Obfuscation techniques for enhancing software security

Details Obfuscation techniques for enhancing software security Obfuscation techniques for enhancing software security

1998-06-09

2003-12-23

Smithers, Matthew (Department: 2132)

Electrical computers and digital processing systems: support

Data processing protection using cryptography

Tamper resistant

C713S152000

Reexamination Certificate

active

06668325

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods and apparatus for preventing, or at least hampering, interpretation, decoding, or reverse engineering of software. More particularly, although not exclusively, the present invention relates to methods and apparatus for increasing the structural and logical complexity of software by inserting, removing, or rearranging identifiable structure or information from the software in such a way as to exacerbate the difficulty of the process of decompilation or reverse engineering.
BACKGROUND
The nature of software renders it susceptible to analysis and copying by third parties. There have been considerable efforts to enhance software security, which have met with mixed success. Such security concerns relate to the need to prevent unauthorized copying of software and a desire to conceal programming techniques in which such techniques can be determined via reverse engineering.
Established legal avenues, such as copyright, provide a measure of legislative protection. However, enforcing legal rights created under such regimes can be expensive and time consuming. Further, the protection afforded to software under copyright does not cover programming techniques. Such techniques (i.e., the function as opposed to the form of the software) are legally difficult to protect. A reverse engineer could escape infringement by rewriting the relevant software, ab initio, based on a detailed knowledge of the function of the software in question. Such knowledge can be derived from analyzing the data structures, abstractions, and organization of the code.
Software patents provide more comprehensive protection. However, it is clearly an advantage to couple legal protection of software with technical protection.
Previous approaches to the protection of proprietary software have either used encryption-based hardware solutions or have been based on simple rearrangements of the source code structure. Hardware-based techniques are non-ideal in that they are generally expensive and are tied to a specific platform or hardware add-on. Software solutions typically include trivial code obfuscators, such as the Crema obfuscator for Java™. Some obfuscators target the lexical structure of the application and typically remove source code formatting and comments and rename variables. However, such an obfuscation technique does not provide sufficient protection against malicious reverse engineering: reverse engineering is a problem regardless of the form in which the software is distributed. Further, the problem is exacerbated when the software is distributed in hardware-independent formats that retain much or all of the information in the original source code. Examples of such formats are Java™ bytecode and the Architecture Neutral Distribution Format (ANDF).
Software development can represent a significant investment in time, effort, and skill by a programmer. In the commercial context, the ability to prevent a competitor from copying proprietary techniques can be critical.
SUMMARY
The present invention provides methods and apparatus for obfuscation techniques for software security, such as computer implemented methods for reducing the susceptibility of software to reverse engineering (or to provide the public with a useful choice). In one embodiment, a computer implemented method for obfuscating code, includes testing for completion of supplying one or more obfuscation transformations to the code, selecting a subset of the code to obfuscate, selecting an obfuscating transform to apply, applying the transformation, and returning to the completion testing step.
In an alternative embodiment, the present invention relates to a method of controlling a computer so that software running on, stored on, or manipulated by the computer exhibits a predetermined and controlled degree of resistance to reverse engineering, including applying selected obfuscating transformations to selected parts of the software, in which a level of obfuscation is achieved using a selected obfuscation transformation so as to provide a required degree of resistance to reverse engineering, effectiveness in operation of the software and size of transformed software, and updating the software to reflect the obfuscating transformations.
In a preferred embodiment, the present invention provides a computer implemented method for enhancing software security, including identifying one or more source code input files corresponding to the source software for the application to be processed, selecting a required level of obfuscation (e.g., potency), selecting a maximum execution time or space penalty (e.g., cost), reading and parsing the input files, optionally along with any library or supplemental files read directly or indirectly by the source code, providing information identifying data types, data structures, and control structures used by the application to be processed, and constructing appropriate tables to store this information, preprocessing information about the application, in response to the preprocessing step, selecting and applying obfuscating code transformations to source code objects, repeating the obfuscating code transformation step until the required potency has been achieved or the maximum cost has been exceeded, and outputting the transformed software.
Preferably, the information about the application is obtained using various static analysis techniques and dynamic analysis techniques. The static analysis techniques include inter-procedural dataflow analysis and data dependence analysis. The dynamic analysis techniques include profiling, and optionally, information can be obtained via a user. Profiling can be used to determine the level of obfuscation, which can be applied to a particular source code object. Transformations can include control transformations created using opaque constructs in which an opaque construct is any mathematical object that is inexpensive to execute from a performance standpoint, simple for an obfuscator to construct, and expensive for a deobfuscator to break. Preferably, opaque constructs can be constructed using aliasing and concurrency techniques. Information about the source application can also be obtained using pragmatic analysis, which determines the nature of language constructs and programming idioms the application contains.
The potency of an obfuscation transformation can be evaluated using software complexity metrics. Obfuscation code transformations can be applied to any language constructs: for example, modules, classes, or subroutines can be split or merged; new control and data structures can be created; and original control and data structures can be modified. Preferably, the new constructs added to the transformed application are selected to be as similar as possible to those in the source application, based on the pragmatic information gathered during preprocessing. The method can produce subsidiary files including information about which obfuscating transformations have been applied and information relating obfuscated code of the transformed application to the source software.
Preferably, the obfuscation transformations are selected to preserve the observable behavior of the software such that if P is the untransformed software, and P′ is the transformed software, P and P′ have the same observable behavior. More particularly, if P fails to terminate or terminates with an error condition, then P′ may or may not terminate, otherwise P′ terminates and produce the same output as P. Observable behavior includes effects experienced by a user, but P and P′ may run with different detailed behavior unobservable by a user. For example, detailed behavior of P and P′ that can be different includes file creation, memory usage, and network communication.
In one embodiment, the present invention also provides a deobfuscating tool adopted to remove obfuscations from an obfuscated application by use of slicing, partial evaluation, dataflow analysis, or statistical analysis.


REFERENCES:
patent: 4644493 (1987-0

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