For the purposes of the present document, the terms and definitions given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905. End-to-End Encryption: User homed in the IWF: Interworking: Interworking function: Interworking group: LMR system: LMR user:

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905. E2EE IWF KEK KMS MC MCPTT LMR LMC LMS OTAK OTAR P25 SDS TETRA UE UKEK URI UUI

Interworking requirements for key management for encrypted interworking include:

1. a mechanism to securely (i.e. authenticity, integrity, confidentiality) share an LMR E2EE traffic key for a private call sessions between a party in an MCPTT system and a party in the LMR system;
2. a mechanism to securely convey to group members, the LMR E2EE key or set of LMR E2EE keys associated with an MC service group or set of MC service groups, to be used for encryption of interworking group calls spanning the multiple systems;
3. a mechanism to securely share with temporary group members in MC systems, the LMR E2EE key(s) associated with a temporary MC service group to be used in interworking group calls spanning the multiple systems;
4. key management solutions shall not preclude the ability of an IWF to allow one or more individual Mission Critical Organizations from having sole control over and sole access to LMR E2EE traffic keys used for the entity's media traffic and users' key encryption keys (UKEKs or KEKs);
5. key management solutions shall support the ability of the IWF to decrypt/reencrypt the media traffic for zero or more groups; and,
6. for deployments where Mission Critical Organizations wish to use LMR E2EE mechanisms when interworking with LMR users:
   1. a mechanism to securely provision an MC service client with the user's UKEK or KEK; and,
   2. a mechanism to convey LMR OTAR or OTAK message contents.

Each LMR technology defines its own packet format for voice media transmission. For interworking sessions, there might be cases where LMR formatted media is required to be transferred between the IWF and LMR aware MCPTT clients. An example of such a case is where E2EE is used and thus the IWF is not able to decrypt the media. In such cases, media that is sent over the IWF-1 interface needs to be routed within MCPTT systems to/from LMR aware MCPTT clients using methods described in TS 23.379. Requirements for media transmission across the IWF-1 interface include:

1. media transmission to carry the LMR formatted media between the IWF and LMR aware MCPTT clients; and
2. the MCPTT system, along with the IWF, may choose to encrypt the LMR formatted media using 3GPP mechanisms.

Figure 7.2-1 shows the functional model for the application plane for interworking between MC systems and LMR systems. Functional entities and interfaces depicted on the right-hand side of the IWF-x interfaces are defined in TS 23.280, TS 23.379, and TS 23.282.

The IWF provides centralised support for interworking between an MCPTT or MCData system and an LMR system. In MCPTT systems, the identity of an LMR user is provided as an MCPTT ID, and the identity of an LMR group is provided as an MCPTT group ID, which can be used by the IWF to derive the corresponding identities used in an LMR system. Similarly, in MCData systems, the identity of an LMR user is provided as an MCData ID, and the identity of an LMR group is provided as an MCData group ID, which can be used by the IWF to derive the corresponding identities used in an LMR system. Identities provided on IWF-x reference points are described in clause 8 of TS 23.280. The IWF can perform the identity mapping between an MCPTT system or MCData system and an LMR system during exchange of signalling and media messages. The assignment of a functional alias that belongs to the MC system to a user homed in the IWF enables the mapping to corresponding role-based addressing schemes applicable in the LMR system.