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Re: [TLS] a slightly different DTLSShortCiphertext

Martin Thomson <martin.thomson@gmail.com> Sun, 04 March 2018 23:11 UTC

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From: Martin Thomson <martin.thomson@gmail.com>
Date: Mon, 05 Mar 2018 10:11:56 +1100
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To: "Fossati, Thomas (Nokia - GB/Cambridge)" <thomas.fossati@nokia.com>
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Subject: Re: [TLS] a slightly different DTLSShortCiphertext
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We are about to remove that bit from the QUIC packet.  I don't see any
advantage in adding it here.

Can you explain in more detail who you think consumes this bit?

On Sun, Mar 4, 2018 at 4:33 AM, Fossati, Thomas (Nokia - GB/Cambridge)
<thomas.fossati@nokia.com> wrote:
> Hi all,
>
> In an off-list discussion on the wire format for DTLS CID Eric raised
> the point that a DTLSShortCiphertext header is completely stuffed, and
> it'd be impossible to grab another bit from the sequence number (already
> down to 12 bits) to signal the presence of a CID.
>
> I made a proposal for a slightly different layout of DTLSShortCiphertext
> that makes room for the CID bit, which I'd like to bring to the list for
> further scrutiny:
>
>  0                   1                   2                   3
>  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> |0|0|1|E|C|X|X|X|            sequence           |               |
> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
> |                                                               |
> +                                                               +
> |                                                               |
> +                   [CID,] encrypted_record                     +
> |                                                               |
> +                                                               +
> |                                                               |
> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>  0                   1                   2                   3
>
> Where:
> - First 4 bits are the same as in current DTLSShortCiphertext (demux +
>   low order epoch bit)
> - Subsequent 4 bits are: C, the connection-id indicator followed by 3
>   reserved bits (to be greased, I suppose)
> - Then, a 16-bit sequence number.
>
> It'd still be 4 bytes shorter than usual DTLSCiphertext, so I guess it's
> OK to keep calling it "short".  There is the question about these 3
> reserved bits, which seem like good candidates for greasing, I guess.
>
> What do people think?
>
> Cheers, thanks
>
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