The present disclosure relates to a method and system for securely transferring master keying material between to a slave dongle (12). Each slave dongle (12) is connected to a data transfer system. The slave dongle (12) contains a public key and a private key and the data transfer system holds a master keying material source that contains master keying material to be transferred securely to the slave dongle (12). The slave dongle's public key is transferred to the master keying material source. The master keying material source encrypts the master keying material with the slave dongle's public key to produce an encrypted master keying material. The encrypted master keying material is sent to the slave dongle (12) and the slave dongle (12) decrypts the encrypted master keying material with the slave dongle's private key. This allows multiple users, each having a slave dongle (12a-n) that has been configured in this manner, to use the same master keying material to securely communicate with one another.