Stand-in Key Identifier and Encrypted Partial IV in the Constrained Application Protocol (CoAP) OSCORE Option

The security protocol Object Security for Constrained RESTful Environments (OSCORE) provides end-to-end protection of messages exchanged with the Constrained Application Protocol (CoAP). Messages protected with OSCORE include a CoAP OSCORE Option, where the "Partial IV" field specifies the sequence number value used by the message sender and the "kid" field specifies the identifier of the message sender. In order to reduce the information exposed on the wire that can be used for fingerprinting traffic and for tracking endpoints, this document defines a lightweight add-on method that obfuscates certain fields of the OSCORE Option, by encrypting the "Partial IV" field and overwriting the "kid" field with a stand-in identifier. Therefore, it updates RFC 8613. With minor adaptations, the defined method is applicable also to the security protocol Group Object Security for Constrained RESTful Environments (Group OSCORE) that protects group communication for CoAP.