Original URL: http://www.theregister.co.uk/2005/11/10/password_hashes/

Rainbow warriors crack password hashes

Is it safe? Nope

By Robert Lemos

Posted in Security, 10th November 2005 15:42 GMT

A trio of entrepreneurial hackers hope to do for the business of password cracking what Google did for search and, in the process, may remove the last vestiges of security from many password systems.

Over the past two years, three security enthusiasts from the United States and Europe set a host of computers to the task of creating eleven enormous tables of data that can be used to look up common passwords. The tables - totaling 500GB - form the core data of a technique known as rainbow cracking, which uses vast dictionaries of data to let anyone reverse the process of creating hashes - the statistically unique codes that, among other duties, are used to obfuscate a user's password.

Last week, the trio went public with their service. Called RainbowCrack Online, the site allows anyone to pay a subscription fee and submit password hashes for cracking.

"Usually people think that a complex, but short, password is very secure, something like $FT%_3^," said Travis, one of the founders of RainbowCrack Online, who asked that his last name not be used. "However, you will find that our tables handle that password quite easily."

While security professionals have questions whether a business can be created by offering access to rainbow tables, the endeavor does highlight the weaknesses in security of password-only authentication. History has shown that password systems are imminently breakable.

In August, a group of Chinese researchers found further breaks in a common hash function, the Secure Hash Algorithm or SHA-1, used by the U.S. government. In September, researchers from the University of California at Berkeley published a paper that demonstrated that the sound of a person typing can reveal the content, including passwords. Those technical breaks do not even account for the human factor: People tend to pick simple passwords and disclose them frequently. In fact, many viruses and worms have successfully spread by trying to log into administrator accounts using a small list of common passwords.

Because of the problems, the U.S. government is requiring that banks move towards two-factor authentication, where the typical password security is augmented by a biometric or a physical security device. Some security researchers maintain that even adding a second type of security check is not enough.

The latest attack focuses on the hash functions used to verify passwords. Because operating systems cannot keep a copy of the password on the disk without weakening system security, the software instead saves a statistically unique code generated from the pasword. While the code, or hash, is computationally easy to create, reversing the process to recover the password is nearly impossible, given a correctly implemented hash function.

Rainbow tables side step the difficulty in cracking a single password by instead creating a large data set of hashes from nearly every possible password. To break a password, the attacker merely looks up the hash to find the password that produces that code.

"Creating the tables takes much more time than cracking a single hash, but then you can use the tables over and over again," said Philippe Oechslin, CEO of Swiss information-technology firm Objectif Sécurité and the inventor of rainbow tables. "The advantage of rainbow tables is that once you have the tables it is faster than a brute force (attack) and it needs less memory than a full dictionary (attack) of the function."

The theory behind rainbow tables extends research by Martin Hellman and Ronald Rivest done in the early 1980s on the performance trade-offs between processing time and the memory needed for cryptoanalysis. In a paper published in 2003, Oechlin refined the techniques and showed the attack could reduce the time to attack 99.9 per cent of Microsoft's LanMan password scheme to 13.6 seconds from 101 seconds. Further refinements have reduced the number of false positives produced by the system.

"This is something that you are never supposed to be able to do with (a good implementation of) crypto - generate every single possible combination," said Dan Moniz, a member of the Shmoo group, a coalition of security researchers and the manager of the groups own rainbow table project.

RainbowCrack Online will offer 11 tables covering six different hash algorithms, including LanMan, MD5, MySQL 323, and SHA-1. Offering the tables in an online service is not about helping attackers, but about helping system administrators secure their systems, said RainbowCrack's Travis.

"Attackers already have tables like these, (so) RainbowCrack serves as a tool to judge what is and what is not a secure password policy," he said.

Making money with rainbow tables is not a new idea. A handful of efforts have been started and then stalled. Zhu Shuanglei, who created the open-source tool that RainbowCrack Online uses to generate its tables, has generated a 64GB LanMan table and advertises it for sale for $400. The Shmoo group created its own rainbow table to crack Microsoft's LanManager tables that offered them for free through BitTorrent, and at the DEF CON hacking convention, Shmoo's Moniz saw several versions of the LanManager tables for sale. People with free computer time would calculate the tables hoping to make a little money, he said.

The experience has Shmoo's Moniz questioning whether there will be demand for a service like RainbowCrack Online. Bruce Schneier, a well-known cryptographer and chief technology officer of network monitoring service Counterpane Internet Security, agrees.

"There could be a criminal business in it," he said. "But I don't see the legitimate business demand for rainbow tables."

To some extent, RainbowCrack Online applies Google's business model to cracking encryption. Like Google, RainbowCrack Online give web access to a large database of information. Both services go through a lot of effort and a lot of memory to give users a quick answer to a query. And both services could be reproduced, barring patent hurdles.

Yet, while searching the web has obvious utility, the usefulness of rainbow tables is questionable, because good programming can make the tables require several magnitudes more memory, rendering the technique essentially useless. Specifically, adding several unpredictable bytes at the beginning of a password before hashing, a technique known as salt, can add several orders of magnitude of complexity to any cryptanalysis of the result.

"Remember that rainbow tables only work for inferior functions that use no salt or initialization vector," Objectif Sécurité's Oechslin said. "If programmers were more careful, there would be no market for a rainbow Google."

RainbowCrack Online's founders disagree. The lion's share of cryptographic hash functions are not well implemented and thus could be broken with their tables quite easily, RainbowCrack's Travis said.

Counterpane's Schneier agrees.

"All we have is anecdotal evidence about development practices, but I would agree that a lot of systems are weak," Schneier said. "The biggest problems that we as cryptographers have to face is bad implementations."

For such insecure password implementations, rainbow-table services may be the sign that it's time to reconsider security.

Copyright © 2005, SecurityFocus