Ok so let's say I have some top secret data ~ 50-100 MB and I need it to be protected. The cipher operation mode and the KDF is irrelevant for this example. Let's just assume we use
The only possibility is either single AES-256 encryption, so 14 rounds with a 256 bit key size or dual AES-128 x2 (cascade) with 20 rounds and an effective key size of 256 bit.
The AES-256 would be 140% slower, while the AES-128 cascade would be 200% slower when measured against AES-128.
So which of the 2 should I take and why?
Is the effective key size of 2 x 128 bit really as strong (or even stronger) than the 256 bits of AES-256?