Vigenère Cipher And Breaking it

It was originally thought to be unbreakable. You use a key such as PASSWORD.  Each Letter in your message is encoded by placing

PASSWORDPASSWORDPASSWORD

THISISATESTOFTHECODE

Each letter is encoded by using choosing the corresponding row from the key letter above your message letter.  The obvious failing is that after a bit you run out of key and start repeating.  This makes the code subject to frequency analysis.  Of course if your key word is a phrase of some length you might be able to avoid that.

It is interesting that only about 150 years ago this would be considered unbreakable.

Research Links

• Vigenère cipher
• Why is Vigenere so strong?
• Vigenere cracking tool   – interesting page that lets you see the role letter frequency plays.  If you read this far you should go play with this.