The disclosure relates to a method (30) for a network node (6, 7, 8) of a wireless network (1) of providing a device (5, 9) access to the wireless network (1). The method (30) comprises receiving (31), from a device (5, 9), an access request comprising a preamble; establishing (32), in response to the access request, a computational puzzle based on the received preamble; and sending (33) the computational puzzle to the device (5, 9). A corresponding method in a device is also disclosed, as are a network node (6, 7, 8), device (5, 9), computer programs and computer program products.