[TLS] certificate type negotiaiton in draft-ietf-tls-oob-pubkey-06

[Nikos Mavrogiannopoulos <[e-mail]>]

Hello,
 There is a bug in the draft. The CertTypeExtension structure is defined as:
   struct {
      select(ClientOrServerExtension)
          case client:
            CertificateType certificate_types<1..2^8-1>;
          case server:
            CertificateType certificate_type;
      }
   } CertTypeExtension;

but the server is expected to send more than one certificate types (as
seen in examples). This cannot happen with the current structure.

Also the whole certificate type negotiation seems complicated. Currently
the certificate types are defined as:

   enum { X.509-Accept (0),
          X.509-Offer (1),
          RawPublicKey-Accept (2),
          RawPublicKey-Offer (3),
          (255)
         } CertificateType;

So a client that accepts X.509 certificates and has a raw key will send
(from the example in the draft)

certificate_type=(X.509-Accept(0), RawPublicKey-Offer(3)) ->

                            <-  server_hello,
                                certificate_type=(X.509-Offer(1),
                                     RawPublicKey-Accept(2)),

Which can be confusing. Why sbd suggest 0,3 and the server reply with 1,2?

I think it would be much simpler to have the certificate type defined as
   enum { X.509 (0),
          RawPublicKey (1),
          (255)
         } CertificateType;

and then modify the extension as:
   struct {
      select(ClientOrServerExtension)
          case client:
            CertificateType client_certificate_types<1..2^8-1>;
            CertificateType server_certificate_types<1..2^8-1>;
          case server:
            CertificateType client_certificate_type;
            CertificateType server_certificate_type;
      }
   } CertTypeExtension;

Which will result in a simpler exchange, and there is no need to know
which side is it in order to interpret the contents of the messages.

client_certificate_type=(raw(1))
server_certificate_type=(x509(0))

                            <-  server_hello,
                                client_certificate_type=(raw(1))
                                server_certificate_type=(x.509(0))

regards,
Nikos