Why the auxiliary cipher in gss_krb5_crypto.c?

[David Howells]

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.

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;
	}