Re: [TLS] DTLS Key Separation PR

["Christopher Wood" <[e-mail]>]

This PR has been merged. Thanks, all!

Best,
Chris

On Thu, Oct 10, 2019, at 4:01 PM, Martin Thomson wrote:
> Thanks, that was my mistake.  I confirmed with our code and we are 
> indeed right up to the line.
> 
> On Thu, Oct 10, 2019, at 23:37, Eric Rescorla wrote:
> > 
> > 
> > On Wed, Oct 9, 2019 at 7:01 PM Martin Thomson <[e-mail]> wrote:
> > > tl;dr keep the space.
> > > 
> > >  I had a little trouble reproducing the 12 from RFC 8446, so I double-checked.
> > > 
> > >  ....
> > > 
> > >  Working from the base for SHA-256:
> > > 
> > >  The last block of SHA-256 is rounded up to 448 bits (56 bytes), less one to allow for padding. Therefore we have 55 bytes to use without having to run two blocks through SHA-256.
> > > 
> > >  HMAC-Hash = H(K XOR opad || H(K XOR ipad || text))
> > > 
> > >  Here `K XOR ipad` is the 32 bytes output size of SHA-256, so we are down to 23 bytes for text before it adds a block.
> > 
> > In HMAC the key consumes an entire input block, so that you are 
> > guaranteed a compression function cycle before the data is added:
> > 
> > 1) append zeros to the end of K to create a B byte string
> >         (e.g., if K is of length 20 bytes and B=64, then K will be
> >          appended with 44 zero bytes 0x00)
> >     (2) XOR (bitwise exclusive-OR) the B byte string computed in step
> >         (1) with ipad
> > 
> > https://tools.ietf.org/rfcmarkup?doc=2104#section-2This shifts 
> > everything to the right 32 bytes, which, because context is 32 bytes, 
> > gets us back to 12, I think, though I haven't re-worked through it. 
> > 
> > Here's Ilari's original analysis, if that's helpful:
> > https://mailarchive.ietf.org/arch/msg/tls/n_TFape7L4HHoKLxGo8CkimZziI
> > 
> > -Ekr
> > 
> > 
> > 
> > 
> > >  HKDF-Expand = HMAC-Hash(PRK, info | 0x01)
> > > 
> > >  This takes one more. Down to 22.
> > > 
> > >  HKDF-Expand-Label passes info in the form of:
> > > 
> > >  struct {
> > >  uint16 length = Length;
> > >  opaque label<7..255> = "tls13 " + Label;
> > >  opaque context<0..255> = Context;
> > >  } HkdfLabel;
> > > 
> > >  which has a minimal overhead of 2 + 1 + len("tls13 ") + 1 = 10. So we get 12.
> > > 
> > >  "c ap traffic" is 12 bytes long, so yeah it *looks* like we're stuck if we care about not adding too many extra hash iterations.
> > > 
> > >  ....
> > > 
> > >  But if you look at the key schedule, we always provide a context for those cases we use "c ap traffic". Those will always spill over into the next iteration as Context is 32 bytes. So for those cases, we have a whole 32 bytes extra to play with. The only cases with an empty Context are "derived" and "res binder"|"ext binder". Those max out at 10, so we seem to have two whole bytes of wiggle room.
> > > 
> > >  You can safely add the space.
> > > 
> > >  On Wed, Oct 2, 2019, at 08:40, Eric Rescorla wrote:
> > >  > Hi folks,
> > >  > 
> > >  > As discussed in Montreal, I've prepared a PR to give us DTLS/TLS key separation.
> > >  > 
> > >  > See: 
> > >  > https://github.com/tlswg/dtls13-spec/pull/99
> > >  > 
> > >  > Sadly. we didn't have enough space for "dtls13 " so I went for "dtls13"
> > >  > 
> > >  > -Ekr
> > >  > 
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