Integer value for the CBOR Object Signing and Encryption (COSE) key identifier

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Table of Contents

1. Introduction

Many Internet of Things (IoT) deployments require technologies which are highly performant in constrained environments [RFC7228]. The connectivity for these settings may exhibit extremely restricted bandwidth constraints, for which byte level optimizations are motivated, see [I-D.ietf-lake-reqs].¶

The use of CBOR [RFC8949] enables a compact encoding of protected data as COSE objects [I-D.ietf-cose-rfc8152bis-struct], which is a basic building block in various IoT security settings such as CWT [RFC8392], OSCORE [RFC8613], and ACE-OAuth [I-D.ietf-ace-oauth-authz]. COSE defines the key identifier parameter kid used to identify keys used in the COSE object.¶

The value of the kid parameter is specified to be encoded as a CBOR byte string, which (with the exception of the empty string) requires at least two bytes on the wire. For comparison, CBOR encoding of small integers (-24, ..., 23) need only one byte on the wire. Since many IoT deployments may use local identifiers for which a few unique identifiers are sufficient, the use of CBOR integers as key identifiers would reduce the overhead due to transport of COSE objects.¶

This specification amends this limitation by extending the COSE parameter kid to allow CBOR integer values. kid is used in two instances in COSE, which both need to be extended to CBOR int encoding:¶

• The kid COSE header parameter, see Section 3.1.¶
• The kid COSE Key Common Parameter, see Section 3.2.¶

2. Security Considerations

There are no additional security considerations compared to key identifiers to being byte strings.¶

3. IANA Considerations

+------+-------+------------+----------------+
| Name | Label | Value Type | Description    |
+------+-------+------------+----------------+
| kid  |   4   | bstr / int | Key identifier |
+------+-------+------------+----------------+

+------+-------+------------+---------------------------+
| Name | Label | Value Type | Description               |
+------+-------+------------+---------------------------+
| kid  |   2   | bstr / int | Key identification value  |
|      |       |            | - to match kid in message |
+------+-------+------------+---------------------------+

Acknowledgments

Authors' Addresses