Why does a one-time-pad key have to be at least as long as a message? I am studying one time pad and find it hard to understand. What happens if the key is one bit or 100 bits shorter than the message?
If you using in one time pad key shorter then message, it means that some part of text will be encrypted with same part of key, two or more time.
In this case, XORing two part encrypted with same key, adversary can receive few information about plaintext. If key significantly shorter then plaintext, adversary can apply frequency analysis to discover whole message, or part of it.