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2

/dev/urandom is only computationally secure, so you won't get information-theoretical security for your OTP if you draw it from /dev/urandom. If you're happy with computational security, you might as well use a stream cipher instead of a OTP. Stream ciphers are much easier to use securely than OTPs. On Linux /dev/random aims for information-theoretical ...


2

Scheme is not IND-CPA for any message longer than one block. I'll include a image of CBC mode below for reference (Source: Wikipedia). Suppose instead of block cipher encryption we have plaintext xor-ed with the key as you propose. You'll note that for message block 1, $M_1$, the ciphertext block $C_1 = M_1 \oplus IV \oplus Key$. Similarly $C_2 = M_2 ...


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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. ...


3

This feels like homework, and so I won't give you the answer; I'll give you hints: If F(x) = x^e mod n, and (n, e) pair is public, will the first 1024 bits of both plaintext and ciphertext be enough for Eve to read the entire message? If Eve knows the first plaintext $M_0$, and the ciphertext $M_0 \oplus (r^e \bmod n)$, how can Eve recover $r^e \bmod n ...


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Re-encrypting with the same pad is insecure, unless what your encrypting is already encrypted with a non-used pad or other system.



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