Data to be output to a removable storage medium is encrypted for sending to an output device by an encryption process based on encryption parameters comprising public data of a trusted party and an encryption key string comprising a policy for allowing the output of the data. The trusted party provides a decryption key to the output device but only after being satisfied that the policy has been met. The decryption key is generated in dependence on the encryption key string and private data of the trusted party. The output device uses the decryption key in decrypting the data to be output. Embodiments are provided that involve multiple policies and trusted parties.