[TLS] Re: Gunter Van de Velde's No Objection on draft-ietf-tls-svcb-ech-07: (with COMMENT)

[Ben Schwartz <[e-mail]>]

________________________________
From: Gunter Van de Velde via Datatracker <[e-mail]>


> 107     ech="AEj+DQBEAQAgACAdd+scUi0IYFsXnUIU7ko2Nd9+F8M26pAGZVpz/KrWPgAEAAEAAWQ
> 108     VZWNoLXNpdGVzLmV4YW1wbGUubmV0AAA="

> GV> I used some tooling to decode this Base64 blob. It seems to decode into hex
> as follows:
>
> 0x0048fe0d004401002000201d77eb1c522d08605b179d4214ee4a3635df7e17c336ea9006655a73fcaad63e00040001000164156563682d73697465732e6578616d706c652e6e65740000
>
> and that decodes as:
>
> Bytes | Meaning
> 0048fe0d | Length of the ECHConfigList (72 bytes total)
> 00440100 | Start of ECHConfig (first part is version 0x0044, meaning draft-13
version of ECH)
> 2000201d77eb1c522d08605b179d4214ee4a3635df7e17c336ea9006655a73fcaad63e0 | Key
> configuration and public keys for encryption (big blob) 0040 | Length of
> public_name 0001 | Cipher suite count (one suite) 0001 | Cipher suite ID
> (indicates a specific cipher, e.g., TLS_AES_128_GCM_SHA256) 64 | Length of
> public_name string (100 bytes) 6563682d73697465732e6578616d706c652e6e6574 |
> "ech-sites.example.net" (this is the public name, in ASCII) 0000 |
> Padding/termination
>
> Is the above is correct?

I'm not sure I understand your description.  I believe it is parsed as

ECHConfigList[0x0048] = [EchConfig{
  version = 0xfe0d
  length = 0x0044
  contents = EchConfigContents{
    key_config = HpkeKeyConfig{
      config_id = 0x01
      kem_id = 0x0020 # DHKEM(X25519, HKDF-SHA256)
      public_key[0x0020] = 0x1d77eb1c522d08605b179d4214ee4a3635df7e17c336ea9006655a73fcaad63e
      cipher_suites[0x0004] = [HpkeSymmetricCipherSuite{
        kdf_id = 0x0001
        aead_id = 0x0001
      }]
    }
    maximum_name_length = 0x64 # 100
    public_name[0x15] = "ech-sites.example.net"
    extensions[0x0000] = []
  }
}]

> maybe it is worthwhile to add this decoding example from informational
> perspective within the draft if this decoding activity adds value?

I don't think it would be sensible in this draft.  For the purposes of draft-ietf-tls-svcb-ech, the ECHConfigList is an opaque byte sequence, and all that matters is how it is conveyed in DNS records and zone files.

Test vectors or examples for EchConfig parsing might be appropriate in draft-ietf-tls-esni.

--Ben