crypto: lib/aesgcm - Add kernel docs for aesgcm_mac

Add kernel documentation for the aesgcm_mac.
This function generates the authentication tag using the AES-GCM algorithm.

Signed-off-by: Sagar Vashnav <sagarvashnav72427@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto: lib/aesgcm - Provide minimal library implementation

Implement a minimal library version of AES-GCM based on the existing
library implementations of AES and multiplication in GF(2^128). Using
these primitives, GCM can be implemented in a straight-forward manner.

GCM has a couple of sharp edges, i.e., the amount of input data
processed with the same initialization vector (IV) should be capped to
protect the counter from 32-bit rollover (or carry), and the size of the
authentication tag should be fixed for a given key. [0]

The former concern is addressed trivially, given that the function call
API uses 32-bit signed types for the input lengths. It is still up to
the caller to avoid IV reuse in general, but this is not something we
can police at the implementation level.

As for the latter concern, let's make the authentication tag size part
of the key schedule, and only permit it to be configured as part of the
key expansion routine.

Note that table based AES implementations are susceptible to known
plaintext timing attacks on the encryption key. The AES library already
attempts to mitigate this to some extent, but given that the counter
mode encryption used by GCM operates exclusively on known plaintext by
construction (the IV and therefore the initial counter value are known
to an attacker), let's take some extra care to mitigate this, by calling
the AES library with interrupts disabled.

[0] https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication800-38d.pdf

Link: https://lore.kernel.org/all/c6fb9b25-a4b6-2e4a-2dd1-63adda055a49@amd.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>