The present invention describes a method for authenticating a user of a mobile device by a verification authority, by making use of at least a personal identification number (PIN) and at least one cryptographic key, such that the PIN and the cryptographic key is known only to the user and the verification authority. The cryptographic key has at least one session key. Firstly, the user encodes the PIN by using at least one session key and then transfers the encoded PIN to a predefined address of the verification authority via the mobile device. Next, the verification authority decodes the PIN by using the cryptographic key authenticates the user if the decoded PIN matches a PIN stored corresponding to the user.