Double Alphabet Code
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The Double Alphabet Code (or cipher) can be used to send moderately secure messages which do not easily fail to frequency analysis. Rather than a simple "ceasar cipher" alphabet shift, it uses two different alphabets alternately. The may be mirror images, or might be shifted by the same, or different amounts.
The recipient must know what the sender is using to easily unscramble the message. The use of numerals, determined by a double alphabet code can further obscure the system being used, making it look like some numeric substitution code is in use. The simplest example is shown below:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Z Y X W V U T S R Q P O N M L K J I H G F E D C B A
HELLO -> 8 21 12 15 15 starting with the top alphabet.
This demonstrates that the double "L" does not appear to be a double, but the "L O" happens to appear to be a double letter, which it is not.
Besides changing the order of the alphabets in use, the choice of starting from the bottom alphabet first, instead of the top, will render the "decoded" text as nonsense.
This has the strength of requiring no codebook or special materials like a "one time pad" to use, nor does it depend on a copy of a particular book, for a "book cipher", where even the wrong edition could make decoding impossible. "No codebook" means it can not be found or captured, as with even the "Enigma" machine in World War II.
Obviously both sender and receiver must have an advance agreement on what alphabet offsets, if any, are in use, how those may change over time, and changing cleartext languages, e.g. from English to Spanish or French to German, on alternate words, will further complicate efforts at decryption whether manual or machine methods are in use.
References[edit]
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