Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8784.¶

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

3. Exchanges

If the initiator is configured to use a PPK with the responder (whether or not the use of the PPK is mandatory), then it MUST include a notification USE_PPK in the IKE_SA_INIT request message as follows:¶

Initiator                       Responder
------------------------------------------------------------------
HDR, SAi1, KEi, Ni, N(USE_PPK)  --->

N(USE_PPK) is a status notification payload with the type 16435; it has a protocol ID of 0, no Security Parameter Index (SPI), and no notification data associated with it.¶

If the initiator needs to resend this initial message with a COOKIE notification, then the resend would include the USE_PPK notification if the original message did (see Section 2.6 of [RFC7296]).¶

If the responder does not support this specification or does not have any PPK configured, then it ignores the received notification (as defined in [RFC7296] for unknown status notifications) and continues with the IKEv2 protocol as normal. Otherwise, the responder replies with the IKE_SA_INIT message, including a USE_PPK notification in the response:¶

Initiator                       Responder
------------------------------------------------------------------
                <--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(USE_PPK)

When the initiator receives this reply, it checks whether the responder included the USE_PPK notification. If the responder did not include the USE_PPK notification and the flag mandatory_or_not indicates that using PPKs is mandatory for communication with this responder, then the initiator MUST abort the exchange. This situation may happen in case of misconfiguration, i.e., when the initiator believes it has a mandatory-to-use PPK for the responder and the responder either doesn't support PPKs at all or doesn't have any PPK configured for the initiator. See Section 6 for discussion of the possible impacts of this situation.¶

If the responder did not include the USE_PPK notification and using a PPK for this particular responder is optional, then the initiator continues with the IKEv2 protocol as normal, without using PPKs.¶

If the responder did include the USE_PPK notification, then the initiator selects a PPK, along with its identifier PPK_ID. Then, it computes this modification of the standard IKEv2 key derivation from Section 2.14 of [RFC7296]:¶

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

 SK_d  = prf+ (PPK, SK_d')
 SK_pi = prf+ (PPK, SK_pi')
 SK_pr = prf+ (PPK, SK_pr')

That is, we use the standard IKEv2 key derivation process, except that the three resulting subkeys SK_d, SK_pi, and SK_pr (marked with primes in the formula above) are then run through the prf+ again, this time using the PPK as the key. The result is the unprimed versions of these keys, which are then used as inputs to subsequent steps of the IKEv2 exchange.¶

Using a prf+ construction ensures that it is always possible to get the resulting keys of the same size as the initial ones, even if the underlying PRF has an output size different from its key size. Note that at the time of this writing, all PRFs defined for use in IKEv2 (see the "Transform Type 2 - Pseudorandom Function Transform IDs" subregistry [IANA-IKEV2]) have an output size equal to the (preferred) key size. For such PRFs, only the first iteration of prf+ is needed:¶

 SK_d  = prf (PPK, SK_d'  | 0x01)
 SK_pi = prf (PPK, SK_pi' | 0x01)
 SK_pr = prf (PPK, SK_pr' | 0x01)

Note that the PPK is used in SK_d, SK_pi, and SK_pr calculations only during the initial IKE SA setup. It MUST NOT be used when these subkeys are calculated as result of IKE SA rekey, resumption, or other similar operations.¶

The initiator then sends the IKE_AUTH request message, including the PPK_ID value as follows:¶

Initiator                       Responder
------------------------------------------------------------------
HDR, SK {IDi, [CERT,] [CERTREQ,]
    [IDr,] AUTH, SAi2,
    TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]}  --->

PPK_IDENTITY is a status notification with the type 16436; it has a protocol ID of 0, no SPI, and notification data that consists of the identifier PPK_ID.¶

A situation may happen when the responder has some PPKs but doesn't have a PPK with the PPK_ID received from the initiator. In this case, the responder cannot continue with the PPK (in particular, it cannot authenticate the initiator), but the responder could be able to continue with the normal IKEv2 protocol if the initiator provided its authentication data computed as in the normal IKEv2 without using PPKs. For this purpose, if using PPKs for communication with this responder is optional for the initiator (based on the mandatory_or_not flag), then the initiator MUST include a NO_PPK_AUTH notification in the above message. This notification informs the responder that PPKs are optional and allows for authenticating the initiator without using PPKs.¶

NO_PPK_AUTH is a status notification with the type 16437; it has a protocol ID of 0 and no SPI. The Notification Data field contains the initiator's authentication data computed using SK_pi', which has been computed without using PPKs. This is the same data that would normally be placed in the Authentication Data field of an AUTH payload. Since the Auth Method field is not present in the notification, the authentication method used for computing the authentication data MUST be the same as the method indicated in the AUTH payload. Note that if the initiator decides to include the NO_PPK_AUTH notification, the initiator needs to perform authentication data computation twice, which may consume computation power (e.g., if digital signatures are involved).¶

When the responder receives this encrypted exchange, it first computes the values:¶

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

The responder then uses the SK_ei/SK_ai values to decrypt/check the message and then scans through the payloads for the PPK_ID attached to the PPK_IDENTITY notification. If no PPK_IDENTITY notification is found and the peers successfully exchanged USE_PPK notifications in the IKE_SA_INIT exchange, then the responder MUST send back an AUTHENTICATION_FAILED notification and then fail the negotiation.¶

If the PPK_IDENTITY notification contains a PPK_ID that is not known to the responder or is not configured for use for the identity from the IDi payload, then the responder checks whether using PPKs for this initiator is mandatory and whether the initiator included a NO_PPK_AUTH notification in the message. If using PPKs is mandatory or no NO_PPK_AUTH notification is found, then the responder MUST send back an AUTHENTICATION_FAILED notification and then fail the negotiation. Otherwise (when a PPK is optional and the initiator included a NO_PPK_AUTH notification), the responder MAY continue the regular IKEv2 protocol, except that it uses the data from the NO_PPK_AUTH notification as the authentication data (which usually resides in the AUTH payload) for the purpose of the initiator authentication. Note that the authentication method is still indicated in the AUTH payload.¶

Table 1 summarizes the above logic for the responder:¶

If a PPK is in use, then the responder extracts the corresponding PPK and computes the following values:¶

 SK_d  = prf+ (PPK, SK_d')
 SK_pi = prf+ (PPK, SK_pi')
 SK_pr = prf+ (PPK, SK_pr')

The responder then continues with the IKE_AUTH exchange (validating the AUTH payload that the initiator included) as usual and sends back a response, which includes the PPK_IDENTITY notification with no data to indicate that the PPK is used in the exchange:¶

Initiator                       Responder
------------------------------------------------------------------
                           <--  HDR, SK {IDr, [CERT,]
                                AUTH, SAr2,
                                TSi, TSr, N(PPK_IDENTITY)}

When the initiator receives the response, it checks for the presence of the PPK_IDENTITY notification. If it receives one, it marks the SA as using the configured PPK to generate SK_d, SK_pi, and SK_pr (as shown above); the content of the received PPK_IDENTITY (if any) MUST be ignored. If the initiator does not receive the PPK_IDENTITY, it MUST either fail the IKE SA negotiation sending the AUTHENTICATION_FAILED notification in the INFORMATIONAL exchange (if the PPK was configured as mandatory) or continue without using the PPK (if the PPK was not configured as mandatory and the initiator included the NO_PPK_AUTH notification in the request).¶

If the Extensible Authentication Protocol (EAP) is used in the IKE_AUTH exchange, then the initiator doesn't include the AUTH payload in the first request message; however, the responder sends back the AUTH payload in the first reply. The peers then exchange AUTH payloads after EAP is successfully completed. As a result, the responder sends the AUTH payload twice -- in the first and last IKE_AUTH reply message -- while the initiator sends the AUTH payload only in the last IKE_AUTH request. See more details about EAP authentication in IKEv2 in Section 2.16 of [RFC7296].¶

The general rule for using a PPK in the IKE_AUTH exchange, which covers the EAP authentication case too, is that the initiator includes a PPK_IDENTITY (and optionally a NO_PPK_AUTH) notification in the request message containing the AUTH payload. Therefore, in case of EAP, the responder always computes the AUTH payload in the first IKE_AUTH reply message without using a PPK (by means of SK_pr'), since PPK_ID is not yet known to the responder. Once the IKE_AUTH request message containing the PPK_IDENTITY notification is received, the responder follows the rules described above for the non-EAP authentication case.¶

   Initiator                         Responder
   ----------------------------------------------------------------
   HDR, SK {IDi, [CERTREQ,]
       [IDr,] SAi2,
       TSi, TSr}  -->
                                <--  HDR, SK {IDr, [CERT,] AUTH,
                                         EAP}
   HDR, SK {EAP}  -->
                                <--  HDR, SK {EAP (success)}
   HDR, SK {AUTH,
       N(PPK_IDENTITY, PPK_ID)
       [, N(NO_PPK_AUTH)]}  -->
                                <--  HDR, SK {AUTH, SAr2, TSi, TSr
                                     [, N(PPK_IDENTITY)]}

Note that the diagram above shows both the cases when the responder uses a PPK and when it chooses not to use it (provided the initiator has included the NO_PPK_AUTH notification); thus, the responder's PPK_IDENTITY notification is marked as optional. Also, note that the IKE_SA_INIT exchange using a PPK is as described above (including exchange of the USE_PPK notifications), regardless of whether or not EAP is employed in the IKE_AUTH.¶

6. Security Considerations

A critical consideration is how to ensure the randomness of this post-quantum preshared key. Quantum computers are able to perform Grover's algorithm [GROVER]; that effectively halves the size of a symmetric key. In addition, an adversary impersonating the server, even with a conventional computer, can perform a dictionary search over plausible post-quantum preshared key values. The strongest practice is to ensure that any post-quantum preshared key contains at least 256 bits of entropy; this will provide 128 bits of post-quantum security, while providing security against conventional dictionary attacks. That provides the security equivalent to Category 5 as defined in the NIST Post-Quantum Cryptography Call for Proposals [NISTPQCFP]. Deriving a post-quantum preshared key from a password, name, or other low-entropy source is not secure because of these known attacks.¶

With this protocol, the computed SK_d is a function of the PPK. Assuming that the PPK has sufficient entropy (for example, at least 2²⁵⁶ possible values), even if an attacker was able to recover the rest of the inputs to the PRF function, it would be infeasible to use Grover's algorithm with a quantum computer to recover the SK_d value. Similarly, all keys that are a function of SK_d, which include all Child SA keys and all keys for subsequent IKE SAs (created when the initial IKE SA is rekeyed), are also quantum secure (assuming that the PPK was of high enough entropy and that all the subkeys are sufficiently long).¶

An attacker with a quantum computer that can decrypt the initial IKE SA has access to all the information exchanged over it, such as identities of the peers, configuration parameters, and all negotiated IPsec SA information (including traffic selectors), with the exception of the cryptographic keys used by the IPsec SAs, which are protected by the PPK.¶

Deployments that treat this information as sensitive or that send other sensitive data (like cryptographic keys) over IKE SAs MUST rekey the IKE SA before the sensitive information is sent to ensure this information is protected by the PPK. It is possible to create a childless IKE SA as specified in [RFC6023]. This prevents Child SA configuration information from being transmitted in the original IKE SA that is not protected by a PPK. Some information related to IKE SA that is sent in the IKE_AUTH exchange, such as peer identities, feature notifications, vendor IDs, etc., cannot be hidden from the attack described above, even if the additional IKE SA rekey is performed.¶

In addition, the policy SHOULD be set to negotiate only quantum-secure symmetric algorithms; while this RFC doesn't claim to give advice as to what algorithms are secure (as that may change based on future cryptographical results), below is a list of defined IKEv2 and IPsec algorithms that should not be used, as they are known to provide less than 128 bits of post-quantum security:¶

• Any IKEv2 encryption algorithm, PRF, or integrity algorithm with a key size less than 256 bits.¶
• Any ESP transform with a key size less than 256 bits.¶
• PRF_AES128_XCBC and PRF_AES128_CBC: even though they can use as input a key of arbitrary size, such input keys are converted into a 128-bit key for internal use.¶

Section 3 requires the initiator to abort the initial exchange if using PPKs is mandatory for it but the responder does not include the USE_PPK notification in the response. In this situation, when the initiator aborts the negotiation, it leaves a half-open IKE SA on the responder (because IKE_SA_INIT completes successfully from the responder's point of view). This half-open SA will eventually expire and be deleted, but if the initiator continues its attempts to create IKE SA with a high enough rate, then the responder may consider it a denial-of-service (DoS) attack and take protective measures (see [RFC8019] for more details). In this situation, it is RECOMMENDED that the initiator cache the negative result of the negotiation and not attempt to create it again for some time. This period of time may vary, but it is believed that waiting for at least few minutes will not cause the responder to treat it as a DoS attack. Note that this situation would most likely be a result of misconfiguration, and some reconfiguration of the peers would probably be needed.¶

If using PPKs is optional for both peers and they authenticate themselves using digital signatures, then an attacker in between, equipped with a quantum computer capable of breaking public key operations in real time, is able to mount a downgrade attack by removing the USE_PPK notification from the IKE_SA_INIT and forging digital signatures in the subsequent exchange. If using PPKs is mandatory for at least one of the peers or if a preshared key mode is used for authentication, then the attack will be detected and the SA won't be created.¶

If using PPKs is mandatory for the initiator, then an attacker able to eavesdrop and inject packets into the network can prevent creation of an IKE SA by mounting the following attack. The attacker intercepts the initial request containing the USE_PPK notification and injects a forged response containing no USE_PPK. If the attacker manages to inject this packet before the responder sends a genuine response, then the initiator would abort the exchange. To thwart this kind of attack, it is RECOMMENDED that, if using PPKs is mandatory for the initiator and the received response doesn't contain the USE_PPK notification, the initiator not abort the exchange immediately. Instead, it waits for more response messages, retransmitting the request as if no responses were received at all, until either the received message contains the USE_PPK notification or the exchange times out (see Section 2.4 of [RFC7296] for more details about retransmission timers in IKEv2). If none of the received responses contains USE_PPK, then the exchange is aborted.¶

If using a PPK is optional for both peers, then in case of misconfiguration (e.g., mismatched PPK_ID), the IKE SA will be created without protection against quantum computers. It is advised that if a PPK was configured but was not used for a particular IKE SA, then implementations SHOULD audit this event.¶

Appendix A. Discussion and Rationale

The primary goal of this document is to augment the IKEv2 protocol to provide protection against quantum computers without requiring novel cryptographic algorithms. The idea behind this document is that while a quantum computer can easily reconstruct the shared secret of an (EC)DH exchange, it cannot as easily recover a secret from a symmetric exchange. This document makes the SK_d (and thus also the IPsec KEYMAT and any subsequent IKE SA's SKEYSEED) depend on both the symmetric PPK and the Diffie-Hellman exchange. If we assume that the attacker knows everything except the PPK during the key exchange and there are 2ⁿ plausible PPKs, then a quantum computer (using Grover's algorithm) would take O(2^n/2) time to recover the PPK. So, even if the (EC)DH can be trivially solved, the attacker still can't recover any key material (except for the SK_ei, SK_er, SK_ai, and SK_ar values for the initial IKE exchange) unless they can find the PPK, which is too difficult if the PPK has enough entropy (for example, 256 bits). Note that we do allow an attacker with a quantum computer to rederive the keying material for the initial IKE SA; this was a compromise to allow the responder to select the correct PPK quickly.¶

Another goal of this protocol is to minimize the number of changes within the IKEv2 protocol, in particular, within the cryptography of IKEv2. By limiting our changes to notifications and only adjusting the SK_d, SK_pi, and SK_pr, it is hoped that this would be implementable, even on systems that perform most of the IKEv2 processing in hardware.¶

A third goal is to be friendly to incremental deployment in operational networks for which we might not want to have a global shared key or for which quantum-secure IKEv2 is rolled out incrementally. This is why we specifically try to allow the PPK to be dependent on the peer and why we allow the PPK to be configured as optional.¶

A fourth goal is to avoid violating any of the security properties provided by IKEv2.¶

Acknowledgements

We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett, Tommy Pauly, Quynh Dang, and the rest of the IPSECME Working Group for their feedback and suggestions for the scheme.¶

Authors' Addresses