Re: [TLS] Fwd: Re: AD review of draft-ietf-tls-dtls-connection-id-07

[Eric Rescorla <[e-mail]>]

On Mon, Oct 26, 2020 at 6:00 PM Benjamin Kaduk <[e-mail]> wrote:

> On Mon, Oct 26, 2020 at 05:38:33PM -0700, Eric Rescorla wrote:
> > On Fri, Oct 23, 2020 at 7:13 PM Benjamin Kaduk <[e-mail]> wrote:
> >
> > > Hi Ekr,
> > >
> > > Thanks for chiming in.
> > >
> > > On Thu, Oct 15, 2020 at 08:59:43AM -0700, Eric Rescorla wrote:
> > > >
> > > > - I agree with Ben that the current construction has some awkward
> > > > properties and that prefixing the length field would remedy that.
> It's
> > > > been quite some time since we had this discussion but as I recall the
> > > > rationale was to protect the bits on the wire as-is rather than some
> > > > reconstructed version of them (see a number of long discussions on
> > > > this topic), so just prefixing the CID length is also not ideal.
> > >
> > > I think the current scheme is unfortunately using fields put together
> in a
> > > rather different order than the actual bits on the wire (more below).
> > >
> >
> > Right. I forgot that we also preserved the TLS order of the seqnum.
> >
> >
> > > >
> > > > This is a little goofy but it has (I think) the properties that (1)
> the
> > > > bytes appear
> > > > in the MAC in the order they appear on the wire (2) fixed-length
> metadata
> > > > appears in
> > > > the front (the seq_num already does) (3) the duplicated tls12_cid in
> the
> > > > front avoids confusion with MAC input for other records.
> > >
> > > I like (1) and (2) and agree with (3), though I'm having a little
> trouble
> > > lining up your figure with the DTLSCiphertext structure, which I'll
> repeat
> > > here so I'm not flipping back and forth as much:
> > >
> > >         struct {
> > >             ContentType special_type = tls12_cid;
> > >             ProtocolVersion version;
> > >             uint16 epoch;
> > >             uint48 sequence_number;
> > >             opaque cid[cid_length];               // New field
> > >             uint16 length;
> > >             opaque enc_content[DTLSCiphertext.length];
> > >         } DTLSCiphertext;
> > >
> > > Ah, your proposal looks more natural if I compare it to the current
> AEAD
> > > "additional_data" from the -07:
> > >
> > > #      additional_data = seq_num +
> > > #                        tls12_cid +
> > > #                        DTLSCipherText.version +
> > > #                        cid +
> > > #                        cid_length +
> > > #                        length_of_DTLSInnerPlaintext;
> > >
> > > If I could try to synthesize your key points (as I understand them),
> hewing
> > > more strictly to the "bits on the wire" philosophy would suggest
> having us
> > > use:
> > >
> > > additional_data:
> > > struct {
> > >   uint8 marker = tls12_cid;
> > >   uint8 cid_len;
> > >   uint8 content_type = tls12_cid;      \
> > >   uint16 DTLSCiphertext.version;       |  appears on wire
> > >   uint64 seq_num; // includes epoch    |
> > >   opaque cid[cid_len];                 /
> > >   uint16 length_of_DTLSInnerPlaintext;
> > > };
> > >
> > >
> > Mostly.
> >
> > I would expect the length here to not be DTLSInnerPlaintext but rather
> the
> > length field that appears on the wire, as in TLS 1.3. Do you agree with
> > that? If not, let's discuss. If so, we can talk about the others.
>
> Good catch, the length on the wire makes more sense.  (I presume I just
> cribbed from the -07 here and didn't think hard enough.
>

So I think that the following is right for MtE.

struct {
  uint8 marker = tls12_cid;
  uint8 cid_len;
  uint8 content_type = tls12_cid;      \
  uint16 DTLSCiphertext.version;       |  appears on wire
  uint64 seq_num; // includes epoch    |
  opaque cid[cid_len];                 /
  uint16 length_of_DTLSInnerPlaintext;
  DTLSInnerPlaintext.content;          \
  DTLSInnerPlaintext.real_type;        |  entirety of DTLSInnerPlaintext
  DTLSInnerPlaintext.zeros;            /
};


As for EtM

Encrypt-then-MAC:
struct {
  uint8 marker = tls12_cid;
  uint8 cid_len;
  uint8 content_type = tls12_cid;      \
  uint16 DTLSCiphertext.version;       |  appears on wire
  uint64 seq_num; // includes epoch    |
  opaque cid[cid_len];                 /
  uint16 iv_length;
  opaque IV[iv_length];
  uint16 enc_content_length;
  opaque enc_content[enc_content_length];
};

This seems kind of unfortunate in that the 7366 version is much more
faithful to the wire, so I still have to think about this one some more, I
think...

-Ekr