Using the Extensible Authentication Protocol (EAP) with Ephemeral Diffie-Hellman over COSE (EDHOC)

Protocol Overview

Overview of the EAP-EDHOC Conversation The EDHOC protocol running between an Initiator and a Responder consists of three mandatory messages (message_1, message_2, message_3), an optional message_4, and an error message.
In an EDHOC session, EAP-EDHOC uses all messages including message_4, which is mandatory and acts as a protected success indication. After receiving an EAP-Request packet with EAP-Type=EAP-EDHOC as described in this document, the conversation will continue with the EDHOC messages transported in the data fields of EAP-Response and EAP-Request packets. When EAP-EDHOC is used, the formatting and processing of EDHOC messages SHALL be done as specified in . This document only lists additional and different requirements, restrictions, and processing compared to . As a reminder of the EAP entities and their roles involved in the EAP exchange, we have the EAP peer, EAP authenticator and EAP server. The EAP authenticator is the entity initiating the EAP authentication. The EAP peer is the entity that responds to the EAP authenticator. The EAP server is the entity that determines the EAP authentication method to be used. If the EAP server is not located on a backend authentication server, the EAP server is part of the EAP authenticator. For simplicity, we will show in the Figures with flows of operation only the EAP peer and EAP server.

Authentication EAP-EDHOC authentication credentials can be of any type supported by COSE and be transported or referenced by EDHOC. EAP-EDHOC provides forward secrecy, by means of the ephemeral Diffie-Hellman public keys exchanged in message_1 and message_2. The optimization combining the execution of EDHOC with the first subsequent OSCORE transaction specified in is not supported in this EAP method. shows an example message flow for a successful execution of EAP-EDHOC.

EAP-EDHOC Message Flow | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC Start) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_1) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_2) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_3) | | ----------------------------------------------------> | | | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_4) | | <--------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | ---------------------------------------------------> | | EAP-Success | | <--------------------------------------------------- | + + ]]> If the EAP-EDHOC peer authenticates successfully, the EAP-EDHOC server MUST send an EAP-Request packet with EAP-Type=EAP-EDHOC containing message_4 as a protected success indication. If the EAP-EDHOC server authenticates successfully, and the EAP-EDHOC peer achieves key confirmation by successfully verifying EDHOC message_4, then the EAP-EDHOC peer MUST send an EAP-Response message with EAP-Type=EAP-EDHOC containing no data. Finally, the EAP-EDHOC server sends an EAP-Success.

Transport and Message Correlation EDHOC is not bound to a particular transport layer and can even be used in environments without IP. Nonetheless, provides a set of requirements for a transport protocol to use with EDHOC. These include: handling the loss, reordering, duplication, correlation, and fragmentation of messages; demultiplexing EDHOC messages from other types of messages; and denial-of-service protection. All these requirements are fulfilled by the EAP protocol, EAP method, or EAP lower layer, as specified in . For message loss, this can be either fulfilled by the EAP layer, or the EAP lower layer, or both. For reordering, EAP relies on the EAP lower layer ordering guarantees, for correct operation. For duplication and message correlation, EAP has the Identifier field, which allows both the EAP peer and EAP authenticator to detect duplicates and match a request with a response. Fragmentation is defined by this EAP method, see . The EAP framework , specifies that EAP methods need to provide fragmentation and reassembly if EAP packets can exceed the minimum MTU of 1020 octets. To demultiplex EDHOC messages from other types of messages, EAP provides the Type field. This method does not provide other mitigation against denial-of-service than EAP .

Termination , , , and illustrate message flows in several cases where the EAP-EDHOC peer or EAP-EDHOC server sends an EDHOC error message. shows an example message flow where the EAP-EDHOC server rejects message_1 with an EDHOC error message.

EAP-EDHOC Server rejection of message_1 | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC Start) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_1) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC error) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | ----------------------------------------------------> | | | | EAP-Failure | | <---------------------------------------------------- | | | ]]> shows an example message flow where the EAP-EDHOC server authentication is unsuccessful and the EAP-EDHOC peer sends an EDHOC error message.

EAP-EDHOC Peer rejection of message_2 | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC Start) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_1) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_2) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC error) | | ----------------------------------------------------> | | EAP-Failure | | <---------------------------------------------------- | ]]> shows an example message flow where the EAP-EDHOC server authenticates to the EAP-EDHOC peer successfully, but the EAP-EDHOC peer fails to authenticate to the EAP-EDHOC server, and the server sends an EDHOC error message.

EAP-EDHOC Server rejection of message_3 | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC Start) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_1) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_2) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_3) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC error) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | ----------------------------------------------------> | | | | EAP-Failure | | <---------------------------------------------------- | | | ]]> shows an example message flow where the EAP-EDHOC server sends the EDHOC message_4 to the EAP peer, but the protected success indication fails, and the peer sends an EDHOC error message.

EAP-EDHOC Peer rejection of message_4 | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC Start) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_1) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_2) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_3) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_4) | | <--------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC error) | | ----------------------------------------------------> | | EAP-Failure | | <---------------------------------------------------- | | | ]]>

Identity It is RECOMMENDED to use anonymous NAIs in the Identity Response as such identities are routable and privacy-friendly. While opaque blobs are allowed by , such identities are NOT RECOMMENDED as they are not routable and should only be considered in local deployments where the EAP-EDHOC peer, EAP authenticator, and EAP-EDHOC server all belong to the same network. Many client certificates contain an identity such as an email address, which is already in NAI format. When the certificate contains an NAI as subject name or alternative subject name, an anonymous NAI SHOULD be derived from the NAI in the certificate; See .

Privacy EAP-EDHOC peer and server implementations supporting EAP-EDHOC MUST support anonymous Network Access Identifiers (NAIs) (Section 2.4 of ). A node supporting EAP-EDHOC MUST NOT send its username (or any other permanent identifiers) in cleartext in the Identity Response (or any message used instead of the Identity Response). Following , it is RECOMMENDED to omit the username (i.e., the NAI is @realm), but other constructions such as a fixed username (e.g., anonymous@realm) or an encrypted username (e.g., xCZINCPTK5+7y81CrSYbPg+RKPE3OTrYLn4AQc4AC2U=@realm) are allowed. Note that the NAI MUST be a UTF-8 string as defined by the grammar in Section 2.2 of . EAP-EDHOC is always used with privacy. This does not add any extra round trips and the message flow with privacy is just the normal message flow as shown in .

Fragmentation EAP-EDHOC fragmentation support is provided through the addition of a flags octet within the EAP-Response and EAP-Request packets, as well as a (conditional) EAP-EDHOC Message Length field that can be one to four octets. To do so, the EAP request and response messages of EAP-EDHOC have a set of information fields that allow for the specification of the fragmentation process (See for the detailed description). Of these fields, we will highlight the one that contains the flag octet, which is used to steer the fragmentation process. If the L bits are set, we are specifying that the message will be fragmented and the length of the message, which is in the EAP-EDHOC Message Length field. Implementations MUST NOT set the L bit in unfragmented messages, but they MUST accept unfragmented messages with and without the L bit set. Some EAP implementations and access networks may limit the number of EAP packet exchanges that can be handled. To avoid fragmentation, it is RECOMMENDED to keep the sizes of EAP-EDHOC peer, EAP-EDHOC server, and trust anchor authentication credentials small and the length of the certificate chains short. In addition, it is RECOMMENDED to use mechanisms that reduce the sizes of Certificate messages. EDHOC is designed to perform well in constrained networks where message sizes are restricted for performance reasons. In the basic message construction, the size of the plaintext in message_2 is limited to the length of the output of the key derivation function, which in turn is determined by the EDHOC hash algorithm of the EDHOC cipher suite that is used in the EDHOC session. For example, with SHA-256 as EDHOC hash algorithm, the maximum size of plaintext in message_2 is 8160 bytes. However, EDHOC also defines an optional backward compatible method for handling arbitrarily long message_2 plaintext sizes, see Appendix G in . The other three EAP-EDHOC messages do not have an upper bound. Furthermore, when an EDHOC message specifies a certificate as the sender's authentication credential and the certificate is transported by value instead of identified by reference, the certificate may in principle be as long as 16 MB. Hence, the EAP-EDHOC messages sent in a single round may be larger than the MTU size or the maximum Remote Authentication Dial-In User Service (RADIUS) packet size of 4096 octets. As a result, an EAP-EDHOC implementation MUST provide its support for fragmentation and reassembly. Since EAP is a simple ACK-NAK protocol, fragmentation support can be easily added. In EAP, fragments that are lost or damaged in transit will be retransmitted, and since sequencing information is provided by the Identifier field in EAP, there is no need for a fragment offset field as is provided in IPv4. EAP-EDHOC fragmentation support is provided through the addition of flags octet within the EAP-Response and EAP-Request packets, as well as an EDHOC Message Length field. Flags include the Length included (L), More fragments (M), and EAP-EDHOC Start (S) bits. The L flag is set to indicate the presence of the EDHOC Message Length field, and MUST be set for the first fragment of a fragmented EDHOC message. The M flag is set on all but the last fragment. The S flag is set only within the EAP-EDHOC start message sent by the EAP server to the peer. The EDHOC Message Length field provides the total length of the EDHOC message that is being fragmented; this simplifies buffer allocation. When an EAP-EDHOC peer receives an EAP-Request packet with the M bit set, it MUST respond with an EAP-Response with EAP-Type=EAP-EDHOC and no data. This serves as a fragment ACK. The EAP server MUST wait until it receives the EAP-Response before sending another fragment. To prevent errors in the processing of fragments, the EAP server MUST increment the Identifier field for each fragment contained within an EAP-Request, and the peer MUST include this Identifier value in the fragment ACK contained within the EAP-Response. Retransmitted fragments will contain the same Identifier value. Similarly, when the EAP-EDHOC server receives an EAP-Response with the M bit set, it MUST respond with an EAP-Request with EAP-Type=EAP-EDHOC and no data. This serves as a fragment ACK. The EAP peer MUST wait until it receives the EAP-Request before sending another fragment. To prevent errors in the processing of fragments, the EAP server MUST increment the Identifier value for each fragment ACK contained within an EAP-Request, and the peer MUST include this Identifier value in the subsequent fragment contained within an EAP- Response. In the case where the EAP-EDHOC mutual authentication is successful, and fragmentation is required, the conversation, illustrated in will appear as follows:

Fragmentation example of EAP-EDHOC Authentication | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC Start, S bit set) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_1) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_2, | | Fragment 1: L,M bits set) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_2, | | (Fragment 2: M bits set) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_2, | | Fragment 3) | | <---------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_3, | | Fragment 1: L,M bits set) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | <--------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_3, | | Fragment 2: M bits set) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | <--------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_3, | | Fragment 3) | | ----------------------------------------------------> | | EAP-Request/ | | EAP-Type=EAP-EDHOC | | (EDHOC message_4) | | <--------------------------------------------------- | | EAP-Response/ | | EAP-Type=EAP-EDHOC | | ---------------------------------------------------> | | EAP-Success | | <--------------------------------------------------- | + + ]]>

Identity Verification The EAP peer identity provided in the EAP-Response/Identity is not authenticated by EAP-EDHOC. Unauthenticated information MUST NOT be used for accounting purposes or to give authorization. The EAP authenticator and the EAP server MAY examine the identity presented in EAP-Response/Identity for purposes such as routing and EAP method selection. EAP-EDHOC servers MAY reject conversations if the identity does not match their policy. The EAP server identity in the EDHOC server certificate is typically a fully qualified domain name (FQDN) in the SubjectAltName (SAN) extension. Since EAP-EDHOC deployments may use more than one EAP server, each with a different certificate, EAP peer implementations SHOULD allow for the configuration of one or more trusted root certificates (CA certificate) to authenticate the server certificate and one or more server names to match against the SubjectAltName (SAN) extension in the server certificate. If any of the configured names match any of the names in the SAN extension, then the name check passes. To simplify name matching, an EAP-EDHOC deployment can assign a name to represent an authorized EAP server and EAP Server certificates can include this name in the list of SANs for each certificate that represents an EAP-EDHOC server. If server name matching is not used, this degrades the confidence that the EAP server with which the EAP peer is interacting is authoritative for the given network. If name matching is not used with a public root CA, then effectively any server can obtain a certificate that will be trusted for EAP authentication by the peer. The process of configuring a root CA certificate and a server name is non-trivial; therefore, automated methods of provisioning are RECOMMENDED. For example, the eduroam federation provides a Configuration Assistant Tool (CAT) to automate the configuration process. In the absence of a trusted root CA certificate (user-configured or system-wide), EAP peers MAY implement a Trust On First Use (TOFU) mechanism where the peer trusts and stores the server certificate during the first connection attempt. The EAP peer ensures that the server presents the same stored certificate on subsequent interactions. The use of a TOFU mechanism does not allow for the server certificate to change without out-of-band validation of the certificate and is therefore not suitable for many deployments including ones where multiple EAP servers are deployed for high availability. TOFU mechanisms increase the susceptibility to traffic interception attacks and should only be used if there are adequate controls in place to mitigate this risk.

Key Hierarchy The key schedule for EDHOC is described in Section 4 of . The Key_Material and Method-Id SHALL be derived from the PRK_exporter using the EDHOC_Exporter interface, see Section 4.2.1 of . Type is the value of the EAP Type field defined in Section 2 of . For EAP-EDHOC, the Type field has the value TBD1. >, 64) EMSK = EDHOC_Exporter(TBD3 ,<< Type >>, 64) Method-Id = EDHOC_Exporter(TBD4, << Type >>, 64) Session-Id = Type || Method-Id ]]> EAP-EDHOC exports the MSK and the EMSK and does not specify how it is used by lower layers.

Parameter Negotiation and Compliance Requirements The EAP-EDHOC peers and EAP-EDHOC servers MUST comply with the compliance requirements (mandatory-to-implement cipher suites, signature algorithms, key exchange algorithms, extensions, etc.) defined in Section 8 of .

EAP State Machines The EAP-EDHOC server sends message_4 in an EAP-Request as a protected success result indication. EDHOC error messages SHOULD be considered failure result indication, as defined in . After sending or receiving an EDHOC error message, the EAP-EDHOC server may only send an EAP-Failure. EDHOC error messages are unprotected. The keying material can be derived after the EDHOC message_2 has been sent or received. Implementations following can then set the eapKeyData and aaaEapKeyData variables. The keying material can be made available to lower layers and the EAP authenticator after the protected success indication (message_4) has been sent or received. Implementations following can set the eapKeyAvailable and aaaEapKeyAvailable variables.