Twin-Hex encoding is much harder to crack than most simple cyphers, as it operates on letter pairs, rather than individual characters - hence the 'twin' part of the name. For a simple substitution cypher, any letter will always be represented as the same symbol in the output. This means that if the original text contains a double letter, such as 'ee' then the output will also contain a double symbol.
However English has very few such double letters, so if the output contains such a pair, the codebreaker immediately knows that it is likely to represent 'ee', 'oo', etc.
so, in this cypher, the original text gets converted two characters at a time, meaning that the output for 'AB' is quite different to the output for 'BA', and the second time that 'AB' appears in the original text, there is a fifty-fifty chance that the A will be part of the previous pair, rather than be paired with the B again.
As a result, while most cyphers have only 26 combinations to work out, this one has literally thousands, making it very much harder to crack, particularly for very short texts. Because there are so many combinations, rather than using decimal numbers as output, this encryption algorithm uses Base36, which is like Hexadecimal, but uses all 26 letters of the alphabet in addition to the digits 0 to 9
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