[tor-commits] [obfsproxy/master] Added a first sample of what should become obfs2's threat model.

[nickm at torproject.org]

commit 368073564058981453e3c7a9e3b175ab463e27f5
Author: George Kadianakis <desnacked at gmail.com>
Date:   Fri May 27 19:48:30 2011 +0200

    Added a first sample of what should become obfs2's threat model.
    
    doc/THREAT_MODEL is a practical list of attacks that obfs2 counters or
    fails to counter.
    
    doc/THREAT_MODEL_PHILOSOPHICAL is a non-practical document of wannabe
    academia rambling that might prove useful in the future.
    
    Of course both files are not in the correct dir since they only refer
    to obfs2 and not to obfsproxy, but hey!
---
 doc/THREAT_MODEL               |   19 ++++++++
 doc/THREAT_MODEL_PHILOSOPHICAL |   99 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 118 insertions(+), 0 deletions(-)

diff --git a/doc/THREAT_MODEL b/doc/THREAT_MODEL
new file mode 100644
index 0000000..d973317
--- /dev/null
+++ b/doc/THREAT_MODEL
@@ -0,0 +1,19 @@
+obfs2 was created to work on top of the Tor protocol (and specifically
+it's TLS handshake phase). Do not use obfs2 to obfuscate other
+protocols, unless you are super-sure of what you are doing.
+
+A list follows, of known attacks that obfs2 protects or does not
+protect against. obfs2 does not protect you against unknown attacks.
+
+Attacks obfs2 protects against:
+* Non-intelligent DPI boxes censoring based on TLS content patterns.
+* In the case of a shared secret, it also protects against intelligent DPI boxes.
+
+Attacks obfs2 does not protect against:
+* Any distinguisher based on the Tor protocol. This includes timing, packet
+  size and replay attacks.
+* Any distinguisher based on packet entropy.
+* In the case of obfs2 without a shared secret, obfs2 does not protect
+  against an intelligent DPI box; a DPI box that simply deobfuscates
+  the obfs2 protocol (for those not familiar with the protocol: this
+  is quite easy.).
diff --git a/doc/THREAT_MODEL_PHILOSOPHICAL b/doc/THREAT_MODEL_PHILOSOPHICAL
new file mode 100644
index 0000000..d67af1b
--- /dev/null
+++ b/doc/THREAT_MODEL_PHILOSOPHICAL
@@ -0,0 +1,99 @@
+        Role of this document
+
+The role of this document is to give a clear image of what obfs2
+provides to a censored Tor user.
+The role of this document is *not* to describe the censorship ecosystem,
+or to analyze the full threat model of Tor.
+
+        Terminology
+* Censorship:
+  'Censorship is the suppression of speech or other public
+  communication which may be considered objectionable, harmful,
+  sensitive, or inconvenient to the general body of people as
+  determined by a government, media outlet, or other controlling
+  body.'
+* Anonymity
+  Anonymity is the state of an individual's personal identity being
+  publicly unknown. Anonymity does not imply Privacy.
+* Tor
+  Tor is a software system and a computer network intended to provide
+  online anonymity. It's main application is 'tor'.
+* Privacy
+  Privacy is used to prevent the disclosure of information to
+  unauthorized individuals or systems. Privacy does not imply
+  Anonymity.
+* Tor bridge
+  A Tor bridge is a component of the tor network that is not publicly
+  announced as part of it. It's main purpose is Censorship
+  circumvention since finding and blocking a bridge is considered
+  non-trivial.
+* obfsproxy
+  obfsproxy is an application that accepts data as input and can
+  reshape it into another form.
+  When used with tor, it can transform tor's traffic into other
+  protocols in the hope of dodging Censorship.
+* obfs2
+  obfs2 is a protocol supported by obfsproxy. It shapes data by
+  encrypting them with a stream cipher with a randomly generated
+  key. It also supports pre-shared passwords.
+
+        Players:
+* Censored tor/obfsproxy user:
+  A censored tor/obfsproxy user is an Internet user that is censored
+  by 'A Man In The Middle'. He wants to use tor to achieve anonymity,
+  but he also needs privacy, to hide the fact that he is using Tor.
+* Bridge Operator:
+  A bridge operator is a person who runs a tor bridge and has
+  successfully given to the 'Censored tor/obfsproxy user' the necessary
+  information so that the latter can access the bridge service. 
+* A Man In The Middle (MITM):
+  A MITM is a person or a group of people who have the power to
+  apply Censorship.
+
+        What Tor does and does not provide:
+
+Once he is a member of the Tor network a censored Tor user can expect
+low-latency anonymity. A Tor user can also expect Privacy with regards
+to A Man In The Middle, when he is a member of the Tor network.
+
+The problem is that before and while becoming a member of the Tor
+network the user is *not* anonymous and his communications with the
+Tor network are *not* private. This allows A Man In The Middle to
+identify the Tor user and censor him/her.
+
+        What obfs2 does and does not provide:
+
+A Tor user using obfs2 with a Tor Bridge *without* a pre-shared password
+can expect low quality privacy on his communications with the Tor
+network before becoming part of it.
+'low quality privacy' means that A Man In The Middle who has studied
+the obfs2 protocol can easily decipher it, effectively extinguishing
+the privacy.
+
+A Tor user using obfs2 with a Tor Bridge *with* a pre-shared password
+can expect modest privacy on his communications with the Tor network
+before becoming part of it.
+'modest privacy' means that A Man In The Middle who has studied the
+obfs2 protocol should not be able to decipher it. 
+Still one should know that:
+* Because of the nature of the protocol, it is susceptible to offline
+  password cracking attacks.
+* It is the responsibility of the user and the bridge operator to
+  decide upon a strong pre-shared password.
+
+After becoming part of the Tor network, the user is considered
+anonymous and his communication private, with regards to A Man In The
+Middle, under normal circumstances.
+
+Since the realm of anti-censorship is quite young and
+not well studied, the obfs2 user should be aware that there are known
+and unknown attacks that can potentially identify obfs2 and can be
+deployed by A Man In The Middle. Known attacks like that include
+entropy counting and timing attacks.
+
+Additionally, a Tor user using obfs2 with a Tor bridge must *not*
+expect additional entity authentication, data integrity, availability
+or deniability.
+
+Finally, a Tor user using obfs2 with a Tor bridge must *not* expect
+protection from flows of the Tor protocol.