Vigenère Cipher Encoder/Decoder

How it is calculated

The Vigenère cipher shifts each letter of the text by an amount taken from a repeating keyword. Each keyword letter is turned into a shift from 0 to 25 (A = 0, B = 1, … Z = 25). For a plaintext letter P and key shift K, encoding gives C = (P + K) mod 26, and decoding gives P = (C − K) mod 26. The key is used letter by letter and starts over when it ends. It only moves forward on letters: spaces, digits and punctuation are copied unchanged and do not use up a key letter. Upper and lower case are kept as typed. Only A to Z count, so accented or non-English letters in the key are ignored and in the text are left as they are. A key with no usable letters returns nothing.

Common mistakes

• Decoding with the encode setting (or the other way round). The same key then shifts letters further instead of undoing the shift.
• Thinking spaces use up key letters. They do not, so a puzzle solved by counting positions that include spaces will not match.
• Putting digits or punctuation in the key and expecting them to count; they are ignored, so "KEY2" acts like "KEY".
• Expecting accented letters such as é or ğ to be enciphered. Only the 26 basic letters are shifted; others pass through unchanged.