Category Archives: Academic papers

An improved clock-skew measurement technique for revealing hidden services

Academic papers, Hardware & signals, Privacy technology

In 2006 I published a paper on remotely estimating a computer’s temperature, based on clock skew. I showed that by inducing load on a Tor hidden service, an attacker could cause measurable changes in clock skew and so allow the computer hosting the service to be re-identified. However, it takes a very long time (hours to days) to obtain a sufficiently accurate clock-skew estimate, even taking a sample every few seconds. If measurements are less granular than the 1 kHz TCP timestamp clock source I used, then it would take longer still.

This limits the attack since in many cases TCP timestamps may be unavailable. In particular, Tor hidden services operate at the TCP layer, stripping all TCP and IP headers. If an attacker wants to estimate clock skew over the hidden service channel, the only directly available clock source may be the 1 Hz HTTP timestamp. The quantization noise in this case is three orders of magnitude above the TCP timestamp case, making the approach I used in the paper effectively infeasible.

While visiting Cambridge in summer 2007, Sebastian Zander developed an improved clock skew measurement technique which would dramatically reduce the noise of clock-skew measurements from low-frequency clocks. The basic idea, shown below, is to only request timestamps very close to a clock transition, where the quantization noise is lowest. This requires the attacker to firstly lock-on to the phase of the clock, then keep tracking it even when measurements are distorted by network jitter.

Synchronized vs random sampling

Sebastian and I wrote a paper — An Improved Clock-skew Measurement Technique for Revealing Hidden Services — describing this technique, and showing results from testing it on a Tor hidden service installed on PlanetLab. The measurements show a large improvement over the original paper, with two orders of magnitude lower noise for low-frequency clocks (like the HTTP case). This approach will allow previous attacks to be executed faster, and make previously infeasible attacks possible.

The paper will be presented at the USENIX Security Symposium, San Jose, CA, US, 28 July – 1 August 2008.

Slow removal of child sexual abuse image websites

Academic papers, Banking security, Legal issues, News coverage, Security economics

On Friday last week The Guardian ran a story on an upcoming research paper by Tyler Moore and myself which will be presented at the WEIS conference later this month. We had determined that child sexual abuse image websites were removed from the Internet far slower than any other category of content we looked at, excepting illegal pharmacies hosted on fast-flux networks; and we’re unsure if anyone is seriously trying to remove them at all!
Continue reading Slow removal of child sexual abuse image websites →

"Covert channel vulnerabilities in anonymity systems" wins best thesis award

Academic papers, Awards, Hardware & signals, Privacy technology

My PhD thesis “Covert channel vulnerabilities in anonymity systems” has been awarded this year’s best thesis prize by the ERCIM security and trust management working group. The announcement can be found on the working group homepage and I’ve been invited to give a talk at their upcoming workshop, STM 08, Trondheim, Norway, 16–17 June 2008.

Update 2007-07-07: ERCIM have also published a press release.

J-PAKE: From Dining Cryptographers to Jugglers

Academic papers, Cryptology

Password Authenticated Key Exchange (PAKE) is one of the central topics in cryptography. It aims to address a practical security problem: how to establish secure communication between two parties solely based on their shared password without requiring a Public Key Infrastructure (PKI).

The solution to the above problem is very useful in practice — in fact, so useful that it spawns a lot “fights” over patents. Many techniques were patented, including the well-known Encrypted Key Exchange (EKE) and Simple Password Exponential Key Exchange (SPEKE). A secondary problem is technical; both the EKE and SPEKE protocols have subtle but worrying technical limitations (see the paper for details).

At the 16th Workshop on Security Protocols held in April 2008, Cambridge, UK, I presented a new solution (joint work with Peter Ryan) called Password Authenticated Key Exchange by Juggling (or J-PAKE). The essence of the protocol design inherits from the earlier work on solving the Dining Cryptographers problem; we adapted the same juggling technique to the two-party case to solve the PAKE problem. To our best knowledge, this design is significantly different from all past PAKE solutions.

Intuitively, the J-PAKE protocol works like a juggling game between two people — if we regard a public key as a “ball”. In round one, each person throws two ephemeral public keys (“balls”) to each other. In round 2, each person combines the available public keys and the password to form a new public key, and throws the new “ball” to each other.

After round 2, the two parties can securely compute a common session key, if they supplied the same passwords. Otherwise, the protocol leaks nothing more than: “the supplied passwords at two sides are not the same”. In other words, one can prove his knowledge of the password without revealing it. A Java implementation of the protocol on a MacBook Pro laptop shows that the total computation time at each side is merely 75 ms.

We hope this protocol is of usefulness to security engineers. For example, compared with SSL/TLS, J-PAKE is potentially much more resistant against phishing attacks, not to mention that it is PKI-free. Since this protocol is the result of an academic research project, we didn’t — and have no intention to — patent it. As explained in the paper, J-PAKE even has technical advantages over the patented EKE and SPEKE in terms of security, with comparable efficiency. It has been submitted as a follow-up to the possible future extension of IEEE P1363.2.

We believe the PAKE research is important and has strong practical relevance. This post is to facilitate discussions on this subject. The paper can be viewed here. Any comments or questions are welcome.

Update

  • 2008-06-28: a crude J-PAKE demo source code (.java) by me. (link broken)
  • 2008-11-04: a more refined J-PAKE in C and OpenSSL by Ben Laurie.
  • 2008-11-11: possible applications of J-PAKE in VPN and browser by James.
  • 2009-02-08: public group parameters for 112-bit and 128-bit security can be found in the comments.
  • 2009-03-15: fixed the broken link to the old Java file. Here is the link to the Java demo code.
  • 2010-04-17: a journal version of the paper available on IACR. No technical change to the protocol.
  • 2010-10-25: the journal version of the paper is accepted to publish on the TCS journal – Springer Transactions on Computational Science, the special issue on “Security in Computing”, 2011.
  • 2010-11-25: Sebastien Martini reported an implementation issue of J-PAKE in OpenSSL and OpenSSH. The issue is not applicable to the Java demo code that I wrote. As stated in the last paragraph of p. 11 in the paper, one shall check the element lies within the specified group. Stefan Arentz implemented a fix in OpenSSL. Official OpenSSL and OpenSSH patches can be found here and here.
  • 2011-01-11: Mozilla built J-PAKE into the base product of Firefox 4 ( beta 8 and later). More details here.
  • 2012-01-18: Today, Mohsen Toorani uploadeda paper on IACR eprint to claim several attacks on J-PAKE. My response can be found here.
  • 2012-07-21: Phil Clay contributed a Java implementation of J-PAKE to bouncycastle.
  • 2013-02-24: J-PAKE included into bouncycastle 1.48.
  • 2013-03-28: a code example to show how to use J-PAKE in bouncycastle
  • 2013-05-21: Submitted two Internet Drafts to IETF (one on J-PAKE and the other one on Schnorr NIZK Proof)
  • 2013-12-30: a code example to show how to implement J-PAKE using Elliptic Curve (or ECDSA-like group setting)
  • 2014-04-17: J-PAKE included into VMware NSS Cryptographic Module
  • 2014-10-27: J-PAKE adopted by the ISO/IEC standard (11770-4) following the ISO/IEC SC27 meeting held in Mexico City, October 20-24, 2014
  • 2014-12-26: My response to Mohsen Toorani’s IEEE ISCC’14 paper “Security Analysis of J-PAKE”.
  • 2015-04-29: J-PAKE included into Python
  • 2015-05-08: Standardization of J-PAKE in ISO/IEC 11770-4 in process. The first working draft (WD1) passed reviews by ISO/IEC SC27 at Kuching Malaysia, May 5-8, 2015.
  • 2015-05-19: Here is an independent formal analysis of J-PAKE by other researchers published at Oakland’2015. Their results are consistent with the original J-PAKE paper.
  • 2015-05-30: J-PAKE included in BarcoSilex BA414E Public Key crypto engine
  • 2015-05-31: Firefox is upgrading Sync 1.1 (using J-PAKE to transfer a full-entropy AES key between sync devices) to new Sync 1.5 (using user-defined passwords as encryption keys). But Pale moon decides to stay with Sync 1.1.
  • 2015-07-28: the Thread Technical white paper is public. It describes a technique that securely enrols a new device to the network based on J-PAKE. The technique is used in Google Nest thermostat products.
  • 2017-10-06: J-PAKE published in ISO/IEC 11770-4 as an international standard and also published in RFC 8236. See here.

PED vulnerability paper receives "Most Practical Paper" award at Oakland

Academic papers, Awards, Banking security

In February, Steven Murdoch, Ross Anderson and I reported our findings on system-level failures of widely deployed PIN Entry Devices (PED) and the Chip and PIN scheme as a whole. Steven is in Oakland presenting the work described in our paper at the IEEE Symposium on Security and Privacy (slides).

We are very pleased that we are the recipients of the new “Most Practical Paper” award of the conference, given to “the paper most likely to immediately improve the security of current environments and systems”. Thanks to everyone who supported this work!

IEEE Security & Privacy Magazine Award

Hardened stateless session cookies

Academic papers, Cryptology, Meta, Security engineering

The root cause behind the last-but-one WordPress cookie debacle was that the authors invented their own password hashing and cookie generation scheme. This is generally a bad idea, since it’s hard even for experts to get these right. Instead, whenever possible, a well-studied proposal should be chosen. It is for this reason that I suggested the phpass library for password hashing, and the Fu et al. stateless session cookie proposal.

These choices would be a substantial improvement on the previous custom design (had they been implemented correctly), but I still was not quite satisfied. The Fu et al. scheme has the property that an attacker who can read the cryptographic key stored in the database can create spoofed cookies. Given the history of WordPress security, it seems likely that there will eventually be a vulnerability discovered which allows the key, which authenticates cookies, to be leaked.

It’s good practice in security engineering to design systems with the widest possible range of attacker capabilities in mind. I therefore designed a cookie scheme which would do all that the Fu et al. design did, but also maintained some of its security properties if the attacker has read-access to the authentication database and knows the cookie authentication key. I published a paper on this topic — Hardened stateless session cookies — at the 2008 Security Protocols Workshop.

The trick behind my scheme is to store the hash of the user’s password in the cookie, and the hash of that in the authentication database. This means that it’s possible for the server to verify cookies, but the authentication database doesn’t contain enough information to create a fake cookie. Thus an attacker with read-access to the database still needs to know the user’s password to log in, and that isn’t stored. There are some additional subtleties to resist different attacks, and those are described in the paper.

I hope this proposal will trigger discussion over this important problem and lead to improved cookie authentication schemes.

WordPress 2.5 cookie integrity protection vulnerability

Academic papers, Cryptology, Meta

Recently, I was preparing to give a talk on web authentication so was looking at the source code of WordPress, which I had just upgraded to version 2.5. Unfortunately, I found a rather nasty security hole, which has now been disclosed. If a WordPress installation is configured to permit account creation, the vulnerability allows an attacker to gain administrator access.

The problem is to do with how cookies are generated. The authentication code was substantially overhauled for WordPress 2.5, in part to deal with security problems in the password database. Now, the authentication cookies take the form of:

wordpress_.COOKIEHASH = USERNAME . | . EXPIRY_TIME . | . MAC

Where:
COOKIEHASH
MD5 hash of the site URL (to maintain cookie uniqueness)
USERNAME
The username for the authenticated user
EXPIRY_TIME
When cookie should expire, in seconds since start of epoch
MAC
HMAC-MD5(USERNAME . EXPIRY_TIME) under a key derived from a secret and USERNAME . EXPIRY_TIME.

This scheme is based on two papers: Dos and Don’ts of Client Authentication on the Web by Fu et al. and A Secure Cookie Protocol by Liu et al. However, there is a small difference and, as is common in cryptographic systems, small changes can have big impact.

The problem is that USERNAME and EXPIRY_TIME are not delimited when calculating the MAC. This means that a MAC for one cookie is valid for any other, provided that USERNAME . EXPIRY_TIME is unchanged. So an attacker can register a username starting with “admin”, log in as usual, then modify their cookie so it’s valid for the administrator account.

Fu et al. called this the “cryptographic splicing” attack in their paper (Section 3.3), and is one of the many ways they show how people can slip up when implementing web authentication. Unfortunately, dynamic website frameworks, especially PHP, offer little assistance to programmers trying to implement secure applications.

I will expand on this topic in a future post but in the mean time, if you run WordPress, you really should upgrade to 2.5.1. While WordPress 2.3.3 doesn’t have the problem described here, it is still not secure.

There is some more detail on the cookie vulnerability in my disclosure (CVE-2008-1930). WordPress have mentioned it in their release announcement and I’ve also just sent it to the usual mailing lists.

Award Winners #2

Academic papers, Awards

Two years ago, almost exactly, I wrote:

Congratulations to Steven J. Murdoch and George Danezis who were recently awarded the Computer Laboratory Lab Ring (the local alumni association) award for the “most notable publication” (that’s notable as in jolly good) for the past year, written by anyone in the whole lab.

Well this year, it’s the turn of Tyler Moore and myself to win, for our APWG paper: Examining the Impact of Website Take-down on Phishing.

The obligatory posed photo, showing that we both own ties (!), is courtesy of the Science Editor of the Economist.

Tyler Moore and Richard Clayton, most notable publication 2008
Tyler Moore and Richard Clayton, most notable publication 2008

Securing Network Location Awareness with Authenticated DHCP

Academic papers, Operating systems, Privacy technology, Security engineering

During April–June 2006, I was an intern at Microsoft Research, Cambridge. My project, supervised by Tuomas Aura and Michael Roe, was to improve the privacy and security of mobile computer users. A paper summarizing our work was published at SecureComm 2007, but I’ve only just released the paper online: “Securing Network Location Awareness with Authenticated DHCP”.

How a computer should behave depends on its network location. Existing security solutions, like firewalls, fail to adequately protect mobile users because they assume their policy is static. This results in laptop computers being configured with fairly open policies, in order to facilitate applications appropriate for a trustworthy office LAN (e.g. file and printer sharing, collaboration applications, and custom servers). When the computer is taken home or roaming, this policy leaves an excessively large attack surface.

This static approach also harms user privacy. Modern applications broadcast a large number of identifiers which may leak privacy sensitive information (name, employer, office location, job role); even randomly generated identifiers allow a user to be tracked. When roaming, a laptop should not broadcast identifiers unless necessary, and on moving location either pseudonymous identifiers should be re-used or anonymous ones generated.

Both of these goals require a computer to be able to identify which network it is on, even when an attacker is attempting to spoof this information. Our solution was to extend DHCP to include an network location identifier, authenticated by a public-key signature. I built a proof-of-concept implementation for the Microsoft Windows Server 2003 DHCP server, and the Vista DHCP client.

A scheme like this should ideally work on both small PKI-less home LANs and still permit larger networks to aggregate multiple access points into one logical network. Achieving this requires some subtle naming and key management tricks. These techniques, and how to implement the protocols in a privacy-preserving manner are described in our paper.

Chip & PIN terminals vulnerable to simple attacks

Academic papers, Banking security, Hardware & signals, Security economics, Security engineering

Steven J. Murdoch, Ross Anderson and I looked at how well PIN entry devices (PEDs) protect cardholder data. Our paper will be published at the IEEE Symposium on Security and Privacy in May, though an extended version is available as a technical report. A segment about this work will appear on BBC Two’s Newsnight at 22:30 tonight.

We were able to demonstrate that two of the most popular PEDs in the UK — the Ingenico i3300 and Dione Xtreme — are vulnerable to a “tapping attack” using a paper clip, a needle and a small recording device. This allows us to record the data exchanged between the card and the PED’s processor without triggering tamper proofing mechanisms, and in clear violation of their supposed security properties. This attack can capture the card’s PIN because UK banks have opted to issue cheaper cards that do not use asymmetric cryptography to encrypt data between the card and PED.

Ingenico attack Dione attack

In addition to the PIN, as part of the transaction, the PED reads an exact replica of the magnetic strip (for backwards compatibility). Thus, if an attacker can tap the data line between the card and the PED’s processor, he gets all the information needed to create a magnetic strip card and withdraw money out of an ATM that does not read the chip.

We also found that the certification process of these PEDs is flawed. APACS has been effectively approving PEDs for the UK market as Common Criteria (CC) Evaluated, which does not equal Common Criteria Certified (no PEDs are CC Certified). What APACS means by “Evaluated” is that an approved lab has performed the “evaluation”, but unlike CC Certified products, the reports are kept secret, and governmental Certification Bodies do not do quality control.

This process causes a race to the bottom, with PED developers able to choose labs that will approve rather than improve PEDs, at the lowest price. Clearly, the certification process needs to be more open to the cardholders, who suffer from the fraud. It also needs to be fixed such that defective devices are refused certification.

We notified APACS, Visa, and the PED manufactures of our results in mid-November 2007 and responses arrived only in the last week or so (Visa chose to respond only a few minutes ago!) The responses are the usual claims that our demonstrations can only be done in lab conditions, that criminals are not that sophisticated, the threat to cardholder data is minimal, and that their “layers of security” will detect fraud. There is no evidence to support these claims. APACS state that the PEDs we examined will not be de-certified or removed, and the same for the labs who certified them and would not even tell us who they are.

The threat is very real: tampered PEDs have already been used for fraud. See our press release and FAQ for basic points and the technical report where we discuss the work in detail.

Update 1 (2008-03-09): The segment of Newsnight featuring our contribution has been posted to Google Video.

Update 2 (2008-03-21): If the link above doesn’t work try YouTube: part1 and part 2.