YHBW.

[maradydd]

Observation just now from Radu Sion during the FC rump session: in the cloud, it costs about $5 million to brute-force 64 bits of symmetric key.

Comments

[docstrange.livejournal.com]

Given a crypto algo without known weaknesses, it should double per extra bit, no?

[docstrange.livejournal.com]

(And given we're talking brute force, the weakness isn't relevant to the measurement...)

[jrtom.livejournal.com]

That's what I'd expect, yes. But what I expect is not always what is true, so it's good to have data that confirm (or deny) my understanding. Also, I'm not a cryptanalyst, so I don't know whether there are any nuances to the "double per extra bit" rule of thumb.

[vatine]

I think the right answer is "roughly". It depends on how much the extra key space influences the actual encryption. A typical example would be 3DES, with triple the number of key bits, for squaring the amount of effort to brute-force (as to exactly why that is, ask a cryptographer, I can sorta see it but not explain it).