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

[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).