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Versions: 00 01 02

Network Working Group                                          E. Nygren
Internet-Draft                                                    S. Erb
Intended status: Standards Track                     Akamai Technologies
Expires: July 1, 2017                                        A. Biryukov
                                                         D. Khovratovich
                                                University of Luxembourg
                                                                A. Juels
                                                      Cornell University
                                                       December 28, 2016


                      TLS Client Puzzles Extension
                   draft-nygren-tls-client-puzzles-02

Abstract

   Client puzzles allow a TLS server to defend itself against asymmetric
   DDoS attacks.  In particular, it allows a server to request clients
   perform a selected amount of computation prior to the server
   performing expensive cryptographic operations.  This allows servers
   to employ a layered defense that represents an improvement over pure
   rate-limiting strategies.

   Client puzzles are implemented as an extension to TLS 1.3
   [I-D.ietf-tls-tls13] wherein a server can issue a HelloRetryRequest
   containing the puzzle as an extension.  The client must then resend
   its ClientHello with the puzzle results in the extension.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 1, 2017.







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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.

Table of Contents

   1.  Overview and rationale  . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Handshake Changes . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  The ClientPuzzleExtension Message . . . . . . . . . . . .   5
   4.  Usage by Servers  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Proposed Client Puzzles . . . . . . . . . . . . . . . . . . .   6
     5.1.  Cookie Client Puzzle Type . . . . . . . . . . . . . . . .   7
     5.2.  SHA-256 CPU Puzzle Type . . . . . . . . . . . . . . . . .   7
     5.3.  SHA-512 CPU Puzzle Type . . . . . . . . . . . . . . . . .   8
     5.4.  Equihash: Memory-hard Generalized Birthday Problem Puzzle
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Overview and rationale

   Adversaries can exploit the design of the TLS protocol to craft
   powerful asymmetric DDOS attacks.  Once an attacker has opened a TCP
   connection, the attacker can transmit effectively static content that
   causes the server to perform expensive cryptographic operations.
   Rate limiting offers one possible defense against this type of
   attack; however, pure rate limiting systems represent an incomplete
   solution:





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   1.  Rate limiting systems work best when a small number of bots are
       attacking a single server.  Rate limiting is much more difficult
       when a large number of bots are directing small amounts of
       traffic to each member of a large distributed pool of servers.

   2.  Rate limiting systems encounter problems where a mixture of
       "good" and "bad" clients are hidden behind a single NAT or Proxy
       IP address and thus are all stuck being treated on equal footing.

   3.  Rate limiting schemes often penalize well-behaved good clients
       (which try to complete handshakes and may limit their number of
       retries) much more heavily than they penalize attacking bad
       clients (which may try to disguise themselves as good clients,
       but which otherwise are not constrained to behave in any
       particular way).

   Client puzzles are complementary to rate-limiting and give servers
   another option than just rejecting some fraction of requests.  A
   server can provide a puzzle (of varying and server-selected
   complexity) to a client as part of a HelloRetryRequest extension.
   The client must choose to either abandon the connection or solve the
   puzzle and resend its ClientHello with a solution to the puzzle.
   Puzzles are designed to have asymmetric complexity such that it is
   much cheaper for the server to generate and validate puzzles than it
   is for clients to solve them.

   Client puzzle systems may be inherently "unfair" to clients that run
   with limited resources (such as mobile devices with batteries and
   slow CPUs).  However, client puzzle schemes will typically only be
   evoked when a server is under attack and would otherwise be rejecting
   some fraction of requests.  The overwhelming majority of transactions
   will never involve a client puzzle.  Indeed, if client puzzles are
   successful in forcing adversaries to use a new attack vector, the
   presence of client puzzles will be completely transparent to end
   users.

   It is likely that not all clients will choose to support this
   extension.  During attack scenarios, servers will still have the
   option to apply traditional rate limiting schemes (perhaps with
   different parameters) to clients not supporting this extension or
   using a version of TLS prior to 1.3.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].




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   Messages are formatted with the notation as described within
   [I-D.ietf-tls-tls13].

3.  Handshake Changes

   Client puzzles are implemented as a new ClientPuzzleExtension to TLS
   1.3 [I-D.ietf-tls-tls13].  A client supporting the
   ClientPuzzleExtension MUST indicate support by sending a
   ClientPuzzleExtension along with their ClientHello containing a list
   of puzzle types supported, but with no puzzle response.  When a
   server wishes to force the client to solve a puzzle, it MAY send a
   HelloRetryRequest with a ClientPuzzleExtension containing a puzzle of
   a supported puzzle type and with associated parameters.  To continue
   with the handshake, a client MUST resend their ClientHello with a
   ClientPuzzleExtension containing a response to the puzzle.  The
   ClientHello must otherwise be identical to the initial ClientHello,
   other than for attributes that are defined by specification to not be
   identical.

   Puzzles issued by the server contain a token that the client must
   include in their response.  This allows a server to issue puzzles
   without retaining state, which is particularly useful when used in
   conjunction with DTLS.

   If a puzzle would consume too many resources, a client MAY choose to
   abort the handshake with the new fatal alert "puzzle_too_hard" and
   terminate the connection.

   A typical handshake when a puzzle is issued will look like:






















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      Client                                               Server

      ClientHello
        + ClientPuzzleExtension
        + ClientKeyShare        -------->
                                <--------       HelloRetryRequest
                                          + ClientPuzzleExtension
      ClientHello
        + ClientPuzzleExtension
        + ClientKeyShare        -------->
                                                      ServerHello
                                                   ServerKeyShare
                                           {EncryptedExtensions*}
                                           {ServerConfiguration*}
                                                   {Certificate*}
                                            {CertificateRequest*}
                                             {CertificateVerify*}
                                <--------              {Finished}
      {Certificate*}
      {CertificateVerify*}
      {Finished}                -------->
      [Application Data]        <------->     [Application Data]

   Figure 1.  Message flow for a handshake with a client puzzle

   * Indicates optional or situation-dependent messages that are not
   always sent.

   {} Indicates messages protected using keys derived from the ephemeral
   secret.

   [] Indicates messages protected using keys derived from the master
   secret.

   Note in particular that the major cryptographic operations (starting
   to use the ephemeral secret and generating the CertificateVerify) are
   performed _after_ the server has received and validated the
   ClientPuzzleExtension response from the client.

3.1.  The ClientPuzzleExtension Message

   The ClientPuzzleExtension message contains an indication of supported
   puzzle types during the initial ClientHello, a selected puzzle type
   and puzzle challenge during HelloRetryRequest, and the puzzle type
   and puzzle response in the retried ClientHello:






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         struct {
             ClientPuzzleType type<1..255>;
             opaque client_puzzle_challenge_response<0..2^16-1>;
         } ClientPuzzleExtension;

         enum {
            cookie (0),
            sha256_cpu (1),
            sha512_cpu (2),
            birthday_puzzle (3),
            (0xFFFF)
         } ClientPuzzleType;

   type  During initial ClientHello, a vector of supported client puzzle
      types.  During the HelloRetryRequest, a vector of exactly one
      element containing the proposed puzzle.  During the retried
      ClientHello, a vector containing exactly one element with the type
      of the puzzle being responded to.

   client_puzzle_challenge_response  Data specific to the puzzle type,
      as defined in Section (#puzzles).  In the initial ClientHello,
      this MUST be empty (zero-length).  During HelloRetryRequest, this
      contains the challenge.  During the retried ClientHello, this
      contains a response to the challenge.  Puzzles containing a token
      may have it within this field.

4.  Usage by Servers

   Servers MAY send puzzles to clients when under duress, and the
   percentage of clients receiving puzzles and the complexity of the
   puzzles both MAY be selected as a function of the degree of duress.

   Servers MAY also occasionally send puzzles to clients under normal
   operating circumstances to ensure that the extension works properly.

   Servers MAY use additional factors, such as client IP reputation
   information, to determine when to send a puzzle as well as the
   complexity.

5.  Proposed Client Puzzles

   Having multiple client puzzle types allows good clients a choice to
   implement puzzles that match with their hardware capabilities
   (although this also applies to bad clients).  It also allows "broken"
   puzzles to be phased out and retired, such as when cryptographic
   weaknesses are identified.





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5.1.  Cookie Client Puzzle Type

   The "cookie" ClientPuzzleType is intended to be trivial.  The
   client_puzzle_challenge_response data field is defined to be a token
   that the client must echo back.

   During an initial ClientHello, this MUST be empty (zero-length).
   During HelloRetryRequest, the server MAY send a cookie challenge of
   zero or more bytes as client_puzzle_challenge_response .  During the
   retried ClientHello, the client MUST respond by resending the
   identical cookie sent in the HelloRetryRequest.

5.2.  SHA-256 CPU Puzzle Type

   This puzzle forces the client to calculate a SHA-256 [RFC5754]
   multiple times.  In particular, the server selects a difficulty and a
   random salt.  The client solves the puzzle by finding any nonce where
   a SHA-256 hash across the nonce, the salt and a label contains
   difficulty leading zero bits.

         struct {
             opaque token<0..2^16-1>;
             uint16 difficulty;
             uint8 salt<0..2^16-1>;
         } SHA256CPUPuzzleChallenge;

         struct {
             opaque token<0..2^16-1>;
             uint64 challenge_solution;
         } SHA256CPUPuzzleResponse;

   token  The token allows the server to encapsulate and drop state, and
      also acts as a cookie for DTLS.  During an initial ClientHello,
      this MUST be empty (zero-length).  During HelloRetryRequest, the
      server MAY send a token challenge of zero or more bytes.  During
      the retried ClientHello, the client MUST respond by resending the
      identical token sent in the HelloRetryRequest.  Servers MAY
      included an authenticated version of difficulty and salt in this
      token if they wish to be stateless.

   difficulty  filter affecting the time to find solution.

   salt  A server selected variable-length bytestring.

   challenge_solution  The solution response to the puzzle, as solved by
      the client.





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   To find the response, the client must find a numeric value of
   challenge_solution where:

   SHA-256(challenge_solution || salt || label) contains difficulty
   leading zeros.

   where "||" denotes concatenation and where label is the NUL-
   terminated value "TLS SHA256CPUPuzzle" (including the NUL
   terminator).

   Clients offering to support this puzzle type SHOULD support a
   difficulty value of at least 18.  [[TODO: is this a good value?
   https://en.bitcoin.it/wiki/Non-specialized_hardware_comparison has a
   comparison of SHA256 on various hardware.]]

5.3.  SHA-512 CPU Puzzle Type

   The SHA-512 CPU Puzzle Type is identical to the "SHA256 CPU Puzzle
   Type" except that the SHA-512 [RFC5754] hash function is used instead
   of SHA-256.  The label used is the value "TLS SHA512CPUPuzzle".

   Clients offering to support this puzzle type SHOULD support
   difficulty values of at least 17.  [[TODO: is this a good value?]]

5.4.  Equihash: Memory-hard Generalized Birthday Problem Puzzle Type

   Using Equihash, the asymmetric memory-hard generalized birthday
   problem PoW [NDSS2016], this puzzle will force a client to use a
   significant amount of memory to solve.  The solution to this puzzle
   can be trivially verified.

         struct {
            opaque token<0..2^16-1>;
            uint16 n;
            uint16 k;
            uint16 difficulty;
            uint8 salt<0..2^16-1>;
        } BirthdayPuzzleChallenge;

        struct {
            opaque token<0..2^16-1>;
            uint8 V<20>;
            uint8 solution<0..2^16-1>;
        } BirthdayPuzzleResponse;

   token  The token allows the server to encapsulate and drop state, and
      also acts as a cookie for DTLS.  During an initial ClientHello,
      this MUST be empty (zero-length).  During HelloRetryRequest, the



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      server MAY send a token challenge of zero or more bytes.  During
      the retried ClientHello, the client MUST respond by resending the
      identical token sent in the HelloRetryRequest.  Servers MAY
      included an authenticated version of n, k, difficulty and salt in
      this token if they wish to be stateless.

   salt  A server selected variable-length bytestring.

   n, k  parameters affecting the complexity of Wagner's algorithm.

   difficulty  secondary filter affecting the time to find solution.

   V  20 byte nonce used in solution.

   solution  list of 2^k (n/(k+1)+1)-bit nonces used in solution,
      referred to as xi below.

   In the further text, the output of blake2b is treated as a 512-bit
   register with most significant bits coming from the last bytes of
   blake2b output (i.e. little-endian conversion).

   To compute the response, the client must find a V and 2^k solutions
   such that:

   blake2b(salt||V||x1) XOR blake2b(salt||V||x2) XOR ... XOR
   blake2b(I||V||x(2^k)) = 0
   blake2b(label||salt||V||x1||x2||...||x(2^k)) has difficulty leading
   zero bits.

   where "||" denotes concatenation and where label is the NUL-
   terminated value "TLS BirthdayPuzzle" (including the NUL terminator).
   Incomplete bytes in nonces xi are padded with zero bits, which occupy
   the most significant bits.

   The client MUST provide the solution list in an order that allows a
   server to verify the solution was created using Wagner's algorithm:

   blake2b(salt||V||x(w_2^l+1)) XOR blake2b(salt||V||x(w_2^l+2)) XOR ...
   XOR blake2b(I||V||x(w*2^l+2^l)) has nl/(k+1) leading zero bits for
   all w,l.

   and two 2^(l-1)(n/(k+1)+1)-bit numbers Z1 and Z2 must satisfy Z1<Z2
   where

   Z1 = x(w_2^l+1)||x(w_2^l+2)||...||x(w_2^l+2^(l-1)) Z2 =
   x(w_2^l+2^(l-1)+1)||x(w_2^l+2)||...||x(w_2^l+2^l) as in([NDSS2016]
   section 4A, 5C).  The server MUST verify these intermediate
   equations.



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   A solution can be found using Wagner's algorithm as described in
   [NDSS2016].  The amount of memory required to find a solution is 2 ^
   (n/(k+1)+k) bytes.  A solution requires (k+1)2^(n/(k+1)+d) calls to
   the blake2b hash function.

   Clients offering to support this puzzle type SHOULD support n, k
   values such that 2^(n/(k+1)+k) is at least 20MB.

   Servers SHOULD look to minimize the value of k as 2^k blake2b hash
   operations will be required to verify a solution.

6.  IANA Considerations

   The IANA will need to assign an extension codepoint value for
   ClientPuzzleExtension.

   The IANA will need to assign an AlertDescription codepoint value for
   puzzle_too_hard.

   The IANA will also need to maintain a registry of client puzzle
   types.

7.  Security Considerations

   A hostile server could cause a client to consume unbounded resources.
   Clients MUST bound the amount of resources (cpu/time and memory) they
   will spend on a puzzle.

   A puzzle type with economic utility could be abused by servers,
   resulting in unnecessary resource usage by clients.  In the worst
   case, this could open up a new class of attacks where clients might
   be directed to malicious servers to get delegated work.  As such, any
   new puzzle types SHOULD NOT be ones with utility for other purposes
   (such as mining cryptocurrency or cracking password hashes).
   Including fixed labels in new puzzle definitions may help mitigate
   this risk.

   Depeding on the structure of the puzzles, it is possible that an
   attacker could send innocent clients to a hostile server and then use
   those clients to solve puzzles presented by another target server
   that the attacker wishes to attack.  There may be ways to defend
   against this by including IP information in the puzzles (not
   currently proposed in this draft), although that introduces
   additional issues.

   All extensions add complexity, which could expose additional attack
   surfaces on the client or the server.  Using cryptographic primitives




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   and patterns already in-use in TLS can help reduce (but certainly not
   eliminate) this complexity.

   An attacker that can force a server into client puzzle mode could
   result in a denial of service to clients not supporting puzzles or
   not having the resources to complete the puzzles.  This is not
   necessarily worse than if the server was overloaded and forced to
   deny service to all clients or to a random selection of clients.  By
   using client puzzles, clients willing to rate-limit themselves to the
   rate at which they can solve puzzles should still be able to obtain
   service while the server is able to stay available for these clients.

   It is inevitable that attackers will build hardware optimized to
   solve particular puzzles.  Using common cryptographic primitives
   (such as SHA-256) also means that commonly deployed clients may have
   hardware assistance, although this also benefits legitimate clients.

8.  Privacy Considerations

   Measuring the response time of clients to puzzles gives an indication
   of the relative capabilities of clients.  This could be used as an
   input for client fingerprinting.

   Client's support for this extension, as well as which puzzles they
   support, could also be used as an input for client fingerprinting.

9.  Acknowledgments

   The story of client puzzles dates back to Dwork and Naor [DN92] and
   Juels and Brainard [JB99].  Some of this draft was inspired by work
   done by Kyle Rose in 2001, as well as a 2001 paper by Drew Dean
   (Xerox PARC) and Adam Stubblefield (Rice) [SEC2001.DEAN].
   Discussions with Eric Rescorla, Yoav Nir, Richard Willey, Rich Salz,
   Kyle Rose, Brian Sniffen, and others on the TLS working group have
   heavily influenced this proposal and contributed to its content.  An
   alternate approach was proposed in [I-D.nir-tls-puzzles].  Some
   similar mechanisms for protecting IKE are discused in
   [I-D.ietf-ipsecme-ddos-protection].

10.  References

10.1.  Normative References

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-13 (work in progress),
              May 2016.




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   [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>.

   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
              2010, <http://www.rfc-editor.org/info/rfc5754>.

10.2.  Informative References

   [DN92]     Dwork, C. and M. Naor, "Pricing via Processing or
              Combatting Junk Mail", Proceedings of Crypto'92 , 1992,
              <http://www.wisdom.weizmann.ac.il/~naor/PAPERS/
              pvp_abs.html>.

   [I-D.ietf-ipsecme-ddos-protection]
              Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
              Protocol version 2 (IKEv2) Implementations from
              Distributed Denial of Service Attacks", draft-ietf-
              ipsecme-ddos-protection-06 (work in progress), April 2016.

   [I-D.josefsson-scrypt-kdf]
              Percival, C. and S. Josefsson, "The scrypt Password-Based
              Key Derivation Function", draft-josefsson-scrypt-kdf-05
              (work in progress), May 2016.

   [I-D.nir-tls-puzzles]
              Nir, Y., "Using Client Puzzles to Protect TLS Servers From
              Denial of Service Attacks", draft-nir-tls-puzzles-00 (work
              in progress), April 2014.

   [JB99]     Juels, A. and J. Brainard, "Client Puzzles: A
              Cryptographic Defense Against Connection Depletion
              Attacks", Proceedings of NDSS'99 , 1999,
              <http://www.wisdom.weizmann.ac.il/~naor/PAPERS/
              pvp_abs.html>.

   [NDSS2016]
              Biryukov, A. and D. Khovratovich, "Equihash: Asymmetric
              proof-of-work based on the Generalized Birthday problem",
              February 2016,
              <https://www.internetsociety.org/sites/default/files/
              blogs-media/equihash-asymmetric-proof-of-work-based-
              generalized-birthday-problem.pdf>.






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   [SEC2001.DEAN]
              Dean, D. and A. Stubblefield, "Using Client Puzzles to
              Protect TLS", Proceedings of the 10th USENIX Security
              Symposium , August 2001,
              <https://www.usenix.org/legacy/events/sec2001/full_papers/
              dean/dean.pdf>.

Authors' Addresses

   Erik Nygren
   Akamai Technologies

   EMail: erik+ietf@nygren.org
   URI:   http://erik.nygren.org/


   Samuel Erb
   Akamai Technologies

   EMail: serb@akamai.com


   Alex Biryukov
   University of Luxembourg

   EMail: alex.biryukov@uni.lu


   Dmitry Khovratovich
   University of Luxembourg

   EMail: khovratovich@gmail.com


   Ari Juels
   Cornell University

   EMail: juels@cornell.edu













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