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

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Date:   Tue, 19 Mar 2019 17:35:23 +0000
From:   Vakul Garg <vakul.garg@....com>
To:     Boris Pismenny <borisp@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Aviad Yehezkel <aviadye@...lanox.com>,
        "davejwatson@...com" <davejwatson@...com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "doronrk@...com" <doronrk@...com>
Subject: RE: [PATCH net-next] net/tls: Add support of AES128-CCM based ciphers



> -----Original Message-----
> From: Boris Pismenny <borisp@...lanox.com>
> Sent: Tuesday, March 19, 2019 10:36 PM
> To: Vakul Garg <vakul.garg@....com>; netdev@...r.kernel.org
> Cc: Aviad Yehezkel <aviadye@...lanox.com>; davejwatson@...com;
> davem@...emloft.net; doronrk@...com
> Subject: Re: [PATCH net-next] net/tls: Add support of AES128-CCM based
> ciphers
> 
> 
> On 3/19/2019 7:15 AM, Vakul Garg wrote:
> > Added support for AES128-CCM based record encryption. AES128-CCM is
> > similar to AES128-GCM. Both of them have same salt/iv/mac size. The
> > notable difference between the two is that while invoking AES128-CCM
> > operation, the salt||nonce (which is passed as IV) has to be prefixed
> > with a hardcoded value '2'. Further, CCM implementation in kernel
> > requires IV passed in crypto_aead_request() to be full '16' bytes.
> > Therefore, the record structure 'struct tls_rec' has been modified to
> > reserve '16' bytes for IV. This works for both GCM and CCM based cipher.
> >
> 
> Can you explain what is the source of the hardcoded '2'? e.g. Why do we
> need a hardcoded constant?

The first bytes of IV is called B0 byte. It encodes width of 'length' field in CCM-IV.
(which defines length of payload that can be encrypted). In this case, width of 'length' field = 3 bytes.

IV[16 bytes] = B0 (1byte) || Fixed implicit Salt (4 bytes) || Explicit IV (8 bytes) || length field (3 bytes)

The 'length' field, is encoded as 'length field width - 1' in B0.
Hence B0 contains '2'.