| 摘要 |
<p>A directory DIR 112 stores identifying titles and pointers to areas 111 of a memory 15 storing respective messages. To protect the messages against unauthorized changes, a MAC (message authentication code) is calculated for them in known manner and stored in a register 116 in a secure unit 16. This involves processing the whole of each message every time the MAC is checked or, if a message has been changed, a fresh MAC has to be calculated. To avoid this, a separate MAC is calculated for each message and stored in the directory (in section 114), and a global MAC is calculated for the individual MAC's (treating them as if they were a message) and stored in a secure register 115. To check a stored message, the global MAC is recalculated (thus verifying the MAC of the message), and the MAC of the message is recalculated (thus verifying the message). If the message is changed, its new MAC and a new global MAC are calculated. The system can be extended to a hierarchy of sub-global MAC's.</p> |
