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Crypto Archives: Selective DoS Attacks: Remailer Vulnerabilities

Selective DoS Attacks: Remailer Vulnerabilities

Robert Hettinga (rahshipwright.com)
Sat, 25 Sep 1999 20:53:24 -0400

--- begin forwarded text

Date: 24 Sep 1999 15:20:12 -0000
From: RProcess <rprocessnym.alias.net>
To: mail2news_nospamanon.lcs.mit.edu, potatoware.reliablelistbot.com,
          potatowarelistbot.com, remailer-operatorsanon.lcs.mit.edu
Subject: Selective DoS Attacks: Remailer Vulnerabilities
CC: cypherpunkscyberpass.net
Newsgroups: alt.privacy.anon-server,alt.security.pgp,alt.privacy
Sender: owner-cypherpunkscyberpass.net
Reply-To: RProcess <rprocessnym.alias.net>

The following is a (somewhat long) discussion of what I consider to be
some of the primary vulnerabilities in the current Cypherpunk/Mixmaster
anonymous remailer system, and how I suspect some of these
vulnerabilities are being quietly exploited.

It is not my intention to undermine confidence in the current system,
which does offer substantial security and usefulness, but to discuss
potential and probable threats with the goal of long-term improvement.

To date I have written three remailer clients (Potato, JBN 1 and 2) and
a Windows remailer (Reliable), and in attempting to test the features
thoroughly over several years time, have observed various problematic
aspects of the remailer system, and have attempted to analyze anomalies
very carefully. I am sharing these observations now because I am fairly
confident that if they are not accurate, they are at least plausible
fiction, and thus worthy of consideration.


      THE HOLE


Briefly, I have observed the same thing that many remailer users have
complained about for some time - lost mail. The possible difference is
that I have investigated it a little more deliberately than most users
would care to do. Some mail sent through remailers vanishes. This has
previously been explained as unreliable software, hardware, networking,
etc. However I have found these explanations to be insufficient to
explain my observations.

One of the goals of writing Potato (and likewise JBN) was to provide a
remailer client that would be very accurate and dependable, thus
providing a means to produce consistent and dependably formatted mail.
By allowing users to save templates of message constructions, it removes
a lot of human error. It also provides an artificial memory by which a
user can verify whether or not a missing message was constructed

Likewise, Reliable (as the name implies) was designed to do everything
possible to eliminate disappearing messages. Reliable is very reluctant
to discard messages, and has several tiers of message disposal, with the
ability to requeue messages which are unprocessed due to
misconfiguration or other questionable problems. In addition, the
Freedom and Mixmaster remailers have undergone a lot of work by Johannes
Kroeger and Ulf Moller which I believe has made them much more
dependable (and secure) than remailers operating some years ago.

As a result of these improvements we have clients and servers which run
consistently and with a large degree of reliability. So why is it so
very difficult to generate a dependable and secure reply-block?

Some of my observations:

      Some mail vanishes, particularly if unique (not sent through a
      previously established encrypted reply-block).

      Mail which is securely encrypted is more likely to vanish or be
      delayed than are simple messages.
      Mail which is well-chained is far more likely to be lost, even
      allowing for increased probability of serial failure and other
      causes, even when all remailers in the chain test ok.
      Ping messages are more reliable than real mail. The new version 2
      stats reveal that most remailers are very consistent and reliable in
      processing pings, and true network and server down time is very
      obvious and quite rare. Yet actual use of the remailers in chains
      paints a very different picture than these stats.
      Established reply-blocks may work flawlessly, while a newly created
      reply-block with the same remailers will be delayed or lost.
      New reply-block chains are delayed for the first several uses.
      There is a correspondence between nym accounts and new reply-block
      unreliability. (ie a new reply-block for xxxnym.alias.net will be
      consistently more unreliable than the same or similar reply-block
      for yyynym.alias.net)

      Remailer operators complain of their stats being low despite their
      remailer functioning normally.

      Conduction of some news messages between servers is impeded beyond

Isolated incidents can be dismissed as coincidence, due to traffic
levels, and other variations, but over time the trends become very clear
and disturbing, the reasons ever diminishing. I think most remailer and
nym users are well acquainted with this behavior, to the point where
they take it for granted, and I think some of those who work in
providing remailer services or software are unaware of the problem
simply because they don't actually use the services extensively.

I think one of three basic possibilities exist for an explanation of the
unreliability. One, unreliable remailer software (servers discarding
the messages despite their delivery and clients generating malformatted
messages). Two, an unreliable network which is not delivering the
messages. Three, a third influence, deliberate or otherwise, which is
preventing their delivery or processing.

I have satisfied my own questions regarding the first. While I admit
that software problems do occur, they are not widespread and subtle to
the extent which would cause this much lost mail and unpredictability,
and predictable loss without discernable and reproducible cause.

Regarding the second possibility, internet reliability has increased.
How often does plain email disappear into a void? Not often. Nor would
this explain the 'intelligent' nature of the losses I have observed.

Despite trying to resist the paranoid alternative, I have come to
conclude that the third alternative is the case - that something or
someone is interfering between the users and remailers, and between
remailers, engaging in a very subtle and selective DoS attack.


DoS refers to Denial of Service. One kind of attack on encrypted or
secure communication involves sabotaging the communication, or denying
the service. An example is a mail bomb sent to a remailer. While
recovering from the mail bomb the remailer is unable to process
messages, and thus users are denied service, are unable to communicate.

By selective denial of service I refer to the ability to inhibit or stop
some kinds or types of messages while allowing others. If done
carefully, and perhaps in conjunction with compromised keys, this can be
used to inhibit the use of some kinds of services while promoting the
use of others. An example:

User X attempts to create a nym account using remailers A and B. It
doesn't work. He recreates his nym account using remailers A and C.
This works so he uses it. Thus he has chosen remailer C and avoided
remailer B. If the attacker runs remailers A and C, or has the keys for
these remailers, but is unable to compromise B, he can make it more
likely that users will use A and C by sabotaging B's messages. He may
do this by running remailer A and refusing certain kinds of messages
chained to B, or he may do this externally by interrupting the
connections to B.

User Y attempts to create a very secure reply-block using advanced anti-
analysis features (perhaps long latency, random hops, superior
encryption, etc). His reply-block fails outright or is so unreliable
that he decides to change it. He forgoes some of his advanced features
for a more reliable (yet less secure) reply-block. Thus the attacker
has induced the user to use less secure features, or perhaps has
discouraged the use of remailers completely.

User Z attempts to create a reply-block, or send a message. Again and
again he has failures so he sends himself test messages using similar
remailers and features, or if he is inexperienced he may mail his own
account directly. By inhibiting the success of his mail, the attacker
has made it more likely that the user will expose himself through
testing, repetition, and security oversights. How many new users are
creating new reply-blocks at any given time? Not that many, especially
given a thorough statistical analysis.

Another example: It is somewhat possible to tell how long a reply-block
is by examining the size of the encrypted reply-block. Thus the
attacker kills some or all messages with longer chains, reducing their

Thus through selective denial of service, the following can be

      Choice of remailers influenced
      Choice of features influenced
      Cause testing and repetition to be required
      Longer chains inhibited
      General dissuasion of remailer use due to unpredictability


How plausible is the scenario that someone is doing this?

Cutting to the chase, I think the most plausible organizations
responsible for such an attack would be the usual suspects - NSA, CIA,
and other military and intelligence organizations. They have the means
and the motives to make such an attack worth their while. Given
operations like Echelon (NSA's widespread interception of electronic
communication in Europe), and other intelligence networks such as the
CIA's meddlesome habits, it is reasonable to assume they are doing
*something* with regard to remailers. Remailers potentially allow
organizations to communicate in an untraceable fashion worldwide. A lot
of intelligence and counterintelligence is based on interrupting
untraceable communication. So it is reasonable to assume they are doing

What is it likely that they are doing? Given their habits, it is most
likely they would do something as subtle as possible. They would prefer
their intrusion to be undetected and unverifiable, with no traces left
behind. They would prefer to leave the illusion that the system has not
been compromised.


The reliability hole in the remailer system provides a way, perhaps the
best way, to accomplish these goals. A lost message only provides
evidence by its absence, so there is no positive evidence of a third
party at work. It provides a way to influence remailer use, and it uses
other genuine remailer problems as cover. It also uses the difficulty
of producing reliable remailer messages manually as cover, although this
has become less of an issue with reliable clients.

As long as remailers have been around, unreliability has been a problem.
Further, as remailers became more reliable, chaining reliability began
showing inexplicable deficiencies. We take stats lists for granted, but
why do we need such an elaborate fix? Why have software and network
problems remained for so long? Or have they? After extensive analysis,
I see very little evidence of software or network problems causing these
losses, and the only alternative explanation remaining is deliberate

I think remailer users and developers have become complacent about the
reliability problem, accepting it, and thus accepting a large
vulnerability in the system. But is it understandable, because it is a
very difficult problem to pinpoint and document.


What are the alternatives to a selective DoS attack?

I will not say these things don't exist or haven't been done, but I have
not observed the following directly or indirectly.

      A Non-Selective DoS - A remailer or group of remailers being
      completely switched off or blocked.

      False Keys - Distribution of false keys and false signatures could
      be used in a man-in-the-middle or other attack, but again this
      leaves possible evidence that someone has compromised the system or
      attempted to do so.
      Hacking Attacks - There is little evidence to indicate a coordinated
      hacking attack.
      Trojan Viruses - Planting software which can steal keys or otherwise
      damage the system is plausible. I have read the usual bits which
      indicate this does go on in software, but I have not witnessed this
      problem directly or related to remailers. Again note that this
      leaves tracks. I think the more plausible method in encryption and
      security products would be for weaknesses to be introduced which
      could later be claimed to be programmer or design errors.


Of course there continue to be other vulnerabilities. Anyone can run a
remailer, and it stands to reason that one method of intercepting some
mail, and selectively deleting some mail, would be for the attacker to
run one or more remailers.

There is also the threat of key theft. Based on what I have observed, I
consider it a very strong possibility that at least some keys are
available to those performing what I suspect is a selective DoS. Given
nym.alias.net's location at MIT (home of half of the DOD), a college
computer with only reasonable security, and given the fact that the key
has not been changed for years, I find it reasonable to suppose that the
key has been stolen.

There remains the possibility of keys or messages being cracked. This
takes time and could explain some of the curious delay trends observed
by myself and others. However one has to suppose that some of the
encryption schemes are somewhat secure or there would be no incentive to
perform a selective DoS.

I find it plausible that intelligence agencies would construct a
campaign to report and generate abuses, causing remailers to be shut
down by ISPs, etc., and to generate mail bombs and other attacks when
suited to their purposes. This is in general keeping with the history
of their indirect involvement.


One argument is that unless you are a spy or a terrorist, it may not
matter much that the intelligence agencies read your mail or trace your
identity. They don't involve themselves much with law enforcement,
preferring to break the laws themselves, except when it suits their

However these people are unlawfully and often unethically exerting an
influence, and most users concerned with privacy and free expression
issues are concerned by this kind of interference.

Further, the anonymous remailer system has grown from being vulnerable
to the type of attack used against anon.penet.fi, to using nym-servers
where even the operator does not know the identity of account owners.
One hopes that other vulnerabilities will be addressed as the need for
privacy and free expression grows.

Finally, this is of concern to all remailer users as it makes the
remailer system devilish to use.

I realize that contemplation of conspiracies are awkward, but in the
case of remailers I think it is safe to assume intelligence agencies are
involved in some way. The only remaining question is how.


I would like to enumerate several short-term and long-term ideas for how
the remailer system can be improved and made less vulnerable to
selective DoS and other attacks.

      Unreliable software and network connectivity provides a cover for
      selective DoS and other attacks. Remailer and client programmers,
      and remailer operators, need to do everything possible to improve
      the reliability of their systems and reduce lost mail, and to insure
      their connectivity to other remailers. The correct keys need to be
      used by remailers which increasingly depend on repgp and remix. It
      is my opinion that anti-SPAM features in some remailers have grown
      out of control and contribute to message loss, so these techniques
      must be applied carefully and only to a necessary extent. Remailers
      need to provide clear feedback to operators on why mail is being
      discarded, and operators need to pay careful attention to this
      information. Most of all, those involved in providing remailer
      services (programmers, operators, etc.) need to become less
      complacent about unreliability. It is a significant problem and
      security threat.

      The habit of keeping the same remailer keys indefinitely represents
      a significant security threat. Keys should be replaced and
      destroyed regularly, especially for remailers located in
      institutions and other areas where people come and go. Larger keys
      should also be used when possible.

      The remailers located at nym-servers need to support Remix-To. Too
      much information about reply-block size and count is leaked between
      the nym-server's Cypherpunk and the first remailer.


A few thoughts on what next generation remailers should address.

      The idea of sending messages individually between remailers (and
      even from/to individuals?) should be discarded. Message packets
      should be made smaller (1k perhaps, interleaved) or larger (2-10 meg
      digests) and constructed in such a way that it is impossible to
      remove one packet or message without detection and correction.
      Multiple messages to a final destination might be sent in digests.
      An automatic system for detecting and replacing, possibly rerouting,
      lost packets should be implemented.
      More random data (continuous if feasible) should be introduced to
      cover the number and size of messages between locations.

      The assumption that an attacker has a remailer's private key or is
      able to decrypt its messages should be used in building a model
      which, while not preventing some eavesdropping in such a
      circumstance, prevents selective DoS.

      Email as the preferred method of transfer between remailers should
      be questioned. Although it provides for greater cross-platform
      interoperability, an encrypted direct connection between remailers,
      with error detection and correction is needed.
      Remailer clients need the (optional) ability to connect to the
      remailers directly, without going through SMTP and other servers,
      and need to receive immediate feedback that the remailer received
      the data.
      An automated and secure method for distributing and changing
      remailer keys regularly, and a system for updating client
      configuration (based on remailers' supported features) is needed.
      Remailers need to be less dependent on the DNS system to prevent
      hijacking and extended downtime (such as when nym.alias.net lost its
      domain for several months)


I do not intend to imply that all lost mail is due to involvement of a
third party. Especially if you are new to remailers, realize that there
are many mistakes which will legitimately cause remailers to discard
messages. By design they are quite unforgiving in this regard, and
users should not jump to conclusions.

I also wish to add that I still consider anonymous remailers to be one
of the few methods on the net for achieving genuinely high levels of
strong anonymity and security. It is exactly because of this that I
consider it plausible that they are the target of such a subtle and
resource-intensive attack.

--- end forwarded text

Robert A. Hettinga <mailto: rahibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

This archive was generated by hypermail 2.0b3 on Sat Sep 25 1999 - 22:52:00 CDT