Posted by: David Harley | May 21, 2012

PIN Holes: Passcode Selection Strategies

This is the second paper I presented at the EICAR 2012 conference in Lisbon. As before, It’s posted here rather than on the ESET resources page for conference papers in accordance with EICAR’s copyright stipulation that EICAR conference papers be posted on personal web sites.

PIN Holes: Numeric Passcodes and Mnemonic Strategies 

And here’s the abstract:

Recommendations on how to select and/or memorize a four-digit PIN (Personal Identification Number) can be found all over the Internet, but while we have learned a great deal from analyses of mixed-character passwords and passphrases revealed by high-profile breaches like the highly publicized Gawker and Rockyou.com attacks, there are no exactly equivalent attack-derived data on PIN usage. However, a sample of 204,508 anonymized passcodes for a smartphone application, by ranking 4-digit strings by popularity, gives us a starting point for mapping that ranking to known selection and mnemonic strategies.

Memorization strategies summarized by Rasmussen and Rudmin include rote learning; memorization according to keypad pattern; passcode re-use from other security contexts; code with personal meaning; code written down or stored electronically (as on mobile phone) – possibly using various concealment and transformation strategies.

The data provided by Amitay allows us to assess the degree to which memorization strategies are used in relation to a standard smartphone numeric keypad, but also to engage in some informed speculation on the extent to which they might be modified on other keypads, including QWERTY phone keypads, ATM keypads, security tokens requiring initial PIN entry, and hardware using an inverted (calculator-type) numeric layout. The ranking allows evaluation of the entropic efficacy of these strategies: the more popular the sequence, the likelier it is to be guessed.

These considerations are used to assess the validity of commonly recommended strategies in a diversity of contexts and generate a set of recommendations based on the findings of this analysis. These recommendations are placed into the context of more general mixed-character passwords and passphrases. They will provide a starting point for security managers and administrators responsible for the education and protection by policy of end users and customers using the kinds of device and application that require numeric passcodes for authentication.

David Harley CITP FBCS CISSP
ESET Senior Research Fellow

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