Methods and systems for performing electronic transactions using dynamic password authentication involve, for example, sending, using a backend processor, a unique random or pseudorandom character string to the user's mobile device processor. Thereafter, also using the backend processor, a user identifier and a challenge string consisting at least in part of the user identifier and the random or pseudorandom character string encrypted with a unique encryption key may be received from the user's mobile device processor. Using the backend processor, a cipher input consisting at least in part of the user identifier and the random or pseudorandom character string is encrypted with the unique encryption key. The received encrypted challenge string is authenticated if the received encrypted challenge string matches the encrypted cipher input.