Prevention Downgrade Attacks on the Internet Key Exchange Protocol Version 2 (IKEv2)

The Internet Key Exchange version 2 protocol (IKEv2) defined in provides authenticated key exchange in the IP Security (IPsec) architecture. The cryptographic design of IKEv2 is based on SIGMA protocol defined in . The protocol allows peers to mutually authenticate themselves and to derive session keys, that are used to protect traffic.

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. It is assumed that readers are familiar with the IKEv2 protocol .

The details of how authentication is performed in IKEv2 are defined in Section 2.15 of . Peers sign (or MAC) some blobs that consist of various parts of protocol data (see also for the rationale). The definition of these blobs is provided below for convenience.

In particular, initiator authenticates the IKE_SA_INIT request (RealMessage1) and the responder authenticates the IKE_SA_INIT response (RealMessage2). Thus, each side authenticates only the initial message it has sent and not the initial message it has received.

The way authentication is performed in IKEv2 allows some kind of downgrade attacks. These attacks are require the set of preconditions that are not common, but still not unrealistic. In particular:

The attacker must be on the path with the ability to intercept communications between the peers and to modify their messages.
Security policies for both initiator and responder must include both "strong" and "weak" key exchange methods (with some definition of "strong" and "weak") and the attacker must be able to break "weak" key exchange methods in real time.
The attacker must either has a long-term authentication key for one of the peers or must be able to break authentication algorithm used by one of the peers in real time.

Having these preconditions the goal of the attacker is to eavesdrop a communication between the peers (note that impersonating of the peer, whose key is compromised, is not a goal for the attacker). In case the attacker knows the initiator's long-term authentication key, the attack can be mount as follows.

The initiator sends the IKE_SA_INIT request message with a list of proposed algorithms that includes both "weak" and "strong" key exchange methods.
The attacker intercepts this message and re-injects a modified message without "strong" key exchange methods. Note that this may require an additional step for the attack to succeed if the initiator includes a public key for a "strong" key exchange method in the request. In this case the attacker intercepts this message and responses with the INVALID_KE_PAYLOAD notification indicating that the initiator must include public key for a "weak" key exchange method. Then this message is intercepted and re-injected without "strong" key exchange methods.
The responder receives this message and selects one of the "weak" key exchange methods (since the message does not include any "strong" one), then it sends back a response message, which attacker lets to pass through without modifications.
Since the attacker has seen both public keys and can break the selected "weak" key exchange method in real time, it calculates SK_* session keys that allow the attacker to read and modify the content of the encrypted IKE messages.
The initiator receives the IKE_SA_INIT response message, accepts the responder's selected algorithms, including the "weak" key exchange method (since it is allowed by its policy), and starts the IKE_AUTH exchange. It computes the AUTH payload, thus authenticating the IKE_SA_INIT request message it has sent.
The attacker intercepts this message, decrypts it and modifies the AUTH payload in such a way, that it allegedly authenticates the IKE_SA_INIT request message that was modified and injected by the attacker. The attacker is able to do this because it knows the session keys and the initiator's long-term authentication key.
The responder receives this message, verifies the AUTH payload and sends back the IKE_AUTH response message, which the attacker allows to pass through.
At this point the peers have established a secure connection using "weak" key exchange method. Note, that this is allowed by their security policies, but without the attacker's intervention they would have used a more secure "strong" key exchange method. The attacker essentially forced the peers to use a "weak" method that it is able to break, thus downgrading the security properties of the connection so that it can read the peers' communication.

A variant of this attack can also be mounted on the hybrid post-quantum key exchange defined in , where the attacker able to break traditional key exchange method (e.g. by means of a quantum computer) prevents peers from executing additional quantum resistant key exchange method(s). Another variant of this attack can be mounted if the attacker has a long-term authentication key for the responder. In this case the attacker cannot change the algorithms selected by the responder, but still may be able to force peers not to use some protocol extensions, in particular those that are initially proposed by the responder. Similar attacks are also described in .

This document defines an IKEv2 extension that aims to detect attempts to mount the downgrade attacks described in . If both peers support this extension and are configured to use it and if at least one non-compromised authentication key is used by the peers in the protocol run then:

An attacker cannot fool any protocol participant that its peer does not support this extension without being detected.
An attacker cannot modify the IKE_SA_INIT messages without being detected.

If this extension is not supported by both peers, then the protocol runs as defined in . The idea is that both the IKE_SA_INIT request and the IKE_SA_INIT response messages must be directly authenticated by both peers. Thus, if at least one non-compromised key is used in the IKE SA establishing, then any modification of the IKE_SA_INIT messages will be detected.

The initiator supporting this extension includes a new status type notification IKE_SA_INIT_AUTH in the IKE_SA_INIT request message. The Notify Message Type for this notification is <TBA1 by IANA>, Protocol ID and SPI Size are both set to 0 and the notification data is empty. If the responder supports this extension then it also includes this notification in the response message regardless of whether it was received in the request or not.

<--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(IKE_SA_INIT_AUTH) ]]> If a peer sent and received the IKE_SA_INIT_AUTH notification, then it usese the modified construction of the blobs to be signed (or MAC'ed) compared to the definition from Section 2.15 of :

where RealMessage1, RealMessage2, NonceIData, NonceRData, MACedIDForI and MACedIDForR are defined in Section 2.15 of .

The IKE_INTERMEDIATE exchange defined in also modifies blobs to be signed (or MAC'ed). This modification is described in Section 3.3.2 of and can be summarized as an addition of new piece of data (IntAuth) to the end of the blobs from Section 2.15 of . If peers support extension defined in this document, then they MUST treat modified blobs to be signed (or MAC'ed) defined in as replacement for blobs defined in Section 2.15 of , so that in case of IKE_INTERMEDIATE the IntAuth is added to these modified blobs. Note, that authentication of the IKE_INTERMEDIATE exchange includes messages sent in both directions, thus the attacker cannot change its messages without being detected.

The IKEv2 extension defined in this document aims to protect against downgrade attacks on IKEv2.

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

IKE_SA_INIT_AUTH ]]>

The attack was originally described by Christopher Patton.