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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 8019
IPSecME Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track V. Smyslov
Expires: February 18, 2017 ELVIS-PLUS
August 17, 2016
Protecting Internet Key Exchange Protocol version 2 (IKEv2)
Implementations from Distributed Denial of Service Attacks
draft-ietf-ipsecme-ddos-protection-08
Abstract
This document recommends implementation and configuration best
practices for Internet Key Exchange Protocol version 2 (IKEv2)
Responders, to allow them to resist Denial of Service and Distributed
Denial of Service attacks. Additionally, the document introduces a
new mechanism called "Client Puzzles" that help accomplish this task.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://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 February 18, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. The Vulnerability . . . . . . . . . . . . . . . . . . . . . . 3
4. Defense Measures while the IKE SA is being created . . . . . 6
4.1. Retention Periods for Half-Open SAs . . . . . . . . . . . 6
4.2. Rate Limiting . . . . . . . . . . . . . . . . . . . . . . 6
4.3. The Stateless Cookie . . . . . . . . . . . . . . . . . . 7
4.4. Puzzles . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.5. Session Resumption . . . . . . . . . . . . . . . . . . . 10
4.6. Keeping computed Shared Keys . . . . . . . . . . . . . . 11
4.7. Preventing "Hash and URL" Certificate Encoding Attacks . 11
4.8. IKE Fragmentation . . . . . . . . . . . . . . . . . . . . 12
5. Defense Measures after an IKE SA is created . . . . . . . . . 12
6. Plan for Defending a Responder . . . . . . . . . . . . . . . 13
7. Using Puzzles in the Protocol . . . . . . . . . . . . . . . . 15
7.1. Puzzles in IKE_SA_INIT Exchange . . . . . . . . . . . . . 15
7.1.1. Presenting a Puzzle . . . . . . . . . . . . . . . . . 16
7.1.2. Solving a Puzzle and Returning the Solution . . . . . 18
7.1.3. Computing a Puzzle . . . . . . . . . . . . . . . . . 19
7.1.4. Analyzing Repeated Request . . . . . . . . . . . . . 19
7.1.5. Deciding if to Serve the Request . . . . . . . . . . 21
7.2. Puzzles in an IKE_AUTH Exchange . . . . . . . . . . . . . 22
7.2.1. Presenting Puzzle . . . . . . . . . . . . . . . . . . 22
7.2.2. Solving Puzzle and Returning the Solution . . . . . . 23
7.2.3. Computing the Puzzle . . . . . . . . . . . . . . . . 24
7.2.4. Receiving the Puzzle Solution . . . . . . . . . . . . 24
8. Payload Formats . . . . . . . . . . . . . . . . . . . . . . . 25
8.1. PUZZLE Notification . . . . . . . . . . . . . . . . . . . 25
8.2. Puzzle Solution Payload . . . . . . . . . . . . . . . . . 25
9. Operational Considerations . . . . . . . . . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 27
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
13.1. Normative References . . . . . . . . . . . . . . . . . . 28
13.2. Informative References . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
Denial of Service (DoS) attacks have always been considered a serious
threat. These attacks are usually difficult to defend against since
the amount of resources the victim has is always bounded (regardless
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of how high it is) and because some resources are required for
distinguishing a legitimate session from an attack.
The Internet Key Exchange protocol version 2 (IKEv2) described in
[RFC7296] includes defense against DoS attacks. In particular, there
is a cookie mechanism that allows the IKE Responder to defend itself
against DoS attacks from spoofed IP-addresses. However, bot-nets
have become widespread, allowing attackers to perform Distributed
Denial of Service (DDoS) attacks, which are more difficult to defend
against. This document presents recommendations to help the
Responder counter (D)DoS attacks. It also introduces a new mechanism
-- "puzzles" -- that can help accomplish this task.
2. 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].
3. The Vulnerability
The IKE_SA_INIT Exchange described in Section 1.2 of [RFC7296]
involves the Initiator sending a single message. The Responder
replies with a single message and also allocates memory for a
structure called a half-open IKE Security Association (SA). This
half-open SA is later authenticated in the IKE_AUTH Exchange. If
that IKE_AUTH request never comes, the half-open SA is kept for an
unspecified amount of time. Depending on the algorithms used and
implementation, such a half-open SA will use from around 100 bytes to
several thousands bytes of memory.
This creates an easy attack vector against an IKE Responder.
Generating the IKE_SA_INIT request is cheap. Sending large amounts
of IKE_SA_INIT requests can cause a Responder to use up all its
resources. If the Responder tries to defend against this by
throttling new requests, this will also prevent legitimate Initiators
from setting up IKE SAs.
An obvious defense, which is described in Section 4.2, is limiting
the number of half-open SAs opened by a single peer. However, since
all that is required is a single packet, an attacker can use multiple
spoofed source IP addresses.
If we break down what a Responder has to do during an initial
exchange, there are three stages:
1. When the IKE_SA_INIT request arrives, the Responder:
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* Generates or re-uses a Diffie-Hellman (D-H) private part.
* Generates a Responder Security Parameter Index (SPI).
* Stores the private part and peer public part in a half-open SA
database.
2. When the IKE_AUTH request arrives, the Responder:
* Derives the keys from the half-open SA.
* Decrypts the request.
3. If the IKE_AUTH request decrypts properly:
* Validates the certificate chain (if present) in the IKE_AUTH
request.
The fourth stage where the Responder creates the Child SA is not
reached by attackers who cannot pass the authentication step.
Stage #1 is pretty light on CPU power, but requires some storage, and
it's very light for the Initiator as well. Stage #2 includes
private-key operations, so it is much heavier CPU-wise. Stage #3 may
include public key operations if certificates are involved. These
operations are often more computationly expensive than those
performed at stage #2.
To attack such a Responder, an attacker can attempt either to exhaust
memory or to exhaust CPU. Without any protection, the most efficient
attack is to send multiple IKE_SA_INIT requests and exhaust memory.
This is easy because IKE_SA_INIT requests are cheap.
There are obvious ways for the Responder to protect itself without
changes to the protocol. It can reduce the time that an entry
remains in the half-open SA database, and it can limit the amount of
concurrent half-open SAs from a particular address or prefix. The
attacker can overcome this by using spoofed source addresses.
The stateless cookie mechanism from Section 2.6 of [RFC7296] prevents
an attack with spoofed source addresses. This doesn't completely
solve the issue, but it makes the limiting of half-open SAs by
address or prefix work. Puzzles, introduced in Section 4.4,
accomplish the same thing only more of it. They make it harder for
an attacker to reach the goal of getting a half-open SA. Puzzles do
not have to be so hard that an attacker cannot afford to solve a
single puzzle; it is enough that puzzles increase the cost of
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creating a half-open SAs, so the attacker is limited in the amount
they can create.
Reducing the lifetime of an abandoned half-open SA also reduces the
impact of such attacks. For example, if a half-open SA is kept for 1
minute and the capacity is 60 thousand half-open SAs, an attacker
would need to create one thousand half-open SAs per second. If the
retention time is reduced to 3 seconds, the attacker would need to
create 20 thousand half-open SAs per second to get the same result.
By introducing a puzzle, each half-open SA becomes more expensive for
an attacker, making it more likely to prevent an exhaustion attack
against Responder memory.
At this point, filling up the half-open SA database is no longer the
most efficient DoS attack. The attacker has two alternative attacks
to do better:
1. Go back to spoofed addresses and try to overwhelm the CPU that
deals with generating cookies, or
2. Take the attack to the next level by also sending an IKE_AUTH
request.
If an attacker is so powerfull that it is able to overwhelm the
Responder's CPU that deals with generating cookies, then the attack
cannot be dealt with at the IKE level and must be handled by means of
the Intrusion Prevention System (IPS) technology.
On the other hand, the second alternative of sending an IKE_AUTH
request is very cheap. It requires generating a proper IKE header
with the correct IKE SPIs and a single Encrypted payload. The
content of the payload is irrelevant and might be junk. The
Responder has to perform the relatively expensive key derivation,
only to find that the MAC on the Encrypted payload on the IKE_AUTH
request fails the integrity check. If a Responder does not hold on
to the calculated SKEYSEED and SK_* keys (which it should in case a
valid IKE_AUTH comes in later) this attack might be repeated on the
same half-open SA. Puzzles make attacks of such sort more costly for
an attacker. See Section 7.2 for details.
Here too, the number of half-open SAs that the attacker can achieve
is crucial, because each one allows the attacker to waste some CPU
time. So making it hard to make many half-open SAs is important.
A strategy against DDoS has to rely on at least 4 components:
1. Hardening the half-open SA database by reducing retention time.
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2. Hardening the half-open SA database by rate-limiting single IPs/
prefixes.
3. Guidance on what to do when an IKE_AUTH request fails to decrypt.
4. Increasing the cost of half-open SAs up to what is tolerable for
legitimate clients.
Puzzles are used as a solution for strategy #4.
4. Defense Measures while the IKE SA is being created
4.1. Retention Periods for Half-Open SAs
As a UDP-based protocol, IKEv2 has to deal with packet loss through
retransmissions. Section 2.4 of [RFC7296] recommends "that messages
be retransmitted at least a dozen times over a period of at least
several minutes before giving up". Many retransmission policies in
practice wait one or two seconds before retransmitting for the first
time.
Because of this, setting the timeout on a half-open SA too low will
cause it to expire whenever even one IKE_AUTH request packet is lost.
When not under attack, the half-open SA timeout SHOULD be set high
enough that the Initiator will have enough time to send multiple
retransmissions, minimizing the chance of transient network
congestion causing an IKE failure.
When the system is under attack, as measured by the amount of half-
open SAs, it makes sense to reduce this lifetime. The Responder
should still allow enough time for the round-trip, enough time for
the Initiator to derive the D-H shared value, and enough time to
derive the IKE SA keys and the create the IKE_AUTH request. Two
seconds is probably as low a value as can realistically be used.
It could make sense to assign a shorter value to half-open SAs
originating from IP addresses or prefixes that are considered suspect
because of multiple concurrent half-open SAs.
4.2. Rate Limiting
Even with DDoS, the attacker has only a limited amount of nodes
participating in the attack. By limiting the amount of half-open SAs
that are allowed to exist concurrently with each such node, the total
amount of half-open SAs is capped, as is the total amount of key
derivations that the Responder is forced to complete.
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In IPv4 it makes sense to limit the number of half-open SAs based on
IP address. Most IPv4 nodes are either directly attached to the
Internet using a routable address or are hidden behind a NAT device
with a single IPv4 external address. For IPv6, ISPs assign between a
/48 and a /64, so it does not make sense for rate-limiting to work on
single IPv6 IPs. Instead, ratelimits should be done based on either
the /48 or /64 of the misbehaving IPv6 address observed.
The number of half-open SAs is easy to measure, but it is also
worthwhile to measure the number of failed IKE_AUTH exchanges. If
possible, both factors should be taken into account when deciding
which IP address or prefix is considered suspicious.
There are two ways to rate-limit a peer address or prefix:
1. Hard Limit - where the number of half-open SAs is capped, and any
further IKE_SA_INIT requests are rejected.
2. Soft Limit - where if a set number of half-open SAs exist for a
particular address or prefix, any IKE_SA_INIT request will be
required to solve a puzzle.
The advantage of the hard limit method is that it provides a hard cap
on the amount of half-open SAs that the attacker is able to create.
The disadvantage is that it allows the attacker to block IKE
initiation from small parts of the Internet. For example, if an
network service provider or some establishment offers Internet
connectivity to its customers or employees through an IPv4 NAT
device, a single malicious customer can create enough half-open SAs
to fill the quota for the NAT device external IP address. Legitimate
Initiators on the same network will not be able to initiate IKE.
The advantage of a soft limit is that legitimate clients can always
connect. The disadvantage is that an adversary with sufficient CPU
resources can still effectively DoS the Responder.
Regardless of the type of rate-limiting used, legitimate initiators
that are not on the same network segments as the attackers will not
be affected. This is very important as it reduces the adverse impact
caused by the measures used to counteract the attack, and allows most
initiators to keep working even if they do not support puzzles.
4.3. The Stateless Cookie
Section 2.6 of [RFC7296] offers a mechanism to mitigate DoS attacks:
the stateless cookie. When the server is under load, the Responder
responds to the IKE_SA_INIT request with a calculated "stateless
cookie" - a value that can be re-calculated based on values in the
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IKE_SA_INIT request without storing Responder-side state. The
Initiator is expected to repeat the IKE_SA_INIT request, this time
including the stateless cookie. This mechanism prevents DoS attacks
from spoofed IP addresses, since an attacker needs to have a routable
IP address to return the cookie.
Attackers that have multiple source IP addresses with return
routability, such as in the case of bot-nets, can fill up a half-open
SA table anyway. The cookie mechanism limits the amount of allocated
state to the number of attackers, multiplied by the number of half-
open SAs allowed per peer address, multiplied by the amount of state
allocated for each half-open SA. With typical values this can easily
reach hundreds of megabytes.
4.4. Puzzles
The puzzle introduced here extends the cookie mechanism of [RFC7296].
It is loosely based on the proof-of-work technique used in Bitcoins
[bitcoins]. Puzzles set an upper bound, determined by the attacker's
CPU, to the number of negotiations the attacker can initiate in a
unit of time.
A puzzle is sent to the Initiator in two cases:
o The Responder is so overloaded that no half-open SAs may be
created without solving a puzzle, or
o The Responder is not too loaded, but the rate-limiting method
described in Section 4.2 prevents half-open SAs from being created
with this particular peer address or prefix without first solving
a puzzle.
When the Responder decides to send the challenge to solve a puzzle in
response to a IKE_SA_INIT request, the message includes at least
three components:
1. Cookie - this is calculated the same as in [RFC7296], i.e. the
process of generating the cookie is not specified.
2. Algorithm, this is the identifier of a Pseudo-Random Function
(PRF) algorithm, one of those proposed by the Initiator in the SA
payload.
3. Zero Bit Count (ZBC). This is a number between 8 and 255 (or a
special value - 0, see Section 7.1.1.1) that represents the
length of the zero-bit run at the end of the output of the PRF
function calculated over the cookie that the Initiator is to
send. The values 1-8 are explicitly excluded, because they
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create a puzzle that is too easy to solve. Since the mechanism
is supposed to be stateless for the Responder, either the same
ZBC is used for all Initiators, or the ZBC is somehow encoded in
the cookie. If it is global then it means that this value is the
same for all the Initiators who are receiving puzzles at any
given point of time. The Responder, however, may change this
value over time depending on its load.
Upon receiving this challenge, the Initiator attempts to calculate
the PRF output using different keys. When enough keys are found such
that the resulting PRF output calculated using each of them has a
sufficient number of trailing zero bits, that result is sent to the
Responder.
The reason for using several keys in the results, rather than just
one key, is to reduce the variance in the time it takes the initiator
to solve the puzzle. We have chosen the number of keys to be four
(4) as a compromise between the conflicting goals of reducing
variance and reducing the work the Responder needs to perform to
verify the puzzle solution.
When receiving a request with a solved puzzle, the Responder verifies
two things:
o That the cookie is indeed valid.
o That the results of PRF of the transmitted cookie calculated with
the transmitted keys has a sufficient number of trailing zero
bits.
Example 1: Suppose the calculated cookie is
739ae7492d8a810cf5e8dc0f9626c9dda773c5a3 (20 octets), the algorithm
is PRF-HMAC-SHA256, and the required number of zero bits is 18.
After successively trying a bunch of keys, the Initiator finds the
following four 3-octet keys that work:
+--------+----------------------------------+----------+
| Key | Last 32 Hex PRF Digits | # 0-bits |
+--------+----------------------------------+----------+
| 061840 | e4f957b859d7fb1343b7b94a816c0000 | 18 |
| 073324 | 0d4233d6278c96e3369227a075800000 | 23 |
| 0c8a2a | 952a35d39d5ba06709da43af40700000 | 20 |
| 0d94c8 | 5a0452b21571e401a3d00803679c0000 | 18 |
+--------+----------------------------------+----------+
Table 1: Three solutions for 18-bit puzzle
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Example 2: Same cookie, but modify the required number of zero bits
to 22. The first 4-octet keys that work to satisfy that requirement
are 005d9e57, 010d8959, 0110778d, and 01187e37. Finding these
requires 18,382,392 invocations of the PRF.
+----------+-------------------------------+
| # 0-bits | Time to Find 4 keys (seconds) |
+----------+-------------------------------+
| 8 | 0.0025 |
| 10 | 0.0078 |
| 12 | 0.0530 |
| 14 | 0.2521 |
| 16 | 0.8504 |
| 17 | 1.5938 |
| 18 | 3.3842 |
| 19 | 3.8592 |
| 20 | 10.8876 |
+----------+-------------------------------+
Table 2: The time needed to solve a puzzle of various difficulty for
the cookie = 739ae7492d8a810cf5e8dc0f9626c9dda773c5a3
The figures above were obtained on a 2.4 GHz single core i5. Run
times can be halved or quartered with multi-core code, but would be
longer on mobile phone processors, even if those are multi-core as
well. With these figures 18 bits is believed to be a reasonable
choice for puzzle level difficulty for all Initiators, and 20 bits is
acceptable for specific hosts/prefixes.
Using puzzles mechanism in the IKE_SA_INIT exchange is described in
Section 7.1.
4.5. Session Resumption
When the Responder is under attack, it SHOULD prefer previously
authenticated peers who present a Session Resumption ticket
[RFC5723]. However, the Responder SHOULD NOT serve resumed
Initiators exclusively because dropping all IKE_SA_INIT requests
would lock out legitimate Initiators that have no resumption ticket.
When under attack the Responder SHOULD require Initiators presenting
Session Resumption Tickets to pass a return routability check by
including the COOKIE notification in the IKE_SESSION_RESUME response
message, as described in Section 4.3.2. of [RFC5723]. Note that the
Responder SHOULD cache tickets for a short time to reject reused
tickets (Section 4.3.1), and therefore there should be no issue of
half-open SAs resulting from replayed IKE_SESSION_RESUME messages.
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Several kinds of DoS attacks are possible on servers supported IKE
Session Resumption. See Section 9.3 of [RFC5723] for details.
4.6. Keeping computed Shared Keys
Once the IKE_SA_INIT exchange is finished, the Responder is waiting
for the first message of the IKE_AUTH exchange from the Initiator.
At this point the Initiator is not yet authenticated, and this fact
allows an attacker to perform an attack, described in Section 3. The
attacker can just send garbage in the IKE_AUTH message forcing the
Responder to perform costly CPU operations to compute SK_* keys.
If the received IKE_AUTH message failed to decrypt correctly (or
failed to pass ICV check), then the Responder SHOULD still keep the
computed SK_* keys, so that if it happened to be an attack, then an
attacker cannot get advantage of repeating the attack multiple times
on a single IKE SA. The responder can also use puzzles in the
IKE_AUTH exchange as decribed in Section 7.2.
4.7. Preventing "Hash and URL" Certificate Encoding Attacks
In IKEv2 each side may use the "Hash and URL" Certificate Encoding to
instruct the peer to retrieve certificates from the specified
location (see Section 3.6 of [RFC7296] for details). Malicious
initiators can use this feature to mount a DoS attack on the
responder by providing an URL pointing to a large file possibly
containing garbage. While downloading the file the responder
consumes CPU, memory and network bandwidth.
To prevent this kind of attack, the responder should not blindly
download the whole file. Instead, it SHOULD first read the initial
few bytes, decode the length of the ASN.1 structure from these bytes,
and then download no more than the decoded number of bytes. Note,
that it is always possible to determine the length of ASN.1
structures used in IKEv2, if they are DER-encoded, by analyzing the
first few bytes. However, since the content of the file being
downloaded can be under the attacker's control, implementations
should not blindly trust the decoded length and SHOULD check whether
it makes sense before continuing to download the file.
Implementations SHOULD also apply a configurable hard limit to the
number of pulled bytes and SHOULD provide an ability for an
administrator to either completely disable this feature or to limit
its use to a configurable list of trusted URLs.
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4.8. IKE Fragmentation
IKE Fragmentation described in [RFC7383] allows IKE peers to avoid IP
fragmentation of large IKE messages. Attackers can mount several
kinds of DoS attacks using IKE Fragmentation. See Section 5 of
[RFC7383] for details on how to mitigate these attacks.
5. Defense Measures after an IKE SA is created
Once an IKE SA is created there usually are only a limited amount of
IKE messages exchanged. This IKE traffic consists of exchanges aimed
to create additional Child SAs, IKE rekeys, IKE deletions and IKE
liveness tests. Some of these exchanges require relatively little
resources (like liveness check), while others may be resource
consuming (like creating or rekeying Child SA with D-H exchange).
Since any endpoint can initiate a new exchange, there is a
possibility that a peer would initiate too many exchanges that could
exhaust host resources. For example, the peer can perform endless
continuous Child SA rekeying or create an overwhelming number of
Child SAs with the same Traffic Selectors etc. Such behavior can be
caused by broken implementations, misconfiguration, or as an
intentional attack. The latter becomes more of a real threat if the
peer uses NULL Authentication, as described in [RFC7619]. In this
case the peer remains anonymous, allowing it to escape any
responsibility for its behaviour. See Section 3 of [RFC7619] for
details on how to mitigate attacks when using NULL Authentication.
The following recommendations apply especially for NULL Authenticated
IKE sessions, but also apply to authenticated IKE sessions, with the
difference that in the latter case, the identified peer can be locked
out.
o If the IKEv2 window size is greater than one, peers are able to
initiate multiple simultaneous exchanges that increase host
resource consumption. Since there is no way in IKEv2 to decrease
window size once it has been increased (see Section 2.3 of
[RFC7296]), the window size cannot be dynamically adjusted
depending on the load. It is NOT RECOMMENDED to allow an IKEv2
window size greater than one when NULL Authentication has been
used.
o If a peer initiates an abusive amount of CREATE_CHILD_SA exchanges
to rekey IKE SAs or Child SAs, the Responder SHOULD reply with
TEMPORARY_FAILURE notifications indicating the peer must slow down
their requests.
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o If a peer creates many Child SA with the same or overlapping
Traffic Selectors, implementations MAY respond with the
NO_ADDITIONAL_SAS notification.
o If a peer initiates many exchanges of any kind, the Responder MAY
introduce an artificial delay before responding to each request
message. This delay would decrease the rate the Responder needs
to process requests from any particular peer, and frees up
resources on the Responder that can be used for answering
legitimate clients. If the Responder receives retransmissions of
the request message during the delay period, the retransmitted
messages MUST be silently discarded. The delay must be short
enough to avoid legitimate peers deleting the IKE SA due to a
timeout. It is believed that a few seconds is enough. Note
however, that even a few seconds may be too long when settings
rely on an immediate response to the request message, e.g. for the
purposes of quick detection of a dead peer.
o If these counter-measures are inefficient, implementations MAY
delete the IKE SA with an offending peer by sending Delete
Payload.
In IKE, a client can request various configuration attributes from
server. Most often these attributes include internal IP addresses.
Malicious clients can try to exhaust a server's IP address pool by
continuously requesting a large number of internal addresses. Server
implementations SHOULD limit the number of IP addresses allocated to
any particular client. Note, this is not possible with clients using
NULL Authentication, since their identity cannot be verified.
6. Plan for Defending a Responder
This section outlines a plan for defending a Responder from a DDoS
attack based on the techniques described earlier. The numbers given
here are not normative, and their purpose is to illustrate the
configurable parameters needed for surviving DDoS attacks.
Implementations are deployed in different environments, so it is
RECOMMENDED that the parameters be settable. For example, most
commercial products are required to undergo benchmarking where the
IKE SA establishment rate is measured. Benchmarking is
indistinguishable from a DoS attack and the defenses described in
this document may defeat the benchmark by causing exchanges to fail
or take a long time to complete. Parameters SHOULD be tunable to
allow for benchmarking (if only by turning DDoS protection off).
Since all countermeasures may cause delays and additional work for
the Initiators, they SHOULD NOT be deployed unless an attack is
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likely to be in progress. To minimize the burden imposed on
Initiators, the Responder should monitor incoming IKE requests, for
two scenarios:
1. A general DDoS attack. Such an attack is indicated by a high
number of concurrent half-open SAs, a high rate of failed
IKE_AUTH exchanges, or a combination of both. For example,
consider a Responder that has 10,000 distinct peers of which at
peak 7,500 concurrently have VPN tunnels. At the start of peak
time, 600 peers might establish tunnels within any given minute,
and tunnel establishment (both IKE_SA_INIT and IKE_AUTH) takes
anywhere from 0.5 to 2 seconds. For this Responder, we expect
there to be less than 20 concurrent half-open SAs, so having 100
concurrent half-open SAs can be interpreted as an indication of
an attack. Similarly, IKE_AUTH request decryption failures
should never happen. Supposing that the tunnels are established
using EAP (see Section 2.16 of [RFC7296]), users may be expected
to enter a wrong password about 20% of the time. So we'd expect
125 wrong password failures a minute. If we get IKE_AUTH
decryption failures from multiple sources more than once per
second, or EAP failures more than 300 times per minute, this can
also be an indication of a DDoS attack.
2. An attack from a particular IP address or prefix. Such an attack
is indicated by an inordinate amount of half-open SAs from a
specific IP address or prefix, or an inordinate amount of
IKE_AUTH failures. A DDoS attack may be viewed as multiple such
attacks. If these are mitigated successfully, there will not be
a need to enact countermeasures on all Initiators. For example,
measures might be 5 concurrent half-open SAs, 1 decrypt failure,
or 10 EAP failures within a minute.
Note that using counter-measures against an attack from a particular
IP address may be enough to avoid the overload on the half-open SA
database. In this case the number of failed IKE_AUTH exchanges will
never exceed the threshold of attack detection.
When there is no general DDoS attack, it is suggested that no cookie
or puzzles be used. At this point the only defensive measure is to
monitor the number of half-open SAs, and setting a soft limit per
peer IP or prefix. The soft limit can be set to 3-5. If the puzzles
are used, the puzzle difficulty should be set to such a level (number
of zero-bits) that all legitimate clients can handle it without
degraded user experience.
As soon as any kind of attack is detected, either a lot of
initiations from multiple sources or a lot of initiations from a few
sources, it is best to begin by requiring stateless cookies from all
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Initiators. This will This will mitigate attacks based on IP address
spoofing, and help avoid the need to impose a greater burden in the
form of puzzles on the general population of Initiators. This makes
the per-node or per-prefix soft limit more effective.
When cookies are activated for all requests and the attacker is still
managing to consume too many resources, the Responder MAY start to
use puzzles for these requests or increase the difficulty of puzzles
imposed on IKE_SA_INIT requests coming from suspicious nodes/
prefixes. This should still be doable by all legitimate peers, but
the use of puzzles at a higher difficulty may degrade the user
experience, for example by taking up to 10 seconds to solve the
puzzle.
If the load on the Responder is still too great, and there are many
nodes causing multiple half-open SAs or IKE_AUTH failures, the
Responder MAY impose hard limits on those nodes.
If it turns out that the attack is very widespread and the hard caps
are not solving the issue, a puzzle MAY be imposed on all Initiators.
Note that this is the last step, and the Responder should avoid this
if possible.
7. Using Puzzles in the Protocol
This section describes how the puzzle mechanism is used in IKEv2. It
is organized as follows. The Section 7.1 describes using puzzles in
the IKE_SA_INIT exchange and the Section 7.2 describes using puzzles
in the IKE_AUTH exchange. Both sections are divided into subsections
describing how puzzles should be presented, solved and processed by
the Initiator and the Responder.
7.1. Puzzles in IKE_SA_INIT Exchange
IKE Initiator indicates the desire to create a new IKE SA by sending
an IKE_SA_INIT request message. The message may optionally contain a
COOKIE notification if this is a repeated request performed after the
Responder's demand to return a cookie.
HDR, [N(COOKIE),] SA, KE, Ni, [V+][N+] -->
According to the plan, described in Section 6, the IKE Responder
should monitor incoming requests to detect whether it is under
attack. If the Responder learns that a (D)DoS attack is likely to be
in progress, then its actions depend on the volume of the attack. If
the volume is moderate, then the Responder requests the Initiator to
return a cookie. If the volume is so high, that puzzles need to be
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used for defense, then the Responder requests the Initiator to solve
a puzzle.
The Responder MAY choose to process some fraction of IKE_SA_INIT
requests without presenting a puzzle while being under attack to
allow legacy clients, that don't support puzzles, to have a chance to
be served. The decision whether to process any particular request
must be probabilistic, with the probability depending on the
Responder's load (i.e. on the volume of attack). The requests that
don't contain the COOKIE notification MUST NOT participate in this
lottery. In other words, the Responder must first perform a return
routability check before allowing any legacy client to be served if
it is under attack. See Section 7.1.4 for details.
7.1.1. Presenting a Puzzle
If the Responder makes a decision to use puzzles, then it includes
two notifications in its response message - the COOKIE notification
and the PUZZLE notification. Note that the PUZZLE notification MUST
always be accompanied with the COOKIE notification, since the content
of the COOKIE notification is used as an input data when solving
puzzle. The format of the PUZZLE notification is described in
Section 8.1.
<-- HDR, N(COOKIE), N(PUZZLE), [V+][N+]
The presence of these notifications in an IKE_SA_INIT response
message indicates to the Initiator that it should solve the puzzle to
have a better chance to be served.
7.1.1.1. Selecting the Puzzle Difficulty Level
The PUZZLE notification contains the difficulty level of the puzzle -
the minimum number of trailing zero bits that the result of PRF must
contain. In diverse environments it is next to impossible for the
Responder to set any specific difficulty level that will result in
roughly the same amount of work for all Initiators, because
computation power of different Initiators may vary by an order of
magnitude, or even more. The Responder may set the difficulty level
to 0, meaning that the Initiator is requested to spend as much power
to solve a puzzle as it can afford. In this case no specific value
of ZBC is required from the Initiator, however the larger the ZBC
that Initiator is able to get, the better the chance is that it will
be served by the Responder. In diverse environments it is
RECOMMENDED that the Initiator set the difficulty level to 0, unless
the attack volume is very high.
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If the Responder sets a non-zero difficulty level, then the level
should be determined by analyzing the volume of the attack. The
Responder MAY set different difficulty levels to different requests
depending on the IP address the request has come from.
7.1.1.2. Selecting the Puzzle Algorithm
The PUZZLE notification also contains identifier of the algorithm,
that must be used by Initiator to compute puzzle.
Cryptographic algorithm agility is considered an important feature
for modern protocols ([RFC7696]). Algorithm agility ensures that a
protocol doesn't rely on a single built-in set of cryptographic
algorithms, but has a means to replace one set with another, and
negotiate new algorithms with the peer. IKEv2 fully supports
cryptographic algorithm agility for its core operations.
To support crypto agility in case of puzzles, the algorithm that is
used to compute a puzzle needs to be negotiated during the
IKE_SA_INIT exchange. The negotiation is performed as follows. The
initial request message sent by the Initiator contains an SA payload
with the list of transforms the Initiator supports and is willing to
use in the IKE SA being established. The Responder parses the
received SA payload and finds a mutually supported PRFs. The
Responder selects the preferred PRF from the list of mutually
supported ones and includes it into the PUZZLE notification. There
is no requirement that the PRF selected for puzzles be the same as
the PRF that is negotiated later for use in core IKE SA crypto
operations. If there are no mutually supported PRFs, then IKE SA
negotiation will fail anyway and there is no reason to return a
puzzle. In this case the Responder returns a NO_PROPOSAL_CHOSEN
notification. Note that PRF is a mandatory transform type for IKE SA
(see Sections 3.3.2 and 3.3.3 of [RFC7296]) and at least one
transform of this type must always be present in the SA payload in an
IKE_SA_INIT request message.
7.1.1.3. Generating a Cookie
If the Responder supports puzzles then a cookie should be computed in
such a manner that the Responder is able to learn some important
information from the sole cookie, when it is later returned back by
Initiator. In particular - the Responder should be able to learn the
following information:
o Whether the puzzle was given to the Initiator or only the cookie
was requested.
o The difficulty level of the puzzle given to the Initiator.
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o The number of consecutive puzzles given to the Initiator.
o The amount of time the Initiator spent to solve the puzzles. This
can be calculated if the cookie is timestamped.
This information helps the Responder to make a decision whether to
serve this request or demand more work from the Initiator.
One possible approach to get this information is to encode it in the
cookie. The format of such encoding is an implementation detail of
Responder, as the cookie would remain an opaque blob to the
Initiator. If this information is encoded in the cookie, then the
Responder MUST make it integrity protected, so that any intended or
accidental alteration of this information in the returned cookie is
detectable. So, the cookie would be generated as:
Cookie = <VersionIDofSecret> | <AdditionalInfo> |
Hash(Ni | IPi | SPIi | <AdditionalInfo> | <secret>)
Note, that according to the Section 2.6 of [RFC7296], the size of the
cookie cannot exceed 64 bytes.
Alternatively, the Responder may continue to generate a cookie as
suggested in Section 2.6 of [RFC7296], but associate the additional
information, using local storage identified with the particular
version of the secret. In this case the Responder should have
different secrets for every combination of difficulty level and
number of consecutive puzzles, and should change the secrets
periodically, keeping a few previous versions, to be able to
calculate how long ago a cookie was generated.
The Responder may also combine these approaches. This document
doesn't mandate how the Responder learns this information from a
cookie.
7.1.2. Solving a Puzzle and Returning the Solution
If the Initiator receives a puzzle but it doesn't support puzzles,
then it will ignore the PUZZLE notification as an unrecognized status
notification (in accordance to Section 3.10.1 of [RFC7296]). The
Initiator MAY ignore the PUZZLE notification if it is not willing to
spend resources to solve the puzzle of the requested difficulty, even
if it supports puzzles. In both cases the Initiator acts as
described in Section 2.6 of [RFC7296] - it restarts the request and
includes the received COOKIE notification into it. The Responder
should be able to distinguish the situation when it just requested a
cookie from the situation where the puzzle was given to the
Initiator, but the Initiator for some reason ignored it.
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If the received message contains a PUZZLE notification and doesn't
contain a COOKIE notification, then this message is malformed because
it requests to solve the puzzle, but doesn't provide enough
information to allow the puzzle to be solved. In this case the
Initiator MUST ignore the received message and continue to wait until
either a valid PUZZLE notification is received or the retransmission
timer fires. If it fails to receive a valid message after several
retransmissions of IKE_SA_INIT requests, then it means that something
is wrong and the IKE SA cannot be established.
If the Initiator supports puzzles and is ready to solve them, then it
tries to solve the given puzzle. After the puzzle is solved the
Initiator restarts the request and returns back to the Responder the
puzzle solution in a new payload called a Puzzle Solution payload
(denoted as PS, see Section 8.2) along with the received COOKIE
notification.
HDR, N(COOKIE), [PS,] SA, KE, Ni, [V+][N+] -->
7.1.3. Computing a Puzzle
General principals of constructing puzzles in IKEv2 are described in
Section 4.4. They can be summarized as follows: given unpredictable
string S and pseudo-random function PRF find N different keys Ki
(where i=[1..N]) for that PRF so that the result of PRF(Ki,S) has at
least the specified number of trailing zero bits. This specification
requires that the solution to the puzzle contain 4 different keys
(i.e. N=4).
In the IKE_SA_INIT exchange it is the cookie that plays the role of
unpredictable string S. In other words, in the IKE_SA_INIT the task
for the IKE Initiator is to find the four different, equal-sized keys
Ki for the agreed upon PRF such that each result of PRF(Ki,cookie)
where i = [1..4] has a sufficient number of trailing zero bits. Only
the content of the COOKIE notification is used in puzzle calculation,
i.e. the header of the Notification payload is not included.
Note, that puzzles in the IKE_AUTH exchange are computed differently
than in the IKE_SA_INIT_EXCHANGE. See Section 7.2.3 for details.
7.1.4. Analyzing Repeated Request
The received request must at least contain a COOKIE notification.
Otherwise it is an initial request and it must be processed according
to Section 7.1. First, the cookie MUST be checked for validity. If
the cookie is invalid, then the request is treated as initial and is
processed according to Section 7.1. It is RECOMMENDED that a new
cookie is requested in this case.
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If the cookie is valid, then some important information is learned
from it, or from local state based on identifier of the cookie's
secret (see Section 7.1.1.3 for details). This information helps the
Responder to sort out incoming requests, giving more priority to
those which were created by spending more of the Initiator's
resources.
First, the Responder determines if it requested only a cookie, or
presented a puzzle to the Initiator. If no puzzle was given, this
means that at the time the Responder requested a cookie it didn't
detect the (D)DoS attack or the attack volume was low. In this case
the received request message must not contain the PS payload, and
this payload MUST be ignored if the message contains a PS payload for
any reason. Since no puzzle was given, the Responder marks the
request with the lowest priority since the Initiator spent little
resources creating it.
If the Responder learns from the cookie that the puzzle was given to
the Initiator, then it looks for the PS payload to determine whether
its request to solve the puzzle was honored or not. If the incoming
message doesn't contain a PS payload, this means that the Initiator
either doesn't support puzzles or doesn't want to deal with them. In
either case the request is marked with the lowest priority since the
Initiator spent little resources creating it.
If a PS payload is found in the message, then the Responder MUST
verify the puzzle solution that it contains. The solution is
interpreted as four different keys. The result of using each of them
in the PRF (as described in Section 7.1.3) must contain at least the
requested number of trailing zero bits. The Responder MUST check all
of the four returned keys.
If any checked result contains fewer bits than were requested, this
means that the Initiator spent less resources than expected by the
Responder. This request is marked with the lowest priority.
If the Initiator provided the solution to the puzzle satisfying the
requested difficulty level, or if the Responder didn't indicate any
particular difficulty level (by setting ZBC to zero) and the
Initiator was free to select any difficulty level it can afford, then
the priority of the request is calculated based on the following
considerations:
o The Responder must take the smallest number of trailing zero bits
among the checked results and count it as the number of zero bits
the Initiator solved for.
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o The higher number of zero bits the Initiator provides, the higher
priority its request should receive.
o The more consecutive puzzles the Initiator solved, the higher
priority it should receive.
o The more time the Initiator spent solving the puzzles, the higher
priority it should receive.
After the priority of the request is determined the final decision
whether to serve it or not is made.
7.1.5. Deciding if to Serve the Request
The Responder decides what to do with the request based on the
request's priority and the Responder's current load. There are three
possible actions:
o Accept request.
o Reject request.
o Demand more work from the Initiator by giving it a new puzzle.
The Responder SHOULD accept an incoming request if its priority is
high - this means that the Initiator spent quite a lot of resources.
The Responder MAY also accept some low-priority requests where the
Initiators don't support puzzles. The percentage of accepted legacy
requests depends on the Responder's current load.
If the Initiator solved the puzzle, but didn't spend much resources
for it (the selected puzzle difficulty level appeared to be low and
the Initiator solved it quickly), then the Responder SHOULD give it
another puzzle. The more puzzles the Initiator solves the higher its
chances are to be served.
The details of how the Responder makes a decision for any particular
request are implementation dependent. The Responder can collect all
of the incoming requests for some short period of time, sort them out
based on their priority, calculate the number of available memory
slots for half-open IKE SAs and then serve that number of requests
from the head of the sorted list. The remainder of requests can be
either discarded or responded to with new puzzle requests.
Alternatively, the Responder may decide whether to accept every
incoming request with some kind of lottery, taking into account its
priority and the available resources.
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7.2. Puzzles in an IKE_AUTH Exchange
Once the IKE_SA_INIT exchange is completed, the Responder has created
a state and is waiting for the first message of the IKE_AUTH exchange
from the Initiator. At this point the Initiator has already passed
the return routability check and has proved that it has performed
some work to complete IKE_SA_INIT exchange. However, the Initiator
is not yet authenticated and this allows a malicious Initiator to
perform an attack, described in Section 3. Unlike a DoS attack in
the IKE_SA_INIT exchange, which is targeted on the Responder's memory
resources, the goal of this attack is to exhaust a Responder's CPU
power. The attack is performed by sending the first IKE_AUTH message
containing garbage. This costs nothing to the Initiator, but the
Responder has to perform relatively costly operations when computing
the D-H shared secret and deriving SK_* keys to be able to verify
authenticity of the message. If the Responder doesn't keep the
computed keys after an unsuccessful verification of the IKE_AUTH
message, then the attack can be repeated several times on the same
IKE SA.
The Responder can use puzzles to make this attack more costly for the
Initiator. The idea is that the Responder includes a puzzle in the
IKE_SA_INIT response message and the Initiator includes a puzzle
solution in the first IKE_AUTH request message outside the Encrypted
payload, so that the Responder is able to verify puzzle solution
before computing the D-H shared secret. The difficulty level of the
puzzle should be selected so that the Initiator would spend
substantially more time to solve the puzzle than the Responder to
compute the shared secret.
The Responder should constantly monitor the amount of the half-open
IKE SA states that receive IKE_AUTH messages that cannot be decrypted
due to integrity check failures. If the percentage of such states is
high and it takes an essential fraction of Responder's computing
power to calculate keys for them, then the Responder may assume that
it is under attack and SHOULD use puzzles to make it harder for
attackers.
7.2.1. Presenting Puzzle
The Responder requests the Initiator to solve a puzzle by including
the PUZZLE notification in the IKE_SA_INIT response message. The
Responder MUST NOT use puzzles in the IKE_AUTH exchange unless a
puzzle has been previously presented and solved in the preceding
IKE_SA_INIT exchange.
<-- HDR, SA, KE, Nr, N(PUZZLE), [V+][N+]
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7.2.1.1. Selecting Puzzle Difficulty Level
The difficulty level of the puzzle in the IKE_AUTH exchange should be
chosen so that the Initiator would spend more time to solve the
puzzle than the Responder to compute the D-H shared secret and the
keys needed to decrypt and verify the IKE_AUTH request message. On
the other hand, the difficulty level should not be too high,
otherwise legitimate clients will experience an additional delay
while establishing the IKE SA.
Note, that since puzzles in the IKE_AUTH exchange are only allowed to
be used if they were used in the preceding IKE_SA_INIT exchange, the
Responder would be able to estimate the computational power of the
Initiator and select the difficulty level accordingly. Unlike
puzzles in the IKE_SA_INIT, the requested difficulty level for
IKE_AUTH puzzles MUST NOT be zero. In other words, the Responder
must always set a specific difficulty level and must not let the
Initiator to choose it on its own.
7.2.1.2. Selecting the Puzzle Algorithm
The algorithm for the puzzle is selected as described in
Section 7.1.1.2. There is no requirement that the algorithm for the
puzzle in the IKE_SA INIT exchange be the same as the algorithm for
the puzzle in IKE_AUTH exchange; however, it is expected that in most
cases they will be the same.
7.2.2. Solving Puzzle and Returning the Solution
If the IKE_SA_INIT response message contains the PUZZLE notification
and the Initiator supports puzzles, it MUST solve the puzzle. Note,
that puzzle construction in the IKE_AUTH exchange differs from the
puzzle construction in the IKE_SA_INIT exchange and is described in
Section 7.2.3. Once the puzzle is solved the Initiator sends the
IKE_AUTH request message, containing the Puzzle Solution payload.
HDR, PS, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SA, TSi, TSr} -->
The Puzzle Solution (PS) payload MUST be placed outside the Encrypted
payload, so that the Responder is able to verify the puzzle before
calculating the D-H shared secret and the SK_* keys.
If IKE Fragmentation [RFC7383] is used in IKE_AUTH exchange, then the
PS payload MUST be present only in the first IKE Fragment message, in
accordance with the Section 2.5.3 of [RFC7383]. Note, that
calculation of the puzzle in the IKE_AUTH exchange doesn't depend on
the content of the IKE_AUTH message (see Section 7.2.3). Thus the
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Initiator has to solve the puzzle only once and the solution is valid
for both unfragmented and fragmented IKE messages.
7.2.3. Computing the Puzzle
A puzzle in the IKE_AUTH exchange is computed differently than in the
IKE_SA_INIT exchange (see Section 7.1.3). The general principle is
the same; the difference is in the construction of the string S.
Unlike the IKE_SA_INIT exchange, where S is the cookie, in the
IKE_AUTH exchange S is a concatenation of Nr and SPIr. In other
words, the task for IKE Initiator is to find the four different keys
Ki for the agreed upon PRF such that each result of PRF(Ki,Nr | SPIr)
where i=[1..4] has a sufficient number of trailing zero bits. Nr is
a nonce used by the Responder in the IKE_SA_INIT exchange, stripped
of any headers. SPIr is the IKE Responder's SPI from the IKE header
of the SA being established.
7.2.4. Receiving the Puzzle Solution
If the Responder requested the Initiator to solve a puzzle in the
IKE_AUTH exchange, then it MUST silently discard all the IKE_AUTH
request messages without the Puzzle Solution payload.
Once the message containing a solution to the puzzle is received, the
Responder MUST verify the solution before performing computationlly
intensive operations i.e. computing the D-H shared secret and the
SK_* keys. The Responder MUST verify all four of the returned keys.
The Responder MUST silently discard the received message if any
checked verification result is not correct (contains insufficient
number of trailing zero bits). If the Responder successfully
verifies the puzzle and calculates the SK_* key, but the message
authenticity check fails, then it SHOULD save the calculated keys in
the IKE SA state while waiting for the retransmissions from the
Initiator. In this case the Responder may skip verification of the
puzzle solution and ignore the Puzzle Solution payload in the
retransmitted messages.
If the Initiator uses IKE Fragmentation, then it is possible, that
due to packet loss and/or reordering the Responder could receive non-
first IKE Fragment messages before receiving the first one containing
the PS payload. In this case the Responder MAY choose to keep the
received fragments until the first fragment containing the solution
to the puzzle is received. In this case the Responder SHOULD NOT try
to verify authenticity of the kept fragments until the first fragment
with the PS payload is received and the solution to the puzzle is
verified. After successful verification of the puzzle, the Responder
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can then calculate the SK_* key and verify authenticity of the
collected fragments.
8. Payload Formats
8.1. PUZZLE Notification
The PUZZLE notification is used by the IKE Responder to inform the
Initiator about the need to solve the puzzle. It contains the
difficulty level of the puzzle and the PRF the Initiator should use.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PRF | Difficulty |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Protocol ID (1 octet) -- MUST be 0.
o SPI Size (1 octet) - MUST be 0, meaning no Security Parameter
Index (SPI) is present.
o Notify Message Type (2 octets) -- MUST be <TBA by IANA>, the value
assigned for the PUZZLE notification.
o PRF (2 octets) -- Transform ID of the PRF algorithm that must be
used to solve the puzzle. Readers should refer to the section
"Transform Type 2 - Pseudo-Random Function Transform IDs" in
[IKEV2-IANA] for the list of possible values.
o Difficulty (1 octet) -- Difficulty Level of the puzzle. Specifies
the minimum number of trailing zero bits (ZBC), that each of the
results of PRF must contain. Value 0 means that the Responder
doesn't request any specific difficulty level and the Initiator is
free to select an appropriate difficulty level on its own (see
Section 7.1.1.1 for details).
This notification contains no data.
8.2. Puzzle Solution Payload
The solution to the puzzle is returned back to the Responder in a
dedicated payload, called the Puzzle Solution payload and denoted as
PS in this document.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Puzzle Solution Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Puzzle Solution Data (variable length) -- Contains the solution to
the puzzle - four different keys for the selected PRF. This field
MUST NOT be empty. All of the keys MUST have the same size,
therefore the size of this field is always a mutiple of 4 bytes.
If the selected PRF accepts only fixed-size keys, then the size of
each key MUST be of that fixed size. If the agreed upon PRF
accepts keys of any size, then then the size of each key MUST be
between 1 octet and the preferred key length of the PRF
(inclusive). It is expected that in most cases the keys will be 4
(or even less) octets in length, however it depends on puzzle
difficulty and on the Initiator's strategy to find solutions, and
thus the size is not mandated by this specification. The
Responder determines the size of each key by dividing the size of
the Puzzle Solution Data by 4 (the number of keys). Note that the
size of Puzzle Solution Data is the size of Payload (as indicated
in Payload Length field) minus 4 - the size of Payload Header.
The payload type for the Puzzle Solution payload is <TBA by IANA>.
9. Operational Considerations
The puzzle difficulty level should be set by balancing the
requirement to minimize the latency for legitimate Initiators with
making things difficult for attackers. A good rule of thumb is for
taking about 1 second to solve the puzzle. A typical Initiator or
bot-net member in 2014 can perform slightly less than a million
hashes per second per core, so setting the difficulty level to n=20
is a good compromise. It should be noted that mobile Initiators,
especially phones are considerably weaker than that. Implementations
should allow administrators to set the difficulty level, and/or be
able to set the difficulty level dynamically in response to load.
Initiators should set a maximum difficulty level beyond which they
won't try to solve the puzzle and log or display a failure message to
the administrator or user.
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10. Security Considerations
Care must be taken when selecting parameters for the puzzles, in
particular the puzzle difficulty. If the puzzles are too easy for
the majority of attacker, then the puzzle mechanism wouldn't be able
to prevent (D)DoS attacks and would only impose an additional burden
on legitimate Initiators. On the other hand, if the puzzles are too
hard for the majority of Initiators, then many legitimate users would
experience unacceptable delays in IKE SA setup (and unacceptable
power consumption on mobile devices), that might cause them to cancel
the connection attempt. In this case the resources of the Responder
are preserved, however the DoS attack can be considered successful.
Thus a sensible balance should be kept by the Responder while
choosing the puzzle difficulty - to defend itself and to not over-
defend itself. It is RECOMMENDED that the puzzle difficulty be
chosen so, that the Responder's load remains close to the maximum it
can tolerate. It is also RECOMMENDED to dynamically adjust the
puzzle difficulty in accordance to the current Responder's load.
Solving puzzles requires a lot of CPU power that increases power
consumption. This additional power consumption can negatively affect
battery-powered Initiators, e.g. mobile phones or some IoT devices.
If puzzles are too hard, then the required additional power
consumption may appear to be unacceptable for some Initiators. The
Responder SHOULD take this possibility into consideration while
choosing the puzzle difficulty, and while selecting which percentage
of Initiators are allowed to reject solving puzzles. See
Section 7.1.4 for details.
If the Initiator uses NULL Authentication [RFC7619] then its identity
is never verified. This condition may be used by attackers to
perform a DoS attack after the IKE SA is established. Responders
that allow unauthenticated Initiators to connect must be prepared to
deal with various kinds of DoS attacks even after the IKE SA is
created. See Section 5 for details.
To prevent amplification attacks implementations must strictly follow
the retransmission rules described in Section 2.1 of [RFC7296].
11. IANA Considerations
This document defines a new payload in the "IKEv2 Payload Types"
registry:
<TBA> Puzzle Solution PS
This document also defines a new Notify Message Type in the "IKEv2
Notify Message Types - Status Types" registry:
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<TBA> PUZZLE
12. Acknowledgements
The authors thank Tero Kivinen, Yaron Sheffer, and Scott Fluhrer for
their contributions to the design of the protocol. In particular,
Tero Kivinen suggested the kind of puzzle where the task is to find a
solution with a requested number of zero trailing bits. Yaron
Sheffer and Scott Fluhrer suggested a way to make puzzle difficulty
less erratic by solving several weaker puzles. The authors also
thank David Waltermire and Paul Wouters for their careful reviews of
the document, Graham Bartlett for pointing out to the possibility of
the "Hash & URL" related attack, and all others who commented the
document.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
DOI 10.17487/RFC5723, January 2010,
<http://www.rfc-editor.org/info/rfc5723>.
[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, <http://www.rfc-editor.org/info/rfc7296>.
[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014,
<http://www.rfc-editor.org/info/rfc7383>.
[IKEV2-IANA]
"Internet Key Exchange Version 2 (IKEv2) Parameters",
<http://www.iana.org/assignments/ikev2-parameters>.
13.2. Informative References
[bitcoins]
Nakamoto, S., "Bitcoin: A Peer-to-Peer Electronic Cash
System", October 2008, <https://bitcoin.org/bitcoin.pdf>.
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[RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication
Method in the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015,
<http://www.rfc-editor.org/info/rfc7619>.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<http://www.rfc-editor.org/info/rfc7696>.
Authors' Addresses
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 6789735
Israel
EMail: ynir.ietf@gmail.com
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
Russian Federation
Phone: +7 495 276 0211
EMail: svan@elvis.ru
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