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Eric Rescorla's No Objection on draft-ietf-httpbis-replay-03: (with COMMENT)

From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 07 Jun 2018 00:49:54 -0700
To: "The IESG" <iesg@ietf.org>
Cc: httpbis-chairs@ietf.org, draft-ietf-httpbis-replay@ietf.org, mcmanus@ducksong.com, ietf-http-wg@w3.org, Patrick McManus <mcmanus@ducksong.com>
Message-ID: <152835779440.30863.14852363402377741220.idtracker@ietfa.amsl.com>
Eric Rescorla has entered the following ballot position for
draft-ietf-httpbis-replay-03: No Objection

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COMMENT:
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Rich version of this review at:
https://mozphab-ietf.devsvcdev.mozaws.net/D4635



COMMENTS
S 3.
>          basis.
>   
>      5.  The server can cause a client to retry a request and not use
>          early data by responding with the 425 (Too Early) status code
>          (Section 5.2), in cases where the risk of replay is judged too
>          great.

I think the point you want to bring out here is that this allows you
to bounce individual requests, as opposed to point #2. In fact, you
might want to say "at the TLS layer" in point #2.


S 3.
>          (Section 5.2), in cases where the risk of replay is judged too
>          great.
>   
>      For a given request, the level of tolerance to replay risk is
>      specific to the resource it operates upon (and therefore only known
>      to the origin server).  In general, if processing a request does not

This is partly true, but note that the client knows whether it would
replay it.


S 3.
>   
>      For a given request, the level of tolerance to replay risk is
>      specific to the resource it operates upon (and therefore only known
>      to the origin server).  In general, if processing a request does not
>      have state-changing side effects, the consequences of replay are not
>      significant.

This seems to contradict the claims in the TLS 1.3 security
considerations about side channels.


S 3.
>      A server can limit the amount of early data with the
>      "max_early_data_size" field of the "early_data" TLS extension.  This
>      can be used to avoid committing an arbitrary amount of memory for
>      deferred requests.  A server SHOULD ensure that when it accepts early
>      data, it can defer processing of requests until after the TLS
>      handshake completes.

I don't understand this last line. You don't have to defer, so why
should you ensure that?


S 4.
>      By their nature, clients have control over whether a given request is
>      sent in early data - thereby giving the client control over risk of
>      replay.  Absent other information, clients MAY send requests with
>      safe HTTP methods (see [RFC7231], Section 4.2.1) in early data when
>      it is available, and SHOULD NOT send unsafe methods (or methods whose
>      safety is not known) in early data.

Is that what browsers plan to do?


S 4.
>      If the server rejects early data at the TLS layer, a client MUST
>      start sending again as though the connection was new.  This could
>      entail using a different negotiated protocol [ALPN] than the one
>      optimistically used for the early data.  Any requests sent in early
>      data MUST be sent again, unless the client decides to abandon those
>      requests.

This MUST in connection with the "unless the client"... seems
confusing. Perhaps "will need to be..."



S 4.
>      were sent in early data, the request will be processed twice.
>   
>      Replays are also possible if there are multiple server instances that
>      will accept early data, or if the same server accepts early data
>      multiple times (though this would be in violation of requirements in
>      Section 8 of [TLS13]).

To be clear, only the second part of this is in violation


S 5.1.
>      An intermediary that forwards a request prior to the completion of
>      the TLS handshake with its client MUST send it with the "Early-Data"
>      header field set to "1" (i.e., it adds it if not present in the
>      request).  An intermediary MUST use the "Early-Data" header field if
>      it might have forwarded the request prior to handshake completion
>      (see Section 6.2 for details).

I don't actually see these details, so perhaps this can be rewritten
more clearly?


S 5.1.
>      A server cannot make a request that contains the Early-Data header
>      field safe for processing by waiting for the handshake to complete.
>      A request that is marked with Early-Data was sent in early data on a
>      previous hop.  Requests that contain the Early-Data field and cannot
>      be safely processed MUST be rejected using the 425 (Too Early) status
>      code.

I think in order for this to be true you need to prohibit clients
sending with Early-Data=1
Received on Thursday, 7 June 2018 07:50:18 UTC

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