Cryptographic File Systems, Part Two: Implementation

Source: Security Focus

This is the second article in a two-part series looking at cryptographic filesystems. The first article in this series covered the background on cryptographic filesystems from the underlying concepts to some of the mechanics of those systems. This article will cover implementation. The focus will be on implementing the Microsoft's EFS under Windows 2000 and the Linux CryptoAPI.

One point to clarify from the first article involves the note that Microsoft's EFS does not support using a password-based symmetric algorithm. This is due to the concern that such schemes are weaker because of their susceptibility to dictionary attacks. While technically accurate, the fact remains that the public portion of the user's X.509v3 certificate (which is used to encrypt the File Encryption Key, or FEK, used by EFS) is used to encrypt the FEK. To decrypt the FEK requires the use of password or passphrase and unless password-based logon is disabled completely this password or passphrase is typically the user's domain password.