Re: [RFC PATCH 03/24] crypto: Add 'krb5enc' hash and cipher AEAD algorithm

Simon Horman
2025-01-20T13:57:59Z

On Fri, Jan 17, 2025 at 06:35:12PM +0000, David Howells wrote:
> Add an AEAD template that does hash-then-cipher (unlike authenc that does
> cipher-then-hash). This is required for a number of Kerberos 5 encoding
> types.
>
> [!] Note that the net/sunrpc/auth_gss/ implementation gets a pair of
> ciphers, one non-CTS and one CTS, using the former to do all the aligned
> blocks and the latter to do the last two blocks if they aren't also
> aligned. It may be necessary to do this here too for performance reasons -
> but there are considerations both ways:
>
> (1) firstly, there is an optimised assembly version of cts(cbc(aes)) on
> x86_64 that should be used instead of having two ciphers;
>
> (2) secondly, none of the hardware offload drivers seem to offer CTS
> support (Intel QAT does not, for instance).
>
> However, I don't know if it's possible to query the crypto API to find out
> whether there's an optimised CTS algorithm available.
>
> Signed-off-by: David Howells <dhowells@redhat.com>

...

> diff --git a/crypto/krb5enc.c b/crypto/krb5enc.c

...

> +static int krb5enc_verify_hash(struct aead_request *req, void *hash)
> +{
> + struct crypto_aead *krb5enc = crypto_aead_reqtfm(req);
> + struct aead_instance *inst = aead_alg_instance(krb5enc);
> + struct krb5enc_instance_ctx *ictx = aead_instance_ctx(inst);
> + struct krb5enc_request_ctx *areq_ctx = aead_request_ctx(req);
> + struct ahash_request *ahreq = (void *)(areq_ctx->tail + ictx->reqoff);
> + unsigned int authsize = crypto_aead_authsize(krb5enc);
> + u8 *ihash = ahreq->result + authsize;
> +
> + scatterwalk_map_and_copy(ihash, req->src, ahreq->nbytes, authsize, 0);
> +
> + if (crypto_memneq(ihash, ahreq->result, authsize))
> + return -EBADMSG;
> + return 0;
> +}
> +
> +static void krb5enc_decrypt_hash_done(void *data, int err)
> +{
> + struct aead_request *req = data;
> +
> + if (err)
> + return krb5enc_request_complete(req, err);
> +
> + err = krb5enc_verify_hash(req, 0);

Hi David,

Sparse complains that the second argument to krb5enc_verify_hash should be
a pointer rather than an integer. So perhaps this would be slightly better
expressed as (completely untested!):

err = krb5enc_verify_hash(req, NULL);

> + krb5enc_request_complete(req, err);

...
> +}