Hardware obfuscation
Encyclopedia
Hardware obfuscation is a technique by which the description or the structure of electronic hardware is modified to intentionally conceal its functionality
, which makes it significantly more difficult to reverse-engineer. In essence, it is different from digital watermarking
(where the ownership is concealed in the digital content itself), or from hardware intellectual property
(IP) watermarking where the ownership information is embedded and concealed in the description of a circuit
. It is also different from cryptography-based hardware IP protection techniques common in the design flow of Field Programmable Gate Array.
The importance of hardware watermarking has increased in the recent years due to widespread adoption of hardware IP based design practices for modern integrated circuit
s (ICs) such as system on chips
(SoCs). Major security issues associated with hardware IPs include: (a) hardware intellectual property infringement
during SoC design; (b) reverse engineering
the manufactured ICs or the IC design database (in fabrication facilities) to produce counterfeit
or clone ICs; and (c) malicious modifications of an IP through the insertion of hardware Trojan
to cause in-field functional failure
. Hardware obfuscation aims at minimizing these threats at IP or chip level by making it difficult for an adversary
to comprehend the actual functionality of a design.
Hardware obfuscation techniques can be classified into two main categories: (a) the "passive" techniques, which do not directly affect the functionality of the electronic system
, and (b) the "active" techniques, which directly alter the functionality of the system. Often the active hardware obfuscation techniques are "key-based", such that normal functionality of the obfuscated design can only be enabled by the successful application of a single pre-determined key or a sequence of secret keys at the input; otherwise the circuit operates in a mode, which exhibits incorrect functionality. This can be done by embedding a well-hidden finite state machine
(FSM) in the circuit to control the functional modes based on application of key. The technique of key-based, active hardware obfuscation is similar in principle to private-key cryptographic
approaches for information
protection, since the "key sequence" for the obfuscated design plays a similar role as the cryptographic key. The technique can be applied at different levels of hardware description, namely gate-level or register transfer level (RTL) design and hence can be used to protect soft, firm and hard IP cores. Obfuscation can also help to effectively hide security features in an IC and thus enable protection of ICs from counterfeiting and cloning in fabrication facilities.
In contrast, the passive techniques modify the circuit description in a soft form (e.g. syntactic changes), such that it becomes difficult for a human reader to understand the functionality of the circuit. These approaches typically employ either string-substitution (including variable name change, comment removal, etc.) , or structural change in the hardware description language
(HDL) description of a circuit (including loop unrolling
, register renaming
, etc.). A major shortcoming of the passive approaches is that they do not modify the black box
functionality of a circuit, and hence cannot prevent potential usage of an IP as black-box in a design. Moreover, the actual strength of such passive obfuscation is debatable, since, in general, black-box obfuscation does not exist, at least for software programs computing certain mathematical functions
.
Hardware watermarking can be used in conjunction with hardware obfuscation. In an obfuscated design, watermarking can be effective in providing a second line of defense against piracy efforts.
Function (engineering)
In engineering, a function is interpreted as a specific process, action or task that a system is able to perform .-In engineering design:In the lifecycle of engineering projects, there are usually distinguished subsequently: Requirements and Functional specification documents. The Requirements...
, which makes it significantly more difficult to reverse-engineer. In essence, it is different from digital watermarking
Digital watermarking
Digital watermarking is the process of embedding information into a digital signal which may be used to verify its authenticity or the identity of its owners, in the same manner as paper bearing a watermark for visible identification. In digital watermarking, the signal may be audio, pictures, or...
(where the ownership is concealed in the digital content itself), or from hardware intellectual property
Intellectual property
Intellectual property is a term referring to a number of distinct types of creations of the mind for which a set of exclusive rights are recognized—and the corresponding fields of law...
(IP) watermarking where the ownership information is embedded and concealed in the description of a circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
. It is also different from cryptography-based hardware IP protection techniques common in the design flow of Field Programmable Gate Array.
The importance of hardware watermarking has increased in the recent years due to widespread adoption of hardware IP based design practices for modern integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s (ICs) such as system on chips
System-on-a-chip
A system on a chip or system on chip is an integrated circuit that integrates all components of a computer or other electronic system into a single chip. It may contain digital, analog, mixed-signal, and often radio-frequency functions—all on a single chip substrate...
(SoCs). Major security issues associated with hardware IPs include: (a) hardware intellectual property infringement
Intellectual property infringement
An intellectual property infringement is the infringement or violation of an intellectual property right. There are several types of intellectual property rights, such as copyrights, patents, and trademarks...
during SoC design; (b) reverse engineering
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...
the manufactured ICs or the IC design database (in fabrication facilities) to produce counterfeit
Counterfeit
To counterfeit means to illegally imitate something. Counterfeit products are often produced with the intent to take advantage of the superior value of the imitated product...
or clone ICs; and (c) malicious modifications of an IP through the insertion of hardware Trojan
Hardware Trojan (computing)
A hardware Trojan is a malicious modification of the circuitry of an integrated circuit. A hardware Trojan is completely characterized by its physical representation and its behavior....
to cause in-field functional failure
Failure
Failure refers to the state or condition of not meeting a desirable or intended objective, and may be viewed as the opposite of success. Product failure ranges from failure to sell the product to fracture of the product, in the worst cases leading to personal injury, the province of forensic...
. Hardware obfuscation aims at minimizing these threats at IP or chip level by making it difficult for an adversary
Adversary (cryptography)
In cryptography, an adversary is a malicious entity whose aim is to prevent the users of the cryptosystem from achieving their goal...
to comprehend the actual functionality of a design.
Hardware obfuscation techniques can be classified into two main categories: (a) the "passive" techniques, which do not directly affect the functionality of the electronic system
Electronic System
Electronic System is a Belgian synthesizer group .Disco Machine is an album of seven original compositions from Dan Lacksman, who’s better known as being part of the group, TELEX. This material was first recorded in 1977.- External links :*...
, and (b) the "active" techniques, which directly alter the functionality of the system. Often the active hardware obfuscation techniques are "key-based", such that normal functionality of the obfuscated design can only be enabled by the successful application of a single pre-determined key or a sequence of secret keys at the input; otherwise the circuit operates in a mode, which exhibits incorrect functionality. This can be done by embedding a well-hidden finite state machine
Finite state machine
A finite-state machine or finite-state automaton , or simply a state machine, is a mathematical model used to design computer programs and digital logic circuits. It is conceived as an abstract machine that can be in one of a finite number of states...
(FSM) in the circuit to control the functional modes based on application of key. The technique of key-based, active hardware obfuscation is similar in principle to private-key cryptographic
Symmetric-key algorithm
Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both encryption of plaintext and decryption of ciphertext. The encryption key is trivially related to the decryption key, in that they may be identical or there is...
approaches for information
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
protection, since the "key sequence" for the obfuscated design plays a similar role as the cryptographic key. The technique can be applied at different levels of hardware description, namely gate-level or register transfer level (RTL) design and hence can be used to protect soft, firm and hard IP cores. Obfuscation can also help to effectively hide security features in an IC and thus enable protection of ICs from counterfeiting and cloning in fabrication facilities.
In contrast, the passive techniques modify the circuit description in a soft form (e.g. syntactic changes), such that it becomes difficult for a human reader to understand the functionality of the circuit. These approaches typically employ either string-substitution (including variable name change, comment removal, etc.) , or structural change in the hardware description language
Hardware description language
In electronics, a hardware description language or HDL is any language from a class of computer languages, specification languages, or modeling languages for formal description and design of electronic circuits, and most-commonly, digital logic...
(HDL) description of a circuit (including loop unrolling
Loop unwinding
Loop unwinding, also known as loop unrolling, is a loop transformation technique that attempts to optimize a program's execution speed at the expense of its binary size...
, register renaming
Register renaming
In computer architecture, register renaming refers to a technique used to avoid unnecessary serialization of program operations imposed by the reuse of registers by those operations.-Problem definition:...
, etc.). A major shortcoming of the passive approaches is that they do not modify the black box
Black box
A black box is a device, object, or system whose inner workings are unknown; only the input, transfer, and output are known characteristics.The term black box can also refer to:-In science and technology:*Black box theory, a philosophical theory...
functionality of a circuit, and hence cannot prevent potential usage of an IP as black-box in a design. Moreover, the actual strength of such passive obfuscation is debatable, since, in general, black-box obfuscation does not exist, at least for software programs computing certain mathematical functions
Function (mathematics)
In mathematics, a function associates one quantity, the argument of the function, also known as the input, with another quantity, the value of the function, also known as the output. A function assigns exactly one output to each input. The argument and the value may be real numbers, but they can...
.
Hardware watermarking can be used in conjunction with hardware obfuscation. In an obfuscated design, watermarking can be effective in providing a second line of defense against piracy efforts.