To install this package, run in Emacs:
M-x package-install RET vigenere RET
A vigenere cipher is similar to a series of shift ciphers. A key is repeated through the plain text, then a shift cipher is used for each letter depending on the letter in the key at that position. Example: The following uses the key EMACS (because what else would my example key be?). Plain text: This is some plain text First, we repeat the key through the plain text This is some plain text EMAC SE MACS EMACS EMAC Then each letter in the plain text is shifted based on the key character at that point: This is some plain text EMAC SE MACS EMACS EMAC Xtiu aw eoow txakf xqxv The specific shift amount for each character is the 0-based offset from A. Therefore, A is 0, B is 1, etc. The key in our case is case-insensitive. EMACS is Emacs is emacs is EmACs. One more note, is that this package only operates on letters, and the case of the original text is preserved. A vigenere cipher should not be considered cryptographically secure. This package is for recreational use only, not for securing sensitive information.