Data is transmitted between a first user and a second user via an information technology communications network, in a method comprising the steps of: generating a first hash value for a selected one of the data items; digitally signing and encrypting the first hash value with a secret identifier associated with the first user; transmitting to a second user the encrypted first hash value; receiving and storing the transmitted encrypted first hash value for audit purposes and generating a second hash value for the received encrypted first hash value; encrypting the second hash value with a private identifier associated with a second user and a public identifier associated with the first user; and returning the encrypted second hash value to the first user.