On Fri, Aug 09, 2019 at 11:39:12AM +0000, Pascal Van Leeuwen wrote: > Herbert, Eric, > > While working on the XTS template, I noticed that it is being used > (e.g. from testmgr, but also when explictly exported from other drivers) > as e.g. "xts(aes)", with the generic driver actually being > "xts(ecb(aes-generic))". > > While what I would expect would be "xts(ecb(aes))", the reason being > that plain "aes" is defined as a single block cipher while the XTS > template actually efficiently wraps an skcipher (like ecb(aes)). > The generic driver reference actually proves this point. > > The problem with XTS being used without the ecb template in between, > is that hardware accelerators will typically advertise an ecb(aes) > skcipher and the current approach makes it impossible to leverage > that for XTS (while the XTS template *could* actually do that > efficiently, from what I understand from the code ...). > Advertising a single block "aes" cipher from a hardware accelerator > unfortunately defeats the purpose of acceleration. > > I also wonder what happens if aes-generic is the only AES > implementation available? How would the crypto API know it needs to > do "xts(aes)" as "xts(ecb(aes))" without some explicit export? > (And I don't see how xts(aes) would work directly, considering > that only seems to handle single cipher blocks? Or ... will > the crypto API actually wrap some multi-block skcipher thing > around the single block cipher instance automatically??) > "xts(aes)" is the cra_name for AES-XTS, while everything else (e.g. "xts(ecb(aes-generic))", "xts-aes-aesni", "xts(ecb-aes-aesni)") is a cra_driver_name for AES-XTS. "xts(ecb(aes))" doesn't make sense, as it's neither the cra_name nor does it name a specific implementation. See create() in crypto/xts.c. It allows the XTS template to be passed either the string "aes", *or* a string which names an *implementation* of "ecb(aes)", like "ecb(aes-generic)" or "ecb-aes-aesni". In the first case it allocates "ecb(aes)" so it gets the highest priority AES-ECB implementation. So in both cases the XTS template uses AES-ECB via the skcipher API. - Eric
