Re: [Ace] Security of the Communication Between C and RS

Hannes Tschofenig <Hannes.Tschofenig@arm.com> Wed, 19 December 2018 11:36 UTC

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From: Hannes Tschofenig <Hannes.Tschofenig@arm.com>
To: Stefanie Gerdes <gerdes@tzi.de>, Ludwig Seitz <ludwig.seitz@ri.se>, Jim Schaad <ietf@augustcellars.com>, "ace@ietf.org" <ace@ietf.org>
Thread-Topic: [Ace] Security of the Communication Between C and RS
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Date: Wed, 19 Dec 2018 11:36:11 +0000
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Subject: Re: [Ace] Security of the Communication Between C and RS
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Hi Steffi,

Are you focused on token expiry or the case where a token + symmetric key has been leaked?

Is the threat you are describing the case where the client uses DTLS/TLS with the RS (potentially long after a token has been presented), or the case where the client contacts the RS and presents a token?

Ciao
Hannes

-----Original Message-----
From: Stefanie Gerdes <gerdes@tzi.de>
Sent: Mittwoch, 19. Dezember 2018 12:26
To: Hannes Tschofenig <Hannes.Tschofenig@arm.com>om>; Ludwig Seitz <ludwig.seitz@ri.se>se>; Jim Schaad <ietf@augustcellars.com>om>; ace@ietf.org
Subject: Re: [Ace] Security of the Communication Between C and RS

Hi Hannes,

On 12/18/2018 04:26 PM, Hannes Tschofenig wrote:
> Hi Steffi,
>
>> In OAuth, the expires_in field is usually used to inform the client how long the access token is valid. If the client uses the access token in a request after the token expired, the RS will reject the request, which usually is not a big problem for the client.
>
>> In ACE, AS provides the client with keying material for the RS, which is a completely different situation. If the client does not know how long the keying material is valid, it may use outdated keying material to communicate with RS. The expires_in field can be used to inform the client how long the keying material for RS is valid, if the keying material is valid as long as the access token.
>
>> The main reason to change keying material from time to time is to avoid that it is compromised. I therefore think that it is necessary for the security of the solution that C knows how long the keying material is supposed to be valid.
>
> If a client uses either an old key or an old token then the request will fail. I don't see much difference here between OAuth and ACE.

The difference is that C cannot authenticate RS with an outdated key.
And even if the correct RS rejects the request, the confidentiality of the request may already have been breached, i.e., the damage is already done. C therefore must know how long the keying material is valid.

Viele Grüße
Steffi
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