Re: [OAUTH-WG] Benjamin Kaduk's Discuss on draft-ietf-oauth-jwt-bcp-06: (with DISCUSS and COMMENT)

Yaron Sheffer <yaronf.ietf@gmail.com> Sun, 13 October 2019 16:50 UTC

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Date: Sun, 13 Oct 2019 19:50:50 +0300
From: Yaron Sheffer <yaronf.ietf@gmail.com>
To: Benjamin Kaduk <kaduk@mit.edu>, The IESG <iesg@ietf.org>
CC: <draft-ietf-oauth-jwt-bcp@ietf.org>, Hannes Tschofenig <hannes.tschofenig@arm.com>, <oauth-chairs@ietf.org>, <oauth@ietf.org>
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Thread-Topic: Benjamin Kaduk's Discuss on draft-ietf-oauth-jwt-bcp-06: (with DISCUSS and COMMENT)
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Subject: Re: [OAUTH-WG] Benjamin Kaduk's Discuss on draft-ietf-oauth-jwt-bcp-06: (with DISCUSS and COMMENT)
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Hi Ben,

Sorry the responding to you retroactively (and with such delay). As you can imagine, most of the changes in the latest version are related to your review. See below for detailed comments.

Thanks,
	Yaron

On 25/06/2019, 2:20, "Benjamin Kaduk via Datatracker" <noreply@ietf.org> wrote:

    Benjamin Kaduk has entered the following ballot position for
    draft-ietf-oauth-jwt-bcp-06: Discuss
    
    When responding, please keep the subject line intact and reply to all
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    Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
    for more information about IESG DISCUSS and COMMENT positions.
    
    
    The document, along with other ballot positions, can be found here:
    https://datatracker.ietf.org/doc/draft-ietf-oauth-jwt-bcp/
    
    
    
    ----------------------------------------------------------------------
    DISCUSS:
    ----------------------------------------------------------------------
    
    Thank you for assembling this document; it will be very valuable to the
    community.  I intend to ballot Yes once the following items are
    resolved:
    
    Section 2.6 notes:
       Previous versions of the JSON format such as the obsoleted [RFC7159]
       allowed several different character encodings: UTF-8, UTF-16 and UTF-
       32.  This is not the case anymore, with the latest standard [RFC8259]
       only allowing UTF-8.  [...]
    
    The actual situation is a bit more subtle than this text makes it seem;
    interoperable JSON can only use non-UTF-8 with explicit mutual
    prearrangement in a closed ecosystem.  So, while this statement is true
    for Internet JWT usage, it may not be true for *all* JWT usage.
    (I do see that in Section 3.7 of this document we do mandate UTF-8 for
    JWT, which makes things unambiguous, even if this text here is not
    correct.)

Fixed.
    
    Section 3.2 notes:
       JWT libraries SHOULD NOT generate JWTs using "none" unless explicitly
       requested to do by the caller.
    
    I couldn't find anywhere where we have matching guidance about "SHOULD
    NOT consume JWTs using 'none' unless explicitly requested"; this seems
    important enough to get called out explicitly.
    	
Fixed.
    
    ----------------------------------------------------------------------
    COMMENT:
    ----------------------------------------------------------------------
    
    I also have some non-Discuss-level substantive comments in the section-by-section notes,
    in addition to the usual editorial nits.
    
    Section 1
    
       and/or encrypted.  The JWT specification has seen rapid adoption
       because it encapsulates security-relevant information in one, easy to
       protect location, and because it is easy to implement using widely-
    
    nit: "one easy-to-protect location".

OK
    
    Section 2.2
    
    I'd consider rewording the text here to make it more poignant; perhaps:
    
      In addition, some applications use a keyed MAC algorithm such as
      "HS256" to sign tokens, but supply a weak symmetric key with
      insufficient entropy (such as a human memorable password).  Such keys
      are vulnerable to offline brute-force or dictionary attacks once an
      attacker possesses such a token.

OK
    
    Section 2.4
    
    I'd suggest noting that the compression attacks are particularly
    powerful when there is attacker-controlled data in the same compression
    space as secret data.

OK
    
    Section 3.2
    
       Therefore, applications MUST only allow the use of cryptographically
       current algorithms that meet the security requirements of the
       application.  This set will vary over time as new algorithms are
       introduced and existing algorithms are deprecated due to discovered
       cryptographic weaknesses.  Applications MUST therefore be designed to
       enable cryptographic agility.
    
    This seems to have high overlap with BCP 201; a reference is probably in
    order.

Since the JWT format itself enables crypto agility, we decided that a further reference to BCP 201 would not add clarity.
    
    Section 3.4
    
       Some cryptographic operations, such as Elliptic Curve Diffie-Hellman
       key agreement ("ECDH-ES") take inputs that may contain invalid
       values, such as points not on the specified elliptic curve or other
       invalid points (see e.g.  [Valenta], Sec. 7.1).  Either the JWS/JWE
       library itself must validate these inputs before using them or it
       must use underlying cryptographic libraries that do so (or both!).
    
    side note: A phrasing like "JWS/JWE libraries MUST ensure that such
    input validation occurs" would leave the same wiggle room for the
    validation to occur at the underlying crypto layer, while leaving it
    crystal clear what entity is responsible for ensuring that the checks
    occur".  But since I don't expect a change of this nature to actually
    cause different behavior by implementors, I'm not very tied to it.

Left unchanged.
    
    Section 3.8
    
    When we say "[o]ther applications may use different means of binding
    keys to issuers", is there any value in noting that certification by a
    trusted authority is a common way to perform this binding (in some
    contexts)?

We couldn't find related use cases in the JWT space, left the text as-is.
    
    Section 3.9
    
       If the same issuer can issue JWTs that are intended for use by more
       than one relying party or application, the JWT MUST contain an "aud"
       (audience) claim that can be used to determine whether the JWT is
       being used by an intended party or was substituted by an attacker at
       an unintended party.  Furthermore, the relying party or application
       MUST validate the audience value and if the audience value is not
       present or not associated with the recipient, it MUST reject the JWT.
    
    (grammar nit?) Is the "Furthermore" sentence supposed to still be scoped
    to the case where the issuer can issue JWTs for more than one audience?
    If not, it seems like we're requiring the rejection of all "aud"-less
    JWTs but not requiring "aud" to be present when generating them.

Clarified.
    
    Section 3.10
    
                                                    Applications should
       protect against such attacks, e.g., by matching the URL to a
       whitelist of allowed locations, and ensuring no cookies are sent in
       the GET request.
    
    This could probably be a SHOULD (or even a MUST?).

Went for a SHOULD.
    
    Section 3.11
    
       When applying explicit typing to a Nested JWT, the "typ" header
       parameter containing the explicit type value MUST be present in the
       inner JWT of the Nested JWT (the JWT whose payload is the JWT Claims
       Set).  The same "typ" header parameter value MAY be present in the
       outer JWT as well, to explicitly type the entire Nested JWT.
    
    This is an interesting recommendation, as it is in some sense
    *introducing* type confusion by using the same type name for JWTs with
    different structures (the inner and outer JWTs)!  My sense is that it is
    not practical to change current usage, though, so I think this should be
    treated as a side note and not an actionable recommendation.

We discussed it, agree with your assessment and reworded a bit, indeed as a side-note.
    
    Section 3.12
    
       -  Use different keys for different kinds of JWTs.  Then the keys
          used to validate one kind of JWT will fail to validate other kinds
          of JWTs.
    
    It might be worth calling back an analogy to RFC 8037's security
    considerations (where we advise to keep an association between key
    material and key algorithm), in effect extending the scope of
    "algorithm" to include application usage.

We felt this would be more confusing than clarifying for our target audience.
    
       Given the broad diversity of JWT usage and applications, the best
       combination of types, required claims, values, header parameters, key
       usages, and issuers to differentiate among different kinds of JWTs
       will, in general, be application specific.  For new JWT applications,
       the use of explicit typing is RECOMMENDED.
    
    This last recommendation seems to duplicate one from the end of Section
    3.11.  While it's important and worth reiterating, we do usually try to
    avoiding using normative RFC 2119 language when repeating ourselves, to
    make it very clear which requirement is the binding one.

Added a reference to 3.11 to clarify.