—This paper presents an eavesdropper-proof algorithm that is capable of fast generating symmetric (secret) keys. Instead of literally exchanging secret keys, both the sender and receiver adopt a mirroring process based on an improved Hebbian rule that uses identical random inputs to separately train on their reciprocal outputs to generate a pair of exactly identical secret key strings. Important parameters are elaborately characterized to implement a fast information transmission for ephemeral key exchanging. We show that performance optimization can be achieved by coordinating the parameters. One essential feature of the proposed algorithm is that even an eavesdropper who acquires entire structure of the algorithm and the transmission data still has no chance to decrypt the encrypted message, thus ensuring security in the subsequent encryption task. Moreover, computation load is well bounded in an acceptable range despite the increasing key length.