Re: [OAUTH-WG] Last Call: <draft-ietf-oauth-jwsreq-11.txt> (The OAuth 2.0 Authorization Framework: JWT Secured Authorization Request (JAR)) to Proposed Standard

Nat Sakimura <sakimura@gmail.com> Wed, 15 February 2017 17:04 UTC

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From: Nat Sakimura <sakimura@gmail.com>
Date: Thu, 16 Feb 2017 02:04:23 +0900
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To: Denis <denis.ietf@free.fr>
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Cc: oauth <oauth@ietf.org>, draft-ietf-oauth-jwsreq@ietf.org
Subject: Re: [OAUTH-WG] Last Call: <draft-ietf-oauth-jwsreq-11.txt> (The OAuth 2.0 Authorization Framework: JWT Secured Authorization Request (JAR)) to Proposed Standard
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Hi Denis,

Thought John's response went to you as well but apparently not.

My replies inline:

On Fri, Feb 10, 2017 at 6:15 AM, Denis <denis.ietf@free.fr>; wrote:

> Hi Nat,
>
> My replies to your proposed disposition of comments are embedded in the
> text.
>
[snip]

>  Section 4 states:
>>
>>
>>
>>
>> *   A Request Object (Section 2.1) is used to provide authorization
>> request parameters for an OAuth 2.0 authorization request.  It    contains
>> OAuth 2.0 [RFC6749] authorization request parameters    including extension
>> parameters**.  *
>>
>> RFC 6749 contains 75 pages, but does not contain a single occurrence of
>> the wording "authorization request parameter" nor of "extension parameter".
>> There should be either references to one or more specific sections of
>> this document or, even better, a list of the mandatory/recommended/possible
>> authorization request parameters as well as a list of
>> mandatory/recommended/possible extension parameters should be included in
>> this document.
>>
>> A clear distinction should be made between the parameters used to
>> authenticate the request and the other ones.
>>
>
> Reject.
> There are 4 flows in RFC6749. In each flow, there is a sub-section
> dedicated to the Authorization request.
> In them, the parameters used in the authorization request are very clearly
> indicated. For example,
>
> 4.1.1 <https://tools.ietf.org/html/rfc6749#section-4.1.1>;.  Authorization Request
>
>    The client constructs the request URI by adding the following
>    parameters to the query component of the authorization endpoint URI ...
>
> It is very difficult to miss.
>
> Then, the possibility for the extension parameters are discussed in 8.2.
> Needless to say, those extension parameters are going to be discussed in
> other specifications.
> Thus, it would be misleading just to say the parameters defined in 4.1.1,
> 4.2.1, etc.
> As an editor, I feel better with the current language because it is at
> least not wrong nor misleading.
>
>
> draft-ietf-oauth-jwsreq-11 states on page 7.
>
>
>
>    To sign, JSON Web Signature (JWS) [RFC7515] is used.  The result is a
>
>    JWS signed JWT [RFC7519].  If signed, the Authorization Request
>
>    Object SHOULD contain the Claims "iss" (issuer) and "aud" (audience)
>
>    as members, with their semantics being the same as defined in the JWT
>
>    [RFC7519] specification.
>
>
>
> This should be changed into:
>
>
>
>    To sign, JSON Web Signature (JWS) [RFC7515] is used.  The result is a
>
>    JWS signed JWT [RFC7519].  If signed, the Authorization Request
>
>    Object *MUST contain a client_id parameter* and SHOULD contain a
>    "iss" (issuer) *parameter* and an "aud" (audience) *parameter*, with
>    their semantics being the same as defined in the JWT RFC7519]
>    specification.
>
>
>
 I am kind of ok with the proposed text but if we do you want to single out
`client_id`, perhpas a reason should be added.
There are other REQIURED parameters in the Auhtorization Request defined in
RFC6749, you know.

> In section 5.2. Message Signature or MAC Validation, the text states:
>
>
>
>    When validating a JWS, the following steps are performed.
>
>
>
> (...)
>
>        See Section 10.6 for security considerations on algorithm
>
>        validation.
>
>
>
> There is no section 10.6 in this document. It seems to be section 10.3
>
> Anyway, it is not the right place to place requirements in a security
> considerations section and the appropriate text
> should be moved in the main body of the document.
>

Sorry, I cannot find the text you are refering to.


>
>
> RFC 6749 states in clause 4.  Obtaining Authorization on page
>
>
>
> 6.2.  JWS Signed Request Object
>
>
>
>    To perform JWS Signature Validation, the "alg" Header Parameter in
>
>    the JOSE Header MUST match the value of the pre-registered algorithm.
>
>    The signature MUST be validated against the appropriate key for that
>
>    "client_id" and algorithm.
>
>
>
> The important point is to provide guidance on how to map the client_id
> parameter with the appropriate key.
> There is none at the present time.
>
>
>
> Add:
>
>
>
>    Identifying the appropriate key MUST be done according to section 6
>    of RFC 7515 and using the Registered Header Parameter Names defined
>    in section 4.1 of RFC 7515, e.g. using the Header Parameters "jku",
>    "jwk", "kid", "x5u", "x5c", "x5t", or "x5t#S256".
>
>
>
>  4. The introduction states on page 4:
>
>>
>>
>>      (a) (integrity protection) The request can be signed so that the
>> integrity of the request can be checked ;
>>
>>
>>
>> This should be changed into:
>>
>>
>>
>>      (a) (integrity protection) The request can be authenticated either
>> using a digital signature or using encryption under a secret key
>>           so that the integrity of the request can be checked ;
>>
>
> Reject.
> This paragraph is talking about the integrity protection and not the
> source authentication.
> And even for source authentication, saying that encryption under a secret
> key is not accurate as it was discussed earlier in the WG mail.
>
> I am not sure if "Introduction" needs to state everything that is
> explained later. The idea of introduction probably is to give main points.
> The list is not an exhaustive list of the benefit of using JWT as the
> authorization request format. For example, being able to encrypt the
> request, which is not listed there, has an advantage of preventing MITB to
> eavesdrop the request. So I think it is ok as is.
>
> Integrity protection cannot be verified without knowing the source of the
> information.
>
Using encryption (which supports at the same time
> an integrity service when secret keys are being used) is another way to be
> able to check the integrity of the request.
>
>
>
> So I maintain may comment.
>

I think the issue is that if you encrypt with a asymmetric algorithm then
the receiver has no idea who encrypted it.
If encrypted with a symmetric key (not secret key) then you know that it
came from someone who has access to that key.
That works because we only support AEAD encryption.

You can use asymmetric encryption but you need to sign first if you want to
know who it is from.


>
>  5. The introduction states on page 4:
>
>>
>>
>> (d) (collection minimization) The request can be *signed* by a third
>> party attesting that the authorization request is compliant to certain
>> policy.
>>
>> The request is not *signed* by a third party.
>>
> However, later on, there is the following explanation:
>>
>>    In addition, it allows requests to be prepared by a third party so
>> that a client application cannot request
>>    more permissions than previously agreed.
>>
>>  If it is the intent, the sentence should be rephrased as:
>>
>> (d) (collection minimization) The request can be *verified* by a third
>> party attesting that the authorization request is compliant to certain
>> policy.
>>
> Reject
> The third party indeed signs the request on behalf of the client as the
> result of verification that the permission is the same as previously
> agreed.
>
>
> If it were the case, the client_id would indicate the name of the third
> party and the name of the user would be missing (or vice versa).
>

The value of `client_id` will be the requesting party.
The value of `iss` can be the third party.
But setting aside that, I guess your point actually is on the use of the
word "request". Authorization request is the entire thing that travels from
the client and not a part of it, and that is a fair point. Having said
that, I have a problem with your use of the word "verified". What about
this?

(d) (collection minimization) The data being requested can be *attested *by
> a third party that is compliant to collection minimization principle.
>

>
>  6. Section 10.1. the text states:
>>
>>
>>
>> *   When sending the authorization request object through "request"
>> parameter, it MUST either be signed using JWS [RFC7515] or encrypted
>> using JWE [RFC7516] with then considered appropriate algorithm.*
>>
>>  The wording" with then considered appropriate algorithm" is too vague.
>> This should be changed into:
>>
>>
>>
>> *   When sending the authorization request object through "request"
>> parameter, it MUST either be signed using JWS [RFC7515] or encrypted
>> using JWE [RFC7516] using a symmetric key algorithm.*
>>
>>  Reject.
>>
> In the above sentence, "*with then considered appropriate algorithm*"
>  applies both on JWS and JWE.
> The intent of the phrase is that a vulnerable algorithm should not be
> used.
>
> Also, I do not understand why the algorithm has to be symmetric key
> algorithm.
>
>
> Maybe, this explains why you didn't understand the previous comment. With
> public key encryption, it is not possible to authenticate
> the source of the request, while it is possible with secret key encryption
> when the encrypted data includes a cryptographic checksum
> like a hash value and an error propagation method for the encryption
> algorithm.
>

I understand this. My point is that this subsection is not talking about
what you just stated. This is a security consideration pointing out that an
alogrithm which has not become vulnerable must be used.

What you describe should instead go below the list (a)(b)(c) in section 5
or section 10.3.
"when symmetric keys are being used" probably is a bit too open to
interpretation. John is now creating a text on it.


>
>
> So I maintain my comment.
>
>  7. Section 10.2 states:
>>
>>    This means that the request object is going to be prepared fresh each
>>    time an authorization request is made and caching cannot be used.
>>
>>  What are the implications ? Is it required/recommended to use a nonce ?
>> The text should be made clearer.
>>
>> Reject.
> The implication is given right after the sentence. There is no variable
> called "nonce" in RFC6749. Since this document is just defining
> another encoding method for OAuth 2.0 authorization request as a
> framework, it does not mandate these.
> An extension specification should define those requirements.
>
>
> Note that this section belongs to the security considerations section
> which SHOULD NOT be normative and should only provide guidance.
>
>
>
> The sentence right after is the following:
>
>
>
>    It has a performance disadvantage, but where such disadvantage is
>
>    permissible, it should be considered.
>
>
>
> It does not provide any guidance.
>

Does it not? It is providing a guidance that the implementation should
consider not using cached request and create the request afresh each time
so that the entire request can be signed etc.


>
>
> The key point is that a parameter able to detect replay needs to be
> included in the request. This should be indicated in the normative part.
>

This security consideration is not about the replay attack but request
tampering.

It is unfortunate that RFC 7515 has not addressed replay protection of JWS
> and only mentions the problem is section 10.10 which is in the
> security considerations section. Here it is:
>

>
> 10.10.  Replay Protection
>
>    While not directly in scope for this specification, note that
>
>    applications using JWS (or JWE) objects can thwart replay attacks by
>
>    including a unique message identifier as integrity-protected content
>
>    in the JWS (or JWE) message and having the recipient verify that the
>
>    message has not been previously received or acted upon.
>
>
>
> The text on page 7 should be changed into:
>
>
>
>    To sign, JSON Web Signature (JWS) [RFC7515] is used.  The result is a
>    JWS signed JWT [RFC7519].  If signed, the Authorization Request
>    Object *MUST contain a client_id parameter* *and a "nonce"*
> *extension *   *parameter* *allowing to detect replay attacks *and SHOULD
> contain an "iss"
>    (issuer) *parameter* and an "aud" (audience) *parameter*, with their
>    semantics being the same as defined in the JWT specification[RFC7519].
>
>
>
> Note that Page 7 uses the "nonce" parameter in the example.
>

I agree that inclusion of nonce etc. to thwart the replay attack has to be
done in the normative section and not in the security consideration.
Having said that, as I stated before, this specification is just defining
another encoding for RFC6749. As the result, the replay protection etc. has
to be deferred to an extension spec, such as OIDC.


> JSON Web Token Claims are listed at: https://www.iana.org/
> assignments/jwt/jwt.xhtml
>
> "Nonce" is mentioned in OpenID Connect Core 1.0 incorporating errata set
> 1.
>
>
>
> It is described as :
>
>
>
> nonce
>
> Value used to associate a Client session with an ID Token
>
>
> This is too restrictive since now a nonce should be included in a JWS
> token.
>
>
>
> The registration is as follows:
>
>    - Parameter name: nonce
>    - Parameter usage location: Authorization Request
>    - Change controller: OpenID Foundation Artifact Binding Working Group
>    - openid-specs-ab@lists.openid.net
>    - Specification document(s): Section 3.1.2
>    <http://openid.net/specs/openid-connect-core-1_0.html#AuthorizationEndpoint>
>    of this document
>    - Related information: None
>
>
> Section 3.1.2 states:
> 3.1.2.  Authorization Endpoint
>
> The Authorization Endpoint performs Authentication of the End-User. This
> is done by sending the User Agent to the Authorization Server's
> Authorization Endpoint for Authentication and Authorization, using request
> parameters defined by OAuth 2.0 and additional parameters
> and parameter values defined by OpenID Connect.
>
>
>
> Communication with the Authorization Endpoint MUST utilize TLS. See
> Section 16.17
> <http://openid.net/specs/openid-connect-core-1_0.html#TLSRequirements>
> for more information on using TLS
>
>
>
> This has nothing to do with the nonce. Hence the nonce registration
> information has been badly defined.
>
>
>
> The OpenID specification also states:
>
>
> "The Client SHOULD check the nonce value for replay attacks. The precise
> method for detecting replay attacks is Client specific".
>
>
>
> This does not allow to interoperate.
>
>
>
> Rather than correcting the registration information in the OpenID
> specification, it would be better to suppress it from the OpenID
> specification
> and incorporate it within an IETF RFC.
>

Out of scope for this specification.
Also, you should discuss something on OIDC on a sperarate list, not here.


>
>
> In order to avoid nonces to be kept in a memory for ever, a good practice
> is to split the nonce in two parts:
>
>    - one of them includes a UTC NumericDate using the format defined in
>    RFC 7519, and
>    - the other one includes a random number.
>
>
> In this way only recent nonces (e.g. received during the last 5 minutes)
> need to be kept in memory.
> Three or four bytes for the random number will be sufficient.
>
>
>
> In order to *allow for interoperability,* a format should be specified.
>
>
> I propose a NumericDate defining the UTC time concatenated with a random
> number with three bytes.
>
>
>
> "Nonce" has not been officially registered by IANA. An IANA Considerations
> section should be added in draft-ietf-oauth-jwsreq-
> to register the "nonce" parameter.
>

Everything related to nonce is out of scope. You should write a new I-D.


>
>
> On page 14, section 6.2., after the previous proposed text which is:
>
>
>
>    Identifying the appropriate key MUST be done according to section 6
>    of RFC 7515 and using the Registered Header Parameter Names defined
>    in section 4.1 of RFC 7515, e.g. using the Header Parameters "jku",
>    "jwk", "kid", "x5u", "x5c", "x5t", or "x5t#S256".
>
>
>
> I proposed to add the following text:
>
>
>
>    To perform JWS Signature Validation, the "nonce" Header Parameter in
>
>    the JOSE Header MUST be present and MUST be checked to verify that
>    the signed request is not the replay of a previous signed request.
>
>
> A section defining the nonce parameter should be added.
>

[snip]

>
>  9. Section 10.3 states at its very end:
>>
>>     An extension specification
>>    should be created as a preventive measure to address potential
>>    vulnerabilities that have not yet been identified.
>>
>>
>> Writing a document for vulnerabilities that have not yet been identified
>> is speculative. It would rather be better
>> either to remove this sentence or to explain what is meant by it.
>>
> Reject.
> It is referring to the first paragraph of the sub-section. Also,
> precaution when security is in question is a good thing.
>
>
> This sentence is simply useless and thus should be deleted. Hence, I
> maintain this comment.
>
> Agree to disagree.


>
> 10. Section 11.1 states:
>>
>>  *11.1.  Collection limitation*
>>
>>
>>
>>
>> *    When the Client is being granted access to a protected resource
>> containing personal data, the Client SHOULD limit the collection of
>> personal data to that which is within the bounds of applicable law    and
>> strictly necessary for the specified purpose(s).*
>>
>>  The *presentation* of personal data should be limited whether or not
>> the protected resource contains personal data.
>>
> It is proposed to change this text into:
>>
>>
>>
>>
>> *   When the Client requests an access to a protected resource, the
>> Client    SHOULD limit the presentation of personal data to that which is
>> within    the bounds of applicable law and strictly necessary for the
>> specified    purpose(s).*
>>
> Reject.
> You are not getting what OAuth does. The party that holds personal data is
> the authorization server / resource.
> It is not the client. The client is the party who is getting those
> "resources" which may contain personal data.
> Yes, the client can provide some personal data to the resource depending
> on what that resource endpoint is, but that is out of scope for OAuth.
> As far as OAuth is concerned, what is being sent from the client to the
> resource is the access token.
>
>
> The dispute is whether the protected resource contains or not personal
> data.
> The data contained by the protected resource may well be public data
> (or/and personal data).
> It does not need to be only "personal data".
>
>
>
> Hence, I maintain my comment.
>
> I do not understand your comment now. Your previous proposeal seems to be
unrelated to the above comment.


>
>  11. Section 11.2.1 states:
>>
>> 11.2.1.  Request Disclosure
>>
>>    This specification allows extension parameters.
>>
>>  It would be useful to name either all of them or some of them. RFC 6749
>> is not crystal clear about this.
>>
> Noted.
> RFC6749 only defines how to define extension parameters.
> This specification draws from OpenID Connect for some examples of
> extension parameters such as nonce.
> See section 4 for example.
>
>
> See my earlier comments where client_id and nonce shall be mandatory.
>
>
client_id is mandatory in RFC6749. Nonce is not defined in RFC6749 and
hence out of scope for this specification.


> Denis
>

[snip]

-- 
Nat Sakimura (=nat)
Chairman, OpenID Foundation
http://nat.sakimura.org/
@_nat_en