Why the auxiliary cipher in gss_krb5_crypto.c?

[Chuck Lever]

> On Dec 4, 2020, at 10:46 AM, Bruce Fields <bfields at fieldses.org> 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.

I can't add more here, this design comes from well before I started
working on this body of code (though, I worked near Kevin when he
implemented it).


--
Chuck Lever