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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?

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Welcome to CSE, but sorry: that question is too trivial for our standards; and also is a mild form of this one and that one. Some food for thought: Does your definition of the OTP define the ciphertext for the plaintext beyond the key size? If not, is not that reason enough? Otherwise, how is that ciphertext defined? And assume you know the beginning of the plaintext, over the length of the key. – fgrieu Feb 5 '14 at 12:23
Then you will be sending one or 100 unencrypted bits. – Vitor Jul 14 at 17:41

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.

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