VIC Cipher (Straddling Checkerboard) Encoder

How it is calculated

The tool performs only the first step of the VIC cipher, a straddling checkerboard substitution. Letters are turned into digits; there is no chain addition and no transposition, so it is an encoding demonstration, not secure encryption.

Board: the first 8 different letters of the key phrase get one-digit codes 0 to 7 in order. If the key phrase is empty it defaults to ATONESIR; if it has fewer than 8 different letters, the board is filled up from A to Z. The remaining alphabet letters follow in A to Z order and take two-digit codes: the first ten use the prefix 8 (80 to 89), the rest use the prefix 9 (90 and up). Only letters are encoded. Spaces, digits and punctuation are dropped, case is ignored, and a text with no letters returns nothing. The board and the number of digits produced are shown below the result.

Common mistakes

• Believing the result is a full VIC cipher. It is a plain letter-for-digits substitution, so do not use it to protect anything that matters.
• Expecting numbers to be encoded. Digits in the plaintext are ignored, so a text such as 1234 returns nothing and 5 in a sentence simply disappears.
• Decoding the output with a board built from a different key phrase. The board changes with the first eight different letters, so the receiver needs the same key phrase.
• Splitting the digit string wrongly when decoding: a digit 8 or 9 always starts a two-digit code, every other digit is a letter on its own.
• Typing a key phrase with repeated letters and expecting each to count. Repeats are skipped, so SWEDISH gives the top row SWEDIHAB, not SWEDISH.