1. A trust center, comprising: a services provider; a routing component comprising a processor and a memory storing program instructions that are executed by the processor to: receive, from a requester, a request message for performing a tax calculation, the request message including a first mark-up language document containing transaction data signed with an electronic signature of the requester; when the services provider is unavailable, forward the request message to a replacement system; when the services provider is available, forward the request message to the services provider; wherein the services provider is configured to: verify the electronic signature of the requester, associated with the request message, using a public key of the requestor retrieved from a public key server; perform the tax calculation based on at least the transaction data to determine a result of the tax calculation; and generate a response message, the response message comprising: a second mark-up language document containing the result of the tax calculation signed with an electronic signature of the provider server; and the first mark-up language document received from the requester, signed with the electronic signature of the requester, wherein the entire response message is further digitally signed with the electronic signature of the provider server.