Method for protecting on-chip memory (flash and RAM) against attacks

• US 6,331,946 B1
• Filed: 07/10/1998
• Issued: 12/18/2001
• Est. Priority Date: 07/15/1997
• Status: Expired due to Term

• Current Assignee: Memjet Technology Ltd. (Memjet Limited)
• Original Assignee: Silverbrook Research Pty Limited
• Inventors: Silverbrook, Kia, Walmsley, Simon Robert
• Primary Examiner(s): Follansbee, John A.
• Application Number:
  US09/112,737
• Time in Patent Office: 1,257 Days
• Field of Search: 365/180, 365/185.03, 365/185.04, 365/203, 365/185.22, 365/189.01, 365/185.21, 365/149, 365/226, 365/45, 365/183, 365/154, 711/103, 711/163, 711/164, 713/200
• US Class Current: 365/185.04
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• CPC Class Codes: G06F 21/79 : in semiconductor storage media, e.g. directly-addressable memories
• CPC Class Codes: G06F 21/86 : Secure or tamper-resistant housings
• CPC Class Codes: G06F 2212/2022 : Flash memory
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• CPC Class Codes: G06K 19/06037 : multi-dimensional coding
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• CPC Class Codes: G06K 7/10 : by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation
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• CPC Class Codes: G06Q 20/3829 : involving key management
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• CPC Class Codes: G11C 11/56 : using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
• CPC Class Codes: G11C 16/22 : Safety or protection circuits preventing unauthorised or accidental access to memory cells
• CPC Class Codes: H01L 23/576 : using active circuits
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• CPC Class Codes: H04L 9/32 : including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials
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• CPC Class Codes: H04N 1/2154 : the still video camera incorporating a hardcopy reproducing device, e.g. a printer
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• CPC Class Codes: H04N 2201/3269 : of machine readable codes or marks, e.g. bar codes or glyphs
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• CPC Class Codes: H05K 1/14 : Structural association of two or more printed circuits providing electric connection to or between printed circuits H05K1/11, H01R12/00
• CPC Class Codes: H05K 1/189 : characterised by the use of a flexible or folded printed circuit H05K3/326 takes precedence
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• CPC Class Codes: Y10S 977/94 : in a logic circuit

Abstract

In security applications, an authentication key may be stored in non-volatile memory on a semiconductor chip. If a supply of these chips with the same authentication key is available, the key may be cracked by successively setting bits of the key using a focussed ion beam, laser, or other means of locally modifying a chip. This invention prevents discovery of the key by storing the key in multi-level memory (e.g. 4 level flash or E-EPROM) with the minimum and maximum levels representing invalid states, and the authentication key being stored on intermediate levels.

Claims

1. A method of providing for resistance to monitoring of an memory circuit having multiple level states corresponding to different output states, said method comprising utilizing the intermediate states only for valid output states.

2. A method as claimed in claim 1 wherein said memory comprises flash memory.

3. A method as claimed in claim 1 wherein said memory includes a parity bit.