FPGA or stock embedded CPU. I didn't mean literally any Big-2 CPU, I was thinking of desktop CPU's when I wrote that. I doubt the 8051's that you can buy by the spindle from electronics suppliers are backdoored, though its technically possible.
8051s also seem pretty under-powered for a task like that.
One thing with encryption on a USB-connected security dongle is that your scp / rsync / git pull invocation is going to go through it, and through its crypto engine. To make it not painfully slow, the crypto engine has to be pretty fast. It's likely not very cheap, either using a (fast) general purpose CPU core, or using custom / specialized circuitry.
Paying extra for high security at high speed may make complete sense in some cases. For a cheap mass-market product, it's less likely.
> AES on any of the Big-2 CPU's is compromised
In that case, which CPU could the Yubikey use?