System and method for data transfer authentication

A transaction initialization method serving first and second devices, interconnected by a communication network, each having a speaker and a microphone, which wish to perform a transaction, the method comprising:

• providing a client which controls the first device to initiate the transaction including (a) commanding the first device's speaker to generate a transaction-heralding sound and (b) notifying the server, via said network, of generation of the transaction-heralding sound;
• and which controls the second device, using a processor, to notify the server, via said network, that a particular transaction-heralding sound has been heard;
• and, wherein, at least once, when a server notes that the same transaction-heralding sound has been reported as having been sent and as having been received, the server responsively validates a transaction between the two devices;
• wherein the transaction-heralding sound is a superposition of n frequencies, wherein the first device is randomly selects the n frequencies for different transactions;
• wherein the second device is arranged to transmit to the server values of frequency peaks received by the second device;
• and wherein the server notes that the same transaction-heralding sound has been reported as having been sent and as having been received when the n frequencies and the frequency peaks fall within the same frequency bins;
• and
• storing in the first device a credit card number without spreading the credit card number to one or more service providers.