Your backdoored DMA capable NIC can also just copy the plaintext out of RAM without futzing around with the key.

This isn't just about surreptitiously installing a compromised PCI card into a desktop PC. It affects every machine with a FireWire, Thunderbolt, or PCMCIA/ExpressCard port, i.e. most stolen corporate laptops.

https://en.wikipedia.org/wiki/DMA_attack

VT-d addresses this. Standard on most Intel Core processors for a couple years.

I've heard that it could address it in theory but hasn't actually been used for that yet. Do you have a source saying it does this by default, or failing that, how to set it up to do that?

IOMMU with VT-d is present, but not utilized by some major commercial operating systems and hypervisors.

There are also some implementation vulnerabilities in specific OSes which are not yet publicly disclosed.

Copy the plaintext of what? The password? The encryption programs usually don't keep the password in the RAM longer than required, but the key must be always there.

I just wanted to say that keeping the decryption keys out of RAM and Disks is not that paranoid because there are techniques which allow extraction of data from the RAM: cold boot, ordinary malware/rootkits, DMA malware.

The plaintext of whatever you're encrypting. Presumably you're going to actually use these debug register AES keys to encrypt or decrypt something more interesting.

The sneaky thing is to do like TreVisor, and I believe the commercial company PrivateCore, to encrypt all memory outside the CPU die (L1/L2, maybe L3?), by pinning the hypervisor and encryption routines to something running inside, and encrypting everything which leaves (and presumably doing some integrity protection). HN user sweis works for PrivateCore; I've talked to them a few times and they seem really interesting, although I think a more conventional HSM makes more sense for some applications, and Intel SGX is going to make the whole thing a lot more interesting in 2-3 years.

Here's a video of a malicious PCIe device extracting both plaintext and encrypted main memory: http://www.youtube.com/watch?v=chvJpEmXvDk