Authorizing transactions by an authentication provider involves at least one preparatory phase and an authorization phase. The preparatory phase includes registering a user account with several personal devices, each with an authentication application installed. The authorization phase receives knowledge of the transaction; determines the user account related to the transaction; determines at least one personal device registered with the user account related to the transaction; receives a request for details specific to the transaction from at least one personal device; provides the authentication application of the at least one personal device with the requested details specific to the transaction; receives from the authentication application a digitally signed transmission which indicates transaction-specific instructions received by the authentication application; and authorizes or denies the transaction based on the received transaction-specific instructions.