EAP Re-authentication Protocol Extensions for Authenticated Anticipatory Keying (ERP/AAK)

The Extensible Authentication Protocol (EAP) is a generic framework supporting multiple types of authentication methods. In systems where EAP is used for authentication, it is desirable to not repeat the entire EAP exchange with another authenticator. The EAP Re-authentication Protocol (ERP) specifies extensions to EAP and the EAP keying hierarchy to support an EAP method-independent protocol for efficient re-authentication between the EAP re-authentication peer and an EAP re-authentication server through any authenticator. The re-authentication server may be in the home network or in the local network to which the mobile host (i.e., the EAP re-authentication peer) is connecting. Authenticated Anticipatory Keying (AAK) is a method by which cryptographic keying materials may be established prior to handover upon one or more candidate attachment points (CAPs). AAK utilizes the AAA infrastructure for key transport. This document specifies the extensions necessary to enable AAK support in ERP.

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 following acronyms are used in this document; see the references for more details. Authentication, Authorization and Accounting Candidate Attachment Point Abbreviation for "ERP/AAK" An EAP peer that supports the ERP/AAK. Note that all references to "peer" in this document imply an EA peer, unless specifically noted otherwise. Serving Attachment Point

ERP/AAK is intended to allow the establishment of cryptographic keying materials on a single Candidate Attachment Point prior to the arrival of the peer at the Candidate Access Network (CAN) upon request by the peer. In this document, ERP/AAK support by the peer is assumed. Also it is assumed that the peer has previously completed full EAP authentication and that either the peer or SAP knows the identities of neighboring attachment points. Note that the behavior of a peer that does not support the ERP-AAK scheme defined in this specification is out of the scope of this document. shows the general protocol exchange by which the keying material is established on the CAP.

+------+ +-----+ +-----+ +-----------+ | Peer | | SAP | | CAP | | EA Server | +--+---+ +--+--+ +--+--+ +-----+-----+ | | | | a. | [EAP-Initiate/ | | | | Re-auth-start | | | | (E-flag)] | | | |<---------------| | | | | | | b. | EAP-Initiate/ | | | | Re-auth | | | | (E-flag) | | | |--------------->| | | c. | | AAA(EAP-Initiate/Re-auth(E-flag))| | |--------------------------------->| | | | +---------+---------+ | | | | CA authorized & | d. | | | | and EA Keying | | | | | Distribution | | | | +---------+---------+ | | | | | | | | f. | | AAA (EAP-Finish/Re-auth(E-flag)) | | |<---------------------------------| g. | EAP-Finish/ | | | | Re-auth(E-flag)| | | |<---------------| | | | | | |

+-----------+ +---------+ | | | | | EA Server | | CAP | | | | | +-----|-----+ +----|----+ | | | | | AAA Request(pMSK) | e.1|------------------------->| | | | | | | | AAA Response (Success) | e.2|<-------------------------| | | | | | | ERP/AAK re-uses the packet format defined by ERP, but specifies a new flag to differentiate EAP early-authentication from EAP re-authentication. The peer initiates ERP/AAK itself, or does so in response to an EAP-Initiate/Re-Auth-Start message from the SAP. In the latter case, the SAP MAY send the identity of one or more Candidate Attachment Points to which the SAP is adjacent to the peer in the EAP-Initiate/Re-auth-Start message (see a. in ). The Peer SHOULD override the identity of CAP(s) carried in EAP-Initiate/Re-auth-Start message by sending EAP-Initiate/Re-auth with the 'E' flag set if it knows to which CAP it will move. If the EAP-Initiate/ Re-auth-Start packet is not supported by the peer, it MUST be silently discarded. If the peer initiates ERP/AAK, the peer MAY send an early-authentication request message (EAP-Initiate/Re-auth with the 'E' flag set) containing the keyName-NAI, the CAP-Identifier, rIK and sequence number (see b. in ). The realm in the keyName-NAI field is used to locate the peer's ERP/AAK server. The CAP-Identifier is used to identify the CAP. The rIK is defined in Narayanan & Dondeti and used to protect the integrity of the message. The sequence number is used for replay protection. The SAP SHOULD verify the integrity of thus message at step b. If this verification fails, the SAP MUST send an EAP-Finish/Re-auth message with the Result flag set to '1' (Failure). If the verification succeeds, the SAP SHOULD encapsulate the early-authentication message into a AAA message and send it to the peer's ERP/AAK server in the realm indicated in the keyName-NAI field (see c. in ). Upon receiving the message, the ERP/AAK server MUST first use the keyName indicated in the keyName-NAI to look up the rIK and check the integrity and freshness of the message. Then the ERP/AAK server MUST verify the identity of the peer by checking the username portion of the KeyName-NAI. If any of the checks fail, the server MUST send an early- authentication finish message (EAP-Finish/Re-auth with E-flag set) with the Result flag set to '1'. Next, the server MUST authorize the CAP specified in the CAP-Identifier TLV. In the success case, the server MUST derive a pMSK from the pRK for the CAP carried in the the CAP-Identifier field using the sequence number associated with CAP-Identifier as an input to the key derivation. (see d. in ). Then the ERP/AAK server MUST transport the pMSK to the authorized CAP via AAA as illustrated above (see e.1 and e.2 in ). Note that key distribution in is one part of step d. in . Finally, in response to the EAP-Initiate/Re-auth message, the ERP/AAK server SHOULD send the early-authentication finish message (EAP-Finish/ Re-auth with E-flag set) containing the identity of the authorized CAP to the peer via the SAP along with the lifetime of the pMSK. If the peer also requests the rRK lifetime, the ERP/AAK server SHOULD send the rRK lifetime in the EAP-Finish/Re-auth message. (see f. and g. in ).

ERP/AAK uses a key hierarchy similar to that of ERP. The ERP/AAK pre-established Root Key (pRK) is derived from either the EMSK or the DSRK as specified below (see ). In general, the pRK is derived from the EMSK if the peer is located in the home AAA realm and derived from the DRSK if the peer is in a visited realm. The DSRK is delivered from the EAP server to the ERP/AAK server as specified in . If the peer has previously been authenticated by means of ERP or ERP/AAK, the DSRK SHOULD be directly re-used.

DSRK EMSK | | +---+---+---+---+ | pRK ... Similarly, the pre-established Master Session Key (pMSK) is derived from the pRK. The pMSK is established for the CAP when the peer early authenticates to the network. The hierarchy relationship is illustrated ,

pRK | +--------+--------+ | pMSK ... below.

The rRK is derived as specified in . pRK = KDF (K, S), where K = EMSK or K = DSRK and S = pRK Label | "\0" | length The pRK Label is an IANA-assigned 8-bit ASCII string: EAP Early-Authentication Root Key@ietf.org assigned from the "USRK key labels" name space in accordance with Salowey, et al. . The KDF and algorithm agility for the KDF are also defined in RFC 5295. The KDF algorithm is indicated in the cryptosuit field or list of cryptosuits TLV payload as specified in the section 5.2 and section 5.3. The pMSK uses the same PDF as pRK and is derived as follows: pMSK = KDF (K, S), where K = pRK and S = pMSK label | "\0" | SEQ | length The pMSK label is the 8-bit ASCII string: EAP Early-Authentication Master Session Key@ietf.org The length field refers to the length of the pMSK in octets encoded as specified in RFC 5295. SEQ is sent by either the peer or the server in the ERP/AAK message using the SEQ field or the Sequence number TLV and encoded as an 16-bit number as specified in and .

This section describes the packet and TLV extensions for the ERP/AAK exchange.

shows the new parameters contained in the EAP-Initiate/Re-auth-Start packet defined in RFC 5296.

illustrates the new parameters contained in the EAP-Initiate/Re-auth packet defined in RFC 5296.

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |R|x|L|E|Resved | SEQ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 or more TVs or TLVs ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cryptosuite | Authentication Tag ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Flags 'x' - The x flag is reserved. It MUST be ignored on receipt. 'L' - As defined in section 5.3.2 of , this bit is used to request the key lifetimes from the server. 'E' - The E flag is used to indicate early-authentication. The first bit(R) and final 4 bits (Resved) MUST be set to 0 and ignored on reception. SEQ As defined in Section 5.3.2 of ,this field is 16-bit sequence number and used for replay protection. TVs and TLVs keyName-NAI: As defined in RFC 5296, this is carried in a TLV payload. The Type is 1. The NAI is variable in length, not exceeding 253 octets. The username part of the NAI is the EMSKname used to identify the peer. The realm part of the NAI is the peer's home domain name if the peer communicates with the home EA server or the domain to which the peer is currently attached (i.e., local domain name) if the peer communicates with a local EA server. The SAP knows whether the KeyName-NAI carries the local domain name by comparing the domain name carried in KeyName-NAI with the local domain name which is associated with the SAP. Exactly one keyName-NAI attribute SHALL be present in an EAP-Initiate/Re-auth packet and the realm part of it SHOULD follow the use of internationalized domain names defined in RFC5890 . CAP-Identifier: Carried in a TLV payload.The Type is TBD (less than 128). This field is used to indicate the FQDN of a CAP. The value field MUST be encoded as specified in Section 8 of RFC 3315 . Exactly one instance of the CAP-Identifier TLV MUST be present in the ERP/AAK-Key TLV. Sequence number: The Type is TBD (less than 128). The value field is a 16-bit field and used in the derivation of the pMSK for a CAP. Cryptosuite This field indicates the integrity algorithm used for ERP/AAK. Key lengths and output lengths are either indicated or obvious from the cryptosuite name, e.g., HMAC-SHA256-128 denotes HMAC computed using the SHA-256 function with 256-bit key length and the output truncated to 128 bits . We specify some cryptosuites below: RESERVED HMAC-SHA256-128 HMAC-SHA256-256 HMAC-SHA256-128 is REQUIRED to implement and SHOULD be enabled in the default configuration. Authentication Tag This field contains an integrity checksum over the ERP/AAK packet from the first bit of the Code field to the last bit of the Cryptosuite field, excluding the Authentication Tag field itself. The value field is calculated using the integrity algorithm indicated in the Cryptosuite field and rIK specified in as the secret key. The length of the field is indicated by the Cryptosuite. The peer uses the Authentication Tag to determine the validity of the EAP-Finish/Re-auth message from the server. If the message doesn't pass verification or the Authentication Tag is not included in the message, the message SHOULD be discarded silently. If the EAP-Initiate/Re-auth packet is not supported by the SAP, it SHOULD be discarded silently. The peer MUST maintain retransmission timers for reliable transport of the EAP-Initiate/Re-auth message. If there is no response to the EAP-Initiate/Re-auth message from the server after the necessary number of retransmissions (see ), the peer MUST assume that ERP/AAK is not supported by the SAP.

shows the new parameters contained in the EAP-Finish/Re-auth packet defined in .

ERP/AAK-Key ::= { sub-TLV: CAP-Identifier } { sub-TLV: pMSK-lifetime } { sub-TLV: pRK-lifetime } { sub-TLV: Cryptosuites } Carried in a sub-TLV payload. The Type is TBD (less than 128). This field is used to indicate the identifier of the candidate authenticator. The value field MUST be encoded as specified in Section 8 of RFC 3315 . There at least one instance of the CAP-Identifier TLV MUST be present in the ERP/AAK-Key TLV. Carried in a sub-TLV payload of the EAP-Finish/Re-auth message. The Type is TBD. The value field is an unsigned 32-bit field and contains the lifetime of the pMSK in seconds. This value is calculated by the server after pRK-lifetime computation upon receiving the EAP-Initiate/Re-auth message. The rIK SHOULD share the same lifetime as the pMSK. If the 'L' flag is set, the pMSK-Lifetime attribute MUST be present. Carried in a sub-TLV payload of EAP-Finish/Re-auth message. The Type is TBD. The value field is an unsigned 32-bit field and contains the lifetime of the pRK in seconds. This value is calculated by the server before pMSK-lifetime computation upon receiving a EAP-Initiate/Re-auth message. If the 'L' flag is set, the pRK-Lifetime attribute MUST be present. Carried in a sub-TLV payload. The Type is 5 . The value field contains a list of cryptosuites (at least one cryptosuite SHOULD be included), each 1 octet in length. The allowed cryptosuite values are as specified in , above. The server SHOULD include this attribute if the cryptosuite used in the EAP-Initiate/Re-auth message was not acceptable and the message is being rejected. The server MAY include this attribute in other cases. The server MAY use this attribute to signal to the peer about its cryptographic algorithm capabilities. Cryptosuite This field indicates the integrity algorithm and PRF used for ERP/ AAK. HMAC-SHA256-128 is REQUIRED to implement and SHOULD be enabled in the default configuration. Key lengths and output lengths are either indicated or obvious from the cryptosuite name. Authentication Tag This field contains the integrity checksum over the ERP/AAK packet from the first bit of the Code field to the last bit of the Cryptosuite field excluding the Authentication Tag field itself. The value field is calculated using the integrity algorithm indicated in the Cryptosuite field and the rIK as the integrity key. The length of the field is indicated by the corresponding Cryptosuite. The peer uses the authentication tag to determine the validity of the EAP-Finish/Re-auth message from a server. If the message doesn't pass verification or the authentication tag is not included in the message, the message SHOULD be discarded silently. If the EAP-Initiate/Re-auth packet is not supported by the SAP, it is discarded silently. The peer MUST maintain retransmission timers for reliable transport of EAP-Initiate/Re-auth message. If there is no response to the EAP-Initiate/Re-auth message from the server after the necessary number of retransmissions (See ), the peer MUST assume that ERP/AAK is not supported by the SAP.

With the exception of the rRK-Lifetime and rMSK-Lifetime TV payloads, the attributes specified in Section 5.3.4 of also apply to this document. In this document, new attributes which may be present in the EAP-Initiate and EAP-Finish messages are defined as below: Sequence number: This is a TV payload. The type is 7. ERP/AAK-Key: This is a TLV payload. The type is 8. pRK-Lifetime: This is a TV payload. The type is 9. pMSK-Lifetime: This is a TV payload. The type is 10. CAP-Identifier: This is a TLV payload. The type is 11.

Similar to ERP, some lower layer specifications may need to be revised to support ERP/AAK; refer to Section 6 of for additional guidance.

The AAA transport of ERP/AAK messages is the same as that of the ERP message . In addition, this document requires AAA transport of the ERP/AAK keying materials delivered by the ERP/AAK server to the CAP. Hence, a new AAA message for the ERP/AAK application should be specified to transport the keying materials.

This section provides an analysis of the protocol in accordance with the AAA key management requirements specified in RFC 4962 . Cryptographic algorithm independence: ERP-AAK satisfies this requirement. The algorithm chosen by the peer for calculating the authentication tag is indicated in the EAP-Initiate/Re-auth message. If the chosen algorithm is unacceptable, the EAP server returns an EAP-Finish/Re-auth message with a Failure indication. Strong, fresh session keys: ERP-AAK results in the derivation of strong, fresh keys that are unique for the given CAP. An pMSK is always derived on-demand when the peer requires a key with a new CAP. The derivation ensures that the compromise of one pMSK does not result in the compromise of a different pMSK at any time. Limit key scope: The scope of all the keys derived by ERP-AAK is well defined. The pRK is used to derive the pMSK for the CAP. Different sequence numbers for each CAP MUST be used to derive a unique pMSK. Replay detection mechanism: For replay protection a sequence number associated with the pMSK is used.The peer increments the sequence number by one after it sends an ERP/AAK message. The server sets the expected sequence number to the received sequence number plus one after verifying the validity of the received message and responds to the message. Authenticate all parties: The EAP Re-auth Protocol provides mutual authentication of the peer and the server. The peer and SAP are authenticated via ERP. The CAP is authenticated and trusted by the SAP. Peer and authenticator authorization: The peer and authenticator demonstrate possession of the same keying material without disclosing it, as part of the lower layer secure authentication protocol. Keying material confidentiality: The peer and the server derive the keys independently using parameters known to each entity. Uniquely named keys: All keys produced within the ERP context can be referred to uniquely as specified in this document. Prevent the domino effect: Different sequence numbers for each CAP MUST be used to derive the unique pMSK. So the compromise of one pMSK does not hurt any other CAP. Bind key to its context: the pMSK are bound to the context in which the sequence numbers are transmitted. Confidentiality of identity: this is the same as with the ERP protocol . Authorization restriction: All the keys derived are limited in lifetime by that of the parent key or by server policy. Any domain-specific keys are further restricted to be used only in the domain for which the keys are derived. Any other restrictions of session keys may be imposed by the specific lower layer and are out of scope for this specification.

IANA is requested to assign four TLV type values from the registry of EAP Initiate and Finish Attributes maintained at http://www.iana.org/assignments/eap-numbers/eap-numbers.xml. with the following numbers: Sequence number: This is a TV payload. The type is 7. ERP/AAK-Key: This is a TLV payload. The type is 8. pRK Lifetime: This is a TLV payload. The type is 9. pMSK Lifetime: This is a TLV payload. The type is 10. CAP-Identifier: This is a TLV payload. The type is 11. This document reuses the crytosuites have already created for 'Re-authentication Cryptosuites' in . Further, this document instructs IANA to add a new label in the User Specific Root Keys (USRK) Key Labels of the Extended Master Session Key (EMSK) Parameters registry, as follows: EAP Early-Authentication Root Key@ietf.org This document creates a new registry for the flags in the EAP Initiate/Re-auth-Start message called the "EAP Initiate/Re-auth-Start Flags" and assigns a new flag (E) as follows: (E) 0x80 The rest of the values in the 8-bit field are reserved. New values can be assigned by Standards Action or IESG approval. This document also creates a new registry for the flags in the EAP Initiate/Re-auth message called the "EAP Initiate/Re-auth Flags". The following flag are reserved: (B) 0x40 [RFC5296] (L) 0x20 [RFC5296] This document assigns a new flag (E) as follows: (E) 0x10 The rest of the values in the 8-bit field are reserved. New values can be assigned by Standards Action or IESG approval. Further,this document creates a new registry for the flags in the EAP Finish/Re-auth message called the "EAP Finish/Re-auth Flags". The following values are reserved. (R) 0x80 [RF5296] (B) 0x40 [RFC5296] (L) 0x20 [RFC5296] This document assigns a new flag (E) as follows: (E) 0x10 The rest of the values in the 8-bit field are reserved. New values can be assigned by Standards Action or IESG approval.

In writing this document, Yungui Wang contributed to early versions of this document and we have received reviews from many experts in the IETF, including Tom Taylor, Tena Zou, Tim Polk, Tan Zhang, Semyon Mizikovsky, Stephen Farrell, Radia Perlman, Miguel A. Garcia and Sujing Zhou. We apologize if we miss some of those who have helped us.