Monday, November 25, 2013

Code 58 : Larrabee Cipher

Larrabee Cipher


This cipher has been registered with the Office of the Librarian of Congress in the year 1874, by C.S. Larrabee,. It was designed to be used for the telegraphy in the United States Navy.
Fig. 1: Larrabee cipher1


The cipher is based on the Vigenère cipher, with the main difference that also numbers can be encoded. Therefore, the numbers "1, 2, 3, … , 9, 0" are replaced by "A, B, C,…, I, J". Sequences of numbers are preceded by a 'Q' and a character that implies how many numbers will follow. A implies one number, B implies two numbers and I implies nine numbers, the maximum of allowed numbers in a sequence.
"1874", for example, would be replaced by "QDAHGD" and then encoded together with the rest of the plaintext with the Vigenère cipher.


The security of the Larrabee cipher is identical to the security of the Vigenère cipher.




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