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I have a homework problem:

Explain how to find $m_{0}$ and $c$ such that $P[c=c': k \leftarrow K, c' \leftarrow E(k, m_{0})] > 0$

where P is probability and k is chosen uniformly.

I do not know if I understand the question correctly. The notation should imply that what is the probability of the condition $c=c'$ given a program that evaluates c' by selecting $k$ uniformly from all keys and encrypts the message $m_{0}$ with this random key. My understanding is that to get a probability of encrypting to $c'$ that is greater than 0, it would be sufficient to guess a $m_{0}$ and $c$ that are of equal length.

Any hints?

share|improve this question
I suppose there is something missing here. (Instead of equal length you might need to consider some padding, but otherwise, as the question stands, your answer is around right.) – Paŭlo Ebermann Mar 6 '13 at 17:58

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