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Halting Password Puzzles: Hard-to-break Encryption from Human-memorable Keys
We revisit the venerable question of “pure password”- based key derivation and encryption, and expose security weaknesses in current implementations that stem from structural flaws in Key Derivation Functions (KDF). We advocate a fresh redesign, named Halting KDF (HKDF), which we thoroughly motivate on these grounds:
- By letting password owners choose the hash iteration count, we gain operational flexibility and eliminate the rapid obsolescence faced by many existing schemes.
- By throwing a Halting-Problem wrench in the works of guessing that iteration count, we widen the security gap with any attacker to its theoretical optimum.
- By parallelizing the key derivation, we let legitimate users exploit all the computational power they can muster, which in turn further raises the bar for attackers.
HKDFs are practical and universal: they work with any password, any hardware, and a minor change to the user interface. As a demonstration, we offer realworld implementations for the TrueCrypt and GnuPG packages, and discuss their security benefits in concrete terms.
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