quantum computing and symmetric algotithms

Robert J. Hansen rjh at sixdemonbag.org
Fri May 1 03:04:54 CEST 2026


> It would be useful in discussions on quantum computing and
> cryptography not to miss that vulnerability (if and to the extent it
> exists) only pertains to the asymmetric algorithms. As far as we
> know, no modern symmetric block  cipher is affected.

True, with 3DES as a possible exception. There's a meet in the middle 
attack that means with truly ludicrous amounts of hardware it could be 
reduced to complexity 2**112, which is ... not practical for Grover's 
algorithm for many reasons, but is definitely not as wildly impractical 
as I'd like.

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