Sunday, March 7, 2010

Passwords for USB Keypads

Bruce Schneier recently posted about a new USB stick that comes with its own on-board numeric keypad, permitting a password consisting of digits to be entered directly into the USB device to authorize unlocking. Such a stick and keypad would circumvent the recent USB password vulnerability that was derived from a poor implementation of password verification on the desktop.


The stick in question from Corsair (shown above) also uses AES-256 encryption to protect the data on the stick. The AES-256 key for the stick is then likely to be derived from the user-supplied password (say using PKCS #5 or RFC 2898), or used to protect a file which contains a full-length 256-bit key. In either case the 256-bit key will be derived from, or protected by, a password which has a much lower entropy.

Bruce points out that a 77-digit password would be needed to produce the same entropy as a 256-bit key (since the logarithm to the base 10 of 2^{256} is about 77 ). I made the same point in Are AES 256-bit keys too large? where I calculated that a password based on the 94 printable ASCII characters would need to be 40 characters in length to achieve the same entropy of a 256-bit key (since the logarithm to the base 94 of 2^{256} is about 40). Deriving or bootstrapping AES keys from passwords is really an exercise in self-deception, especially when considering 256-bit keys. The discrepancy between the low entropy of passwords and the astronomical keyspace of AES-256 simply cannot be reconciled.

Perhaps the situation would improve if a biometric such as a fingerprint was used to bootstrap a 256-bit key. I did some research about a year ago and posted what I found in On the Entropy of Fingerprints. Some work has been done by IBM researchers who estimate the entropy of fingerprints to be at most 85 bits, or approximately the same as a length 13 password based on the 94 printable ASCII characters. An improvement, but still a long way from 256 bits of entropy.

1 comment:

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