Secure computing device with monotonic counter and method therefor
A secure computing device ( 14 ) includes a secure processing section ( 30 ) having a tamper detection circuit ( 58 ) and a monotonic counter ( 68 ). The tamper detection circuit ( 58 ) detects an event which suggests that the trust associated with the secure processing section ( 30 ) may have been compromised. When such an event is detected, a security breach is declared and trusted software ( 38 ) is disabled. After a security breach is declared, the monotonic counter ( 68 ) may be reclaimed. The monotonic counter ( 68 ) provides a monotonic count value ( 70 ) that includes an LSB portion ( 80 ) and an MSB portion ( 82 ). The LSB portion ( 80 ) is obtained from a binary counter ( 72 ). The MSB portion ( 82 ) is obtained from a register ( 84 ) of independent one-time-programmable bits. The monotonic counter ( 68 ) is reclaimed by programming one of the one-time programmable bits to guarantee that future counting of the monotonic counter will be monotonic relative to all past counting.
1. A secure computing device which produces a monotonic count value in response to a read operation, said device comprising:
a counter configured to count in response to a count signal and to provide least significant bits of said monotonic count value;
a roll-over detection circuit coupled to said counter and configured to detect advancement of a value presented in said least significant bits beyond a maximum value;
a tamper detection circuit coupled to said roll-over detection circuit, said tamper detection circuit being configured to declare a security breach in response to detection of a roll over in said least significant bits of said monotonic count value;
a register of at least one one-time-programmable bit configured to provide one or more most significant bits of said monotonic count value; and
a synchronizer coupled to said counter and to said register, said synchronizer being configured to supply both of said least significant bits of said monotonic count value and said one or more most significant bits of said monotonic count value in response to said read operation.
2. A secure computing device as claimed in claim 1 wherein said counter is a binary counter.
3. A secure computing device as claimed in claim 1 additionally comprising a battery coupled to said counter so that said counter is continuously energized.
4. A secure computing device as claimed in claim 3 wherein:
said register of one-time-programmable bits is a non-volatile register; and
said counter includes volatile memory elements which remain continuously energized by said battery.
5. A secure computing device as claimed in claim 1 wherein said register is provided by at least one fusible link.
6. A secure computing device as claimed in claim 1 wherein:
said counter is an L-bit counter configured to present up to 2 L unique values, where L is a first integer number; and
said register is an M-bit register configured to present up to M+1 unique values, where M is a second integer number.
7. A secure computing device as claimed in claim 6 where said integer L is greater than said integer M.
8. A method of operating a secure computing device having a secure processing section which provides data security services including a monotonic count value, said method comprising:
controlling a counter to produce a least significant bits portion of said monotonic count value in response to a count signal;
forming a most significant bits portion of said monotonic count value in a register of one-time-programmable bits;
supplying both of said first and second portions of said monotonic count value in response to a read operation;
evaluating said counter to detect any roll-over event from said least significant bits portion of said monotonic count value; and
configuring said secure processing section to disable at least a portion of said data security services and exit an operational phase in response to said any roll-over event.
9. A method as claimed in claim 8 additionally comprising synchronizing said count signal and said read operation to prevent changes in said least significant bits portion of said monotonic count value during said read operation.
10. A method as claimed in claim 8 additionally comprising programming one of said one-time-programmable bits in response to said any roll-over event to reclaim said monotonic counter.
11. A method as claimed in claim 8 wherein:
said register of one-time-programmable bits is configured as a non-volatile register; and
said counter includes volatile memory elements which remain continuously energized by a battery.
12. A method as claimed in claim 8 wherein:
said counter is an L-bit counter which presents up to 2 L unique values, where L is a first integer number; and
said register is an M-bit register which presents up to M+1 unique values, where M is a second integer number.
13. A method as claimed in claim 8 additionally comprising detecting a security breach, wherein one of said one-time-programmable bits is programmed in response to detecting said security breach to reclaim said monotonic counter.
14. A method as claimed in claim 13 wherein:
said one of said one-time-programmable bits is programmed to cause said monotonic count value to count; and
said method additionally comprises using said secure computing device after detecting said security breach.
15. A method as claimed in claim 14 wherein said secure computing device executes trusted software during said evaluating operation, and said method additionally comprises:
disabling execution of said trusted software in said secure computing device in response to said any roll-over event; and
said method additionally comprises refurbishing said secure computing device after disabling execution of said trusted software to permit execution of said trusted software, wherein said one of said one-time-programmable bits is programmed during said refurbishing operation.
16. A method of operating a secure computing device having a secure processing section which provides data security services including a monotonic count value, said method comprising:
controlling a counter to produce a least significant bits portion of said monotonic count value in response to a count signal;
forming a most significant bits portion of said monotonic count value in a register of independent one-time-programmable bits;
programming one of said one-time-programmable bits;
supplying both of said least significant bits portion and most significant bits portion of said monotonic count value in response to a read operation;
synchronizing said count signal and said read operation to prevent changes in said least significant bits portion of said monotonic count value during said read operation; and
configuring said secure processing section to disable at least a portion of said data security services in response to any roll-over from said least significant bits portion of said monotonic count value.
17. A method as claimed in claim 16 wherein:
said register of independent one-time-programmable bits is configured as a non-volatile register; and
said counter includes volatile memory elements which remain continuously energized by a battery.
18. A method as claimed in claim 16 additionally comprising detecting, prior to programming said one of said one-time-programmable bits, a security breach in said secure computing device.
19. A method as claimed in claim 12 wherein said integer L is greater than said integer M.
20. A method as claimed in claim 16 wherein:
said counter is an L-bit counter which presents up to 2 L unique values, where L is a first integer number;
said register is an M-bit register which presents up to M+1 unique values, where M is a second integer number; and
said integer L is greater than said integer M.