Re: [Ace] draft-ietf-ace-dtls-authorize-01

[Olaf Bergmann <[e-mail]>]

Hello Hannes,

Thank you very much for your comments. I am replying to the comment that
Ludwig did not yet address:

Ludwig Seitz <[e-mail]> writes:

> On 2017-10-01 11:35, Hannes Tschofenig wrote:

>> - What is the reasoning behind this statement:
>>
>>     "This specification mandates that at least the key derivation
>>     algorithm "HKDF SHA-256" as defined in [I-D.ietf-cose-msg] MUST be
>>     supported."
>>
>> I would have assumed at the session key provided by the AS to the client
>> and the key embedded in the access token is used directly within TLS as
>> a PSK.

Yes, you could embed the session key in the access token. But then, you
would always have to encrypt the access token and ensure that is never
decrypted by unauthorized parties. Key derivation allows you to transfer
the access token unencrypted (as long as the privacy objectives are met,
of course). This could even save some bytes in the token as the
encrypted session key does not have to be transferred.

This mechanism has previously been discussed in section 6 of [1] but now
has been adjusted from the simple ad-hoc syntax in DCAF to the more
flexible COSE method.

[1] https://tools.ietf.org/html/draft-gerdes-ace-dcaf-authorize-04#section-6

Grüße
Olaf