I can't make the jump from the existance of unbreakable encryption to doom of SETI.

I think that as intellegences advance, they are probably less likely to use encryption.

I don't believe that our current search relies at all on decoding the message. Only detecting sequences of ones and zeroes that look like they might carry information.

To the extent that the message may be indistinguishable from random noise it would indeed be impossible to detect. Any conjecture that ETIs interchange of info is impossible to detect is in the realm of opinion at this point.

We only know how TIs communicate, and we know we are quite detectable. So much so, that our own cummunications (with satellites, etc.) keep getting detected as potential ETI messages.