Category Archives: Cryptology

Cryptographic primitives, cryptanalysis

Hidden Assumptions in Cryptographic Protocols

At the end of last week, Microsoft Research hosted a meeting of “Cryptoforma”, a proposed new project (a so-called “network of excellence”) to bring together researchers working on applying formal methods to security. They don’t yet know whether or not this project will get funding from the EPSRC, but I wish them good luck.

There were several very interesting papers presented at the meeting, but today I want to talk about the one by Liqun Chen, “Parsing ambiguities in authentication and key establishment protocols”.

Some of the protocol specifications published by ISO specify how the protocol should be encoded on the wire, in sufficient detail to enable different implementations to interoperate. An example of a standard of this type is the one for the public key certificates that are used in SSL authentication of web sites (and many other applications).

The security standards produced by one group within ISO (SC27) aren’t like that. They specify the abstract protocols, but give the implementor considerable leeway in how they are encoded. This means that you can have different implementations that don’t interoperate. If these implementations are in different application domains, the lack of interoperability doesn’t matter. For example, Tuomas Aura and I recently wrote a paper in which we presented a protocol for privacy-preserving wireless LAN authentication, which we rather boldly claim to be based on the abstract protocol from ISO 9798-4.

You could think of these standards as separating concerns: the SC27 folks get the abstract crypto protocol correct, and then someone else standardises how to encode it in a particular application. But does the choice of concrete encoding affect the protocol’s correctness?

Liqun Chen points out one case where it clearly does. In the abstract protocols in ISO 9798-4 and others, data fields are joined by a double vertical bar operator. If you want to find out what that double vertical bar really means, you have to spend another 66 Swiss Francs and get a copy of ISO 9798-1, which tells you that Y || Z means “the result of the concatenation of the data items Y and Z in that order”.

Oops.

When we specify abstract protocols, it’s generally understood that the concrete encoding that gets signed or MAC’d contains enough information to unambigously identify the field boundaries: it contains length fields, a closing XML tag, or whatever. A signed message {Payee, Amount} K_A should not allow a payment of $3 to Bob12 to be mutated by the attacker into a payment of $23 to Bob1. But ISO 9798 (and a bunch of others) don’t say that. There’s nothing that says a conforming implementation can’t send the length field without authentication.

No of course, an implementor probably wouldn’t do that. But they might.

More generally: do these abstract protocols make a bunch of implicit, undocumented assumptions about the underlying crypto primitives and encodings that might turn out not to be true?

See also: Boyd, C. Hidden assumptions in cryptographic protocols. Computers and Digital Techniques, volume 137, issue 6, November 1990.

J-PAKE: From Dining Cryptographers to Jugglers

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)
  • 2014-10-27: 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”.

Hardened stateless session cookies

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

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.

Relay attacks on card payment: vulnerabilities and defences

At this year’s Chaos Communication Congress (24C3), I presented some work I’ve been doing with Saar Drimer: implementing a smart card relay attack and demonstrating that it can be prevented by distance bounding protocols. My talk (abstract) was filmed and the video can be found below. For more information, we produced a webpage and the details can be found in our paper.

[ slides (PDF 9.6M) | video (BitTorrent — MPEG4, 106M) ]

Update 2008-01-15:
Liam Tung from ZDNet Australia has written an article on my talk: Bank card attack: Only Martians are safe.

Other highlights from the conference…

A cryptographic hash function reading guide

After a few years of spectacular advances in breaking cryptographic hash function NIST has announced a competition to determine the next Secure Hash Algorithm, SHA-3. SHA-0 is considered broken, SHA-1 is still secure but no one knows for how long, and the SHA-2 family are desperately slow. (Do not even think about using MD5, or MD4 for which Prof. Wang can find collisions by hand, but RIPEMD-160 still stands.) Cryptographers are ecstatic about this development: as if they were a bit bored since the last NIST AES competition and depressed by the prospect of not having to design another significant block cipher for the next few years.

The rest of us should expect the next four years to be filled with news, first about advances in the design, then advances in the attacks against Hash functions, as teams with candidate hash algorithms will bitterly try to find flaws in each other’s proposals to ensure that their function becomes SHA-3. To fully appreciate the details of this competition, some of us may want a quick refresher on how to build secure hash function.

Here is a list of on-line resources for catching up with the state of the art:

  1. A very quick overview of hash functions and their applications is provided by Ilya Mironov. This is very introductory material, and does not go into the deeper details of what makes these functions secure, or how to break them.
  2. Chapter 9 on Hash Functions and Data Integrity of the Handbook of Applied Cryptography (Alfred J. Menezes, Paul C. van Oorschot and Scott A. Vanstone) provides a very good first overview of the properties expected from collision resistant hash function. It also presents the basic constructions for such functions from block ciphers (too slow for SHA-3), as well as from dedicated compression functions. Chapter 3 also quickly presents Floyd’s cycle finding algorithm to find collisions with negligible storage requirements.
  3. If your curiosity has not been satisfied, the second stop is Prof. Bart Preneel’s thesis entitled “Analysis and Design of Cryptographic Hash Functions“. This work provides a very good overview of the state of the art in hash function design up to the middle of the nineties (before SHA-1 was commissioned.) The back to the basics approach is very instructive, and frankly the thesis could be entitled “everything you wanted to know about hash functions and never dared ask.” Bart is one of the authors of RIPEMD-160 that is still considered secure, an algorithm worth studying.
  4. Hash functions do look like block ciphers under the hood, and an obvious idea might be to adapt aspects of AES and turn it into such a function. Whirlpool does exactly this, and is worth reading about. One of its authors, Paulo Barreto, also maintains a very thorough bibliography of hash function proposals along with all known cryptanalytic results against them (and a cute health status indicating their security.)
  5. Prof. Wang’s attacks that forced NIST to look for better functions are a must-read, even though they get very technical very soon. A gentler introduction to these attacks is provided in Martin Schlaffer’s Master’s thesis describing how the attacks are applied to MD4.
  6. Finally it is no fun observing a game without knowing the rules: the NIST SHA-3 requirements provide detailed descriptions of what the algorithm should look like, as well as the families of attacks it should resist. After reading it you might even be tempted to submit your own candidate!

WordPress cookie authentication vulnerability

In my previous post, I discussed how I analyzed the recent attack on Light Blue Touchpaper. What I did not disclose was how the attacker gained access in the first place. It turned out to incorporate a zero-day exploit, which is why I haven’t mentioned it until now.

As a first step, the attacker exploited an SQL injection vulnerability. When I noticed the intrusion, I upgraded WordPress then restored the database and files from off-server backups. WordPress 2.3.1 was released less than a day before my upgrade, and was supposed to fix this vulnerability, so I presumed I would be safe.

I was therefore surprised when the attacker broke in again, the following day (and created himself an administrator account). After further investigation, I discovered that he had logged into the “admin” account — nobody knows the password for this because I set it to a long random string. Neither me nor other administrators ever used that account, so it couldn’t have been XSS or another cookie stealing attack. How was this possible?

From examining the WordPress authentication code I discovered that the password hashing was backwards! While the attacker couldn’t have obtained the password from the hash stored in the database, by simply hashing the entry a second time, he generated a valid admin cookie. On Monday I posted a vulnerability disclosure (assigned CVE-2007-6013) to the BugTraq and Full-Disclosure mailing lists, describing the problem in more detail.

It is disappointing to see that people are still getting this type of thing wrong. In their 1978 summary, Morris and Thompson describe the importance of one way hashing and password salting (neither of which WordPress does properly). The issue is currently being discussed on LWN.net and the wp-hackers mailing list. Hopefully some progress will be made at getting it right this time around.

Counters, Freshness, and Implementation

When we want to check freshness of cryptographically secured messages, we have to use monotonic counters, timestamps or random nonces. Each of these mechanisms increases the complexity of a given system in a different way. Freshness based on counters seems to be the easiest to implement in the context of ad-hoc mesh wireless networks. One does not need to increase power consumption for an extra message for challenge (containing a new random number), nor there is need for precise time synchronisation. It sounds easy but people in the real world are … creative. We have been working with TinyOS, an operating system that was designed for constrained hardware. TinyOS is a quite modular platform and even mesh networking is not part of the system’s core but is just one of the modules that can be easily replaced or not used at all.

Frame structures for TinyOS and TinySec on top of 802.15.4
Fig.: Structures of TinyOS and TinySec frames with all the counters. TinySec increases length of “data” field to store initialisation vector. Continue reading Counters, Freshness, and Implementation

Time to forget?

In a few hours time Part III of the Regulation of Investigatory Powers Act 2000 will come into effect. The commencement order means that as of October 1st a section 49 notice can be served which requires that encrypted data be “put into an intelligible form” (what you and I might call “decrypted”). Extended forms of such a notice may, under the provisions of s51, require you to hand over your decryption key, and/or under s54 include a “no tipping off” provision.

If you fail to comply with a notice (or breach a tipping off requirement by telling someone about it) then you will have committed an offence, for which the maximum penalty is two years and a fine or both. It’s five years for “tipping off” and also five years (an amendment in s15 of the Terrorism Act 2006) if the case relates to “national security”.

By convention, laws in the UK very seldom have retrospective effect, so that if you do something today, Parliament is very loth to pass a law tomorrow to make your actions illegal. However, the offences in Part III relate to failing to obey a s49 notice and that notice could be served on you tomorrow (or thereafter), but the material may have been encrypted by you today (or before).

Potentially therefore, the police could start demanding the putting into an intelligible form, not only of information that they seize in a raid tomorrow morning, but also of material that they seized weeks, months or years ago. In the 1995 Smith case (part of Operation Starburst), the defendant only received a suspended sentence because the bulk of the material was encrypted. In this particular example, the police may be constrained by double jeopardy or the time that has elapsed from serving a notice on Mr Smith, but there’s nothing in RIP itself, or the accompanying Code of Practice, to prevent them serving a s49 notice on more recently seized encrypted material if they deem it to be necessary and proportionate.

In fact, they might even be nipping round to Jack Straw’s house demanding a decryption key — as this stunt from 1999 makes possible (when the wording of a predecessor bill was rather more inane than RIP was (eventually) amended to).

There are some defences in the statute to failing to comply with a notice — one of which is that you can claim to have forgotten the decryption key (in practice, the passphrase under which the key is stored). In such a case the prosecution (the burden of proof was amended during the passage of the Bill) must show beyond a reasonable doubt that you have not forgotten it. Since they can’t mind-read, the expectation must be that they would attempt to show regular usage of the passphrase, and invite the jury to conclude that the forgetting has been faked — and this might be hard to manage if a hard disk has been in a police evidence store for over a decade.

However, if you’re still using such a passphrase and still have access to the disk, and if the contents are going to incriminate you, then perhaps a sledgehammer might be a suitable investment.

Me? I set up my alibi long ago :)

The role of software engineering in electronic elections

Many designs for trustworthy electronic elections use cryptography to assure participants that the result is accurate. However, it is a system’s software engineering that ensures a result is declared at all. Both good software engineering and cryptography are thus necessary, but so far cryptography has drawn more attention. In fact, the software engineering aspects could be just as challenging, because election systems have a number of properties which make them almost a pathological case for robust design, implementation, testing and deployment.

Currently deployed systems are lacking in both software robustness and cryptographic assurance — as evidenced by the English electronic election fiasco. Here, in some cases the result was late and in others the electronic count was abandoned due to system failures resulting from poor software engineering. However, even where a result was returned, the black-box nature of auditless electronic elections brought the accuracy of the count into doubt. In the few cases where cryptography was used it was poorly explained and didn’t help verify the result either.

End-to-end cryptographically assured elections have generated considerable research interest and the resulting systems, such as Punchscan and Prêt à Voter, allow voters to verify the result while maintaining their privacy (provided they understand the maths, that is — the rest of us will have to trust the cryptographers). These systems will permit an erroneous result to be detected after the election, whether caused by maliciousness or more mundane software flaws. However should this occur, or if a result is failed to be returned at all, the election may need to fall back on paper backups or even be re-run — a highly disruptive and expensive failure.

Good software engineering is necessary but, in the case of voting systems, may be especially difficult to achieve. In fact, such systems have more similarities to the software behind rocket launches than more conventional business productivity software. We should thus expect the consequential high costs and, despite all this extra effort, that the occasional catastrophe will be inevitable. The remainder of this post will discuss why I think this is the case, and how manually-counted paper ballots circumvent many of these difficulties.

Continue reading The role of software engineering in electronic elections