# How long does it take a quantum computer to brute force AES?

I understand that using Grover's algorithm it only requires $2^{64}$ lookups for a 128 bit AES encryption, leading people to say we need to increase to 256 bit keys. But how long would it actually take a quantum computer to do $2^{64}$ lookups?

I can't say I fully understand quantum computers, but aren't people making the assumption that 128 bit is only unsafe to quantum computers because 64 bit (which 128 bit becomes using a quantum computer) can be broken through brute force on our current computers...? Even if a quantum computer only needs to do $2^{64}$ lookups, presumably it doesn't do them at the exact speed of current computers.

When people have asked in the past how long a quantum computer would take to break AES 128 bit, people always answer that it would take $2^{64}$ lookups (which some people take to mean the amount of time we currently take to break 64 bit), but there's never any indication of an actual time.

I understand that quantum computers are highly theoretical at this stage in terms of large scale implementation, but can anyone offer any ideas?

• Take a look at this
– rath
Commented Oct 13, 2013 at 4:48
• the biggest number factored so far by a quantum algorithm is something like 64. That's six four. Don't hold your breath. Commented Oct 15, 2013 at 0:41
• If it's a Microsoft Core-Q, it'll be 30 seconds...4379 years...53 minutes
– Leo
Commented Jul 1, 2016 at 8:41

However I could explain why people recommend 256-bit security in the face of quantum computing using some numbers. If you feel that $2^{128}$ is a comfortable security against bruteforcing, remember that a $2^{64}$ security level is $18446744073709551616$ times faster to bruteforce.