A network helper is provided that assists verifiers in executing a puzzle-based protocol for authentication of a token. A token stores a secret key and one or more puzzle-generating algorithms. The helper stores a plurality of puzzles associated with a particular token. When requested to do so by a verifier, the helper provides a plurality of pseudorandomly selected puzzles for the token to a verifier. The puzzles are encoded with information that is used between the verifier and token to establish a secured symmetric key. The verifier selects one or a few of the encoded puzzles and breaks them by a brute force attack. Because the helper does not know which puzzles have been selected, it has to break all puzzles to attempt to figure out the symmetric key. However, if a large number of puzzles are utilized, say millions, then breaking all of them becomes a computationally prohibitive task.