Subject: Re: Why the auxiliary cipher in gss_krb5_crypto.c?

From: Bruce Fields <[email-masked]>
Date: Fri, 4 Dec 2020 10:46:26 -0500

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.

--b.

> 
> David
> ---
> 	nbytes = buf->len - offset - GSS_KRB5_TOK_HDR_LEN;
> 	nblocks = (nbytes + blocksize - 1) / blocksize;
> 	cbcbytes = 0;
> 	if (nblocks > 2)
> 		cbcbytes = (nblocks - 2) * blocksize;
> 
> 	memset(desc.iv, 0, sizeof(desc.iv));
> 
> 	if (cbcbytes) {
> 		SYNC_SKCIPHER_REQUEST_ON_STACK(req, aux_cipher);
> 
> 		desc.pos = offset + GSS_KRB5_TOK_HDR_LEN;
> 		desc.fragno = 0;
> 		desc.fraglen = 0;
> 		desc.pages = pages;
> 		desc.outbuf = buf;
> 		desc.req = req;
> 
> 		skcipher_request_set_sync_tfm(req, aux_cipher);
> 		skcipher_request_set_callback(req, 0, NULL, NULL);
> 
> 		sg_init_table(desc.infrags, 4);
> 		sg_init_table(desc.outfrags, 4);
> 
> 		err = xdr_process_buf(buf, offset + GSS_KRB5_TOK_HDR_LEN,
> 				      cbcbytes, encryptor, &desc);
> 		skcipher_request_zero(req);
> 		if (err)
> 			goto out_err;
> 	}
> 
> 	/* Make sure IV carries forward from any CBC results. */
> 	err = gss_krb5_cts_crypt(cipher, buf,
> 				 offset + GSS_KRB5_TOK_HDR_LEN + cbcbytes,
> 				 desc.iv, pages, 1);
> 	if (err) {
> 		err = GSS_S_FAILURE;
> 		goto out_err;
> 	}