Secure time on computers with insecure clocks

Details Secure time on computers with insecure clocks Secure time on computers with insecure clocks

1999-09-17

2004-04-27

Barrón, Gilberto (Department: 2132)

Electrical computers and digital processing systems: support

Multiple computer communication using cryptography

Particular communication authentication technique

C713S503000, C713S400000

Reexamination Certificate

active

06728880

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to time identification on a computer.
Computers generally contain clock circuits, and these local clock circuits operate to maintain time on the computer. The local clock circuits can be reset by users so that a computer's local time can be any value. Therefore, a computer may have a local time that is different from time on any other clock. In some situations, it is necessary for a computer program to obtain a trusted time, for example, Greenwich Mean Time (GMT), that is, a time that can be relied upon as being correct in the sense that it is not subject to user manipulation. Ordinarily, computers obtain a trusted time by being actively connected to a special device or to a network that can provide the trusted time in a secure way to the computer.
SUMMARY OF THE INVENTION
In general, in one aspect, the invention provides techniques that can be implemented as methods, systems, or apparatus, including computer program apparatus, for providing a trusted time. The techniques include sending a first local time from a computer to a trusted server, receiving trusted time data protected by a digital signature from the trusted server, storing the trusted time data on the computer, checking the validity of the trusted time data, and using the trusted time data to compute a trusted time. Advantageously, in one implementation, the computer has a read-only local counter securely coupled to its local clock circuit, the local counter and the local clock circuit operating to change a counter value of the local counter whenever the local clock circuit is reset in a way that resets the local time provided by the local clock circuit.
Advantages that can be seen in implementations of the invention include one or more of the following. A process running on a computer can determine a trusted time securely without being continuously connected to a special device or a network even in a situation where the computer's local clock circuit is not secure. A process running on a computer can determine securely the difference between a trusted time and a local time. A process running on a computer can check the a validity of a time difference, saved on the computer, between a trusted time and a local time. A process running on a computer can maintain the security of a document security scheme that depends on knowing a correct time, such as GMT, without requiring the computer to be tethered (i.e., actively connected) to a secure time source.


REFERENCES:
patent: 5327468 (1994-07-01), Edblad et al.
patent: 5392421 (1995-02-01), Lennartsson
patent: 5444780 (1995-08-01), Hartman, Jr.
patent: 5500897 (1996-03-01), Hartman, Jr.
patent: 5521887 (1996-05-01), Loomis
patent: 5826066 (1998-10-01), Jardine et al.

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