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

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Table of Contents

2. Terminology and Notation

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 [RFC2119] [RFC8174] 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 [RFC8784] and a term Alternative Approach to refer to this specification.¶

3. Alternative Approach Description

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.¶

Initiator                       Responder
------------------------------------------------------------------
HDR, SAi1, KEi, Ni,
N(INTERMEDIATE_EXCHANGE_SUPPORTED),
N(USE_PPK)              --->
                        <---    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 [RFC8784].¶

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 Figure 1.¶

                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                             PPK_ID                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                        PPK Confirmation                       +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Where:¶

• PPK_ID (variable) -- PPK_ID as defined in Section 5.1 of [RFC8784].¶
• 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) MUST 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.¶

Initiator                         Responder
------------------------------------------------------------------
HDR, SK { ... N(PPK_IDENTITY_KEY, PPK_ID_1)
           [, N(PPK_IDENTITY_KEY, PPK_ID_2)] ...
           [, N(PPK_IDENTITY_KEY, PPK_ID_n)]}   --->

Depending on the responder's capabilities and policy the following situations are possible.¶

1. 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.¶

   Initiator                       Responder
   ---------------------------------------------------------------
                           <---    HDR, SK { ... }
   

   ¶

   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 MUST abort creating IKE SA. Otherwise, the initiator continues with the IKE_AUTH exchange as described in [RFC8784].¶

2. 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 MUST return the PPK_IDENTITY notification containing no data.¶

   Initiator                       Responder
   ---------------------------------------------------------------
                           <---    HDR, SK { N(PPK_IDENTITY) }
   

   ¶

   In this case the initiator cannot achieve quantum computer resistance using the proposed PPKs. If this is a requirement for the initiator, then it MUST abort creating IKE SA. Otherwise, the initiator continues with the IKE_AUTH exchange as defined in [RFC7296]. The initiator SHOULD NOT fall back to the IKE_AUTH exchange defined in [RFC8784] 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 [RFC8784]). Otherwise the IKE SA will be created as per [RFC7296].¶

3. 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.¶

   Initiator                       Responder
   ---------------------------------------------------------------
                  <---    HDR, SK { ... N(PPK_IDENTITY, PPK_ID_i)}
   

   ¶

   In this case the IKE_AUTH exchange is performed as defined in [RFC7296], 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 Section 4. If the responder returns PPK identity that was not proposed by the initiator, then the initiator must treat this as a fatal error and MUST abort the IKE SA establishment.¶

Note, that in the last two cases the IKE_AUTH exchange is performed as per [RFC7296], despite the fact that peers exchanged the USE_PPK notifications in the IKE_SA_INIT exchange, and, according to the [RFC8784], they were required to use the modified IKE_AUTH exchnage defined there. This requirement is relaxed if peers follow this specification.¶

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.¶

4. Computing IKE SA Keys

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 [I-D.ietf-ipsecme-ikev2-multiple-ke], then applying PPK MUST 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 [RFC8784]. 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 [RFC8784], and the result is used as SKEYSEED'.¶

SKEYSEED' = prf+ (PPK, SK_d)

Then the SKEYSEED' is used to recalculate all SK_* keys as defined in Section 2.14 of [RFC7296].¶

{SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr}
                           = prf+ (SKEYSEED', Ni | Nr | SPIi | SPIr )

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 [RFC8784].¶

The resulting keys are then used in the IKE_AUTH exchange and in the created IKE SA.¶

5. Comparison of the Conventional and the Alternative Approaches

This specification isn't intended to be a replacement for [RFC8784]. 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:¶

1. 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 [I-D.ietf-ipsecme-g-ikev2].¶
2. 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.¶
3. 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.¶

6. Security Considerations

Security considerations of using Post-quantum Preshared Keys in the IKEv2 protocol are discussed in [RFC8784]. This specification defines an alternative way of exchanging PPK identity information.¶

7. IANA Considerations

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

<TBA>       PPK_IDENTITY_KEY

8. Acknowledgements

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

Author's Address