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Pseudorandom permutations

hu flag
pepsilon

So I am trying to solve some exercises about pseudorandom permutations.

Assume that keyed-permuation $E_k(x)$ is a pseudorandom permutation, where $|x|=|k|=n$. Using $E_k(x)$, we construct an encryption sheme as follows.
$$ c=m\oplus E_k(0^n)\\ m=c\oplus E_k(0^n) $$ where $k$ is a random key.

The task is to show if this sheme either provides OT-IND-CPA or IND-CPA.

So if I understand pseudorandom permutations correctly this sheme should not provide any of those two. My argument would be that no mather what key $k$ one uses the output of $E_k(0^n)$ will always be $0^n$ since every permutation of $0^n$ is $0^n$ and therefore every message $m$ gets encrypted as its plaintext: $c=m$ which obiously does not provide IND-CPA or OT-IND-CPA.

Am I missing something or is it really that easy?

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r3mainer avatar
us flag
r3mainer
No, the permutation $E$ is a mapping from all values in $\{01\}^n$ to a different ordering of the same set of values. So the probability that $E_k(0^n)=0^n$ is negligible if $n$ is large.
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fgrieu avatar
ng flag
fgrieu
Hint: when $k$ is fixed and $m$ varies, what part(s) of the expression defining $c$ varies? What about (various variants of) the IND-CPA property under that condition?
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