It must be possible for a computer to simply not have an adequate supply of randomness though, no? Sure, urandom will use hardware sources of randomness if they're available - but what if they're not?
No, this is a misconception. Once the LRNG is seeded, it can generate a more or less unbounded amount of high-quality random bytes from that seed. Entropy can't be "depleted".
Then how can a 4096-byte key be meaningfully safer than a 1024-byte key? Couldn't you just use the 1024-byte key to seed a RNG, generate a 4096-byte key from that, and have 4096-byte safety?
(Also what if the linux RNG isn't seeded e.g. in a VM scenario? Or is the answer just "don't do that"?)
