netdev - Re: [PATCH net-next] net/tls: Add support of AES128-CCM based ciphers

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Date:   Tue, 19 Mar 2019 13:50:50 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     vakul.garg@....com
Cc:     netdev@...r.kernel.org, borisp@...lanox.com, aviadye@...lanox.com,
        davejwatson@...com, doronrk@...com
Subject: Re: [PATCH net-next] net/tls: Add support of AES128-CCM based
 ciphers

From: Vakul Garg <vakul.garg@....com>
Date: Tue, 19 Mar 2019 05:15:20 +0000

> @@ -479,11 +477,18 @@ static int tls_do_encryption(struct sock *sk,
>  	struct tls_rec *rec = ctx->open_rec;
>  	struct sk_msg *msg_en = &rec->msg_encrypted;
>  	struct scatterlist *sge = sk_msg_elem(msg_en, start);
> -	int rc;
> +	int rc, iv_offset = 0;
> +
> +	/* For CCM based ciphers, first byte of nonce+iv is always '2' */
> +	if (prot->cipher_type == TLS_CIPHER_AES_CCM_128) {
> +		rec->iv_data[0] = 2;
> +		iv_offset = 1;
> +	}
 ...
>  	iv = aad + prot->aad_size;
>  
> +	/* For CCM based ciphers, first byte of nonce+iv is always '2' */
> +	if (prot->cipher_type == TLS_CIPHER_AES_CCM_128) {
> +		iv[0] = 2;
> +		iv_offset = 1;
> +	}

Your explanation to Stephen about this '2' is great but belongs in the code
somehow so that the next person reading this will understand too.

Why not make a CPP macro for this value of '2', and explain it in a big
comment about the macro's definition?