JPAKE

JPAKE, short for Password-Authenticated Key Exchange by Juggling (often written as J-PAKE), is a two-party protocol that enables two users who share a password to establish a high-entropy session key over an insecure network. It is a type of PAKE (password-authenticated key exchange) protocol designed to resist eavesdropping and active interference, while ensuring that the password itself is not exposed.

In broad terms, JPAKE operates using public-key cryptography and zero-knowledge proofs. Each participant generates secret exponents and corresponding public values, accompanied by zero-knowledge proofs that they know the related secrets. Through a sequence of rounds, the parties exchange these values and verify the proofs. After validating the other party’s contributions, both sides perform a computation analogous to Diffie-Hellman, in which the password-related components cancel, yielding a shared secret. A key derivation function is then applied to produce the final session key.

Properties and use cases: JPAKE provides mutual authentication and forward secrecy when implemented with ephemeral parameters. It ensures that the established key is known only to parties who possess the correct password, without transmitting the password itself. The protocol relies on the hardness of the discrete logarithm problem in the chosen group and on sound random-number generation. It is implemented in various libraries for secure messaging, VPNs, and other protocols that require password-based key establishment.

Variants exist using modulus groups or elliptic curves. Proper parameter selection, randomization, and secure handling of credentials are essential to maintain security in practical deployments.