Hi Paul,
On 2020-09-17, 17:26, "Paul Kyzivat" <pkyzi...@alum.mit.edu> wrote:
Hi Olaf,
On 9/17/20 4:09 AM, Olaf Bergmann wrote:
> Hi Paul,
>
> Responding to you remaining comments please see inline.
>
> Paul Kyzivat <pkyzi...@alum.mit.edu> writes:
>
>>>>> * Also in section 3.3:
>>>>>
>>>>> All CBOR data types are encoded in CBOR using preferred
serialization
>>>>> and deterministic encoding as specified in Section 4 of
>>>>> [I-D.ietf-cbor-7049bis]. This implies in particular that the
"type"
>>>>> and "L" components use the minimum length encoding. The content
of
>>>>> the "access_token" field is treated as opaque data for the
purpose of
>>>>> key derivation.
>>>>>
>>>>> IIUC the type of serialization and encoding is a requirement. Will
>>>>> need some rewording to make it so.
>>>>
>>>> I take it that you and Ben have agreed that the example description
does
>>>> not necessarily need normative language as the description of this key
>>>> derivation procedure is meant as an example how the authorization
server
>>>> and the resource server can securely agree on a shared secret to be
used
>>>> between the client and the resource server.
>>
>> This still confuses me. IIUC preferred serialization and deterministic
>> encoding are *optional* in CBOR. The text hear seems to require it,
>> but doesn't use normative language to do so.
>>
>> If these are required for things to work then you make a normative
>> statement about it. E.g., "The "type" and "L" components MUST use the
>> minimum length encoding."
>>
>> Or do you intend that some other (non-minimum-length) MAY be used? (In
>> which case both sides would need a side agreement on what encoding is
>> used.)
>
> The text here just gives an example how key derivation may be used by
> the authorization server and the resource server to agree on a shared
> secret (that is used to encrypt the traffic between the resource server
> and the to-be-authorized client).
>
> To that regard, the text is not really normative. The only normative
> language we need here would be to avoid security issues. Commenting on
> the data representation here is to be understood as a suggestion to use,
> e.g., preferred CBOR serialization according to 7049bis.
>
> [...]
Sorry to be so dense, but I'm still not getting it.
I take your point that this is only an example of a way to agree on a
shared secret. But at the end of the day they indeed must somehow agree
on a shared secret. *If* they use this technique then it will only work
if they also agree on a consistent way to do the serialization and
encoding that is otherwise not standardized. So they need a side
agreement, which is not a good situation for a standardized protocol.
At the very least it seems like you should highlight that some sort of
out of band communication is required between the authorization and
resource servers to establish the shared secret or the algorithm to be
used for deriving the shared secret.
[GS]
I'm not sure I understand the issue correctly.
Section 4.1 of ietf-cbor-7049bis states:
'The preferred serialization always uses the shortest form of representing the
argument'
This seems to me to be in coherence with:
"All CBOR data types are encoded in CBOR using preferred serialization and
deterministic encoding"
and
'This implies in particular that the "type" and "L" components use the minimum
length encoding. '
If I understand right you would like to replace the latter sentence with:
'The "type" and "L" components MUST use the minimum length encoding.'
But there are multiple statements in this example which could be replaced with
normative language, e.g.,:
'In this example, HKDF consists of the composition of the HKDF-Extract and
HKDF-Expand steps [RFC5869].'
'The symmetric key is derived from the key identifier, the key derivation key
and other data:'
'type is set to the constant text string "ACE-CoAP-DTLS-key-derivation" '
Would you be happy with only the specific normative statement you proposed
(which BTW is fine to me) or would you like to see all statements like this to
be replaced with normative text (or can we make a normative formulation at the
beginning of the example indicating that "compliance to this example REQUIRES
the following:")?
Thanks,
Göran
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