Abstract: A method and system for authenticating secure transactions between a transacting user (9) and a secure transaction host (15) is provided. The system includes a mobile phone software application (59) installed on a transacting user's mobile phone (7) which is configured to compose a digital fingerprint (13) uniquely associated with the specific mobile phone (7) on which it is installed. The system further includes an authentication service provider with which users of the system may be enrolled by registering at least the digital identifiers composed by the applications installed on their mobile communication devices in an authentication database (5). The authentication service provider is configured to authenticate secure transactions on request from secure transaction hosts by sending transaction confirmation requests to mobile phones of enrolled users requiring them to confirm or deny secure transactions before such transactions are allowed to be finalized.