Re: XTS template wrapping question

[Eric Biggers <ebiggers@kernel.org>]

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