* Cipher A. Deavours and Louis Kruh, " Machine Cryptography and Modern Cryptanalysis ", Artech House, 1985, pp144 – 145 ; 148 – 150.
* Cipher A. Deavours and Louis Kruh, " Machine Cryptography and Modern Cryptanalysis ", Artech House, 1985, chapter IV.
* Cipher A. Deavours and Louis Kruh, " Machine Cryptography and Modern Cryptanalysis ", Artech House, 1985, p199.
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