EAP method based on EDHOC Authentication

EDHOC is a new protocol for autentication and key derivation that has been proposed as an alternative in IoT to provide a secure exchange in an end-to-end fashion. This key material can be futher used to run other protocols such as OSCORE, as well as providing key material to any other protocol that needs pre-shared key material to secure the communications. Provides authentication and key material generation, which are basic pillars to the design of an EAP method. And indeed the most important thing is that it is lightweight and designed for IoT. In addition, the EDHOC implementation that exists on the device can be reused to establish OSCORE Security Associations (SAs) for the authentication process. EAP is a protocol that allows to implement different authentication mechanims, provides a framework for key management and has integration with AAA infrastructures. For these reasons, this new EAP method will allow the different applications and use cases to take advantage of EAP.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The exchange of messages befalls between two entities that EDHOC identifies as Initiator (I) and Responder (R). In this EAP method, we establish equivalence with those terms. On the one hand, EAP peer acts as Initiator while EAP Server takes on the role of Responder. The EAP-EDHOC conversation typically starts with the negotiation of EAP by the EAP authenticator and the EAP peer. The EAP authenticator sends an EAP-Request/Identity packet to the EAP peer, to which the EAP peer answers with an EAP-Response/Identity. This last messages contains the peer's user-Id. From this point on, the authenticator MAY act as a forwarder of the EAP messages between the EAP peer and the server, if the pass-through mode is used, receiving the EAP packets from the peer, encapsulating them for transmission to the EAP server that will act as Authentication Server (AS). Once the EAP server receives the peer's Identity, it MUST respond with an empty EAP-EDHOC/Start message, which is an EAP-Request packet with EAP-Type=EAP-EDHOC and no data. Initiator must initiate the EDHOC conversation. Hence, EAP Server sends this message to indicate that it can start the authentication process. The EAP-EDHOC conversation will then begin, with the peer sending an EAP-Response packet with EAP-Type=EAP-EDHOC. The data field of that packet will encapsulate the "EDHOC Message 1". The EAP server will then respond with an EAP-Request packet with EAP-Type=EAP-EDHOC. The data field of this packet will encapsulate "EDHOC Message 2" message. To this message, the EAP peer will send the and EAP-Response message containing the "EDHOC Message 3" message. In the case where the EDHOC mutual authentication is successful, the conversation will appear as follows:

One of the defining characteristics of EAP is its lock-step procedure. The EAP protocol manages the exchange of messages guaranteeing the order of transmission. In the same way, it manages retransmissions and the detection of duplicate messages. Therefore, EAP ensures the message correlation mechanism in the different EAP layers. Given the above, EDHOC does not need to use its internal mechanism for correlating messages. Then, the value for METHOD_CORR variable must satisfy the formula: METHOD_CORR = 4 * method + corr Where: method = EDHOC Method Type defined in Section 8.2 of EDHOC corr =

It is RECOMMENDED to use NAIs in the Identity Response as identities.

The identity provided in the EAP-Response/Identity is not authenticated by EAP-EDHOC, hence SHALL NOT be used for authorization or accounting purposes. The authenticator and the EAP server MAY examine the identity presented in EAP-Response/Identity for routing and EAP method selection.

EDHOC uses HKDF RFC 5869 to derive keys. HKDF-Extract is used for deriving fixed-length uniformly pseudorandom keys (PRK) from ECDH shared secrets. HKDF-Expand is used for deriving additional output keying material (OKM) from the PRKs. The derivation proceeds as follows: PRK = HKDF-Extract( salt, IKM ) Where: HKDF-Extract = RFC5869 HKDF function salt = The empty byte string IKM (input keying material) = The ECDH shared secret illustrates the EDHOC Key Hierarchy. In EAP-EDHOC, the MSK, EMSK, and Initialization Vector (IV) are derived from the PRK via a hash function. This ensures that the EDHOC PRK cannot be derived from the MSK, EMSK, or IV unless the hash function is defeated. Since the MSK and EMSK are derived from the EDHOC PRK, if the EDHOC PRK is compromised then the MSK and EMSK are also compromised. EAP-EDHOC derives exported keying material and parameters as follows: Type-Code = 0XFF Key_Material = HKDF-Expand(EDHOC PRK, "EAP-EDHOC encryption", 128) MSK = Key_Material(0,63) EMSK = Key_Material(64,127) IV = HKDF-Expand(EDHOC PRK, "EAP-EDHOC IV", 64) Session-Id = Type-Code || Method-Id Method-Id = HKDF-Expand(EDHOC PRK, "EAP_EDHOC_Method-Id", 64) Where: Key_Material(S,F) = Octets S through F inclusive of the key material.

TBD.

This work is possible due the EU Project IoTCrawlwer under grant agreement n.779852 and the EU Project INSPIRE-5Gplus under grant agreement n.871808