1. A computer-implemented method comprising: receiving, by a server, authentication information of a user for a first domain and a second domain from a client system; authenticating an identity of the user based at least in part upon the authentication information; generating a set of instructions for client-side communication between the first domain and the second domain in response to a request received from the client system, the set of instructions indicating instructions that are permitted to be used in the client-side communication between the first domain and the second domain; generating cryptographic construct data, using one or more processors in one or more computer systems, for the set of instructions; and sending the set of instructions and the cryptographic construct data to the client system.