Abstract

Strong password methods verify even small passwords over a network without additional stored keys or certificates with the user, and without fear of network dictionary attack. We describe a new extension to further limit exposure to theft of a stored password-verifier, and apply it to several protocols including the Simple Password Exponential Key Exchange (SPEKE). Alice proves knowledge of a password C to Bob, who has a stored verifier S, where S=g mod p. They perform a SPEKE exchange based on the shared secret S to derive ephemeral shared key K,. Bob chooses a random X and X sends g mod p. Alice computes K2=gxc mod p, and proves knowledge of {K,,K2/. Bob vervies this result to confirm that Alice knows C. Implementation issues are summarized, showing the potential for improved pe$ormance over Bellovin & Merritt's comparably strong Augmented-Encrypted Key Exchange. These methods make the password a strong independent factor in authentication, and are suitable for both Internet and intranet use.

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