Re: [OAUTH-WG] Securing APIs with OAuth 2.0

Justin Richer <jricher@mitre.org> Mon, 05 March 2012 14:15 UTC

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Date: Mon, 5 Mar 2012 09:14:18 -0500
From: Justin Richer <jricher@mitre.org>
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To: Sergey Beryozkin <sberyozkin@gmail.com>
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Cc: oauth@ietf.org
Subject: Re: [OAUTH-WG] Securing APIs with OAuth 2.0
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>>
>> Shane is right; the way you described your problem, the client
>> credential grant type may be appropriate. That's especially true if
>> the client will be accessing resources that don't necessarily belong
>> to specific users. But if the client (web site) will be using the API
>> (OAuth auth/resource server) to access user-specific resources, then
>> the authorization code grant type is a better fit. It doesn't matter
>> that the OAuth server trusts the client without needing user
>> authorization.
>>
>> The authorization code flow offers a solution for user identification
>> that is absent in the client credential flow. In other words, even
>> though the OAuth server trusts the client and will comply with all API
>> requests, how is the client x supposed to identify a user so it can
>> request the right resource from the resource server?
>>
>> By using an authorization code grant, the client can acquire an access
>> token that is bound to a specific user. This is makes the
>> authorization code flow suitable for single sign-on implementations,
>> whereas the client credential flow is not appropriate for user
>> authentication.
>>
>> Don't worry about the fact that the client does not need to be
>> authorized by the user. You can still use the authorization code flow,
>> and the authorization server will not need to prompt the user for
>> authorization because you will have pre-authorized the client for all
>> users.
>>
>
> Can the authorization server return a (pre-authorized) token 
> immediately in this case, despite the fact the client is specifying 
> "response_type=code" ?
No -- the server MUST return an authorization code, even if there is no 
interaction beyond the user logging in (which may be SSO and therefore 
"invisible" to users). This code is bound to the user session and the 
client ID, and it needs to be presented in the back channel with the 
client's ID and secret, away from the user session. These two steps are 
what close the client-server-user triangle in a secure way so that no 
party knows more than they really need to. The Client Credentials flow 
and the Implicit flow collapse this triangle into a line in two 
different ways, both for different use cases and both have their 
tradeoffs. So if you want to get an access token from the authz endpoint 
directly, you use the implicit flow. It puts all of the weight onto the 
user agent, which sounds like the opposite of what you actually want to 
do here.

> If the authorization code, to be exchanged later for this token, has 
> to be returned, how reasonable is it to expect that the authorization 
> code will be bound to the pre-authorized access token (example, the 
> access token's key/id will be returned as the authorization code) ?
>
> I suspect it may not be a good idea given the spec is saying the 
> authorization code should be short-lived, thus the codes and actual 
> tokens will have different life-cycles, however the fact the end user 
> has pre-authorized the client adds some uncertainty to it...

Again, no -- the code shouldn't have anything in it that ties it to the 
access token directly. If it did, then anybody intercepting just the 
code on the wire could guess the access token, which would make the auth 
code a pointless abstraction. The best way to deploy the authz code is 
to have a short, random, opaque blob that is one-time use and expires 
after a set amount of time, probably fairly short (on the order of 
minutes in most cases). Think of it as a one-time-password that is 
generated for the resource owner to give to the client on their behalf. 
Since it's a credential known to both the user agent and the client, you 
really, really don't want it to be copied from any other part of the flow.

  -- Justin


> thanks, Sergey
>
>> As an added bonus, this sets you up perfectly for when you add new
>> clients which are not pre-authorized and need user authorization.
>>
>> I hope that helps.
>>
>> Regards,
>> Andre DeMarre
>>
>> On Wed, Feb 29, 2012 at 6:59 PM, Shane B Weeden<sweeden@au1.ibm.com>  
>> wrote:
>>> 1. Yes, client credentials sounds right for what you described. 
>>> Think of it
>>> as lightweight b2b authentication in that sense (but two steps - one 
>>> to get
>>> a token, and another to use it).
>>> 2. Can't help you with source - but do have a product-based solution :)
>>> 3. Absolutely it should for the resource server, but the answer may 
>>> depend
>>> have same dependency on the implementation you use.
>>>
>>> Regards,
>>> Shane.
>>>
>>>
>>>
>>> From:   Pete Clark<pete@appmuscle.com>
>>> To:     "oauth@ietf.org"<oauth@ietf.org>
>>> Date:   29/02/2012 06:50 PM
>>> Subject:        [OAUTH-WG] Securing APIs with OAuth 2.0
>>> Sent by:        oauth-bounces@ietf.org
>>>
>>>
>>>
>>> Hey all, I've joined the list because I'd like to use OAuth 2 to 
>>> implement
>>> security for a new set of REST APIs I'm developing for a client.  I'm
>>> coding with PHP, but my questions are more general.  Right now, 
>>> there will
>>> be only one web site that uses the APIs, in a server-to-server 
>>> fashion, and
>>> currently we don't have a need for a third party application to gain 
>>> access
>>> to user data, such that a user would need to authorize that app.  We 
>>> do,
>>> however, want to have that ability down the road.  My question is, 
>>> can I
>>> still use OAuth 2 in some way to implement our first phase?  From 
>>> what I've
>>> read, it seems like the "client credentials" flow is the one I want 
>>> to use
>>> for now.  Can someone:
>>>
>>> 1) Confirm that that's what I should use for this first phase?
>>> 2) Point me to an implementation of this flow (in any language) that I
>>> could use or port to PHP?  I've found some libraries for php but can't
>>> really tell, being new, if they offer the "client credentials" flow
>>> 3) Answer one more question.. Will using the client credentials flow 
>>> now
>>> allow me to move to one of the user-authorizes-external-app flows 
>>> down the
>>> road without having to reimplement or throw away the client credentials
>>> flow code?
>>>
>>> I apologize for all the questions, but these would really help point 
>>> me in
>>> the right direction.. Thank you for reading!
>>>
>>> Sincerely,
>>> Pete
>>>
>>>
>>>
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>>>
>>>
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