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20 June 2011, 11:05

Storing passwords in uncrackable form

by Daniel Bachfeld

News about intrusions into the servers of online stores, games vendors and other internet services can now be read on an almost daily basis. Often, the intruders obtain customers' login data including their passwords. As many people use the same password in multiple places, criminals can use the passwords to obtain unauthorised access to further services.

To prevent passwords from being extracted, web site operators usually protect their users' passwords through such cryptographic techniques as one-way hashing. For this purpose, a character string that doesn't allow any conclusions to be drawn about the actual password is derived from the password. The only way of finding out whether a password matches a hash is to rehash the password and compare the results. This method is used by the authentication systems of operating systems and web applications – and also by password crackers.

MD5 hashing was long considered sufficiently resilient for this purpose, because the time that is required to try out all possible combinations made it difficult for attackers to reconstruct a password from a hash. With a strong password, trying out all password combinations (brute force attack) using a cracker such as John the Ripper on conventional hardware used to take months, if not years. But times have changed.

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Cloud, CUDA and multi-core computer technologies are both a blessing and a curse: they can greatly accelerate the processing of data and make even complex simulations available to end users. Unfortunately, crackers use the same high-speed computing power to reconstruct plain-text data from an encrypted password, and then they use the password to log into a system as administrators. In this context, password crackers can take advantage of the fact that the harvested hashes were probably created using the MD5 algorithm, which is optimised for fast processing.

Commercial password crackers such as those by vendor Elcomsoft, and such free tools as Hashcat and BarsWF, can try out several million hashes per second to find out whether one of them matches a specific password. This means that a password of eight characters can be cracked in four days. However, there are even faster ways. As hard disk storage is getting cheaper and cheaper, attackers often use giant tables (rainbow tables) containing billions of pre-calculated hashes to find a password. These tables potentially allow them to determine a password within minutes. The lists required for dictionary attacks are also becoming longer and longer and, with very weak passwords, often enable cracking programs to succeed within hours.

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