Re: [TLS] Working Group Last Call for draft-ietf-tls-tls13-18

[John Mattsson <[e-mail]>]

(This is the same comments as yesterday, just resending them as my mail
client turned my text into an unreadable mess, hopefully this is better).


Hi,

Very well written draft and an excellent protocol. The things I have found
is mostly editorial. I think it’s ready. I will try to make sure that 3GPP
already next year mandates support of both TLS 1.3 and DTLS 1.3 in all the
places they use (D)TLS.

Cheers,
John


- I cannot find any text describing the requirements TLS 1.3 put on the
  lower layers. In fact the only text I can find is "which might be hidden
  by TCP". I don't know if the draft should mandate TCP or not, but
  otherwise it should be stated that reliable, in-order transport is
  required, and that the lower layers must allow identification TLS
  messages in the same session (e.g. the 5-tuple when TCP is used).

- I cannot find any text describing that TLS 1.3 outputs a stream identical
  to the inputstream. The current text only states that the input on the
  sender side is application messages. Should state that the TLS is
  reliable and in-order.

- The are no requirements on the TLS API. I think the following should be
  added:
  - SHALL support an exporter API
  - SHALL support an API allowing application to get information regarding
    the negotiated connection.
  - SHOULD support an API allowing application to influence the connection
    to be negotiated.

- TLS 1.3 also updates RFC5246, but I don't know if you can both update and
  obsolete. If you have to choose, obsolete is better.

- Section 1: This is the only place channel is used in the meaning
  connection. I suggest "channel" -> "connection" as connection is used in
  the rest of the document.

- Section 1: "The TLS standard, however, does not specify ... how to
  interpret the authentication certificates"

  I don't this is really true any more, the draft has significant text on
  certificates. I suggest removing "how to interpret the authentication
  certificates exchanged"

- Figure 1, 4: "pre_shared_key_modes" -> "psk_key_exchange_modes"

- Figure 1. "Server Params" not horizontally aligned with "Key Exch" and
  "Auth" (not sure if this is a feature or a bug).

- Section 2: 
  "CertificateVerify:  a signature over the entire handshake "
  "Finished :  a MAC over the entire handshake "

  Not really true. Maybe add something like "to this point"

- Section 2: The text below Figure 1 should mention psk_key_exchange_modes.
  It talks about all other messages and extensions.

- Section 2.2: OLD "PSKs can be used with (EC)DHE exchange"
               NEW "PSKs SHOULD be used with (EC)DHE exchange"

- Section 2.3: 
  "When PSKs are provisioned out of band, the PSK identity and the KDF to
  be used with the PSK MUST also be provisioned."

  OLD "and the KDF to be used"
  NEW "and the Hash algorithm to be used"
  
  I think this is a little problematic as current PSK provisioning
  mechanisms do not provision a hash algorithm. This means that they need
  to be updated before TLS 1.3 can be used. Otherwise the risk is that one
  endpoint uses SHA-256 and the other SHA-384. To enable PSK systems to
  directly and easily upgrade to TLS 1.3 I suggest the following

  OLD
  "When PSKs are provisioned out of band, the PSK identity and the KDF to
  be used with the PSK MUST also be provisioned."

  NEW
  "When PSKs are provisioned out of band, the PSK identity MUST also be
  provisioned and the Hash algorithm to be used SHOULD be provisioned. If
  no hash algorithm has been provisioned, then SHA-256 SHALL be used."
  
  This would also affect 4.2.6.

- Section 2.3: "the following additional information MUST be provisioned to
  both parties:

   -  The Application-Layer Protocol Negotiation (ALPN) protocol, if any
      is to be used

   -  The Server Name Indication (SNI), if any is to be used"

  I think these two bullets apply to ALL uses of TLS. Or? I suggest moving
  to a general section.

- Section 2.3: "1.  This data is not forward secret, as it is encrypted
  solely under keys derived using the offered PSK."

  This is true for all uses of PSK without (EC)DHE and not only Zero-RTT. I
  suggest moving this text to another more general section. Maybe in
  Section 2 when the three basic key exchange modes are discussed.

- Section 2.3: Zero-RTT with PSK obtained out-of-band is a special case of
  Zero-RTT, with its own requirements and warnings. I think it should be
  moved to its own subsubsection (2.3.1).

- Section 3.5: "enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;"
  The variable 'n' is used for two different purposes. Use n, m or
  something.

- Section 3.8: Same thing here. The variable 'n' is used for two different
  purposes.

- Section 4.0 The cases in Handshake definition in some random order.
  Should keep the same order as the HandshakeType definition.

- Section 4.1.2 "Including a "cookie" extension if one was provided in the
  HelloRetryRequest."
  Could state that this is the HelloRetryRequest cookie echoed back.

- Section 4.1.3 I don't think ServerHello.extensions can be 0 length, but
  maybe there is no well defined minimum length...

- Section 4.1.3
  Should state that Server.random shall be (psuedo-)random, I don't think
  this is stated (except that the name kind of indicates it)
  
  OLD: "random  This structure is generated by the server and MUST be
        generated independently of the ClientHello.random.
  NEW: "random  32 bytes generated by the server. The first 24 bytes are
       generated by a secure random number and MUST be generated
       independently of the ClientHello.random.

- Section 4.1.3
  OLD "as long as ephemeral ciphers are used"
  NEW "as long as ephemeral key exchange are used"

  As the ephemeral part is not part of the cipher suites anymore.

- Section 4.1.3 "It does not provide downgrade protection when static RSA
  is used."
  I don't know the details of ServerKeyExchange, but it this not true for
  non-DHE PSK as well?

- Section 4.2.4
  I do not really understand why the private_use divided and resticted?
  I.e. ffdhe_private_use and ecdhe_private_use. I think it would be
  preferable to just have "private_use" without specifying the exact use.
  Where do I e.g. put my private usE SIDH, and Lattice-based key exchange
  information. Feels like this is the natural place, or is the intention
  that a new extension is required for PQC?

- Section 4.2.8
  OLD "the server MUST have accepted a PSK cipher suite"
  NEW "the server MUST have accepted a PSK key exchange mode"

- Section 5.5 "up to 2^24.5 full-size records"

  It is not well-defined on how to do with non-full-size records. I suggest

  OLD "up to 2^24.5 full-size records"
  NEW “up to 2^24.5 records"

- Section 5.5
  Limits are needed also for AES-CCM. It should also be stated that new
  specifications defining new TLS ciphers SHALL provide lmits on the number
  of records that can be protected with a given key.

  As far as I understand the confidentiality limits for AES-CCM (and CTR
  and CBC) would be the same as for AES-GCM. I suggest
  
  OLD "For AES-GCM, up to 2^24.5 full-size records"
  NEW "For AES-GCM and AES-CCM, up to 2^24.5 records"
 
  These limits are quite hard, but I think that is fine as it should not be
  a problem to rekey frequently.
 
- Section 5.5 "for Authenticated Encryption (AE) security."
  I think it is better to state that this is confidentiality (in constrast
  to integrity or key recovery). Suggestion
  
  OLD "for Authenticated Encryption (AE) security."
  NEW "for an confidentiality attack"

- Section 8
  The sentences "In the absence of an application profile standard
  specifying otherwise" only apply to some parts of Section 8. E.g. it
  applies to cipher suites but not to the Diffie-Hellman groups. Feels like
  this should apply to the whole section. I suggest removing the current
  occurrences of ""In the absence of an application profile standard
  specifying otherwise" and add (at the start of Section 8.)

  "In the absence of an application profile standard specifying otherwise,
  the following requirements apply".

- Section 8.1 and 8.2
  The heading says "MTI" but the text also contains non-mandatory things
  (SHOULD)

  I suggest changing "MTI" to "Requirements"

- Section 8.1
  The title is "cipher suites", but the scope is broader (signatures and
  key exchange). I suggest changing the heading to
  
  "Requirements for cipher suites, signatures, and groups"

- Section 8.2 This section seems to forget psk_key_exchange_modes. If
  pre_shared_key is mandatory then psk_key_exchange_modes should be to.

- Section 9: Should mention that an attacker at any point can terminate the
  session by sending a single malformed packet. This termination feature is
  both positive and negative.

- Section 11.2: [RFC4279] is an informative reference but never referred
  to.

- Section A.4 "TLS 1.2 and lower cipher suites cannot be used with
  TLS 1.3." I cannot find any text in the draft statinn that you can
  negotiate TLS 1.2 cipher suites in TLS 1.3, I think this should be
  stated.
  
  OLD: "TLS 1.2 and lower cipher suites cannot be used with TLS 1.3."
  NEW: "TLS 1.2 and lower cipher suites cannot be used with TLS 1.3, but
       can be negotiated in a TLS 1.3 Client Hello."

- Section D.2
  OLD "weaker than 2048-bit RSA or 224-bit ECDSA are not appropriate"
  NEW "weaker than 2048-bit RSA, 224-bit ECDSA, or 128-bit PSK are not
      appropriate"

- Section D.2 "The reader should refer to the following references for
  analysis of the TLS record layer."
   
  No references given.

 
------------------------------------------------------------------
JOHN MATTSSON
MSc Engineering Physics, MSc Business Administration and Economics
Ericsson IETF Security Coordinator
Senior Researcher, Security