1. A method for authorizing a transaction, the method comprising the following acts performed by a telecommunications server configured to act as an authentication provider: at least one preparatory phase; and at least one authorization phase; wherein the at least one preparatory phase comprises for each of several user accounts: registering a user account via a user terminal; registering a plurality of personal devices with the registered user account, wherein registering of a personal device comprises registering an authentication application installed in that personal device; wherein the authentication application in the registered personal device is configured to: indicate at least a subset of received transaction-specific details via a user interface; receive transaction-specific instructions via the user interface; and digitally sign the transaction-specific instructions by using a cryptographic private key assigned to the user account; wherein the at least one authorization phase performed by the telecommunications server comprises for each of several transactions related to one of the several user accounts: receiving knowledge of a transaction relating to a user of a user terminal; determining, in response to receiving the knowledge, a user account related to the transaction; receiving a request for details specific to the transaction from at least one personal device; checking, whether the at least one personal device wherefrom the request is received belongs to the plurality of personal devices registered with the user account determined to relate to the transaction; providing, in response to the at least one personal device wherefrom the request is received belonging to the plurality of personal devices registered with the user account determined to relate to the transaction, the requested details specific to the transaction to the authentication application in the at least one personal device wherefrom the request for details specific to the transaction were received; receiving, after the providing, from the authentication application in the at least one personal device a digitally signed transmission which indicates transaction-specific instructions received via the user interface by the authentication application in the at least one personal device; and authorizing or denying the transaction based on the received transaction-specific instructions.