Internet DRAFT - draft-dschinazi-ipsecme-sa-init-privacy-addition
draft-dschinazi-ipsecme-sa-init-privacy-addition
ipsecme D. Schinazi
Internet-Draft Apple Inc.
Intended status: Standards Track March 5, 2018
Expires: September 6, 2018
Privacy Addition to the Internet Key Exchange Protocol Version 2 (IKEv2)
IKE_SA_INIT Exchange
draft-dschinazi-ipsecme-sa-init-privacy-addition-00
Abstract
The Internet Key Exchange Protocol version 2 (IKEv2) provides strong
security and privacy properties to both endpoints once they have
authenticated each other. However, before an endpoint has validated
the peer's AUTH payload, it could be divulging information to an
untrusted host. An example of such information is the Identification
payload of the initiator. Another example is the fact that a host is
running an IKEv2 responder. This document introduces a new
"Initialization Authentication Code" notify payload that can be
included in IKE_SA_INIT messages to increase their trustworthiness.
This new protection is meant to be used in addition to current IKEv2
mechanisms and is not meant to replace the AUTH payload in any way.
Status of This Memo
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
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This Internet-Draft will expire on September 6, 2018.
Copyright Notice
Copyright (c) 2018 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Attack Vectors . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. On-Path Attacker Targeting Initiator . . . . . . . . . . 4
2.2. Off-Path Attacker Targeting Responder . . . . . . . . . . 4
3. Initialization Authentication . . . . . . . . . . . . . . . . 5
3.1. Computing Initialization Authentication . . . . . . . . . 5
3.2. Initialization Authentication Notify Payload . . . . . . 6
3.3. Receiving Initialization Authentication . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Timing Attacks . . . . . . . . . . . . . . . . . . . . . 7
4.2. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 7
4.3. Denial of Service Attacks . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Normative References . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
The Internet Key Exchange Protocol version 2 (IKEv2) [RFC7296]
provides strong security and privacy properties to both endpoints
once they have authenticated each other. However, before an endpoint
has validated the peer's AUTH payload, it could be divulging
information to an untrusted host. Examples include:
o The Identification payload of the initiator is sent with the
initiator's first IKE_AUTH request. This payload can be used to
track the owner of the device initiating IKE.
o Some IKEv2 servers may wish to hide their very existence to avoid
being blacklisted by entities that resent the privacy properties
an IKEv2/IPsec tunnel can provide to users. If the IKEv2 server
is accessible over TLS on a TCP port [RFC8229] that is shared with
another protocol, responding to the initiator's IKE_SA_INIT can
disclose the server's existence.
This document introduces a new "Initialization Authentication Code"
(IAC) notify payload that can be included in IKE_SA_INIT messages to
increase their trustworthiness. This new protection is meant to be
used in addition to current IKEv2 mechanisms and is not meant to
replace the AUTH payload in any way.
1.1. Requirements Language
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
[RFC2119] [RFC8174] when, and only when, they appear in all capitals,
as shown here.
1.2. Terminology
This document uses the following terms:
Endpoint One of the two hosts that are involved in an IKE exchange.
Initiator The endpoint that sends the first IKE_SA_INIT request of
the IKE exchange being discussed.
Responder The endpoint that is not the initiator.
Peer When discussing an endpoint, its peer is the other endpoint
participating in the IKE exchange.
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IASS Initialization Authentication Shared Secret, a shared
secret included in the IKEv2 configuration. It is separate
from any shared secret used for computation of the AUTH
payload.
MAC Message Authentication Code, a cryptographic means of
ensuring integrity and authenticity of a message.
IAC Initialization Authentication Code, a MAC of IKE_SA_INIT
nonces with the IASS.
PRF Pseudo-Random Function, a function used to compute the IAC.
2. Attack Vectors
This document only attempts to address the following attack vectors.
2.1. On-Path Attacker Targeting Initiator
This attack vector assumes the presence of an active on-path attacker
that can block and forge any packets between both endpoints. Without
the mechanism described in this document, the attacker can forge an
IKE_SA_INIT reply and get the initiator to send it its IKE_AUTH
request encrypted with the ephemeral shared secret computed between
the initiator and the attacker. This leaks the identity of the
initiator (IDi) and can leak the identity of the responder (IDr) if
the initiator also sent it.
2.2. Off-Path Attacker Targeting Responder
Some network middleboxes may wish block to block IKEv2 negotiation.
This is often done by blocking UDP traffic which can be worked around
using IKEv2 TCP encapsulation [RFC8229]. This obfuscation can even
be improved by encapsulating IKEv2 and IPsec inside TLS. However, a
more persevering middlebox can establish a TLS connection to the
responder and try to send an IKE_SA_INIT to probe the server for
IKEv2 support. Without the mechanism described in this document, the
responder has to send an IKE_SA_INIT reply before it's established
any initiator identity, leaking the presence of the IKEv2 server.
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3. Initialization Authentication
3.1. Computing Initialization Authentication
Each endpoint configuration will include both an IASS and a PRF for
this endpoint, and also IASS and PRF of the peer. It will commonly
be the case (but it is not required) that the same IASS and the same
PRF is used in both directions.
The peers authenticate the IKE_SA_INIT messages by having each MAC
nonces using a padded shared secret as the key. The IAC is computed
as follows:
IAC_i = prf_i( prf_i(IASS_i, "Initialization Authentication Key Pad
for IKEv2 Initiator"), Ni)
IAC_r = prf_r( prf_r(IASS_r, "Initialization Authentication Key Pad
for IKEv2 Responder"), Ni | Nr)
Where IAC_i and IAC_r are the Initialization Authentication Codes of
the initiator and responder respectively. Ni and Nr are the nonces
sent in the IKE_SA_INIT messages that contain the IAC. The strings
are 57 ASCII characters without null termination. prf_i() and prf_r()
denote the PRFs selected in the initiator and responder
configurations respectively. IASS_i and IASS_r denote the
initialization authentication shared secret in the initiator and
responder configurations respectively.
The pad strings are added so that if the IASS are derived from a
password, the IKE implementation need not store the password in
cleartext, which could not be used as a password equivalent for
protocols other than IKEv2. Using different pad strings for each
direction limits the information leakage about the IASS if IASS_i and
IASS_r are equal. IAC_r is based on both Ni and Nr to prevent replay
attacks on the IKE_SA_INIT reply while also preventing a MAC oracle
on the responder, since the responder controls the random generation
of Nr.
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3.2. Initialization Authentication Notify Payload
The Initialization Authentication Notify Payload is defined as
follows:
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| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol ID(=0)| SPI Size (=0) | Notify Message Type (=TBD) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initialization Authentication Code ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'Next Payload', 'C', 'RESERVED', 'Payload Length', 'Protocol ID',
'SPI Size', and 'Notify Message Type' fields are the same as
described in Section 3 of [RFC7296]. The Critical ('C') bit MUST be
set to 0. The 'SPI Size' field MUST be set to 0 to indicate that the
SPI is not present in this message. The 'Protocol ID' MUST be set to
0, since the notification is specific to this IKE_SA_INIT message.
The 'Payload Length' field is set to the length in octets of the
entire payload, including the generic payload header. The 'Notify
Message Type' field is set to indicate
INITIALIZATION_AUTHENTICATION_CODE (TBD). The Initialization
Authentication Code field has a variable length, and is computed
according to Section 3.1.
3.3. Receiving Initialization Authentication
When the responder receives the initiator's IKE_SA_INIT request, it
has not yet established the identity of the initiator, as the
identity payload will come later. If the responder has distributed
the same initialization authentication shared secret for all of its
clients, it can easily verify that incoming IKE_SA_INIT requests come
from clients that possess the shared secret. If the responder uses
different initialization authentication shared secrets per client, it
will have to iterate all of them to find a match since there is no
identity sent with the IKE_SA_INIT request. Care should be taken
with regards to the timing of the IKE_SA_INIT reply to avoid leaking
information. If the responder cannot find a (IASS, PRF) combination
in its configuration that matches the IAC in the incoming IKE_SA_INIT
request, it MUST silently ignore the incoming packet. Not responding
at all is crucial to hiding the fact that the responder is running an
IKEv2 server. The responder SHOULD log the failure to facilitate
debugging.
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When the initiator receives the responder's IKE_SA_INIT reply, it
knows the identity of the responder it is trying to establish a
security association with. It can therefore use the (IASS, PRF) from
its configuration to validate the IAC on the reply. If the IAC in
the reply does not match what was computed from the configuration,
the initiator treats this similarly to receiving and error on the
reply and MUST fail the exchange and MUST NOT send the IKE_AUTH
message it would have normally sent. This is crucial to protect the
initiator identity (IDi) from an active on-path attacker. The
initiator SHOULD log the failure to facilitate debugging.
4. Security Considerations
This document attempts to resolve the attacks described in Section 2
and no other attacks on IKEv2.
4.1. Timing Attacks
An IKEv2 responder wishing to stay hidden needs to ensure it doesn't
leak information via the timing of its responses. In general if it
receives an IKE_SA_INIT message whose IAC does not match, it simply
does not respond. However if IKEv2 is running over TCP, the timing
of when the responder closes the TCP connection can leak information.
Implementors of hidden IKEv2 responders should ensure that they reply
to bad input and to invalid IAC in similar time. In particular, if
the server is also running another application protocol on the same
port, it SHOULD reply to an invalid or missing IAC the same way as it
would reply to an invalid request on that other protocol.
4.2. Replay Attacks
The initiator's IKE_SA_INIT message is sent unencrypted and can be
replayed. The mechanism described in this document is still
vulnerable to replays of the IKE_SA_INIT message. Note however that
an obfuscated IKEv2 server running over TLS can leverage TLS to
ensure the absence of on-path attackers inside the TLS channel
between both endpoints.
The responder's IKE_SA_INIT message is also sent unencrypted and can
also be replayed. However, the Initialization Authentication Code
takes Ni as input so replaying a previous responder IKE_SA_INIT for a
different IKEv2 exchange will have a different IAC and will be
ignored.
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4.3. Denial of Service Attacks
An IKEv2 responder implementing this specification opens themselves
to computing more MACs for IKE_SA_INIT messages. We believe that
downside is negligible compared to other DOS attacks on IKEv2.
5. IANA Considerations
If approved, this document defines a new payload in the IANA "IKEv2
Notify Message Types - Status Types" registry [IKEV2IANA]:
NOTIFY messages: status types Value
-----------------------------------------------------------------
INITIALIZATION_AUTHENTICATION_CODE TBD
6. Normative References
[IKEV2IANA]
"IANA, Internet Key Exchange Version 2 (IKEv2)
Parameters",
<https://www.iana.org/assignments/ikev2-parameters/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
August 2017, <https://www.rfc-editor.org/info/rfc8229>.
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Author's Address
David Schinazi
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
US
Email: dschinazi@apple.com
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