linux-kernel - Re: [PATCH] crypto: x86/aesni - Update aesni_set

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Date: Tue, 12 Mar 2024 08:37:04 -0700
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: Ard Biesheuvel <ardb@...nel.org>
CC: <linux-kernel@...r.kernel.org>, <linux-crypto@...r.kernel.org>,
	<herbert@...dor.apana.org.au>, <davem@...emloft.net>, <ebiggers@...nel.org>,
	<x86@...nel.org>
Subject: Re: [PATCH] crypto: x86/aesni - Update aesni_set_key() to return void

On 3/12/2024 8:18 AM, Ard Biesheuvel wrote:
> 
> I wonder whether we need aesni_set_key() at all.

The following looks to be relevant from the AES-NI whitepaper [1]:

The Relative Cost of the Key Expansion
     ...
     Some less frequent applications require frequent key scheduling. For
     example, some random number generators may rekey frequently to
     achieve forward secrecy. One extreme example is a Davies-Meyer
     hashing construction, which uses a block cipher primitive as a
     compression function, and the cipher is re-keyed for each processed
     data block.

     Although these are not the mainstream usage models of the AES
     instructions, we point out that the AESKEYGENASSIST and AESIMC
     instructions facilitate Key Expansion procedure which is lookup
     tables free, and faster than software only key expansion. In
     addition, we point out that unrolling of the key expansion code,
     which is provided in the previous sections, improves the key
     expansion performance. The AES256 case can also utilize the
     instruction AESENCLAST, for the sbox transformation, that is faster
     than using AESKEYGENASSIST.

[1] 
https://www.intel.com/content/dam/doc/white-paper/advanced-encryption-standard-new-instructions-set-paper.pdf

Thanks,
Chang