Why the auxiliary cipher in gss_krb5_crypto.c?

[Bruce Fields]

On Fri, Dec 04, 2020 at 10:46:26AM -0500, Bruce Fields wrote:
> On Fri, Dec 04, 2020 at 02:59:35PM +0000, David Howells wrote:
> > Hi Chuck, Bruce,
> > 
> > Why is gss_krb5_crypto.c using an auxiliary cipher?  For reference, the
> > gss_krb5_aes_encrypt() code looks like the attached.
> > 
> > >From what I can tell, in AES mode, the difference between the main cipher and
> > the auxiliary cipher is that the latter is "cbc(aes)" whereas the former is
> > "cts(cbc(aes))" - but they have the same key.
> > 
> > Reading up on CTS, I'm guessing the reason it's like this is that CTS is the
> > same as the non-CTS, except for the last two blocks, but the non-CTS one is
> > more efficient.
> 
> CTS is cipher-text stealing, isn't it?  I think it was Kevin Coffman
> that did that, and I don't remember the history.  I thought it was
> required by some spec or peer implementation (maybe Windows?) but I
> really don't remember.  It may predate git.  I'll dig around and see
> what I can find.

Like I say, I've got no insight here, I'm just grepping through
mailboxes and stuff, but maybe some of this history's useful;

Addition of CTS mode:

	https://lore.kernel.org/linux-crypto/20080220202543.3209.47410.stgit@jazz.citi.umich.edu/

This rpc/krb5 code went in with 934a95aa1c9c "gss_krb5: add remaining
pieces to enable AES encryption support"; may be worth looking at that
and the series leading up to it, I see the changelogs have some RFC
references that might explain why it's using the crypto it is.

--b.