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November 14th, 2012, 04:47 AM
 ydan87
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Crypto Algorithm Evaluation - Decrypting + proof insecureness for an encryption scheme

Hello there,
I ran into this question in my homework, wonder if you can help me:
Consider this encryption scheme:
For security parameter n, a key K is a
random sequence K = (i1, i2, ... , in) when each ij is a number from 1 to n (Equivalently, a key is a random permutation of (1, 2, ....., n).
The encryption algorithm EK(m) chooses a random R, a n-bit binary string, and outputs the ciphertext
c = (R, m XOR Ri1Ri2...Rin). That is, the key is used to permute the bits in R and the result is then XOR’ed with m.

- How to decrypt this?
- How to show that this is insecure, by choosing two messages whose encryptions can be easily distinguished?

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