Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum Security

The Internet Key Exchange Protocol version 2, defined in , is used in the IPsec architecture to perform authenticated key exchange. defines an extension of IKEv2 for protecting today's VPN traffic against future quantum computers. At the time this extension was being developed, it was a consensus in the IPSECME WG that only IPsec traffic needs to have such a protection. It was believed that no sensitive information is transferred over IKE SA and extending the protection to also cover IKE SA traffic would require serious modifications to core IKEv2 protocol, that contradicted to one of the goals to minimize such changes. For the cases when this protection is needed it was suggested to immediately rekey IKE SA once it is created. In some situations it is desirable to have this protection for IKE SA from the very beginning, when an initial IKE SA is created. An example of such situation is Group Key Management protocol using IKEv2, defined in . In this protocol session keys are transferred from Group Controller/Key Server (GCKS) to Group Members (GM) immediately once an initial IKE SA is created. While it is possible to postpone transfer of the keys until the IKE SA is rekeyed (and specifies how to do this), the needed sequence of actions introduces an additional delay and adds unnecessary complexity to the protocol. Since was written, a new IKE_INTERMEDIATE exchange for IKEv2 was defined in . While the primary motivation for developing this exchange was to allow multiple key exchanges to be used in IKEv2 (which is defined in ), the IKE_INTERMEDIATE exchange itself can be used for other purposes too. This specification makes use of the IKE_INTERMEDIATE exchange to define an alternative approach to , which allows getting protection against quantum computers for initial IKE SA.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. We will use a term Conventional Approach in the content of using PPK to refer to the and a term Alternative Approach to refer to this specification.

The IKE initiator who supports the IKE_INTERMEDIATE exchange and wants to use PPK includes both the INTERMEDIATE_EXCHANGE_SUPPORTED and the USE_PPK notifications in the IKE_SA_INIT request. If the responder supports the IKE_INTERMEDIATE exchange and is willing to use PPK, it includes both these notifications in the response.

<--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(INTERMEDIATE_EXCHANGE_SUPPORTED), N(USE_PPK) ]]> If this is the case, then the initiator MAY choose to use the IKE_INTERMEDIATE exchange to negotiate PPK identity with the responder. Note, that it is up to the initiator whether to use the alternative or conventional approaches, i.e. whether to send PPK identity in the IKE_INTERMEDIATE exchange or in the IKE_AUTH exchange, as defined in the . If the initiator decides to use alternative approach, it includes one or more PPK_IDENTITY_KEY notification containing PPK identities the initiator believes are appropriate for the IKE SA being created, into the IKE_INTERMEDIATE request. The PPK_IDENTITY_KEY is a Status Type IKEv2 notification. Its Notify Message Type is <TBA by IANA>, Protocol ID and SPI Size fields are both set to 0. The format of the notification data is shown below on .

Where: PPK_ID (variable) -- PPK_ID as defined in Section 5.1 of . PPK Confirmation (8 octest) -- value, which allows the responder to check whether it has the same PPK as the initiator for a given PPK_ID. This field contains the first 8 octets of a string computed as prf( PPK, Ni | Nr | SPIi | SPIr ), where prf is the negotiated PRF; PPK is the key value for a specified PPK_ID; Ni, Nr, SPIi, SPIr -- nonces and IKE SPIs for the SA being established. If a series of the IKE_INTERMEDIATE exchanges takes place, the PPK_IDENTITY_KEY notification(s) MUSTbcp14> be sent in the last one, i.e. in the IKE_INTERMEDIATE exchange immediately preceding the IKE_AUTH exchange. If the last IKE_INTERMEDIATE exchange contains other payloads aimed for some other purpose, then the notification(s) MAY be piggybacked with these payloads.

]]> Depending on the responder's capabilities and policy the following situations are possible. If the responder doesn't support the alternative approach, it will ignore the received PPK_IDENTITY_KEY notification(s) and won't include any additional notifications in the response.

In this case the initiator cannot make an initial IKE SA to be a quantum computer resistant, so if this is a requirement for the initiator, then it MUSTbcp14> abort creating IKE SA. Otherwise, the initiator continues with the IKE_AUTH exchange and MAY try to use PPK as described in . If the responder supports this extension, but doesn't have any of the PPKs which IDs were sent by the initiator or it has some of proposed PPKs, but their values mismatch the initiator's ones (based on the information from the PPK Confirmation field), then it MUSTbcp14> return the PPK_IDENTITY notification containing no data.

In this case the initiator cannot achieve quantum computer resistance using the proposed PPKs. If this is a requirement for the initiator, then it MUSTbcp14> abort creating IKE SA. Otherwise, the initiator continues with the IKE_AUTH exchange as defined in . The initiator SHOULD NOT fall back to the IKE_AUTH exchange defined in in this case, since the initiator already knows that no one of the proposed PPKs is suitable for the responder. If using PPK is mandatory for the responder, then it will send back the AUTHENTICATION_FAILED notification in the IKE_AUTH response (as specified in Table 1 of ). Otherwise the IKE SA will be created as per . If the responder supports this extension and is configured with one of the PPKs which IDs were sent by the initiator and this PPK matches the initiator's one (based on the information from the PPK Confirmation field), then the responder selects this PPK and returns back its identity in the PPK_IDENTITY notification.

In this case the IKE_AUTH exchange is performed as defined in , so that neither PPK_IDENTITY nor NO_PPK_AUTH notifications are sent, since it's already known which PPK to use. However, the keys for the IKE SA are computed using PPK, as described in . If the responder returns PPK identity that was not proposed by the initiator, then the initiator must treat this as a fatal error and MUSTbcp14> abort the IKE SA establishment. Since the responder selects PPK before it knows the identity of the initiator, a situation may occur, when the responder agrees to use some PPK in the IKE_INTERMEDIATE exchange, but later discovers during the IKE_AUTH exchange that this particular PPK is not associated with the initiator's identity in its local policy. Note, that the responder does have this PPK, but it is just not listed among the PPKs for using with this initiator. In this case the responder SHOULD abort negotiation and return back the AUTHENTICATION_FAILED notification to be consistent with its policy. However, if using PPK with this initiator is marked optional in the local policy, then the responder MAY continue creating IKE SA using the negotiated "wrong" PPK.

Once the PPK is negotiated in the last IKE_INTERMEDIATE exchange, the IKE SA keys are recalculated. Note that if the IKE SA keys are also recalculated as the result of the other actions performed in the IKE_INTERMEDIATE exchange (for example, as defined in , then applying PPK MUSTbcp14> be done after all of them, so that recalculating IKE SA keys with PPK is the last action before they are used in the IKE_AUTH exchange. The IKE SA keys are computed differently compared to . A new SKEYSEED' value is computed using the negotiated PPK and the most recently computed SK_d key. Note, that the PPK is applied to SK_d exactly how it is specified in , and the result is used as SKEYSEED'.

Then the SKEYSEED' is used to recalculate all SK_* keys as defined in Section 2.14 of .

In the formula above Ni and Nr are nonces from the IKE_SA_INIT exchange and SPIi, SPIr - SPIs of the IKE SA being created. Note, that SK_d, SK_pi, and SK_pr are not individually recalculated using PPK, as it is defined in . The resulting keys are then used in the IKE_AUTH exchange and in the created IKE SA.

This specification isn't intended to be a replacement for . Instead, it is supposed to be used in situations where the conventional approach has a significant shortcomings. However, if the partners support both approaches, then the alternative approach MAY also be used in situations where convenient approach suffices. The alternative approach has the following advantages: The main advantage of the alternative approach is that it allows an initial IKE SA to be protected against quantum computers. This is important for those IKE extensions which transfer sensitive information, e.g. cryptographic keys, over initial IKE SA. The prominent example of such extensions is . Using the alternative approach allows the initiator to specify several appropriate PPKs and the responder to choose one of them. This feature could simplify PPK rollover. With the alternative approach there is no need for the initiator to calculate the content of the AUTH payload twice (with and without PPK) to support a situation when using PPK is optional for both sides. The main disadvantage of the alternative approach is that it requires an additional round trip (the IKE_INTERMEDIATE exchange) to set up IKE SA. However, if the IKE_INTERMEDIATE exchange has to be used for some other purposes in any case, then PPK stuff can be piggybacked with other payloads, thus eliminating this penalty.

Security considerations of using Post-quantum Preshared Keys in the IKEv2 protocol are discussed in . This specification defines an alternative way of exchanging PPK identity information.

This document defines a new Notify Message Type in the "IKEv2 Notify Message Types - Status Types" registry:

PPK_IDENTITY_KEY ]]>

The author would like to thank Paul Wouters for valuable comments and Tero Kivinen for pointing out to the problem of mismatched preshared keys.