| Internet Engineering Task Force (IETF) | C. Tjhai |
| Internet-Draft | M. Tomlinson |
| Updates: 7296 (if approved) | Post-Quantum |
| Intended status: Standards Track | G. Bartlett |
| Expires: January 11, 2021 | Quantum Secret |
| S. Fluhrer | |
| Cisco Systems | |
| D. Van Geest | |
| ISARA Corporation | |
| O. Garcia-Morchon | |
| Philips | |
| V. Smyslov | |
| ELVIS-PLUS | |
| July 10, 2020 |
Multiple Key Exchanges in IKEv2
draft-ietf-ipsecme-ikev2-multiple-ke-01
This document describes how to extend the Internet Key Exchange Protocol Version 2 (IKEv2) to allow multiple key exchanges to take place while computing a shared secret during a Security Association (SA) setup. The primary application of this feature in IKEv2 is the ability to perform one or more post-quantum key exchanges in conjunction with the classical (Elliptic Curve) Diffie-Hellman key exchange, so that the resulting shared key is resistant against quantum computer attacks. Another possible application is the ability to combine several key exchanges in situations when no single key exchange algorithm is trusted by both initiator and responder.
This document updates RFC7296 by renaming a transform type 4 from "Diffie-Hellman Group (D-H)" to "Key Exchange Method (KE)" and renaming a field in the Key Exchange Payload from "Diffie-Hellman Group Num" to "Key Exchange Method". It also renames an IANA registry for this transform type from "Transform Type 4 - Diffie-Hellman Group Transform IDs" to "Transform Type 4 - Key Exchange Method Transform IDs". These changes generalize key exchange algorithms that can be used in IKEv2.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 11, 2021.
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.
Internet Key Exchange Protocol (IKEv2) as specified in [RFC7296] uses the Diffie-Hellman (DH) or Elliptic Curve Diffie-Hellman (ECDH) algorithm to establish a shared secret between an initiator and a responder. The security of the DH and ECDH algorithms relies on the difficulty to solve a discrete logarithm problem in multiplicative and elliptic curve groups respectively when the order of the group parameter is large enough. While solving such a problem remains difficult with current computing power, it is believed that general purpose quantum computers will be able to solve this problem, implying that the security of IKEv2 is compromised. There are, however, a number of cryptosystems that are conjectured to be resistant against quantum computer attack. This family of cryptosystems is known as post-quantum cryptography (PQC). It is sometimes also referred to as quantum-safe cryptography (QSC) or quantum-resistant cryptography (QRC).
This document describes a method to perform multiple successive key exchanges in IKEv2. It allows integration of QSC in IKEv2, while maintaining backwards compatibility, to derive a set of IKE keys that is resistant to quantum computer attacks. This extension allows the negotiation of one or more QSC algorithm to exchange data, in addition to the existing DH or ECDH key exchange data. We believe that the feature of using more than one post-quantum algorithms is important as many of these algorithms are relatively new and there may be a need to hedge the security risk with multiple key exchange data from several distinct QSC algorithms.
The secrets established from each key exchange are combined in a way such that should the post-quantum secrets not be present, the derived shared secret is equivalent to that of the standard IKEv2; on the other hand, a post-quantum shared secret is obtained if both classical and post-quantum key exchange data are present. This extension also applies to key exchanges in IKE Security Associations (SAs) for Encapsulating Security Payload (ESP) [RFC4303] or Authentication Header (AH) [RFC4302], i.e. Child SAs, in order to provide a stronger guarantee of forward security.
Some post-quantum key exchange payloads may have sizes larger than the standard maximum transmission unit (MTU) size, and therefore there could be issues with fragmentation at the IP layer. IKE does allow transmission over TCP where fragmentation is not an issue [RFC8229]; however, we believe that a UDP-based solution will be required too. IKE does have a mechanism to handle fragmentation within UDP [RFC7383], however that is only applicable to messages exchanged after the IKE_SA_INIT. To use this mechanism, this specification relies on the IKE_INTERMEDIATE exchange as outlined in [I-D.ietf-ipsecme-ikev2-intermediate]. With this mechanism, we do an initial key exchange, using a smaller, possibly non-quantum resistant primitive, such as ECDH. Then, before we do the IKE_AUTH exchange, we perform one or more IKE_INTERMEDIATE exchanges, each of which contains an additional key exchange. As the IKE_INTERMEDIATE exchange is encrypted, the IKE fragmentation protocol [RFC7383] can be used. The IKE SK_* values are updated after each exchange, and so the final IKE SA keys depend on all the key exchanges, hence they are secure if any of the key exchanges are secure.
Note that readers should consider the approach defined in this document as providing a long term solution in upgrading the IKEv2 protocol to support post-quantum algorithms. A short term solution to make IKEv2 key exchange quantum secure is to use post-quantum pre-shared keys as discussed in [RFC8784].
Note also, that the proposed approach of performing multiple successive key exchanges in such a way that resulting session keys depend on all of them is not limited to achieving quantum resistance only. It can also be used when all the performed key exchanges are classical (EC)DH ones, where for some reasons (e.g. policy requirements) it is essential to perform multiple of them.
RFC EDITOR PLEASE DELETE THIS SECTION.
Changes in this draft in each version iterations.
draft-ietf-ipsecme-ikev2-multiple-ke-01
draft-ietf-ipsecme-ikev2-multiple-ke-00
draft-tjhai-ipsecme-hybrid-qske-ikev2-04
draft-tjhai-ipsecme-hybrid-qske-ikev2-03
draft-tjhai-ipsecme-hybrid-qske-ikev2-02
draft-tjhai-ipsecme-hybrid-qske-ikev2-01
draft-tjhai-ipsecme-hybrid-qske-ikev2-00
The remainder of this document is organized as follows. Section 2 summarizes design criteria. Section 3 describes how multiple key exchanges are performed between two IKE peers and how keying materials are derived for both SAs and Child SAs. A summary of alternative approaches that have been considered, but later discarded, are described in Appendix A. Section 4 discusses IANA considerations for the namespaces introduced in this document, and lastly Section 5 discusses security considerations.
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.
The design of the proposed extension is driven by the following criteria:
This design assigns new Transform Type 4 identifiers to the various post-quantum key exchanges (which will be defined later). We specifically do not make a distinction between classical (DH and ECDH) and post-quantum key exchanges, nor post-quantum algorithms which are true key exchanges versus post-quantum algorithms that act as key transport mechanisms; all are treated equivalently by the protocol. To be more specific, this document renames Transform Type 4 from "Diffie-Hellman Group (D-H)" to "Key Exchange Method (KE)" and renames a field in the Key Exchange Payload from "Diffie-Hellman Group Num" to "Key Exchange Method". The corresponding IANA registry is also renamed from "Diffie-Hellman Group Transform IDs" to "Key Exchange Method Transform IDs".
In order to support IKE fragmentation for additional key exchanges that may have long public keys, the proposed framework utilizes the IKE_INTERMEDIATE exchange defined in [I-D.ietf-ipsecme-ikev2-intermediate].
In order to minimize communication overhead, only the key shares that are agreed to be used are actually exchanged. In order to achieve this several new Transform Types are defined, each sharing possible Transform IDs with Transform Type 4. The IKE_SA_INIT message includes one or more newly defined SA transforms that lists the extra key exchange policy required by the initiator; the responder selects a single transform of each type, and returns them in the response IKE_SA_INIT message. Then, provided that additional key exchanges are negotiated, the initiator and the responder perform one or more IKE_INTERMEDIATE exchanges; each such exchange includes a KE payload for one of the negotiated key exchanges.
Here is an overview of the initial exchanges:
Initiator Responder
---------------------------------------------------------------------
<-- IKE_SA_INIT (additional key exchanges negotiation) -->
<-- {IKE_INTERMEDIATE (additional key exchange)} -->
...
<-- {IKE_INTERMEDIATE (additional key exchange)} -->
<-- {IKE_AUTH} -->
The additional key exchanges may use algorithms that are currently considered to be resistant to quantum computer attacks. These algorithms are collectively referred to as post-quantum algorithms in this document. However, it is also possible to use classical (EC)DH primitives for non post-quantum requirements.
Most post-quantum key agreement algorithms are relatively new, and thus are not fully trusted. There are also many proposed algorithms, with different trade-offs and relying on different hard problems. The concern is that some of these hard problems may turn out to be easier to solve than anticipated and thus the key agreement algorithm may not be as secure as expected. A hybrid solution allows us to deal with this uncertainty by combining a classical key exchange with a post-quantum one, as well as leaving open the possibility of multiple post-quantum key exchanges.
The method that we use to perform additional key exchanges also addresses the fragmentation issue. The initial IKE_INIT messages do not have any inherent fragmentation support within IKE; however that can include a relatively short KE payload (e.g. one for group 14, 19 or 31). The rest of the KE payloads are encrypted within IKE_INTERMEDIATE messages; because they are encrypted, the standard IKE fragmentation solution [RFC7383] is available.
The fact that all Additional Key Exchange Transform Types share the same registry with Transform Type 4 allows additional key exchanges to be of any type - either post-quantum ones or classical (EC)DH ones. This approach allows any combination of defined key exchange methods to take place. This also allows performing a single post-quantum key exchange in the IKE_SA_INIT without additional key exchanges, provided that IP fragmentation is not an issue and that hybrid key exchange is not needed.
In the simplest case, the initiator is happy with a single key exchange (and has no interest in supporting multiple), and it is not concerned with possible fragmentation of the IKE_SA_INIT messages (either because the key exchange it selects is small enough not to fragment, or the initiator is confident that fragmentation will be handled either by IP fragmentation, or transport via TCP).
In this case, the initiator performs the IKE_SA_INIT as standard, inserting a preferred key exchange (which is possibly a post-quantum algorithm) as the listed Transform Type 4, and including the initiator KE payload. If the responder accepts the policy, it responds with an IKE_SA_INIT response, and IKE continues as usual.
If the initiator desires to negotiate multiple key exchanges, or it needs IKE to handle any possible fragmentation, then the initiator uses the protocol listed below.
Multiple key exchanges are negotiated using the standard IKEv2 mechanism, via SA payload. For this purpose several new transform types, namely Additional Key Exchange 1, Additional Key Exchange 2, Additional Key Exchange 3, etc., are defined. They are collectively called Additional Key Exchanges and have slightly different semantics than existing IKEv2 transform types. They are interpreted as additional key exchanges that peers agreed to perform in a series of IKE_INTERMEDIATE exchanges. The possible transform IDs for these transform types are the same as IDs for the Transform Type 4, so they all share a single IANA registry for transform IDs.
Key exchange methods negotiated via Transform Type 4 MUST always take place in the IKE_SA_INIT exchange. Additional key exchanges negotiated via newly defined transforms MUST take place in a series of IKE_INTERMEDIATE exchanges, in an order of the values of their transform types, so that key exchange negotiated using Transform Type n always precedes that of Transform Type n + 1. Each IKE_INTERMEDIATE exchange MUST bear exactly one key exchange method. Note that with this semantics, Additional Key Exchanges transforms are not associated with any particular type of key exchange and do not have any specific per transform type transform IDs IANA registry. Instead they all share a single registry for transform IDs - "Key Exchange Method Transform IDs", as well as Transform Type 4. All new key exchange algorithms (both classical or post-quantum) should be added to this registry. This approach gives peers flexibility in defining the ways they want to combine different key exchange methods.
When forming a proposal the initiator adds transforms for the IKE_SA_INIT exchange using Transform Type 4. In most cases they will contain classical key exchange methods (DH or ECDH), however it is not a requirement. Additional key exchange methods are proposed using Additional Key Exchanges transform types. All these transform types are optional, the initiator is free to select any of them for proposing additional key exchange methods. Consequently, if none of Additional Key Exchange transforms are included in the proposal, then this proposal indicates performing standard IKEv2, as defined in [RFC7296]. If the initiator includes any transform of type n (where n is among Additional Key Exchanges) in the proposal, the responder MUST select one of the algorithms proposed using this type. A transform ID NONE may be added to those transform types which contain key exchange methods that the initiator believes are optional.
If the initiator includes any Additional Key Exchanges transform types into SA payload, it MUST also negotiate using IKE_INTERMEDIATE exchange as described in [I-D.ietf-ipsecme-ikev2-intermediate], by including INTERMEDIATE_EXCHANGE_SUPPORTED notification in the IKE_SA_INIT request message. If the responder agrees to use additional key exchanges, it MUST also return this notification, thus confirming that IKE_INTERMEDIATE exchange is supported and will be used for transferring additional key exchange data. The presence of Additional Key Exchanges transform types in SA payload without negotiation of using IKE_INTERMEDIATE exchange MUST be treated as protocol error by both initiator and responder.
The responder performs negotiation using standard IKEv2 procedure described in Section 3.3 of [RFC7296]. However, for the Additional Key Exchange types the responder's choice MUST NOT contain equal transform IDs (apart from NONE), and the ID selected for Transform Type 4 MUST NOT appear in any of Additional Key Exchange transforms. In other words, all selected key exchange methods must be different.
For each extra key exchange agreed to in the IKE_SA_INIT exchange, the initiator and the responder perform one IKE_INTERMEDIATE exchange, as described in [I-D.ietf-ipsecme-ikev2-intermediate].
These exchanges are as follows:
Initiator Responder
---------------------------------------------------------------------
HDR, SK {KEi(n)} -->
<-- HDR, SK {KEr(n)}
The initiator sends key exchange data in the KEi(n) payload. This packet is protected with the current SK_ei/SK_ai keys.
On receiving this, the responder sends back key exchange payload KEr(n); again, this packet is protected with the current SK_er/SK_ar keys.
The former "Diffie-Hellman Group Num" (now called "Key Exchange Method") field in the KEi(n) and KEr(n) payloads MUST match the n-th negotiated additional key exchange. Note that the negotiated transform types (the encryption type, integrity type, prf type) are not modified.
Once this exchange is done, then both sides compute an updated keying material:
SKEYSEED(n) = prf(SK_d(n-1), KE(n) | Ni | Nr)
where KE(n) is the resulting shared secret of this key exchange, Ni and Nr are nonces from the IKE_SA_INIT exchange and SK_d(n-1) is the last generated SK_d, (derived from the previous IKE_INTERMEDIATE exchange, or the IKE_SA_INIT if there have not already been any IKE_INTERMEDIATE exchanges). Then, SK_d, SK_ai, SK_ar, SK_ei, SK_er, SK_pi, SK_pr are updated as:
{SK_d(n) | SK_ai(n) | SK_ar(n) | SK_ei(n) | SK_er(n) | SK_pi(n) |
SK_pr(n)} = prf+ (SKEYSEED(n), Ni | Nr | SPIi | SPIr)
Both the initiator and the responder use these updated key values in the next exchange.
After all IKE_INTERMEDIATE exchanges have completed, the initiator and the responder perform an IKE_AUTH exchange. This exchange is the standard IKE exchange, except that the initiator and responder signed octets are modified as described in [I-D.ietf-ipsecme-ikev2-intermediate].
The CREATE_CHILD_SA exchange is used in IKEv2 for the purpose of creating additional Child SAs, rekeying them and rekeying IKE SA itself. When creating or rekeying Child SAs, the peers may optionally perform a Diffie-Hellman key exchange to add a fresh entropy into the session keys. In case of IKE SA rekey, the key exchange is mandatory.
If the IKE SA was created using multiple key exchange methods, the peers may want to continue using multiple key exchanges in the CREATE_CHILD_SA exchange too. If the initiator includes any Additional Key Exchanges transform in the SA payload (along with Transform Type 4) and the responder agrees to perform additional key exchanges, then the additional key exchanges are performed in a series of new IKE_FOLLOWUP_KE exchanges that follows the CREATE_CHILD_SA exchange. The IKE_FOLLOWUP_KE exchange is introduced as a dedicated exchange type to transfer data of additional key exchanges following the key exchange performed in the CREATE_CHILD_SA. Its Exchange Type is <TBA by IANA>.
These key exchanges are performed in an order of the values of their transform types, so that key exchange negotiated using Transform Type n always precedes key exchange negotiated using Transform Type n + 1. Each IKE_FOLLOWUP_KE exchange MUST bear exactly one key exchange method. Key exchange negotiated via Transform Type 4 always takes place in the CREATE_CHILD_SA exchange, as per IKEv2 specification.
Since after IKE SA is created the window size may be greater than one and multiple concurrent exchanges may be in progress, it is essential to link the IKE_FOLLOWUP_KE exchanges together and with the corresponding CREATE_CHILD_SA exchange. A new status type notification ADDITIONAL_KEY_EXCHANGE is used for this purpose. Its Notify Message Type is <TBA by IANA>, Protocol ID and SPI Size are both set to 0. The data associated with this notification is a blob meaningful only to the responder, so that the responder can correctly link successive exchanges. For the initiator the content of this notification is an opaque blob.
The responder MUST include this notification in a CREATE_CHILD_SA or IKE_FOLLOWUP_KE response message in case the next exchange is expected, filling it with some data that would allow linking this exchange to the next one. The initiator MUST copy the received notification with its content intact into the request message of the next exchange.
Below is an example of three additional key exchanges.
Initiator Responder
---------------------------------------------------------------------
HDR(CREATE_CHILD_SA), SK {SA, Ni, KEi} -->
<-- HDR(CREATE_CHILD_SA), SK {SA, Nr, KEr,
N(ADDITIONAL_KEY_EXCHANGE)(link1)}
HDR(IKE_FOLLOWUP_KE), SK {KEi(1),
N(ADDITIONAL_KEY_EXCHANGE)(link1)} -->
<-- HDR(IKE_FOLLOWUP_KE), SK {KEr(1),
N(ADDITIONAL_KEY_EXCHANGE)(link2)}
HDR(IKE_FOLLOWUP_KE), SK {KEi(2),
N(ADDITIONAL_KEY_EXCHANGE)(link2)} -->
<-- HDR(IKE_FOLLOWUP_KE), SK {KEr(2),
N(ADDITIONAL_KEY_EXCHANGE)(link3)}
HDR(IKE_FOLLOWUP_KE), SK {KEi(3),
N(ADDITIONAL_KEY_EXCHANGE)(link3)} -->
<-- HDR(IKE_FOLLOWUP_KE), SK {KEr(3)}
The former "Diffie-Hellman Group Num" (now called "Key Exchange Method") field in the KEi(n) and KEr(n) payloads MUST match the n-th negotiated additional key exchange.
It is possible that due to some unexpected events (e.g. reboot) the initiator could forget that it is in the process of performing additional key exchanges and never starts next IKE_FOLLOWUP_KE exchanges. The responder MUST handle this situation gracefully and delete the associated state if it does not receive the next expected IKE_FOLLOWUP_KE request after some reasonable period of time.
If responder receives IKE_FOLLOWUP_KE request containing ADDITIONAL_KEY_EXCHANGE notification and the content of this notify does not correspond to any active key exchange state the responder has, it MUST send back a new error type notification STATE_NOT_FOUND. This is a non-fatal error notification, its Notify Message Type is <TBA by IANA>, Protocol ID and SPI Size are both set to 0 and the data is empty. If the initiator receives this notification in response to IKE_FOLLOWUP_KE exchange performing additional key exchange, it MUST cancel this exchange and MUST treat the whole series of exchanges started from the CREATE_CHILD_SA exchange as failed. In most cases, the receipt of this notification is caused by premature deletion of the corresponding state on the responder (the time period between IKE_FOLLOWUP_KE exchanges appeared too long from responder's point of view, e.g. due to a temporary network failure). After receiving this notification the initiator MAY start a new CREATE_CHILD_SA exchange (eventually followed by the IKE_FOLLOWUP_KE exchanges) to retry the failed attempt. If the initiator continues to receive STATE_NOT_FOUND notifications after several retries, it MUST treat this situation as a fatal error and delete IKE SA by sending a DELETE payload.
When rekeying IKE SA or Child SA, it is possible that the peers start doing this at the same time, which is called simultaneous rekeying. Sections 2.8.1 and 2.8.2 of [RFC7296] describes how IKEv2 handles this situation. In a nutshell IKEv2 follows the rule that if in case of simultaneous rekeying two identical new IKE SAs (or two pairs of Child SAs) are created, then one of them should be deleted. Which one is to be deleted is determined by comparing the values of four nonces, that were used in the colliding CREATE_CHILD_SA exchanges - the IKE SA (or pair of Child SAs) that was created by the exchange in which the smallest nonce was used should be deleted by the initiator of this exchange.
With multiple key exchanges the SAs are not yet created when the CRETE_CHILD_SA is completed, they would be created only after the series of IKE_FOLLOWUP_KE exchanges is finished. For this reason if additional key exchanges were negotiated in the CREATE_CHILD_SA initiated by the losing side, there is nothing to delete and this side just stops the rekeying process - this side MUST not initiate IKE_FOLLOWUP_KE exchange with next key exchange.
In most cases, rekey collisions are resolved in the CREATE_CHILD_SA exchange. However, a situation may occur when due to packet loss, one of the peers receives CREATE_CHILD_SA message requesting rekeying SA that is already being rekeyed by this peer (i.e. the CREATE_CHILD_SA exchange initiated by this peer has been already completed and the series of IKE_FOLLOWUP_KE exchanges is in progress). In this case, a TEMPORARY_FAILURE notification MUST be sent in response to such a request.
If multiple key exchanges were negotiated in the CREATE_CHILD_SA exchange, then the resulting keys are computed as follows. In case of IKE SA rekey:
SKEYSEED = prf(SK_d, KE | Ni | Nr | KE(1) | ... KE(n))
In case of Child SA creation or rekey:
KEYMAT = prf+ (SK_d, KE | Ni | Nr | KE(1) | ... KE(n))
In both cases SK_d is from existing IKE SA; KE, Ni, Nr are the shared key and nonces from the CREATE_CHILD_SA respectively; KE(1)...KE(n) are the shared keys from additional key exchanges.
This document adds new exchange type into the "IKEv2 Exchange Types" registry:
<TBA> IKE_FOLLOWUP_KE
This document renames Transform Type 4 defined in "Transform Type Values" registry from "Diffie-Hellman Group (D-H)" to "Key Exchange Method (KE)".
This document renames IKEv2 registry "Transform Type 4 - Diffie-Hellman Group Transform IDs" to "Transform Type 4 - Key Exchange Method Transform IDs".
This document adds the following Transform Types to the "Transform Type Values" registry:
Type Description Used In
-----------------------------------------------------------------
<TBA> Additional Key Exchange 1 (optional in IKE, AH, ESP)
<TBA> Additional Key Exchange 2 (optional in IKE, AH, ESP)
<TBA> Additional Key Exchange 3 (optional in IKE, AH, ESP)
<TBA> Additional Key Exchange 4 (optional in IKE, AH, ESP)
<TBA> Additional Key Exchange 5 (optional in IKE, AH, ESP)
<TBA> Additional Key Exchange 6 (optional in IKE, AH, ESP)
<TBA> Additional Key Exchange 7 (optional in IKE, AH, ESP)
This document defines a new Notify Message Type in the "Notify Message Types - Status Types" registry:
<TBA> ADDITIONAL_KEY_EXCHANGE
and a new Notify Message Type in the "Notify Message Types - Error Types" registry:
<TBA> STATE_NOT_FOUND
The key length of the Encryption Algorithm (Transform Type 1), the Pseudorandom Function (Transform Type 2) and the Integrity Algorithm (Transform Type 3), all have to be of sufficient length to prevent attacks using Grover's algorithm [GROVER]. In order to use the extension proposed in this document, the key lengths of these transforms SHALL be at least 256 bits long in order to provide sufficient resistance to quantum attacks. Accordingly the post-quantum security level achieved is at least 128 bits.
SKEYSEED is calculated from shared KE(x) using an algorithm defined in Transform Type 2. While a quantum attacker may learn the value of KE(x), if this value is obtained by means of a classical key exchange, other KE(x) values generated by means of a quantum-resistant algorithm ensure that the final SKEYSEED is not compromised. This assumes that the algorithm defined in the Transform Type 2 is post-quantum.
The main focus of this document is to prevent a passive attacker performing a "harvest and decrypt" attack. In other words, an attacker that records messages exchanges today and proceeds to decrypt them once he owns a quantum computer. This attack is prevented due to the hybrid nature of the key exchange. Other attacks involving an active attacker using a quantum-computer are not completely solved by this document. This is for two reasons.
The first reason is because the authentication step remains classical. In particular, the authenticity of the SAs established under IKEv2 is protected using a pre-shared key, RSA, DSA, or ECDSA algorithms. Whilst the pre-shared key option, provided the key is long enough, is post-quantum, the other algorithms are not. Moreover, in implementations where scalability is a requirement, the pre-shared key method may not be suitable. Quantum-safe authenticity may be provided by using a quantum-safe digital signature and several quantum-safe digital signature methods are being explored by IETF. For example, if the implementation is able to reliably track state, the hash based method, XMSS has the status of an RFC, see [RFC8391]. Currently, quantum-safe authentication methods are not specified in this document, but are planned to be incorporated in due course.
It should be noted that the purpose of post-quantum algorithms is to provide resistance to attacks mounted in the future. The current threat is that encrypted sessions are subject to eavesdropping and archived with decryption by quantum computers taking place at some point in the future. Until quantum computers become available there is no point in attacking the authenticity of a connection because there are no possibilities for exploitation. These only occur at the time of the connection, for example by mounting a man-in-the-middle (MitM) attack. Consequently there is not such a pressing need for quantum-safe authenticity.
This draft does not attempt to address key exchanges with KE payloads longer than 64k; the current IKE payload format does not allow that as a possibility. If such huge KE payloads are required, a work around (such as making the KE payload a URL and a hash of the real payload) would be needed. At the current time, it appears likely that there will be plenty of key exchanges available that would not require such a workaround.
The authors would like to thanks Frederic Detienne and Olivier Pelerin for their comments and suggestions, including the idea to negotiate the post-quantum algorithms using the existing KE payload. The authors are also grateful to Tobias Heider and Tobias Guggemos for valuable comments.
| [I-D.ietf-ipsecme-ikev2-intermediate] | Smyslov, V., "Intermediate Exchange in the IKEv2 Protocol", Internet-Draft draft-ietf-ipsecme-ikev2-intermediate-04, June 2020. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC7296] | Kaufman, C., Hoffman, P., Nir, Y., Eronen, P. and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 2014. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
This section gives an overview on a number of alternative approaches that we have considered, but later discarded. These approaches are:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C|F| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Diffie-Hellman Group Number | Fragment Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Index | Total Fragments |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total KE Payload Data Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Fragmented KE Payload ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+