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SubjectWhy the auxiliary cipher in gss_krb5_crypto.c?
Date
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;
}

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