--- 1/draft-ietf-ipsecme-ddos-protection-00.txt 2015-03-09 15:14:53.360649416 -0700 +++ 2/draft-ietf-ipsecme-ddos-protection-01.txt 2015-03-09 15:14:53.412650715 -0700 @@ -1,19 +1,20 @@ IPSecME Working Group Y. Nir Internet-Draft Check Point -Intended status: Standards Track October 27, 2014 -Expires: April 30, 2015 +Intended status: Standards Track V. Smyslov +Expires: September 9, 2015 ELVIS-PLUS + March 8, 2015 Protecting Internet Key Exchange (IKE) Implementations from Distributed Denial of Service Attacks - draft-ietf-ipsecme-ddos-protection-00 + draft-ietf-ipsecme-ddos-protection-01 Abstract This document recommends implementation and configuration best practices for Internet-connected IPsec 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 @@ -24,53 +25,69 @@ 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 April 30, 2015. + This Internet-Draft will expire on September 9, 2015. Copyright Notice - Copyright (c) 2014 IETF Trust and the persons identified as the + Copyright (c) 2015 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Conventions Used in This Document . . . . . . . . . . . . 3 2. The Vulnerability . . . . . . . . . . . . . . . . . . . . . . 3 - 3. Puzzles . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 4. Retention Periods for Half-Open SAs . . . . . . . . . . . . . 7 + 3. Puzzles . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 + 3.1. The Keyed-Cookie Notification . . . . . . . . . . . . . . 8 + 3.2. The Puzzle-Required Notification . . . . . . . . . . . . 8 + 4. Retention Periods for Half-Open SAs . . . . . . . . . . . . . 8 5. Rate Limiting . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Plan for Defending a Responder . . . . . . . . . . . . . . . 9 - 7. Operational Considerations . . . . . . . . . . . . . . . . . 11 - 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 - 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 - 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 - 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 - 10.2. Informative References . . . . . . . . . . . . . . . . . 12 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 + 6.1. Session Resumption . . . . . . . . . . . . . . . . . . . 11 + 7. Operational Considerations . . . . . . . . . . . . . . . . . 12 + 8. Using Puzzles in the Protocol . . . . . . . . . . . . . . . . 12 + 8.1. Puzzles in IKE_SA_INIT Exchange . . . . . . . . . . . . . 12 + 8.1.1. Presenting Puzzle . . . . . . . . . . . . . . . . . . 13 + 8.1.2. Solving Puzzle and Returning the Solution . . . . . . 15 + 8.1.3. Analyzing Repeated Request . . . . . . . . . . . . . 16 + 8.1.4. Making Decision whether to Serve the Request . . . . 17 + 8.2. Puzzles in IKE_AUTH Exchange . . . . . . . . . . . . . . 18 + 8.2.1. Presenting Puzzle . . . . . . . . . . . . . . . . . . 19 + 8.2.2. Solving Puzzle and Returning the Solution . . . . . . 20 + 8.2.3. Receiving Puzzle Solution . . . . . . . . . . . . . . 20 + 9. DoS Protection after IKE SA is created . . . . . . . . . . . 21 + 10. Payload Formats . . . . . . . . . . . . . . . . . . . . . . . 22 + 10.1. PUZZLE Notification . . . . . . . . . . . . . . . . . . 22 + 10.2. Puzzle Solution Payload . . . . . . . . . . . . . . . . 23 + 11. Security Considerations . . . . . . . . . . . . . . . . . . . 24 + 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 + 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 + 13.1. Normative References . . . . . . . . . . . . . . . . . . 24 + 13.2. Informative References . . . . . . . . . . . . . . . . . 24 1. Introduction 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 SA (Security Association). This half-open SA is later authenticated in the IKE_AUTH Exchange, but 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 @@ -220,104 +246,112 @@ The puzzle introduced here extends the cookie mechanism from RFC 7296. It is loosely based on the proof-of-work technique used in BitCoins ([bitcoins]). Future versions of this document will have the exact bit structure of the notification payloads, but for now, I will only describe the semantics of the content. A puzzle is sent to the Initiator in two cases: o The Responder is so overloaded, than no half-open SAs are allowed to be created without the puzzle, or + o The Responder is not too loaded, but the rate-limiting in Section 5 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 notification in - response to a IKE_SA_INIT request, the notification includes two + response to a IKE_SA_INIT request, the notification includes three fields: 1. Cookie - this is calculated the same as in RFC 7296. As in RFC - 7296, the process of generating the cookie is not specified, but - this specification does assume that it is fixed-length, meaning - that all cookies produced by a particular responder are of the - same length. - 2. Zero Bit Count. This is a number between 8 and 255 that + 7296, the process of generating the cookie is not specified. + + 2. Algorithm, this is the identifier of a PRF algorithm, one of + those proposed by the Initiator in the SA payload. + + 3. Zero Bit Count. This is a number between 8 and 255 that represents the length of the zero-bit run at the end of the - SHA-256 hash of the Cookie payload that the Initiator is to send. - Since the mechanism is supposed to be stateless for the - Responder, the same value is sent to all Initiators who are - receiving this challenge. The values 0 and 1-8 are explicitly - excluded, because the value zero is meaningless, and the values - 1-8 create a puzzle that is too easy to solve to make any - difference in mitigating DDoS attacks. + output of the PRF function calculated over the Keyed-Cookie + payload that the Initiator is to send. Since the mechanism is + supposed to be stateless for the Responder, the same value is + sent to all Initiators who are receiving this challenge. The + values 0 and 1-8 are explicitly excluded, because the value zero + is meaningless, and the values 1-8 create a puzzle that is too + easy to solve for it to make any difference in mitigating DDoS + attacks. Upon receiving this challenge payload, the Initiator attempts to - append different strings to the Cookie field from the challenge, and - calculates the SHA-256 hash of the result. When a string is found - such that the resulting hash has a sufficient number of trailing zero - bits, that result is sent to the Responder in a Cookie notification, - similar to what is described in RFC 7296. The difference is that the - string in this Cookie notification is longer than the one - transmitted. + calculate the PRF using different keys. When a key is found such + that the resulting PRF output has a sufficient number of trailing + zero bits, that result is sent to the Responder in a Keyed-Cookie + notification, as described in Section 3.1. - When receiving a request with an extended Cookie, the Responder - verifies two things: + When receiving a request with a Keyed-Cookie, the Responder verifies + two things: - o That the first bits of the transmitted cookie are indeed valid. - o That the hash of the transmitted cookie has a sufficient number of - trailing zero bits. + o That the cookie part is indeed valid. + + o That the PRF of the transmitted cookie calculated with the + transmitted key has a sufficient number of trailing zero bits. Example 1: Suppose the calculated cookie is - fdbcfa5a430d7201282358a2a034de0013cfe2ae (20 octets) and the required - number of zero bits is 16. After successively trying a bunch of - strings, the Initiator finds out that appending three octets: 022b3d - yields a 23-octet string whose SHA-256 hash is - 3b4bdf201105e059e09f65219021738b8f6a148896b2e1be2fdc726aeb6e0000. - That has 17 trailing zero bits, so it is an acceptable cookie. + fdbcfa5a430d7201282358a2a034de0013cfe2ae (20 octets), the algorithm + is HMAC-SHA256, and the required number of zero bits is 18. After + successively trying a bunch of keys, the Initiator finds that the key + that is all-zero except for the last three bytes which are 02fc95 + yields HMAC_SHA256(k, cookie) = + 843ab73f35c5b431b1d8f80bedcd1cb9ef46832f799c1d4250a49f683c580000, + which has 19 trailing zero bits, so it is an acceptable solution. Example 2: Same cookie, but this time the required number of zero - bits is 22. The first string to satisfy that requirement is 5c2880, - which yields a hash with 23 trailing zero bits. Finding this - requires 6,105,472 hashes. + bits is 22. The first key to satisfy that requirement ends in + 960cbb, which yields a hash with 23 trailing zero bits. Finding this + requires 9,833,659 invocations of the PRF. - +--------------+--------------------------+---------+---------------+ - | Appended | Last 24 Hex Hash Digits | # | Time To | - | String | | 0-bits | Calculate | - +--------------+--------------------------+---------+---------------+ - | 04 | 2817ae10f20f4e0b0739f5cc | 2 | 0.000 | - | 06 | e540cf315fff88c1c5f362a8 | 3 | 0.000 | - | 0d | 8c459376268f747d7ed40da0 | 5 | 0.000 | - | 1c | 398c49be1babe50576cdae40 | 6 | 0.000 | - | 00f0 | 3f523ad7c0e00252c51ad980 | 7 | 0.000 | - | 0182 | e284296e2ffffa256bdfa800 | 11 | 0.000 | - | 235c | 7dc74302dc8bd695821ab000 | 12 | 0.006 | - | 7186 | a4411c3df3661eff1d574000 | 14 | 0.019 | - | d836 | 498bcd04ab1ae0c2c3a08000 | 15 | 0.036 | - | 022b3d | 96b2e1be2fdc726aeb6e0000 | 17 | 0.136 | - | 0aa679 | 620f48af85428996c1f00000 | 20 | 0.512 | - | 4ffbad | f9ba0ece854cd0fa88e00000 | 21 | 3.602 | - | 5c2880 | d44e6467d8fc37723d800000 | 23 | 4.143 | - | cdafe1 | 0d4058660c3e67be62000000 | 25 | 9.245 | - | 022bffc8 | 5f2d874764a71e2948000000 | 27 | 36.169 | - | 181ac92a | c3b5449fa1019b0580000000 | 31 | 255.076 | - | a987978d | 95a5673968a9b37a00000000 | 33 | 1309.519 | - +--------------+--------------------------+---------+---------------+ + +----------+--------------------------+----------+------------------+ + | key | Last 24 Hex PRF Digits | # 0-bits | Time To | + | | | | Calculate | + +----------+--------------------------+----------+------------------+ + | 00 | 0cbbbd1e105f5a177f9697d4 | 2 | 0.000 | + | 08 | 34cdedf89560f600aab93c68 | 3 | 0.000 | + | 0b | 6153a5131b879a904cd7fbe0 | 5 | 0.000 | + | 2b | 0098af3e9422aa40a6f7b140 | 6 | 0.000 | + | 0147 | c8bf4a65fc8b974046b97c00 | 10 | 0.001 | + | 06e2 | 541487a10cbdf3b21c382800 | 11 | 0.005 | + | 0828 | 48719bd62393fcf9bc172000 | 13 | 0.006 | + | 0204a7 | 3dce3414477c2364d5198000 | 15 | 0.186 | + | 185297 | c19385bb7b9566e5fdf00000 | 20 | 2.146 | + | 69dc34 | 1b61ecb347cb2e0cba200000 | 21 | 9.416 | + | 960cbb | e48274bfac2b7e1930800000 | 23 | 13.300 | + | 01597972 | 39a0141d0fe4b87aea000000 | 25 | 30.749 | + | 0b13cd9a | 00b97bb323d6d33350000000 | 28 | 247.914 | + | 37dc96e4 | 1e24babc92234aa3a0000000 | 29 | 1237.170 | + | 7a1a56d8 | c98f0061e380a49e00000000 | 33 | 2726.150 | + +----------+--------------------------+----------+------------------+ Table 1: COOKIE=fdbcfa5a430d7201282358a2a034de0013cfe2ae 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 I believe that 20 bits is a reasonable choice for puzzle level difficulty for all Initiators, with 24 bits acceptable for specific hosts/prefixes. +3.1. The Keyed-Cookie Notification + + To be added + +3.2. The Puzzle-Required Notification + + To be added + 4. Retention Periods for Half-Open SAs As a UDP-based protocol, IKEv2 has to deal with packet loss through retransmissions. Section 2.4 of RFC 7296 recommends "that messages be retransmitted at least a dozen times over a period of at least several minutes before giving up". Retransmission policies in practice wait at least one or two seconds before retransmitting for the first time. Because of this, setting the timeout on a half-open SA too low will @@ -470,20 +506,32 @@ 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. +6.1. Session Resumption + + When the Responder is under attack, it MAY choose to prefer + previously authenticated peers who present a session resumption + [RFC5723] ticket. The Responder MAY require such Initiators to pass + a return routability check by including the COOKIE notification in + the IKE_SESSION_RESUME response message, as allowed by RFC 5723, Sec. + 4.3.2. Note that the Responder SHOULD cache tickets for a short time + to reject reused tickets (Sec. 4.3.1), and therefore there should be + no issue of half-open SAs resulting from replayed IKE_SESSION_RESUME + messages + 7. Operational Considerations [This section needs a lot of expanding] Not all Initiators support the puzzles, but all initiators are supposed to support stateless cookies. If this notification is sent to a non-supporting but legitimate initiator, the exchange will fail. Responders are advised to first try to mitigate the DoS using stateless cookies, even imposing them generally before resorting to using puzzles. @@ -496,46 +544,627 @@ 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. -8. Security Considerations +8. Using Puzzles in the Protocol + +8.1. Puzzles in IKE_SA_INIT Exchange + + IKE initiator indicates the desire to create new IKE SA by sending + IKE_SA_INIT request message. The message may optionally contain + COOKIE notification if this is a repeated request after the responder + asked initiator to return a cookie. + + HDR, [N(COOKIE),] SA, KE, Ni, [V+][N+] --> + + According to the plan, described in Section 6, IKE responder should + monitor incoming requests to detect whether it is under attack. If + the responder learns that (D)DoS attack is likely to be in progress, + then it either requests the initiator to return cookie or, if the + volume is so high, that puzzles need to be used for defense, it + requests the initiator to solve the puzzle. + + The responder MAY choose to process some fraction of IKE_SA_INIT + requests without presenting a puzzle even being under attack to allow + legacy clients, that don't support puzzles, to have chances 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 the attack). Only those requests, that + contain COOKIE notification, must participate in this lottery. In + other words, the responder MUST first perform return routability + check before allowing any legacy client to be served if it is under + attack. See Section 8.1.3 for details. + +8.1.1. Presenting Puzzle + + If the responder takes a decision to use puzzles, then it includes + two notifications in its response message - the COOKIE notification + and the PUZZLE notification. The format of the PUZZLE notification + is described in Section 10.1. + + <-- HDR, N(COOKIE), N(PUZZLE), [V+][N+] + + The presence of these notifications in IKE_SA_INIT response message + indicates to the initiator that it should solve the puzzle to get + better chances to be served. + +8.1.1.1. Selecting 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 the order of + magnitude, or even more. The responder may set difficulty level to + 0, meaning that the initiator is requested to spend as much power to + solve puzzle, as it can afford. In this case no specific number of + trailing zero bits is required from the initiator, however the more + bits initiator is able to get, the higher chances it will have to be + served by the responder. In diverse environments it is RECOMMENDED + that the initiator sets difficulty level to 0. + + If the responder sets 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 requestd + depending on the IP address the request has come from. + +8.1.1.2. Selecting Puzzle Algorithm + + The PUZZLE notification also contains identificator of the algorithm, + that must be used by initiator in puzzle solution. + + Cryptographic algorithm agility is considered an important feature + for modern protocols ([ALG-AGILITY]). This feature ensures that + protocol doesn't rely on a single build-in set of cryptographic + algorithms, but has a means to replace one set with another and + negotiate new set with the peer. IKEv2 fully supports cryptographic + algorithm agility for its core operations. + + To support this feature in case of puzzles the algorithm, that is + used to compute puzzle, needs to be negotiated during IKE_SA_INIT + exchange. The negotiation is done as follows. The initial request + message sent by initiator contains SA payload with the list of + transforms the initiator supports and is willing to use in the IKE SA + being established. The responder parses received SA payload and + finds mutually supported set of transforms of type PRF. It selects + most preferred transform from this set 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 the + use in core IKE SA crypto operations. If there are no mutually + supported PRFs, then negotiation will fail anyway and there is no + reason to return a puzzle. In this case the responder returns + 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 SA + payload in IKE_SA_INIT exchange. + +8.1.1.3. Generating Cookie + + If responder supports puzzles then 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 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. + + 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 a local matter 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 returned cookie is detectable. So, the cookie + would be generated as: + + Cookie = | | + Hash(Ni | IPi | SPIi | | ) + + Alternatively the responder may continue to generate cookie as + suggested in Section 2.6 of [RFC7296], but associate the additional + information, that would be stored locally, with the particular + version of the secret. In this case the responder should have + different secret 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 + the cookie was generated. + + The responder may also combine these approaches. This document + doesn't mandate how the responder learns this information from the + cookie. + +8.1.2. Solving Puzzle and Returning the Solution + + If initiator receives puzzle but it doesn't support puzzles, then it + will ignore PUZZLE notification as unrecognized status notification + (in accordance to Section 3.10.1 of [RFC7296]). The initiator also + MAY ignore puzzle if it is not willing to spend resources to solve + puzzle of 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 when the puzzle + was given to the initiator, but the initiator for some reason ignored + it. + + If the received message contains PUZZLE notification, but doesn't + contain cookie, then this message is malformed, because it requests + to solve the puzzle, but doesn't provide enough information to do it. + In this case the initiator SHOULD resend IKE_SA_INIT request. If + this situation repeats several times, then it means that something is + wrong and IKE SA cannot be established. + + If initiator supports puzzles and is ready to deal with them, then it + tries to solve the given puzzle. After the puzzle is solved the + initiator restarts the request and returns the puzzle solution in a + new payload called Puzzle Solution payload (denoted as PS, see + Section 10.2) along with the received COOKIE notification back to the + responder. + + HDR, N(COOKIE), [PS,] SA, KE, Ni, [V+][N+] --> + +8.1.2.1. Computing Puzzle + + General principals of constructing puzzles in IKEv2 are described in + Section 3. They can be summarized as follows: given unpredictable + string S and pseudo-random function PRF find the key K for that PRF + so that the result of PRF(K,S) has the specified number of trailing + zero bits. + + In the IKE_SA_INIT exchange it is the cookie that plays the role of + unpredictable string S. In other words, in IKE_SA_INIT the task for + IKE initiator is to find the key K for the agreed upon PRF such that + the result of PRF(K,cookie) has 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. + +8.1.3. Analyzing Repeated Request + + The received request must at least contain COOKIE notification. + Otherwise it is an initial request and it must be processed according + to Section 8.1. 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 8.1. 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 8.1.1.3 for details). + This information would allow the responder to sort out incoming + requests, giving more priority to those of them, which were created + spending more 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, then it + 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 for any reason the message contains + it. Since no puzzle was given, the responder marks the request with + the lowest priority since the initiator spent a 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 PS payload, then it 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 a little resources creating it. + + If PS payload is found in the message then the responder MUST verify + the puzzle solution that it contains. The result must contain at + least the requested number of trailing zero bits (that is also + learned from the cookie, as well as the PRF algorithm used in puzzle + solution). If the result of the solution contais fewer bits, than + were requested, it means that 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 requesting zero level) 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 higher zero bits the initiator got, the higher priority its + request should achieve. + + o The more consecutive puzzles the initiator solved (it must be + learned from the cookie), the higher priority its request should + achieve. + + o The more time the initiator spent solving the puzzles (it must be + learned from the cookie), the higher priority its request should + achieve. + + After the priority of the request is determined the final decision + whether to serve it or not is made. + +8.1.4. Making Decision whether to Serve the Request + + The responder decides what to do with the request based on its + priority and responder's current load. There are three possible + actions: + + o Accept request. + + o Reject request. + + o Demand more work from initiator by giving it a new puzzle. + + The responder SHOULD accept incoming request if its priority is high + - it means that the initiator spent quite a lot of resources. The + responder MAY also accept some of low-priority requests where the + initiators don't support puzzles. The percentage of accepted legacy + requests depends on the responder's current load. + + If 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 solve the higher + would be its chances ro be served. + + The details of how the responder takes decision on any particular + request are implementation dependant. The responder can collect all + the incoming requests for some short period of time, sort them out + based on their priority, calculate the number of alailable memory + slots for half-open IKE SAs and then serve that number of the + requests from the head of the sorted list. The rest of requests can + be either discarded or responded to with new puzzles. + + 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. + +8.2. Puzzles in IKE_AUTH Exchange + + Once the IKE_SA_INIT exchange is completed, the responder has created + a state and is awaiting for the first message of the IKE_AUTH + exchange from initiator. At this point the initiator has already + passed 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 fact allows malicious initiator to + conduct an attack, described in Section 2. Unlike DoS attack in + IKE_SA_INIT exchange, which is targeted on the responder's memory + resources, the goal of this attack is to exhaust 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 do relatively costly operations of computing the + Diffie-Hellman shared secret and deriving SK_* keys to be able to + verify authenticity of the message. If the responder doesn't save + the computed keys after unsuccessful verification of 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 puzzle in the + IKE_SA_INIT response message and the initiator includes 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 Diffie-Hellman 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, but cannot decrypt + them due to the 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 can + assume that it is under attack and can use puzzles to make it harder + for attackers. + +8.2.1. 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 the + puzzle has been previously presented and solved in the preceeding + IKE_SA_INIT exchange. + + <-- HDR, SA, KE, Nr, N(PUZZLE), [V+][N+] + +8.2.1.1. Selecting Puzzle Difficulty Level + + The difficulty level of the puzzle in IKE_AUTH should be chosen so, + that the initiator would spend more time to solve the puzzle, than + the responder to compute Diffie-Hellman shared secret and the keys, + needed to decrypt and verify IKE_AUTH message. On the other hand, + the difficulty level should not be too high, otherwise the legitimate + clients would experience additional delay while establishing IKE SA. + + Note, that since puzzles in the IKE_AUTH exchange are only allowed to + be used if they were used in the preceeding IKE_SA_INIT exchange, the + responder would be able to estimate the computing power of the + initiator and to select the difficulty level accordingly. Unlike + puzzles in IKE_SA_INIT, the requested difficulty level for IKE_AUTH + puzzles MUST NOT be zero. In other words, the responder must always + set specific difficulty level and must not let the initiator to + choose it on its own. + +8.2.1.2. Selecting Puzzle Algorithm + + The algorithm for the puzzle is selected as described in + Section 8.1.1.2. 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. + +8.2.2. 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. Puzzle + construction on the IKE_AUTH exchange differs from the puzzle in the + IKE_SA_INIT exchange and is described in Section 8.2.2.1. 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 payload is placed outside the Encrypted payload, + so that the responder would be able to verify the puzzle before + calculating the Diffie-Hellman shared secret and the SK_* keys. + + If IKE Fragmentation is used, 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 8.2.2.1). Thus the responder has to solve the + puzzle only once and the solution is valid for both unfragmented and + fragmented IKE messages. + +8.2.2.1. Computing Puzzle + + The puzzle in the IKE_AUTH exchange is computed differently, than in + the IKE_SA_INIT exchange (see Section 8.1.2.1). The general + principle is the same, the difference is in constructing 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 key K for the agreed + upon PRF such that the result of PRF(K,Nr | SPIr) has sufficient + number of trailing zero bits. Nr is a nonce used by the responder in + IKE_SA_INIT exchange, stripped of any headers. SPIr is IKE responder + SPI in the SA being established. + +8.2.3. Receiving Puzzle Solution + + If the responder requested the initiator to solve puzzle in the + IKE_AUTH exchange, then it SHOULD silently discard all the IKE_AUTH + request messages without the Puzzle Solution payload. + + Once the message containing solution for the puzzle is received the + responder SHOULD verify the solution before performing computationly + intensive operations - computing the Diffie-Hellman shared secret and + the SK_* keys. The responder MUST silently discard the received + message if the puzzle solution is not correct. If the puzzle is + successfully verified and the SK_* key are calculated, but the + message authenticity check fails, the responder 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 packets loss and/or reordering the responder would 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. However in this case the + responder SHOULD NOT try to verify authenticity (that would require + the calculation of the SK_* keys) untill 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 would + calculate the SK_* key and verify authenticity of the collected + fragments. + +9. DoS Protection after IKE SA is created + + Once IKE SA is created there is usually no much traffic over it. In + most cases this traffic consists of exchanges aimed to create + additional Child SAs, rekey or delete them and check the liveness of + the peer. With a typical setup and typical Child SA lifetimes there + must be no more than a few such exchanges in a minute, often less. + Some of these exchanges require relatively little resources (like + liveness check), while others may be resourse consuming (like + creating or rekeying Child SA with Diffie-Hellman exchange). + + Since any endpoint can initiate 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 overwhelming number of Child SAs with the + same Traffic Selectors etc. Such behaviour may be caused by buggy + implementation, misconfiguration or be intentional. The latter + becomes more real threat if the peer uses NULL Authentication, + described in [NULL-AUTH]. In this case the peer remains anonymous, + that allow it to escape any resposibility for its actions. + + The following recommendations for defense against possible DoS + attacks after IKE SA is established are mostly intended for + implementations that allow unauthenticated IKE sessions. However + they may also be useful in other cases. + + o If the IKEv2 window size is greater than one, then the peer could + initiate multiple simultaneous exchanges, that would potentially + increase host resourse consumption. Since currently there is no + way in IKEv2 to decrease window size once it was increased (see + Section 2.3 of [RFC7296]), the window size cannot be dynamically + adjusted depending on the load. For that reason if is NOT + RECOMMENDED to ever increase IKEv2 window size above its default + value of one if the peer uses NULL Authentication. + + o If the peer initiates requests to rekey IKE SA or Child SA too + often, implementations can respond to some of these requests with + the TEMPORARY_FAILURE notification, indicating that the request + should be retried after some period of time. + + o If the peer creates too many Child SA with the same or overlapping + Traffic Selectors, implementations can respond with the + NO_ADDITIONAL_SAS notification. + + o If the peer initiates too many exchanges of any kind, + implementations can introduce artificial delay before responding + to request messages. This delay would decrease the rate the + implementation need to process requests from any particular peer, + making possible to process requests from the others. The delay + should not be too long not to cause IKE SA to be deleted on the + other end due to timeout. It is believed that a few seconds is + enough. Note, that if the responder receives retransmissions of + the request message during the delay period, the retransmitted + messages should be silently discarded. + + o If these counter-measures are inefficient, implementations can + delete IKE SA with an offending peer by sending Delete Payload. + +10. Payload Formats + +10.1. PUZZLE Notification + + The PUZZLE notification is used by IKE responder to inform the + initiator about the necessity 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 , 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 + minimum number of trailing zero bit, that the result of PRF must + contain. Value 0 means that the responder doesn't request any + specific difficulty level and the initiator is free to select + appropriate difficulty level of its own. + + This notification contains no data. + +10.2. Puzzle Solution Payload + + The solution to the puzzle is returned back to the responder in a + dedicated payload, called Puzzle Solution payload and denoted as PS + in this document. + + 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 - i.e. the key for the PRF. This field MUST NOT be + empty. If the selected PRF has a fixed-size key, then the size of + the Puzzle Solution Data MUST be equal to the size of the key. If + the PRF agreed upon accepts keys of any size, then then the size + of the Puzzle Solution Data MUST be between 1 octet and the + preferred key length of the PRF (inclusive). + + The payload type for the Puzzle Solution payload is . + +11. Security Considerations To be added. -9. IANA Considerations +12. IANA Considerations - IANA is requested to assign a notify message type from the status - types range (16430-40959) of the "IKEv2 Notify Message Types - Status - Types" registry with name "PUZZLE". + This document defines a new payload in the "IKEv2 Payload Types" + registry: -10. References + Puzzle Solution PS -10.1. Normative References + This document also defines a new Notify Message Type in the "IKEv2 + Notify Message Types - Status Types" registry: + + PUZZLE + +13. References + +13.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. - [RFC7296] Kivinen, T., Kaufman, C., Hoffman, P., Nir, Y., and P. - Eronen, "Internet Key Exchange Protocol Version 2 - (IKEv2)", RFC 7296, October 2014. + [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. + Kivinen, "Internet Key Exchange Protocol Version 2 + (IKEv2)", STD 79, RFC 7296, October 2014. -10.2. Informative References + [RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2 + (IKEv2) Message Fragmentation", RFC 7383, November 2014. + + [IKEV2-IANA] + "Internet Key Exchange Version 2 (IKEv2) Parameters", + . + +13.2. Informative References + + [RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange + Protocol Version 2 (IKEv2) Session Resumption", RFC 5723, + January 2010. [bitcoins] Nakamoto, S., "Bitcoin: A Peer-to-Peer Electronic Cash - System", October 2008. + System", October 2008, . -Author's Address + [ALG-AGILITY] + Housley, R., "Guidelines for Cryptographic Algorithm + Agility", draft-iab-crypto-alg-agility-02 (work in + progress), December 2014. + + [NULL-AUTH] + Smyslov, V. and P. Wouters, "The NULL Authentication + Method in IKEv2 Protocol", draft-ietf-ipsecme-ikev2-null- + auth-02 (work in progress), January 2015. + +Authors' Addresses Yoav Nir Check Point Software Technologies Ltd. 5 Hasolelim st. Tel Aviv 6789735 Israel - Email: ynir.ietf@gmail.com + 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