Computer systems and methods for improving security or performance of one or more client computers interacting with a plurality of server computers. In an embodiment, a computer system comprises a first server computer and a second server computer; wherein the first server computer is configured to: generate a challenge nonce, wherein the challenge nonce corresponds to a challenge state; generate the challenge state based on the challenge nonce, wherein the challenge state corresponds to a response state; send, to a first client computer, the challenge nonce and the challenge state, but not the response state; wherein the second server computer is configured to: receive, from the first client computer, a test nonce and a test response state; determine whether the test response state matches the response state based on the test nonce, without: receiving the challenge state from the first server computer; receiving the challenge state from the first client computer.