Sunday, June 14, 2009

Enterprise Password Management Guidelines from NIST

Those industrious people over at NIST have produced another draft publication in the SP-800 series on Guidelines for Enterprise Password Management. At 38 pages it will be a slim addition to your already bulging shelf of NIST reports. The objective of the report is to provide recommendations for password management, including “the process of defining, implementing, and maintaining password policies throughout an enterprise”. The report is consistently sensible and, in places, quite sagely. Overall the message is that passwords are complex to manage effectively, and you will need to spend considerable time and effort on covering all the bases. My short conclusion is that a CPO – a Chief Password Officer – is required.

Let’s begin with a definition. A password is “a secret (typically a character string) that a claimant uses to authenticate its identity”. The definition includes the shorter PIN variants, and the longer passphrase variants of passwords. Passwords are a factor of authentication, and as is well-known, not a very strong factor when used in isolation. Better management of the full password lifecycle can reduce the risks of security exposures from password failures. This NIST document will help your enterprise get there.

Storage and Transmission

NIST begins by discussing password storage and transmission, since enforcing more stringent password policies on users is counterproductive if those passwords are not adequately protected while in flight and at rest. Web browsers, email clients, and other applications may store user passwords for convenience but this may not be done in a secure manner. There is an excellent article on Security Focus by Mikhael Felker from 2006 on password storage risks for IE and FireFox. In general, applications that store passwords and automatically enter them on behalf of a user make unattended desktops more attractive to opportunistic data thieves in the workplace for example. Further, as noted in the recent Data Breach Investigation Report (DBIR) from Verizon, targeted malware is not just extracting passwords from disk locations but directly from RAM and other temporary storage locations. From page 22 of the DBIR, “the transient storage of information within a system’s RAM is not typically discussed. Most application vendors do not encrypt data in memory and for years have considered RAM to be safe. With the advent of malware capable of parsing a system’s RAM for sensitive information in real-time, however, this has become a soft-spot in the data security armour”.

As NIST observes, many passwords and password hashes are transmitted over internal and external networks to provide authentication capabilities between hosts, and the main threat to such transmissions are sniffers. Sniffers today are quite sophisticated, capable of extracting unencrypted usernames and passwords sent by common protocols such as Telnet, FTP, POP and HTTP. NIST states that mitigating against sniffing is relatively easy, beginning with encrypting traffic at the network layer (VPN) or at the transport layer (SSL/TLS). A more advanced mitigation is to use network segregation and fully switched networks to protect passwords transmitted on internal networks. But let’s not forget that passwords can also be captured at source by key loggers and other forms of malware, as noted by NIST and the DBIR.

Guessing and Cracking

NIST then moves on to a discussion of password guessing and cracking. By password guessing NIST means online attacks on a given account, while password cracking is defined as attempting to invert an intercepted password hash in offline mode. Password guessing is further subdivided into brute force attacks and improved dictionary attacks. The main mitigation against guessing attacks is mandating appropriate password length and complexity rules, and reducing the number of possible online guessing attempts. Restricting the number of guesses to a small number like 5 or so is not a winning strategy.

The main mitigation against password cracking is to increase the effort of the attacker by using salting and stretching. Salting increases the amount of storage to invert a password hash by pre-computation (for example using rainbow tables), while stretching increases the time to compute a password guess. Stretching is not a standard term as far as I know, and it is more commonly referred to as the iteration count, or more simply, as password spin.


Next is a discussion of password complexity, and the size of various combinations and length and composition rules as shown in the table below (double click to enlarge). Such computations are common, but security people normally take some pleasure in seeing them recomputed. NIST observes that in general the number of possible passwords increases more rapidly with longer lengths as opposed to permitting additional characters sets.


Of course, the table above does not take into account user bias in password selection. A large fraction of a password space may effectively contain zero passwords that will be selected by a user. And password cracking tools, like the recently upgraded LC6, made their name on that distinction. NIST briefly mentions the issue of password entropy but not in any systematic manner. There is a longer discussion on password entropy in another NIST publication. NIST does suggest several heuristics for strong password selection and more stringent criteria for passwords chosen by administrators.

Get a CPO

The NIST guidelines go on further to discuss some strategies for password reset and also provides an overview of existing password enterprise solutions. A useful glossary is presented as well. Overall the issues surrounding password management are complex and involved, and NIST gives good guidance on the main issues. It would appear that large companies which run big heterogeneous IT environments will require the services of a CPO – Chief Password Officer – to keep their password management under control and within tolerable risk limits.


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