Re: [TLS] Is stateless HelloRetryRequest worthwhile? (was Re: TLS 1.3 Problem?)

[Michael D'Errico <[e-mail]>]

> > You can't possibly implement [stateless HelloRetryRequest] the
> > way the spec suggests with just a hash in a HRR cookie extension. 
> 
> The only thing the server needs to know is the hash of the ClientHello 
> (so it can restore the transcript hash) and that the server has already 
> sent a HelloRetryRequest (which it can detect by presence of the 
> cookie). The only argument I've seen made for what the spec suggests 
> not working is being able to verify which fields changed between 
> ClientHello1 and ClientHello2.

The server also needs to know the entire HelloRetryRequest message
since this goes into the Transcript Hash calculation:

  Transcript-Hash(ClientHello1, HelloRetryRequest, ... Mn) =
      Hash(message_hash ||        /* Handshake type */
           00 00 Hash.length  ||  /* Handshake message length (bytes) */
           Hash(ClientHello1) ||  /* Hash of ClientHello1 */
           HelloRetryRequest  || ... || Mn)

Many of the fields in HelloRetryRequest are fixed or predictable, but
the legacy_session_id_echo is not, for example.  Also, relying on the
client to remind you what the hash of ClientHello1 is seems extremely
"optimistic" (in my opinion).

> I see no language in RFC 8446 that the server MUST enforce that
> the ClientHello2 is conformant with respect to  ClientHello1.

It doesn't, and you could probably argue that the HelloRetryRequest
can be just a "do over" and let the client try again however it wants.

But if the server doesn't do 100% validation of the second ClientHello
as if it never received the first one, then there is room for mischief by
a "curious" client.

Mike