crypto: Add 'krb5enc' hash and cipher AEAD algorithm

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>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: "David S. Miller" <davem@davemloft.net>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-crypto@vger.kernel.org
cc: netdev@vger.kernel.org

crypto/Kconfig

@@ -228,6 +228,18 @@ config CRYPTO_AUTHENC
 
 	  This is required for IPSec ESP (XFRM_ESP).
 
+config CRYPTO_KRB5ENC
+	tristate "Kerberos 5 combined hash+cipher support"
+	select CRYPTO_AEAD
+	select CRYPTO_SKCIPHER
+	select CRYPTO_MANAGER
+	select CRYPTO_HASH
+	select CRYPTO_NULL
+	help
+	  Combined hash and cipher support for Kerberos 5 RFC3961 simplified
+	  profile.  This is required for Kerberos 5-style encryption, used by
+	  sunrpc/NFS and rxrpc/AFS.
+
 config CRYPTO_TEST
 	tristate "Testing module"
 	depends on m || EXPERT