Apparatus for and method of controlling propagation of decryption keys

A method of controlling the propagation of a decryption keys that allows access to encrypted data, the method comprising the steps of:

• associating a propagation control word with a decryption key for an item of data, and in response to an instruction to send the key to a specified recipient, checking the status of the control word to determine if propagation is allowed, and if so, modifying the control word and encrypting the control word and decryption key with a recipient's public key and sending the encrypted key, including as part of the decryption key an identifier indicating the identity of a sender's key, and appending a control word against a user's identity in the decryption key to instruct a security system to initiate a message to them or an agent informing them of the propagation of the key and giving information concerning that propagation.

A security system for controlling access to encrypted information by a plurality of users, comprising a memory for storing at least one data unit comprising a decryption key and an associated security code, in which the decryption key is used in decrypting an encrypted item of information and the security code controls the number of times that the decryption key can be propagated, and the security system examines the security code which code includes a group code as an indication of an acceptable range of recipients to determine whether it is authorized to send encrypted copies of the decryption key to those recipients, wherein if the security system is authorized to send an encrypted copy of the decryption key, it encrypts the decryption key and propagates the encrypted copy of the decryption key.

A method of controlling the propagation of decryption keys to a plurality of users for allowing access to encrypted data, comprising the steps of storing at least one data unit in a memory, the at least one data unit comprising a decryption key, including a propagation control word with the decryption key in the data unit, and in response to an instruction to send the data unit to a specified recipient, checking the status of the control word to determine if propagation is allowed, including checking that the specified recipient is within an acceptable range of recipients indicated as a group code in the control word, and if so, modifying the control word and encrypting the data unit comprising the control word and decryption key with a recipient's public key and sending the data unit.