SIP C. Jennings
Internet-Draft Cisco Systems
Expires: January 16, 2006 July 15, 2005
Computational Puzzles for SPAM Reduction in SIP
draft-jennings-sip-hashcash-02
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Abstract
One of the techniques used in SPAM prevention and various solutions
for denial of service attacks is to force the SIP client requesting a
service to perform a calculation that limits the rate and increases
the cost of the request. This draft defines a way to allow a UAS to
ask the UAC to compute a computationally expensive hash based
function and present the result to the UAS. Although the computation
is expensive for the UAC to compute, it is cheap for the UAS to
verify. The solution also allows for proxies to compute and check
the puzzle on behalf of the UAC or UAS.
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1. Introduction
The SPAM prevention problem is complex and will require many
techniques working in combination to balance reducing SPAM to
acceptable levels while still fostering efficient communication. The
overall problem and various approaches are in [7]. Clearly white
lists are a critical part of dealing with SPAM. Any system would
first check whether an incoming request for communications was from
someone on the white list. The Identity [6] mechanisms are critical
for understanding who the caller is and to check whether the caller
is on the white list. As well, there still needs to be a way for
callers not on the white list to communicate with the user. It is
here that this specification becomes relevant.
The problem is how to permit contacts from people with no previous
relationship to us without receiving undesirable contacts. This
draft uses the idea that it may be possible to make undesirable
contacts more expensive than desirable ones.
Different undesirables are willing to spend different amounts of time
and money on contacting their markets. Founders of acquired startups
are often contacted by random financial companies offering to help
manage the new riches. These companies will send people from New
York to San Jose and spend hours talking to this very narrow target
market. Clothing retailers will mail glossy catalogues worth $1
apiece to houses within the right demographic zip codes. Emails
advertising Viagra are sent to random email addresses. As the costs
go down, the volume of unsolicited contact goes up.
Often people whose contact is desirable are willing to spend much
less than some of the undesirables. The student in Fiji who wants to
ask about this draft will send an email but probably will not fly
here to talk to me. I would like to receive that email.
Increasing the cost of contact will reduce both desirable and
undesirable contact. My assumption is that the cost should be set
very low, so that even a person with a pathetic CPU could still make
contact in, say, 10 seconds. Key to this draft is that the receiver
can set this cost. This low cost will not stop the financial
advisers or the telemarketers, but it might stop the Viagra ads. It
would also probably stop a single user from ringing every phone of
some residential service provider in a five-second window, before any
operator or system can react. Deciding what cost to set constitutes
a classic type I/type II error problem, and the receiver gets to
choose how to balance these two errors.
As is clearly stated in [7], whitelists are the best thing. After
that, this is one of the multiple other options that need
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consideration.
In general there are two arguments about why the computation puzzles
in this specification will not work. The first is that the bad guys
have the most powerful CPUs. This issues was addressed above. The
other argument is that bad guys have infinite CPU time through using
armies of zombie PCs. The problem with this argument is that the
goal is not to block particular bad guys but to reduce the overall
number of undesirable messages. This second argument is, however,
more worrisome than the first.
Assume that some percentage of the world's machines each year get
owned and used as zombies. Let's say that a given machine has 1% of
having this happen to it in a year, that it sends zombie traffic for
24 hours before getting shut down, and that the mechanism described
here limits it to ten messages per second: each machine on the
internet would receive an average of about one undesirable message
per hour. If you assume there are more users than machines, this
looks appealing. If message sending technology detects users that
are sending lots of messages and shuts them down in less than 24
hours, it gets better. It gets better still if you hope for
improvements in operating systems or for users to choose them more
carefully. The next assumption is hard to model statistically but it
is true: the people with the best financial incentives to send
undesirable messages do not want to be subject to the legal and
reputation problems of using zombies to get their message across.
The zombie problem basically comes down to this. If there are a
small percentage of machines in the world that are zombies, they do
not render this computation puzzle approach useless. If 10% of the
machines in the world are zombies, this approach will be useless.
This specification does not attempt to deal with how to make the
world such that a small percentage of computers are zombies - the is
the problem for other work and that work needs to happen for SPAM to
be reduced to reasonable level. This specification does assume that
the zombie problem is solved to the level where a small percentage of
the worlds computers are zombies.
So in summary, white listing is the first and best defense. But for
dealing with messages from people with whom we have not previous
direct or indirect relationship, another approach is necessary.
Hashcash cannot stop all bad messages - that is not the goal - but it
can raise the cost of messages and thus decrease the number of times
it makes economic sense to send undesirable ones. This approach does
assume that bad guys will have more CPU power than good guys and that
zombies will still send lots of messages. This approach will simply
reduce the number of undesirable messages by some amount that cannot
be measured.
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No one knows if this approach would reduce SPAM noticeably. Right
now the only thing that limits the rate at which I can call every SIP
phone in the world is proxies getting overloaded. And of course,
most SIP phones are not connected to the public internet. Of course,
the SPAM problem is one reason why many SIP phones are not connected
to the public internet. There are some other approaches outlined in
[7]. They have different pros and cons, and it is probably necessary
to use most of them to ensure SPAM stays at an acceptable level.
2. Overview
This specification extends RFC 3261 [3] and defines a mechanism for a
proxy or UAS to request that a UAC compute the solution to a puzzle.
The puzzle is based on finding a value called the pre-image that,
when hashed with SHA1 [4], results in a specific value referred to as
the image. The goal is for the UAC to find a pre-image that will
SHA1 hash to the correct image. The UAS provides a partial pre-image
with some of the low order bits set to zero, together with the number
of bits in the pre-image that have been set to zero.
The UAS provides the puzzle information using a 419 response, and the
UAC resubmits the request along with the solution to the puzzle. The
high level flow of information is shown below.
UAC UAS
| Request |
|------------------------->|
| |
| 419 with Puzzle |
|<-------------------------|
| |
| Request with Solution |
|------------------------->|
| |
This specification defines the 419 response code along with a new
header, called Puzzle, to carry the puzzle and solution.
3. Definitions
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 RFC 2119 [2].
4. Puzzles
The normative definition of a puzzle is as follows. A puzzle is four
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values: an integer number referred to as work, a pre-image string, an
image string, and a integer number referred to as value. There MUST
exist a value X such that all but the "work" number of low order bits
of X match the pre-image string, and the SHA1 hash of the string
formed by the concatenation of "z9hG4bK" and X results in a value Y,
where the "value" number of low order bits of Y are the same as those
bits in the image string. The SHA1 hash is computed as described in
RFC 3174 [4]. The value X is the solution to the puzzle. The 'work'
number of low order bits of the pre-image MUST be zero.
This can all be described more mathematically. The notation low(v,x)
returns the low v low order bits of x, and zero(v,x) returns x with
the low v bits set to zero. The | operator signifies string
concatenation. The solution to the puzzle can be considered finding
an X such that both the following are true:
low( value, image ) = low( value, sha1( "z9hG4bK" | X ) )
zero( work, X ) = zero( work, pre-image )
The pre-image forms a constraint on X. The value of X is the same
pre-image, other than the low 'work' bits that are set to zero in the
pre-image. The 'value' is the number of bits that match in the
solution and is typically set to 160, which is the full size of the
SHA1 hash result.
The following is a non-normative way for a UAS or proxy to construct
a puzzle. The following strings are concatenated:
1. a secret that only this device knows. This would typically be a
crypto random string of bits;
2. the current time, rounded to the nearest minute;
3. the URI of the request, the Call-ID, the From tags, and the
branch tag for a proxy or the To tag for a UAS.
The string is hashed with SHA1 to form the pre-image. The pre-image
is appended to the string "z9hG4bK", and the SHA1 hash of this is
computed to get the value of the image. A value 'work' indicates how
many bits of the pre-image are to be removed. The value 'work' could
be a configurable parameter, or it could be dynamically discovered by
the software based on how long a hash should take and the speed of
the computer it was running on. In the latter case, the resulting
software would automatically choose larger values of 'work' as
computers get faster. The low order 'work' bits of the pre-image are
set to zero. The puzzle consists of the chosen value of 'work', the
pre-image (with the low order bits set to zero), the image, and the
'value'. The 'value' would typically be set to 160 as this is the
size of the SHA1 hash.
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5. Semantics
5.1 UAS Creating Puzzle
When a UAS wishes to challenge a request, it MAY create a puzzle,
encode this puzzle in a Puzzle header field value, and return the
puzzle in a 419 response.
5.2 UAC Receiving Puzzle
When a UAC receives a 419 response, it needs to look at the 'work'
and 'value' requested and decide whether or not to try to solve this
puzzle. This decision can be made based on the programmed policy and
possibly human input. The UAC should not tackle a puzzle that will
take longer than the age of the universe to solve. If the UAC
chooses to try to solve the puzzle then it proceeds along the
following steps:
1. Check that the 'work' bottom bits of the pre-image are all zero.
If they are not, this is an invalid puzzle and the 419 response
MUST be considered an error response.
2. Set Y to low( value, image ).
3. Create a loop where X ranges from the value of the pre-image to
the value of the pre-image plus 2 raised to power of the 'work'.
4. For each interaction through the loop, check if low( value, sha1(
"z9hG4bK" | X )) equals Y. If it does, a solution X has been
found and the loop can terminate.
If the loop terminates without a solution being found, the puzzle was
bad and the 419 response MUST be considered as an error response.
Once the solution to the puzzle, X, is found, a new request is formed
by copying the old request and adding an additional puzzle header
field value. The new puzzle header field value MUST have the 'work'
set to 0, the pre-image set to the value X, the image set to the
value of the image in the original puzzle, and the value parameter
set to the same as the value parameter in the original puzzle. Note
that if a request was challenged by one proxy and a new request was
generated with a solution, and then this request was challenged by a
second proxy, a third request would be generated that had two Puzzle
header field values. If a UAC, through some out of band mechanism,
knows that it will be challenged and what the puzzle will be, it MAY
include the appropriate puzzle header field value in the initial
request.
5.3 Proxy Behavior
SIP allows proxies to act as UASs when generating 4xx responses.
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This same mechanism can be used to allow a proxy to generate the
challenge on behalf of a UAS in its domain.
Proxies may also act on behalf of the UAC and compute the solution to
a puzzle on behalf of the UAC in either a request or a response that
passes through the proxy. Typically a proxy would only do this for a
UAC that had authenticated to the proxy and for which the proxy had a
service relationship.
6. Example
TBD
7. Syntax
The Puzzle header field carries the puzzle and solution information.
It has a parameter called 'work' that has the number of bits of the
pre-image that have been set to zero for this puzzle. It has a
parameter called 'pre' that carries the pre-image string base64
encoded, and a parameter called 'image' that carries the image string
base64 encoded. In addition there is a parameter called 'value' that
indicates how many bits of the resulting hash will match the 'image'
string. The base64 encoding is done as described in RFC 3548 [1].
When the header field value is carrying a solution to a puzzle, the
work parameter will be set to zero.
Example:
Puzzle: work=10; pre="XPokF1n0+NG6iwRcYzeXuETrtDo=";
image="XPokF1n0+NG6iwRcYzeXuETrtDo="; value=160
The ABNF for the header is:
Puzzle = "Puzzle" HCOLON puzzle-parm *(COMMA puzzle-param)
puzzle-param = puzzle-bits SEMI puzzle-pre SEMI puzzle-image
SEMI puzzle-value *( SEMI generic-param )
puzzle-work = "work=" 1*DIGIT
puzzle-value = "value=" 1*DIGIT
puzzle-pre = "pre=" quoted-string
puzzle-image = "image=" quoted-string
This document updates the dreaded Table 2 of RFC 3261 to be:
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Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Puzzle amr o o - o o o
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o o o o
8. Open Issues and To Do Items
The current mechanism has poor interaction with the HERFP forking
problem. If several endpoints sent a 419, the proxy would need to
aggregate the results and add something like the realm to the
challenges to keep them sorted out. Need to add this in next
revision. In many cases the solution would work out better if the
proxy that was doing the forking applied the policy and did the 419
before forking. This approach has the usual HERFP problem that if
some UAs do a 419, and some UAs don't, the request will only reach
the UAs that don't do the 419.
9. Security Considerations
Still TBD.
The concatenation with "z9hG4bK" is done so that this mechanism
cannot be used as a distributed computation to reverse arbitrary hash
values, as that would present a security risk for other hash based
security schemes.
TODO - Advice on selecting the size of 'work'.
10. IANA
This specification registers a new header and a new response code.
IANA is requested to make the following updates in the registry at:
http:///www.iana.org/assignments/sip-parameters
10.1 Puzzle Header
Add the following entry to the header sub-registry.
Header Name compact Reference
----------------- ------- ---------
Puzzle [RFCXXXX]
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10.2 419 Response
Add the following entry to the response code sub-registry under the
"Request Failure 4xx" heading.
419 Puzzle Required [RFCXXXX]
11. Acknowledgments
This approach was motivated by [5].
12. References
12.1 Normative References
[1] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[4] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
RFC 3174, September 2001.
12.2 Informational References
[5] Black, A., "http://www.hashcash.org/", February 2005.
[6] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-05 (work in progress), May 2005.
[7] Rosenberg, J., "The Session Initiation Protocol (SIP) and Spam",
draft-ietf-sipping-spam-00 (work in progress), February 2005.
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Author's Address
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 421 9990
Email: fluffy@cisco.com
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