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A Future-Adaptable Password Scheme

Niels Provos and David Mazières
{provos,dm}@openbsd.org
The OpenBSD Project

Abstract:

Many authentication schemes depend on secret passwords. Unfortunately, the length and randomness of user-chosen passwords remain fixed over time. In contrast, hardware improvements constantly give attackers increasing computational power. As a result, password schemes such as the traditional UNIX user-authentication system are failing with time.

This paper discusses ways of building systems in which password security keeps up with hardware speeds. We formalize the properties desirable in a good password system, and show that the computational cost of any secure password scheme must increase as hardware improves. We present two algorithms with adaptable cost--eksblowfish, a block cipher with a purposefully expensive key schedule, and bcrypt, a related hash function. Failing a major breakthrough in complexity theory, these algorithms should allow password-based systems to adapt to hardware improvements and remain secure well into the future.

• Introduction
• Related Work
• Design criteria for password schemes
• Eksblowfish Algorithm
• Bcrypt Algorithm
  • Implementation
• Bcrypt Evaluation
  • Comparison
    • Traditional crypt
    • MD5 crypt
  • Attacks and Vulnerabilities
    • Salt Collisions
    • Precomputing Dictionaries
    • Algorithm Optimization
    • Hardware Improvements
• Conclusion
• Acknowledgments
• References
• About this document ...