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For a random function you'd expect all outputs to be different if you generate fewer than $2^{n/2}$ blocks (birthday problem). Thus PRPs and PRFs are indistinguishable unless you observe about $2^{n/2}$, at which point you'd expect collisions using the PRF but not using the PRP. For a 128 bit cipher this is a lot of data, so we generally don't care about ...



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