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Don't think about xor over an alphabet. Only think of it over a binary encoding of an alphabet. I see what you are saying, and what you are hinting at is why Vigenere Cipher is a bad idea in general (one time pad of length $\ell$ that you use over a message of length $k$ for $\ell < k$. However, there is no "meaningful" way of implementing xor with an ...

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From "Claude Elwood Shannon - Collected Papers" edited by N. J. A. Sloane and Aaron D. Wyner, I understand that Claude Shannon proved that any encryption algorithm possessing these characteristics is absolutely secure: The encryption keys must be random numbers of uniform distribution. The keys must be shared in absolute secrecy by the sender and receiver. ...

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The "XOR cipher" described does not encrypt more than the first block, even if you do not reuse keys. The subsequent blocks can be "decrypted" by the attacker simply by undoing the XOR – there is no secret involved. Decrypting the first block and finding the key does require more than one message. It is a case of the many-time pad and can be solved either ...

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