> Less an issue of the computing power, more an issue of the knowledge.
I think you're mixing up two issues:
(a) Strengthening DES against differential cryptanalysis is a knowledge issue. The NSA knew about that attack, and maybe the Soviets knew too, but the public didn't.
(b) The key length for DES however is entirely a computing power issue; it is not a knowledge issue. It would be obvious to everyone even in 1977 that a 64-bit key as originally proposed would have been better than the 56 bits they ended up using[1]. Furthermore, everyone understood how to do a brute force attack, but it just wasn't feasible for the public. Only the NSA would have had the budget and inclination to develop special-purpose computing hardware to actually do it in the 1980's DES era.
[1]I have to mention one nitpick here, otherwise someone else will point it out. The 56-bit key length was supposedly ideal because DES would have been more easily cracked using differential cryptanalysis than brute force with any key longer than 56 bits. Thus the NSA can claim, "no we didn't weaken DES by shortening the key, we optimized it because 64 bits doesn't buy you extra security over 56 bits". But that argument is bogus. They could have improved DES in such a way that it used 64-bit keys and wasn't vulnerable to differential cryptanalysis. But they did the latter and not the former, because it keeps DES secure against the Soviets (assuming they knew about differential cryptanalysis) and against the public (who can't mount a brute force attack) but does allow the NSA to be able crack it.
I think you're mixing up two issues:
(a) Strengthening DES against differential cryptanalysis is a knowledge issue. The NSA knew about that attack, and maybe the Soviets knew too, but the public didn't.
(b) The key length for DES however is entirely a computing power issue; it is not a knowledge issue. It would be obvious to everyone even in 1977 that a 64-bit key as originally proposed would have been better than the 56 bits they ended up using[1]. Furthermore, everyone understood how to do a brute force attack, but it just wasn't feasible for the public. Only the NSA would have had the budget and inclination to develop special-purpose computing hardware to actually do it in the 1980's DES era.
[1]I have to mention one nitpick here, otherwise someone else will point it out. The 56-bit key length was supposedly ideal because DES would have been more easily cracked using differential cryptanalysis than brute force with any key longer than 56 bits. Thus the NSA can claim, "no we didn't weaken DES by shortening the key, we optimized it because 64 bits doesn't buy you extra security over 56 bits". But that argument is bogus. They could have improved DES in such a way that it used 64-bit keys and wasn't vulnerable to differential cryptanalysis. But they did the latter and not the former, because it keeps DES secure against the Soviets (assuming they knew about differential cryptanalysis) and against the public (who can't mount a brute force attack) but does allow the NSA to be able crack it.