the text "THE OPERATION TO DEMOLISH THE BUNKER IS TOMORROW AT ELEVEN" would be written on grid paper as: TPTTMSEKSOWLN HEIOOHBETRAEA ERODLTURORTVX OANEIHNIMOEET (The above transposition key requires longer messages which would have at least 25 columns and 25 rows). As an additional security measure, the agent would add prearranged errors into the text as security checks. For example, there might be an intentional error in every 18th letter. This was to ensure that, if the agent was captured or the poem was found, the enemy might transmit without the security checks. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Poem code」の詳細全文を読む スポンサード リンク
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