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In classical cryptography, the trifid cipher is a cipher invented around 1901 by Felix Delastelle, which extends the concept of the bifid cipher to a third dimension, allowing each symbol to be fractionated into 3 elements instead of two. While the bifid uses the Polybius square to turn each symbol into coordinates on a 5 × 5 (or 6 × 6) square, the trifid turns them into coordinates on a 3 × 3 × 3 cube. As with the bifid, this is then combined with transposition to achieve diffusion. However a higher degree of diffusion is achieved because each output symbol depends on 3 input symbols instead of two. Thus the trifid was the first practical trigraphic substitution. == Operation == Several variants probably exist of the Trifid cipher, and there are known Cyrillic variations of it as well. Below is one example but most decoders work slightly differently. All Triffid systems use TABLE,ROW,COLUMN or some variation of it. The principle remains the same but the result will be completely different. The Dutch version of this page has another example. And there are also different ways to fill out a keyword in the tables: spread out horizontally or fill up one table first. First, a mixed alphabet cubic analogue of the Polybius square is drawn up: In theory, the message is then converted to its coordinates in this grid; in practice, it is more convenient to write the triplets of trits out in a table, like so: Then the coordinates are written out vertically beneath the message: T R E A T Y E N D S B O E R W A R . 2 1 3 3 2 1 3 3 2 1 1 1 3 1 2 3 1 3 3 1 1 3 3 1 1 1 2 3 2 3 1 1 2 3 1 1 3 2 1 2 3 3 1 2 2 3 3 1 1 2 3 2 2 3 They are then read out in rows: 2 1 3 3 2 1 3 3 2 1 1 1 3 1 2 3 1 3 3 1 1 3 3 1 1 1 2 3 2 3 1 1 2 3 1 1 3 2 1 2 3 3 1 2 2 3 3 1 1 2 3 2 2 3 Then divided up into triplets again, and the triplets turned back into letters using the table: 213 321 332 111 312 313 311 331 112 323 112 311 321 233 122 331 123 223 M U A F N . E Q R K R E U T X Q B W In this way, each ciphertext character depends on three plaintext characters, so the trifid is a trigraphic cipher. To decrypt, the procedure is simply reversed. 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「Trifid cipher」の詳細全文を読む スポンサード リンク
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