The bifid cipher is a cipher which combines the Polybius square with transposition, and uses fractionation to achieve diffusion. It was invented around 1901 by Felix Delastelle.
1 2 3 4 5
1|A B C D E
2|F G H I K
3|L M N O P
4|Q R S T U
5|V W X Y Z
|
To encode a message, you would construct a mixed alphabet Polybius Square, then
convert your message to its row and column coordinates according to this table, and write them vertically beneath the letters, like the example below shows. Then you read the numbers off; all of the rows first and then all of the columns. Using this string of numbers, you then look up the letters on the table again and get the encoded message.
Message: H E L P
Row: 2 1 3 3
Column: 3 5 1 5
The Numbers: 21 33 35 15
Encoded: F N P E
|
All non-letters are ignored and not encoded. The one skipped letter will
be automatically translated if you type it in the box. Numbers, spaces, and
punctuation will remain in place and will not be encoded.
To decrypt, the procedure is simply reversed.
Longer messages are first broken up into blocks of fixed length, called the period. Each block is then encrypted separately. Odd periods are slightly more secure than even periods.
