





Internet Engineering Task Force (IETF)                            Y. Nir
Request for Comments: 8019                                   Check Point
Category: Standards Track                                     V. Smyslov
ISSN: 2070-1721                                               ELVIS-PLUS
                                                           November 2016


      Protecting Internet Key Exchange Protocol Version 2 (IKEv2)
       Implementations from Distributed Denial-of-Service Attacks

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 helps accomplish this
   task.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8019.

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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   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 . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  The Stateless Cookie  . . . . . . . . . . . . . . . . . .   8
     4.4.  Puzzles . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Session Resumption  . . . . . . . . . . . . . . . . . . .  11
     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  . . . . . . . . . . . . . . .  14
   7.  Using Puzzles in the Protocol . . . . . . . . . . . . . . . .  16
     7.1.  Puzzles in IKE_SA_INIT Exchange . . . . . . . . . . . . .  16
       7.1.1.  Presenting a Puzzle . . . . . . . . . . . . . . . . .  17
       7.1.2.  Solving a Puzzle and Returning the Solution . . . . .  19
       7.1.3.  Computing a Puzzle  . . . . . . . . . . . . . . . . .  20
       7.1.4.  Analyzing Repeated Request  . . . . . . . . . . . . .  21
       7.1.5.  Deciding Whether to Serve the Request . . . . . . . .  22
     7.2.  Puzzles in an IKE_AUTH Exchange . . . . . . . . . . . . .  23
       7.2.1.  Presenting the Puzzle . . . . . . . . . . . . . . . .  24
       7.2.2.  Solving the Puzzle and Returning the Solution . . . .  24
       7.2.3.  Computing the Puzzle  . . . . . . . . . . . . . . . .  25
       7.2.4.  Receiving the Puzzle Solution . . . . . . . . . . . .  25
   8.  Payload Formats . . . . . . . . . . . . . . . . . . . . . . .  26
     8.1.  PUZZLE Notification . . . . . . . . . . . . . . . . . . .  26
     8.2.  Puzzle Solution Payload . . . . . . . . . . . . . . . . .  27
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  28
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     12.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32












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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
   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, botnets 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 DoS and DDoS 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", "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 one hundred
   to several thousand 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.



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   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:

       *  Generates or reuses a Diffie-Hellman (DH) 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, the Responder:

       *  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 usage, 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 computationally expensive than those
   performed at stage #2.

   To attack such a Responder, an attacker can attempt to exhaust either
   memory or 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,



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   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
   creating 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 60,000 half-open SAs, an attacker would
   need to create 1,000 half-open SAs per second.  If the retention time
   is reduced to 3 seconds, the attacker would need to create 20,000
   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 powerful 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 Message Authentication Code (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.



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   A strategy against DDoS has to rely on at least 4 components:

   1.  Hardening the half-open SA database by reducing retention time.

   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, for the Initiator
   to derive the DH shared value, and to derive the IKE SA keys and
   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.









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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.

   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 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, rate limits 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 a
   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




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   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 recalculated based on values in the
   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 botnets, 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 Bitcoin
   [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.











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   When the Responder decides to send the challenge to solve a puzzle in
   response to an 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 Pseudorandom 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
       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.






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   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           | # of Zero Bits |
      +--------+----------------------------------+----------------+
      | 061840 | e4f957b859d7fb1343b7b94a816c0000 |       18       |
      | 073324 | 0d4233d6278c96e3369227a075800000 |       23       |
      | 0c8a2a | 952a35d39d5ba06709da43af40700000 |       20       |
      | 0d94c8 | 5a0452b21571e401a3d00803679c0000 |       18       |
      +--------+----------------------------------+----------------+

               Table 1: Four Solutions for the 18-Bit Puzzle

   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.

            +----------------+-------------------------------+
            | # of Zero 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 39ae7492d8a810cf5e8dc0f9626c9dda773c5a3

   The figures above were obtained on a 2.4 GHz single-core Intel i5
   processor in a 2013 Apple MacBook Pro.  Run times can be halved or
   quartered with multi-core code, but they 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.





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   Using the 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 of [RFC5723]); therefore, there should be no
   issue of half-open SAs resulting from replayed IKE_SESSION_RESUME
   messages.

   Several kinds of DoS attacks are possible on servers supported by 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.
   Instead of sending a properly formed and encrypted IKE_AUTH message,
   the attacker can just send arbitrary data, 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 the Integrity Check Value (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 an advantage of
   repeating the attack multiple times on a single IKE SA.  The
   Responder can also use puzzles in the IKE_AUTH exchange as described
   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 a URL pointing to a large file possibly




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   containing meaningless bits.  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 limit its
   use to a configurable list of trusted URLs.

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 is usually 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 a liveness check), while others may be resource
   consuming (like creating or rekeying a Child SA with DH 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 behavior.  See Section 3 of [RFC7619] for
   details on how to mitigate attacks when using NULL Authentication.






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   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.

   o  If a peer creates many Child SAs 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 countermeasures 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
   the 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





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   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 to 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
   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 Extensible Authentication Protocol (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.






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   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 countermeasures 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 set 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
   Initiators.  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.




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7.  Using Puzzles in the Protocol

   This section describes how the puzzle mechanism is used in IKEv2.  It
   is organized as follows.  Section 7.1 describes using puzzles in the
   IKE_SA_INIT exchange and 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

   The 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+]   -->

                   Figure 1: Initial IKE_SA_INIT Request

   According to the plan, described in Section 6, the IKE Responder
   monitors incoming requests to detect whether it is under attack.  If
   the Responder learns that a DoS or DDoS 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 high to such an extent that
   puzzles need to be 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.












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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 the
   puzzle.  The format of the PUZZLE notification is described in
   Section 8.1.

                             <--   HDR, N(COOKIE), N(PUZZLE), [V+][N+]

             Figure 2: IKE_SA_INIT Response Containing Puzzle

   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 nearly 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 the 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.

   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.












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7.1.1.2.  Selecting the Puzzle Algorithm

   The PUZZLE notification also contains an identifier of the algorithm
   that is used by the Initiator to compute the 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 from the Initiator contains an SA payload
   containing a list of transforms of different types.  In that manner,
   the Initiator asserts that it supports all transforms from this list
   and can use any of them in the IKE SA being established.  The
   Responder parses the received SA payload and finds mutually supported
   transforms of type PRF.  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 is always 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
   the 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.

   o  The number of consecutive puzzles given to the Initiator.





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   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
   the Responder, as the cookie would remain an opaque block of data 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 Section 2.6 of [RFC7296], the size of the
   cookie cannot exceed 64 bytes.

   Alternatively, the Responder may 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.

   When selecting cookie generation, algorithm implementations MUST
   ensure that an attacker gains no or insignificant benefit from
   reusing puzzle solutions in several requests.  See Section 10 for
   details.

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 with 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 in it.  The Responder



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   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.

   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 this 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 (PS)
   payload (see Section 8.2) along with the received COOKIE
   notification.

   HDR, N(COOKIE), [PS,] SA, KE, Ni, [V+][N+]   -->

         Figure 3: IKE_SA_INIT Request Containing Puzzle Solution

7.1.3.  Computing a Puzzle

   General principles of constructing puzzles in IKEv2 are described in
   Section 4.4.  They can be summarized as follows: given unpredictable
   string S and 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 puzzle
   solution contains 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 Notify 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.







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7.1.4.  Analyzing Repeated Request

   The received request must at least contain a COOKIE notification.
   Otherwise, it is an initial request and in this case, 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.

   If the cookie is valid, then some important information is learned
   from it or from local state based on the 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 that 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 DoS or DDoS 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 the ZBC to 0) and the



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   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.

   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 Whether 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



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   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.

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 the IKE_SA_INIT exchange.  However, the
   Initiator is not yet authenticated, and this allows a malicious
   Initiator to perform an attack, as 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 arbitrary data.  This costs nothing to
   the Initiator, but the Responder has to perform relatively costly
   operations when computing the DH 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 a puzzle solution
   before computing the DH shared secret.

   The Responder constantly monitors 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 the 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.










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7.2.1.  Presenting the 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+]

         Figure 4: IKE_SA_INIT Response Containing IKE_AUTH Puzzle

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 DH 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 roughly 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 0.  In other words, the Responder must
   always set a specific difficulty level and must not let the Initiator
   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 the IKE_AUTH exchange; however, it is expected that in
   most cases they will be the same.

7.2.2.  Solving the Puzzle and Returning the Solution

   If the IKE_SA_INIT regular response message (i.e., the message
   containing SA, KE, NONCE payloads) 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 PS payload.



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   HDR, PS, SK {IDi, [CERT,] [CERTREQ,]
               [IDr,] AUTH, SA, TSi, TSr}   -->

      Figure 5: IKE_AUTH Request Containing IKE_AUTH Puzzle Solution

   The PS payload MUST be placed outside the Encrypted payload, so that
   the Responder is able to verify the puzzle before calculating the DH
   shared secret and the SK_* keys.

   If IKE fragmentation [RFC7383] is used in the IKE_AUTH exchange, then
   the PS payload MUST be present only in the first IKE Fragment
   message, in accordance with 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
   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 the 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 PS payload.

   Once the message containing a solution to the puzzle is received, the
   Responder MUST verify the solution before performing computationally
   intensive operations, i.e., computing the DH 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



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   Initiator.  In this case, the Responder may skip verification of the
   puzzle solution and ignore the PS payload in the retransmitted
   messages.

   If the Initiator uses IKE fragmentation, then it sends all fragments
   of a message simultaneously.  Due to packets loss and/or reordering,
   it is possible that the Responder receives subsequent fragments
   before receiving the first one that contains 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 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 SPI is present.

   o  Notify Message Type (2 octets) -- MUST be 16434, 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 "Transform
      Type 2 - Pseudorandom Function Transform IDs" subregistry on
      [IKEV2-IANA] for the list of possible values.





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   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 PS payload.

                        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 multiple 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 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 the Payload (as
      indicated in the Payload Length field) minus 4 -- the size of the
      Payload header.

   The payload type for the PS payload is 54.








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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
   taking about 1 second to solve the puzzle.  At the time this document
   was written, a typical Initiator or botnet member can perform
   slightly less than a million hashes per second per core, so setting
   the number of zero bits to 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.

   Until the widespread adoption of puzzles happens, most Initiators
   will ignore them, as will all attackers.  For puzzles to become a
   really powerful defense measure against DDoS attacks, they must be
   supported by the majority of legitimate clients.

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 attackers, then the puzzle mechanism wouldn't be able
   to prevent DoS or DDoS 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.

   If the cookie is generated as suggested in Section 2.6 of [RFC7296],
   then an attacker can use the same SPIi and the same Ni for several
   requests from the same IPi.  This will result in generating the same





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   cookies for these requests until the Responder changes the value of
   its cookie generation secret.  Since the cookies are used as an input
   data for puzzles in the IKE_SA_INIT exchange, generating the same
   cookies allows the attacker to reuse puzzle solutions, thus bypassing
   the proof-of-work requirement.  Note that the attacker can get only
   limited benefit from this situation -- once the half-open SA is
   created by the Responder, all the subsequent initial requests with
   the same IPi and SPIi will be treated as retransmissions and
   discarded by the Responder.  However, once this half-open SA is
   expired and deleted, the attacker can create a new one for free if
   the Responder hasn't changed its cookie generation secret yet.

   The Responder can use various countermeasures to completely eliminate
   or mitigate this scenario.  First, the Responder can change its
   cookie generation secret frequently especially if under attack, as
   recommended in Section 2.6 of [RFC7296].  For example, if the
   Responder keeps two values of the secret (current and previous) and
   the secret lifetime is no more than a half of the current half-open
   SA retention time (see Section 4.1), then the attacker cannot get
   benefit from reusing a puzzle solution.  However, short cookie
   generation secret lifetime could have a negative consequence on weak
   legitimate Initiators, since it could take too long for them to solve
   puzzles, and their solutions would be discarded if the cookie
   generation secret has been already changed few times.

   Another approach for the Responder is to modify the cookie generation
   algorithm in such a way that the generated cookies are always
   different or are repeated only within a short time period.  If the
   Responder includes a timestamp in <AdditionalInfo> as suggested in
   Section 7.1.1.3, then the cookies will repeat only within a short
   time interval equal to timestamp resolution.  Another approach for
   the Responder is to maintain a global counter that is incremented
   every time a cookie is generated and include this counter in
   <AdditionalInfo>.  This will make every cookie unique.

   Implementations MUST use one of the above (or some other)
   countermeasures to completely eliminate or make insignificant the
   possible benefit an attacker can get from reusing puzzle solutions.
   Note that this issue doesn't exist in IKE_AUTH puzzles (Section 7.2)
   since the puzzles in IKE_AUTH are always unique if the Responder
   generates SPIr and Nr randomly in accordance with [RFC7296].

   Solving puzzles requires a lot of CPU usage that increases power
   consumption.  This additional power consumption can negatively affect
   battery-powered Initiators, e.g., mobile phones or some Internet of
   Things (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



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   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:

     54       Puzzle Solution                   PS

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

     16434    PUZZLE

12.  References

12.1.  Normative References

   [IKEV2-IANA]
              IANA, "Internet Key Exchange Version 2 (IKEv2)
              Parameters",
              <http://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,
              <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>.






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   [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>.

12.2.  Informative References

   [BITCOINS] Nakamoto, S., "Bitcoin: A Peer-to-Peer Electronic Cash
              System", October 2008, <https://bitcoin.org/bitcoin.pdf>.

   [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>.

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 puzzles.  The authors also
   thank David Waltermire and Paul Wouters for their careful reviews of
   the document, Graham Bartlett for pointing out the possibility of an
   attack related to "Hash & URL", Stephen Farrell for catching the
   repeated cookie issue, and all others who commented on the document.














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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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