Internet Engineering Task Force (IETF) V. Smyslov
Request for Comments: 7619 ELVIS-PLUS
Updates: 4301 P. Wouters
Category: Standards Track Red Hat
ISSN: 2070-1721 August 2015 The NULL Authentication Method
in the Internet Key Exchange Protocol Version 2 (IKEv2)
This document specifies the NULL Authentication method and the
ID_NULL Identification Payload ID Type for Internet Key Exchange
Protocol version 2 (IKEv2). This allows two IKE peers to establish
single-side authenticated or mutual unauthenticated IKE sessions for
those use cases where a peer is unwilling or unable to authenticate
or identify itself. This ensures IKEv2 can be used for Opportunistic
Security (also known as Opportunistic Encryption) to defend against
Pervasive Monitoring attacks without the need to sacrifice anonymity.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Internet Key Exchange Protocol version 2 (IKEv2), specified in
[RFC7296], provides a way for two parties to perform an authenticated
key exchange. While the authentication methods used by the peers can
be different, there is no method for one or both parties to remain
unauthenticated and anonymous. This document extends the
authentication methods to support unauthenticated and anonymous IKE
In some situations, mutual authentication is undesirable,
superfluous, or impossible. The following three examples illustrate
these unauthenticated use cases:
o A user wants to establish an anonymous secure connection to a
server. In this situation, the user should be able to
authenticate the server without presenting or authenticating to
the server with their own identity. This case uses a single-sided
authentication of the responder.
o A sensor that periodically wakes up from a suspended state wants
to send a measurement (e.g., temperature) to a collecting server.
The sensor must be authenticated by the server to ensure
authenticity of the measurement, but the sensor does not need to
authenticate the server. This case uses a single-sided
authentication of the initiator.
o Two peers without any trust relationship wish to defend against
widespread pervasive monitoring attacks as described in [RFC7258].
Without a trust relationship, the peers cannot authenticate each
other. Opportunistic Security [RFC7435] states that
unauthenticated encrypted communication is preferred over
cleartext communication. The peers want to use IKE to setup an
unauthenticated encrypted connection that gives them protection
against pervasive monitoring attacks. An attacker that is able
and willing to send packets can still launch a man-in-the-middle
(MITM) attack to obtain a copy of the unencrypted communication.
This case uses a fully unauthenticated key exchange.
To meet these needs, this document introduces the NULL Authentication
method and the ID_NULL ID type. This allows an IKE peer to
explicitly indicate that it is unwilling or unable to certify its
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Using the NULL Authentication Method
In IKEv2, each peer independently selects the method to authenticate
itself to the other side. A peer may choose to refrain from
authentication by using the NULL Authentication method. If a host's
local policy requires that the identity of its peer be (non-null)
authenticated, and if that host receives an AUTH payload containing
the NULL Authentication method type, it MUST return an
AUTHENTICATION_FAILED notification. If an initiator uses the
Extensible Authentication Protocol (EAP), the responder MUST NOT use
the NULL Authentication method (in conformance with Section 2.16 of
NULL authentication affects how the Authentication and the
Identification payloads are formed in the IKE_AUTH exchange.
2.1. Authentication Payload
NULL authentication still requires a properly formed AUTH payload to
be present in the IKE_AUTH exchange messages, as the AUTH payload
cryptographically links the IKE_SA_INIT exchange messages with the
other messages sent over this IKE Security Association (SA).
When using NULL authentication, the content of the AUTH payload is
computed using the syntax of pre-shared secret authentication,
described in Section 2.15 of [RFC7296]. The value of SK_pi for the
initiator and SK_pr for the responder is used as the shared secret
for the content of the AUTH payload. Implementers should note this
means that authentication keys used by the two peers are different in
each direction. This is identical to how the contents of the two
last AUTH payloads are generated for the non-key-generating EAP
methods (see Section 2.16 of [RFC7296] for details).
The IKEv2 Authentication Method value for NULL Authentication is 13.
2.2. Identification Payload
When a remote peer is not authenticated, any ID presented in the
Identification Data field of the ID payload cannot be validated. To
avoid the need of sending a bogus ID Type with placeholder data, this
specification defines a new ID Type, ID_NULL. The Identification
Data field of the ID payload for this ID Type MUST be empty.
If NULL authentication is in use and anonymity is a concern, then
ID_NULL SHOULD be used in the Identification payload. Some examples
of cases where a non-null identity type and value with NULL
authentication can be used are logging, troubleshooting, and in
scenarios where authentication takes place out of band after the IKE
SA is created (like in [AUTOVPN]). The content of the Identification
payload MUST NOT be used for any trust and policy checking in
IKE_AUTH exchange when NULL authentication is employed (see
Section 2.4 for details).
ID_NULL is primarily intended to be used with NULL authentication but
could be used in other situations where the content of the
Identification payload is not used. For example, ID_NULL could be
used when authentication is performed via raw public keys and the
identities are the keys themselves. These alternative uses of
ID_NULL should be described in their own respective documents.
The IKEv2 Identification Payload ID Type for ID_NULL is 13.
2.3. INITIAL_CONTACT Notification
The identity of a peer using NULL authentication cannot be used to
find existing IKE SAs created by the same peer, as the peer identity
is not authenticated. For that reason, the INITIAL_CONTACT
notifications MUST NOT be used to delete any other IKE SAs based on
the same peer identity without additional verification that the
existing IKE SAs with matching identity are actually stale.
The standard IKE Liveness Check procedure, described in Section 2.4
of [RFC7296], can be used to detect stale IKE SAs created by peers
using NULL authentication. Inactive, unauthenticated IKE SAs should
be checked periodically. Additionally, the event of creating a new
unauthenticated IKE SA can be used to trigger an out-of-order check
on existing unauthenticated IKE SAs possibly limited to identical or
close-by IP addresses or to identical identities of the just created
Implementations should weigh the resource consumption of sending
Liveness Checks against the memory usage of possible orphaned IKE
SAs. Implementations may choose to handle situations with thousands
of unauthenticated IKE SAs differently from situations with very few
2.4. Interaction with the Peer Authorization Database (PAD)
Section 4.4.3 of [RFC4301] defines the Peer Authorization Database
(PAD), which provides the link between the Security Policy Database
(SPD) and IKEv2. The PAD contains an ordered list of records with
peers' identities along with corresponding authentication data and
Child SA authorization data. When the IKE SA is being established,
the PAD is consulted to determine how the peer should be
authenticated and what Child SAs it is authorized to create.
When using NULL authentication, the peer identity is not
authenticated and cannot be trusted. If ID_NULL is used with NULL
authentication, there is no ID at all. The processing of the PAD
described in Section 4.4.3 of [RFC4301] is updated for NULL
authentication as follows.
NULL authentication is added as one of the supported authentication
methods. This method does not have any authentication data. ID_NULL
is included into the list of allowed ID types. The matching rule for
ID_NULL consists only of whether this type is used, i.e., no actual
ID matching is done as ID_NULL contains no identity data.
When using the NULL Authentication method, those matching rules MUST
include matching of a new flag in the SPD entry specifying whether
unauthenticated users are allowed to use that entry. That is, each
SPD entry needs to be augmented to have a flag specifying whether it
can be used with NULL authentication or not, and only those rules
that explicitly have that flag turned on can be used with
The specific updates of text in Section 4.4.3 of [RFC4301] are listed
in Appendix A.
2.5. Traffic Selectors
Traffic Selectors and narrowing allow two IKE peers to mutually agree
on a traffic range for an IPsec SA. An unauthenticated peer must not
be allowed to use this mechanism to steal traffic that an IKE peer
intended to be for another host. This is especially problematic when
supporting anonymous IKE peers behind NAT, as such IKE peers build an
IPsec SA using their pre-NAT IP address that is different from the
source IP of their IKE packets. A rogue IKE peer could use malicious
Traffic Selectors to trick a remote host into giving it IP traffic
that the remote host never intended to be sent to remote IKE peers.
For example, if the remote host uses 192.0.2.1 as the DNS server, a
rogue IKE peer could set its Traffic Selector to 192.0.2.1 in an
attempt to receive the remote peer's DNS traffic. Implementations
SHOULD restrict and isolate all anonymous IKE peers from each other
and itself and only allow it access to itself and possibly its
intended network ranges.
One method to achieve this is to always assign internal IP addresses
to unauthenticated IKE clients, as described in Section 2.19 of
[RFC7296]. Implementations may also use other techniques such as
internal NAT and connection tracking.
Implementations MAY force unauthenticated IKE peers to single host-
to-host IPsec SAs. When using IPv6, this is not always possible, so
implementations MUST be able to assign a full /64 address block to
the peer as described in [RFC5739], even if it is not authenticated.
3. Security Considerations
If authenticated IKE sessions are possible for a certain Traffic
Selector range between the peers, then unauthenticated IKE SHOULD NOT
be allowed for that Traffic Selector range. When mixing
authenticated and unauthenticated IKE with the same peer, policy
rules should ensure the highest level of security will be used to
protect the communication between the two peers. See [RFC7435] for
If both peers use NULL authentication, the entire key exchange
becomes unauthenticated. This makes the IKE session vulnerable to
active MITM attacks.
Using an ID Type other than ID_NULL with the NULL Authentication
method may compromise the client's anonymity in case of an active
IKE implementations without NULL authentication have always performed
mutual authentication and were not designed for use with
unauthenticated IKE peers. Implementations might have made
assumptions that remote peers are identified. With NULL
authentication, these assumptions are no longer valid. Furthermore,
the host itself might have made trust assumptions or may not be aware
of the network topology changes that resulted from IPsec SAs from
unauthenticated IKE peers.
3.1. Audit Trail and Peer Identification
With NULL authentication, an established IKE session is no longer
guaranteed to provide a verifiable (authenticated) entity known to
the system or network. Any logging of unproven ID payloads that were
not authenticated should be clearly marked and treated as "untrusted"
and possibly accompanied by logging the remote IP address of the IKE
session. Rate limiting of logging might be required to prevent
excessive resource consumption causing system damage.
3.2. Resource Management and Robustness
Section 2.6 of [RFC7296] provides guidance for mitigation of denial-
of-service (DoS) attacks by issuing COOKIES in response to resource
consumption of half-open IKE SAs. Furthermore, [DDOS-PROTECTION]
offers additional countermeasures in an attempt to distinguish
attacking IKE packets from legitimate IKE peers.
These defense mechanisms do not take into account IKE systems that
allow unauthenticated IKE peers. An attacker using NULL
authentication is a fully legitimate IKE peer that is only
distinguished from authenticated IKE peers by having used NULL
Implementers that implement NULL authentication should ensure their
implementation does not make any assumptions that depend on IKE peers
being "friendly", "trusted", or "identifiable". While
implementations should have been written to account for abusive
authenticated clients, any omission or error in handling abusive
clients may have gone unnoticed because abusive clients have been a
rare or nonexistent problem. When adding support for unauthenticated
IKE peers, these implementation omissions and errors will be found
and abused by attackers. For example, an unauthenticated IKE peer
could send an abusive amount of Liveness probes or Delete requests.
3.3. IKE Configuration Selection
Combining authenticated and unauthenticated IKE peers on a single
host can be dangerous, assuming the authenticated IKE peer gains more
or different access from unauthenticated peers (otherwise, why not
only allow unauthenticated peers). An unauthenticated IKE peer MUST
NOT be able to reach resources only meant for authenticated IKE peers
and MUST NOT be able to replace the Child SAs of an authenticated IKE
3.4. Networking Topology Changes
When a host relies on packet filters or firewall software to protect
itself, establishing an IKE SA and installing an IPsec SA might
accidentally circumvent these packet filters and firewall
restrictions, as the Encapsulating Security Payload (ESP, protocol
50) or ESPinUDP (UDP port 4500) packets of the encrypted traffic do
not match the packet filters defined for unencrypted traffic. IKE
peers supporting unauthenticated IKE MUST pass all decrypted traffic
through the same packet filters and security mechanisms as incoming
Appendix A. Update of PAD processing in RFC 4301
This appendix lists the specific updates of the text in Section 4.4.3
of [RFC4301] that should be followed when implementing NULL
A new item is added to the list of supported ID types in
Section 126.96.36.199 of [RFC4301]
o NULL ID (matches ID type only)
and the following text is added at the end of the section:
The NULL ID type is defined as having no data. For this name
type, the matching function is defined as comparing the ID type
A new item is added to the list of authentication data types in
Section 188.8.131.52 of [RFC4301]:
- NULL authentication
and the next paragraph is updated as follows:
For authentication based on an X.509 certificate [...] For
authentication based on a pre-shared secret, the PAD contains the
pre-shared secret to be used by IKE.
For authentication based on an X.509 certificate [...] For
authentication based on a pre-shared secret, the PAD contains the
pre-shared secret to be used by IKE. For NULL authentication the
PAD contains no data.
In addition, the following text is added at the end of
Section 184.108.40.206 of [RFC4301]:
When using the NULL Authentication method, implementations MUST
make sure that they do not mix authenticated and unauthenticated
SPD rules, i.e., implementations need to keep them separately; for
example, by adding a flag in the SPD to tell whether NULL
authentication can be used or not for the entry. That is, each
SPD entry needs to be augmented to have a flag specifying whether
it can be used with NULL authentication or not, and only those
rules that explicitly have that flag set can be used with
The authors would like to thank Yaron Sheffer and Tero Kivinen for
their reviews, valuable comments, and contributed text.
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