What is the difference between a Vigenère cipher and a one time pad cipher? I'm doing a decryption competition at a nearby college.  We are allowed one cheat sheet so I'm trying to make one.  I realized that the Vigenère cipher was very similar to the one time pad and wanted to make sure that I know the difference. 

As I understand it, the only difference is that a one time pad has a key that is as long as the message. The Vigenère cipher can use a key that is shorter than the message, so you repeat the key till the end of the message.

Is above description correct?
The Vigenère cipher is essentially a series of interwoven Caesar ciphers, in which the size of the shift is different for each character and is defined by the keyword.

The one-time pad (OTP) is a very similar technique in which the only difference is that key is generally:


truly random,
at least as long as the message,
and never reused (thus the name),


making it provably confidential if the key is kept secret.

In this way, both ciphers make use of modular addition, which is still in use in most modern digital cryptography. But, in the case of the OTP, the length of the key guarantees that the ciphertext is not vulnerable due to repetition in the way that the Vigenère cipher is.