Network Working Group E. Nygren
Internet-Draft Akamai Technologies
Intended status: Standards Track July 02, 2015
Expires: January 3, 2016
TLS Client Puzzles Extension
draft-nygren-tls-client-puzzles-00
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
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Table of Contents
1. Overview and rationale . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Handshake Changes . . . . . . . . . . . . . . . . . . . . . . 3
3.1. The ClientPuzzleExtension Message . . . . . . . . . . . . 5
4. Usage by Servers . . . . . . . . . . . . . . . . . . . . . . 6
5. Proposed Client Puzzles . . . . . . . . . . . . . . . . . . . 6
5.1. Cookie Client Puzzle Type . . . . . . . . . . . . . . . . 6
5.2. SHA-256 CPU Reverse Puzzle Type . . . . . . . . . . . . . 6
5.3. SHA-512 CPU Reverse Puzzle Type . . . . . . . . . . . . . 8
5.4. SHA-256 Memory Reverse 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
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:
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
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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].
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
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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:
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.
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{} 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:
struct {
ClientPuzzleType type<1..255>;
opaque client_puzzle_challenge_response<0..2^16-1>;
} ClientPuzzleExtension;
enum {
cookie (0),
sha256_reverse_cpu (1),
sha512_reverse_cpu (2),
sha256_reverse_memory (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.
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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.
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 Reverse Puzzle Type
This puzzle forces the client to calculate a SHA-256 [RFC5754]
multiple times. In particular, the server selects a random number
and challenge includes both the maximum possible value that the
random number could be as well as a salt bytestring. The server
communicates the maximum possible value that the number could be,
along with the salt and the result of performing a SHA-256 across a
number, the salt, and a label. The client solves the puzzle by
finding the number (within the range) where the SHA-256 matches the
provided value.
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struct {
opaque token<0..2^16-1>;
uint64 challenge_max;
uint8 salt<0..2^16-1>;
uint8 sha256_result<32>;
} SHA256CPUReversePuzzleChallenge;
struct {
opaque token<0..2^16-1>;
uint64 challenge_solution;
} SHA256CPUReversePuzzleResponse;
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 challenge_max, sha256_result,
and salt in this token if they wish to be stateless.
salt A server selected variable-length bytestring.
sha256_result The expected result of performing a SHA-256 across the
challenge_solution, salt, and label.
challenge_max The upper bound of the range that challenge_solution
is within. This is selected by the server to select the hardness
of the puzzle. The computational work that a client will need to
expend is intended to be O(challenge_max).
challenge_solution The solution response to the puzzle, as solved by
the client. The server guarantees that 0 <= challenge_solution <=
challenge_max.
To find the response, the client must find the numeric value of
challenge_solution where:
sha256_result = SHA-256(challenge_solution + salt + label)
where "+" denotes concatenation and where label is the NUL-terminated
value "TLS SHA256CPUReversePuzzle" (including the NUL terminator).
Clients offering to support this puzzle type SHOULD support
challenge_max values of at least 500,000. [[TODO: is this a good
value? https://en.bitcoin.it/wiki/Non-
specialized_hardware_comparison has a comparison of SHA256 on various
hardware.]]
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(_TODO / Open question: Should this be HMAC-SHA256(salt,
challenge_solution + label) or similar?_)
5.3. SHA-512 CPU Reverse Puzzle Type
The SHA-512 CPU Reverse Puzzle Type is identical to the "SHA256 CPU
Reverse Puzzle Type" except that the SHA-512 [RFC5754] hash function
is used instead of SHA-256. The label used is the value "TLS
SHA512CPUReversePuzzle" and SHA512CPUReversePuzzleChallenge is
updated accordingly:
struct {
opaque token<0..2^16-1>;
uint64 challenge_max;
uint8 salt<0..2^16-1>;
uint8 sha512_result<64>;
} SHA512CPUReversePuzzleChallenge;
Clients offering to support this puzzle type SHOULD support
challenge_max values of at least 250,000. [[TODO: is this a good
value?]]
5.4. SHA-256 Memory Reverse Puzzle Type
[[_TODO: This puzzle is a place-holder not intended to be implemented
until a better and asymmetric memory-hard puzzle is proposed and
specified. Another, and likely better, alternative for a symmetric
memory-hard puzzle would be to leverage the scrypt KDF._
[I-D.josefsson-scrypt-kdf]]]
This puzzle is be more memory-heavy than CPU-heavy which may be a
good option for mobile clients. Unfortunately, the memory-hard
aspect of this puzzle is not yet asymmetric.
This puzzle starts similar to SHA256CPUReversePuzzle but to fold in
another bytestring from information provided by the server.
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struct {
opaque token<0..2^16-1>;
uint64 challenge_max;
uint8 salt<0..2^16-1>;
uint8 seed<0..2^16-1>;
uint64 mem;
unit32 noff;
uint8 sha256_result<32>;
} SHA256MemoryReversePuzzleChallenge;
struct {
opaque token<0..2^16-1>;
uint64 challenge_solution;
} SHA256MemoryReversePuzzleResponse;
Additional parameters to those in SHA256CPUReversePuzzleChallenge and
SHA256CPUReversePuzzleResponse:
seed the seed to a PRF (pseudorandom function) from which a
bytestream is generated
mem the number of bytes to expand from the PRF into memory
noff the number of offsets into the expand to pull from the PRF
So in specific:
expanded = PRF(seed, 0...mem-1)
extraction = SHA-256(expanded[PRF(seed, mem)]
+ expanded[PRF(seed, mem+1)]
+ ...
+ expanded[PRF(seed, mem+noff)])
(_TODO: Formalize the details of the PRF, unless this puzzle is
eliminated or replaced._)
In this case, challenge_solution is searched for in:
sha256_result = SHA-256(challenge_solution + salt
+ label + extraction)
Where label is the NUL-terminated string "TLS
SHA256MemoryReversePuzzle".
Resource usage by clients is then _either_:
- Memory of O(1) and CPU of O(challenge_max) + O(noff * mem)
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- Memory of O(mem) and CPU of O(challenge_max) + O(noff)
Servers MAY precompute sets of extractions and reuse them across
multiple clients.
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
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
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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
Some of this 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, Samuel Erb, 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-tls13-06 (work in progress),
June 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
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10.2. Informative References
[I-D.ietf-ipsecme-ddos-protection]
Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
(IKE) Implementations from Distributed Denial of Service
Attacks", draft-ietf-ipsecme-ddos-protection-01 (work in
progress), March 2015.
[I-D.josefsson-scrypt-kdf]
Percival, C. and S. Josefsson, "The scrypt Password-Based
Key Derivation Function", draft-josefsson-scrypt-kdf-03
(work in progress), May 2015.
[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.
[SEC2001.DEAN]
Xerox PARC and Rice University, "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>.
Author's Address
Erik Nygren
Akamai Technologies
EMail: erik+ietf@nygren.org
URI: http://erik.nygren.org/
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