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TECHNICAL REPORT
CYBER;
Security techniques for protecting software
in a white box model
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2 ETSI TR 103 642 V1.1.1 (2018-10)
Reference
DTR/CYBER-0029
Keywords
security, software
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3 ETSI TR 103 642 V1.1.1 (2018-10)
Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
Introduction . 4
1 Scope . 5
2 References . 5
2.1 Normative references . 5
2.2 Informative references . 5
3 Definitions of terms and abbreviations. 7
3.1 Terms . 7
3.2 Abbreviations . 9
4 Threat security model . 10
5 Description of the different techniques . 11
5.1 Introduction . 11
5.2 White Box Cryptography (WBC) . 12
5.3 Code and data protection . 13
5.3.1 Introduction. 13
5.3.2 Anti-xxx techniques . 13
5.3.3 Code and data obfuscation . 13
5.3.4 Device binding . 15
5.3.5 Watermarking . 15
5.3.6 Software-based exploit mitigation . 15
6 Description of attacks . 16
6.1 Introduction . 16
6.2 General description of attacks on a software application . 16
6.3 Focus on attacks on white box implementations . 18
6.3.1 Cryptanalysis . 18
6.3.2 From Side-channel analysis to Differential Computation Analysis . 18
6.3.3 Fault Analysis . 18
6.3.4 CHES 2017 challenge . 19
7 Use cases . 19
7.1 Digital Rights Management . 19
7.2 Automotive . 20
7.3 Cloud-based payment . 20
8 Advantages and drawbacks . 20
9 Conclusion . 21
Annex A: Classification of the techniques for obfuscation . 22
History . 23
ETSI
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4 ETSI TR 103 642 V1.1.1 (2018-10)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (https://ipr.etsi.org/).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
Foreword
This Technical Report (TR) has been produced by ETSI Technical Committee Cyber Security (CYBER).
Modal verbs terminology
In the present document "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be
interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Introduction
When one tries to protect some sensitive data or code in a device like a mobile, one has to consider that the environment
may be compromised (or if it is not yet, it will be soon). The attacker, trying to get secrets of an application, is able to
read a lot about the software code and applications, and overrule the security measures, etc. This situation makes much
harder the protection of the assets of a mobile application.
To address this, the main strategy is to apply obfuscation and anti-tampering techniques on the software (to hide code,
data, key, etc.) and to deploy on the server side assurance function to detect fraudulent behaviour of the application
and/or the device. One needs to put in place functions to diversify the software, the protection techniques, the assets
(per user or per instance of the application) in order to prevent large scale attacks and functions for quick redeployment,
replenishment of the software.
This methodology relies on two pillars:
• White Box Cryptography (which is about protecting the cryptographic functions in the code, together with the
key); and
• Code and data protection (which is about making sure that the code, used to run the application cannot be
understood, tampered, extracted, exploited, and the data cannot be retrieved and/or modified).
By combining those countermeasures, (and having a correct security design for the application), the level of resistance
of a software application to software attack is increased.
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5 ETSI TR 103 642 V1.1.1 (2018-10)
1 Scope
The present document reports on the application of techniques for protecting software implementations, in the form of
applications and content, using software resident security techniques. The present document makes recommendations
for the application of specific techniques including white box cryptography (WBC), code obfuscation, and other
techniques denoted as anti-xxx and including anti-tampering, anti-reversing, anti-debugging, anti-cloning, etc. These
techniques address the threats presented by attackers of the forms outlined in the present document.
2 References
2.1 Normative references
Normative references are not applicable in the present document.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] C. Collberg, C. Thomborson and D. Low: "A taxonomy of obfuscating transformations",
Technical Report #148, University of Auckland, 1997.
NOTE: Available at http://profs.sci.univr.it/~giaco/download/Watermarking-
Obfuscation/Obfuscation%20Taxonomy.pdf.
[i.2] N. Eyrolles, PhD: "Obfuscation with Mixed Boolean-Arithmetic Expressions: Reconstruction,
Analysis and Simplification Tools", Cryptography and Security, Université Paris-Saclay, 2017.
NOTE: Available at https://tel.archives-ouvertes.fr/tel-01623849/.
[i.3] C. Collberg, C. Thomborson and D. Low: "Manufacturing cheap, resilient, and stealthy opaque
constructs", University of Auckland, 1998.
NOTE: Available at https://www2.cs.arizona.edu/~collberg/content/research/papers/collberg98manufacturing-
clean.pdf.
[i.4] E. Biham and A. Shamir: "Differential cryptanalysis of DES-like cryptosystems", A. Menezes and
S. A. Vanstone, Eds., CRYPTO, LNCS 537, pp. 2-21. Springer, 1990.
[i.5] A. Biryukov, C. De Cannière, A. Braeken, and B. Preneel: "A toolbox for cryptanalysis: Linear
and affine equivalence algorithms", in E. Biham, Ed., EUROCRYPT, LNCS 2656, pp. 33-50.
Springer, 2003.
[i.6] O. Billet, H. Gilbert, and C. Ech-Chatbi: "Cryptanalysis of a White Box Implementation", in
H. Handschuh and A. Hasan, Eds., Selected Areas in Cryptography 2004, LNCS 3357,
pp. 227-240. Springer, 2005.
[i.7] L. Tolhuizen: "Improved cryptanalysis of an AES implementation", Proceedings of Symposium on
Information Theory in the Benelux 2012.
[i.8] T. Lepoint, M. Rivain, Y. De Mulder, P. Roelse, and B. Preneel: "Two attacks on a white-box AES
implementation", in T. Lange, K. Lauter and P. Lisonek, Eds., Selected Areas in
Cryptography 2013, LNCS 8282, pp. 265-285, Springer, 2014.
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[i.9] W. Michiels, P. Gorissen, H.D.L. Hollmann: "Cryptanalysis of a generic class of white-box
implementations", Proceedings of Selected Areas in Cryptography (SAC) 2008, LNCS 5381,
pp. 414-428, 2009.
[i.10] Y. De Mulder, B. Wyseur, and B. Preneel: "Cryptanalysis of a perturbated white-box AES
implementation", in G. Gong and K. C. Gupta, Eds., INDOCRYPT 2010, LNCS 6498,
pp. 292-310. Springer, 2010.
[i.11] Y. De Mulder, P. Roelse, and B. Preneel: "Cryptanalysis of the Xiao-Lai white-box AES
implementation", in L. R. Knudsen and H. Wu, Eds., Selected Areas in Cryptography 2012,
LNCS 7707, pp. 34-49. Springer, 2012.
[i.12] P. Kocher, J Jaffe, and B. Jun: "Differential Power Analysis", Advances in Cryptology - Crypto
'99 Proceedings, LNCS 1666, M. Wiener, Ed., pp. 388-397, Springer 1999.
[i.13] E. Biham and A. Shamir: "Differential fault analysis of secret key cryptosystems", Advances in
Cryptology - Crypto '97 Proceedings, LNCS 1294, B. Kaliski, Ed., pp. 513-525, Springer 1997.
[i.14] J. Bos, C. Hubain, W. Michiels, and P. Teuwen: "Differential Computation Analysis: Hiding Your
White-Box Design is Not Enough", Cryptology ePrint Archive, Report 2015/753, 2015.
NOTE: Available at https://eprint.iacr.org/2015/753.
[i.15] M. Jacob, D. Boneh, E.W. Felten: "Attacking an obfuscated cipher by injecting faults. Proceedings
of security and privacy in Digital Rights Management (DRM)", LNCS 2696, pp. 16-31,
Springer, 2002.
[i.16] CHES 2017: "Capture the Flag Challenge", organised by the ECRYPT-CSA project, Submission
server developed by CryptoExperts, Submission server, hosted by TU Eindhoven.
NOTE: Available at https://ches.2017.rump.cr.yp.to/a905c99d1845f2cf373aad564ac7b5e4.pdf.
[i.17] E. Diehl: "Securing Digital Video - Techniques for DRM and Content Protection", Springer, 2012.
ISBN 978-3-642-17344-8.
[i.18] Motion Picture Laboratories Inc.: "MovieLabs Specification for Enhanced Content Protection",
Version 1.1, 2015.
[i.19] H. Benoit: "Digital Television - Satellite, Cable, Terrestrial, IPTV, Mobile TV in the DVB
rd
Framework", 3 edition. Elsevier, 2008. ISBN 978-0-240-52081-0.
[i.20] ETSI TS 100 289 (V1.1.1): "Digital Video Broadcasting (DVB); Support for use of the DVB
Scrambling Algorithm version 3 within digital broadcasting systems".
[i.21] CI Plus LLP, CI Plus Specification: "Content Security Extensions to the Common Interface",
v1.4.2, 2016.
[i.22] EMVCo: "EMV Mobile Payment - Software-based Mobile Payment Security Requirements",
Version 1.0, December 2016.
NOTE: Available at https://www.emvco.com/wp-content/EMVCo-Software-based-Mobile-Payment-Security-
Requirements_V1.0_20161213.pdf.
[i.23] ISO SC27: "Study Period on Security requirements, test and evaluation methods for White Box
Cryptography (WBC)" October 2016.
[i.24] ISO SC27: "Study Period on Security properties, test and evaluation guidance for White Box
Cryptography (WBC)", April 2018.
[i.25] FIDO Alliance, FIDO Authenticator certification program.
NOTE: Available at https://fidoalliance.org/certification/authenticator-certification-levels/.
[i.26] OWASP, Mobile TOP 10 attacks, 2016.
NOTE: Available at https://www.owasp.org/index.php/Mobile_Top_10_2016-Top_10.
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[i.27] Visa Mobile publications, Cloud-Based Payments Program.
NOTE: Available at https://technologypartner.visa.com/Mobile/MobilePublications.aspx#59.
3 Definitions of terms and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
NOTE: The definitions below are split in two paragraphs, to distinguish software protection techniques from
attacks.
anti-cloning: software protection technique to prevent execution of a binary on a non-genuine device
NOTE: Strictly speaking the binary knows fingerprints of the execution environment like the MAC address, CPU
ID, HDD serial number, etc. and verifies its integrity during execution. If the fingerprints do not match,
the device is seen as "cloned" and the program can react in several ways. This technology can help to
prevent code lifting.
anti-debugging: software protection technique to prevent debugging of an application through a debugger
NOTE: Example of debuggers are GDB, WinDBG. If a debugger is detected the program can react in several
ways like notification of the user or abortion of the execution.
anti-disassembly: method to prevent disassembling of the binary through disassemblers
NOTE: Disassemblers like IDA Pro, Binary Ninja, etc. Anti-Disassembly can be applied directly on assembler
code to let disassemblers generate wrong opcodes or on control flow level to hide the connection of the
basic blocks of a function.
BGE: named after its authors (Billet, Gilbert and Ech-Chatbi), a realistic (in terms of work factor) algebraic attack on
the white-box AES design by Chow, Eisen, Johnson and van Oorschot
NOTE: It has been further generalized to all Substitution-Permutation Network ciphers.
cloning: making an illegal copy of the contents (of the device) to a new device or for analysis
code anti-tampering: software protection technique that prevents modification of a binary
NOTE 1: If a modification of the binary is detected, the program can react in several ways like notification of the
user or abortion of the execution.
NOTE 2: Code anti-Tampering can be divided into two groups:
Static Anti Tampering: Before execution of the binary the integrity of the file will be verified and
loaded into memory if and only if the checksum is valid.
Dynamic Anti Tampering: During execution of the binary random integrity checks will be executed
to self-check if the binary was tampered.
code/data anti-reversing: set of software techniques aiming to protect an attacker having access to a program to
understand its implementation or having access to sensitive data in clear form
NOTE: See also Obfuscation.
code-flow/-pointer integrity: technique which prevents attacks that try to take control of the execution flow of the
program
NOTE: During compilation the compiler injects guards that will verify if the destination address is valid. If the
destination address is not valid the program can detect that and abort the execution.
code lifting: method where the attacker tries to reuse part of the binary code for executing it in an unauthorized way
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code obfuscation: combination of several software based techniques that transform the source or machine code into
code that is very difficult to understand for humans
NOTE: The purpose of obfuscation is to harden the resistance against attacks like reverse engineering or
tampering of the code. Obfuscation can be broken but the amount of time and the expertise to apply
attacks on obfuscation can be significant and is done by skilled experts.
code-pointer separation: technology to verify the integrity of code pointers by splitting the memory into safe memory
and regular memory.
NOTE: Code pointers are placed in the safe memory and the integrity of the memory will be verified during
execution of the binary. This technique prevents attacks where an attacker tries to take control of the
execution flow of the application and has a performance advantage over Code-flow integrity.
code watermarking: technique to deeply embed a unique watermark in a binary and that is very difficult to detect and
remove
NOTE: The watermark can be used to authenticate a genuine software but also to trace back illegal copies.
data anti-tampering: software protection technique that prevents modification of data (e.g. integrity checksum,
redundancy, etc.)
.
NOTE: Data anti-tampering can be static or dynamic
data encodings: reversible method to transform data without the requirement of a secret key
NOTE 1: Typically used in WBC.
NOTE 2: They are external encodings applied at the input and at the output of a crypto functions. They are internal
encodings applied to internal variables and/or tables. The purpose is to manipulate data not in clear form
and to avoid software side channel attacks. They are static encodings that are fixed at compilation time
and they are dynamic encodings that are changing at execution time.
data lifting: method where the attacker tries to reuse data in an unauthorized way, on behalf of the genuine data owner
NOTE: Typically re-using a WBC dynamic key from one instance to another.
data obfuscation: set of software protection techniques to hide data against an attacker trying to reverse it (i.e. find,
detect, and/or localise)
NOTE: This includes hiding sensitive data (encryption/encoding), making, splitting input data into smaller
chunks or using steganography.
data protection: techniques that try to hide data like cryptographic keys, debug strings, application strings, etc. against
an attacker and tools like "strings" or an (hex-) editor
data watermarking: technique to deeply embed a unique watermark in a data that is very difficult to detect and remove
NOTE: An example is a movie file watermarked to track back illegal copies.
device binding: software and/or a hardware technique to bind a data or code to a device, meaning if one tries to use the
bound item in another device, it will fail
Differential Computation Analysis (DCA): side-channel attack derived from DPA aiming at key extraction from a
WBC where measuring noisy power consumption is replaced by noiseless intermediate computation value collection
during a cryptographic software executions
Differential Fault Analysis (DFA): family of attacks that relies on the analysis of the difference in outputs when a
perturbation is injected during a cryptographic computation
NOTE: Such a perturbation can be a laser shot on a hardware chip, or a value modification in the context of a
software implementation.
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Differential Power Analysis (DPA): attack that exploits the information leakage of the manipulation of a secret value
by a hardware, through its power consumption
NOTE 1: Information is collected for multiple variable inputs with an oscilloscope. Difference of means is then
computed, after splitting the dataset based on a guessed value computed from the actual inputs and a key
hypothesis. The good key stands out as it correctly splits the data.
NOTE 2: Power consumption is not the only side-channel that can be used. The same attack based on electro-
magnetic radiation would be called DEMA, for example.
hooking: range of techniques used to alter or augment the behaviour of an operating system, of applications, or of other
software components by intercepting function calls or messages or events passed between software components
stack cookies: method used to protect against buffer overflows
NOTE: The compiler integrates known values, "stack cookies", between a buffer and the control data. When the
stack cookie gets overwritten during a buffer overflow the code can detect that by verifying the stack
cookie with the hardcoded value in the code and abort the execution of the program.
White Box Cryptography (WBC): set of software protection techniques aiming at protecting software
implementations of cryptographic algorithms against key recovery
NOTE: The specificity of a white-box cryptography attacker is that he is assumed to have full control over the
whole execution of an implementation. In this highly unfavourable setup, security by obscurity was the
initial industry response, with the deployment of private algorithms in real-world WBC. The academic
world effort has increased over the past years, with a focus on standard cryptographic algorithms.
3.2 Abbreviations
For the purposes of the present document, the following abbreviations apply:
AES Advanced Encryption Standard
Anti-xxx All methods starting with "anti-"
NOTE: It includes at least anti-tampering, anti-cloning, anti-reversing, and anti-debugging.
BGE Billet, Gilbert and Ech-Chatbi
NOTE: From the name of the authors of a well-known attack against a white-box implementation of AES.
CA Conditional Access
CA/DRM Conditional Access/Digital Rights Management
CBP Cloud Based Payment
CHES Cryptographic Hardware and Embedded Systems
NOTE: A conference on cryptographic hardware and embedded systems.
CPE Customer Premises Equipment
CPUID derived from CPU (Central Processing Unit) IDentification
DCA Differential Computation Analysis
DES Data Encryption Standard
DFA Differential Fault Analysis
DPA Differential Power Analysis
DRM Digital Rights Management
DVB Digital Video Broadcasting
ECI Embedded Common Interface
EMV Europay Mastercard Visa
IC Integrated Card
ISG Industry Specification Group
MAC Medium Access Control
MBA Mixed Boolean-Arithmetic
OS Operating System
OTT Over-The-Top
SE Secure Element
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SIM Subscriber Identity Module
SoC System on Chip
VCK Virtual Car Key
WBC White Box Cryptography
4 Threat security model
There are different threat security models used in the field of software security.
The classical threat model involves a malicious third party Eve attempting to decrypt the communication between Alice
and Bob. In this case, Alice and Bob are friendly parties communicating, and not attacking the system (Eve is an
external attacker). There are situations where this is not true, and where the attacker may be one of the two
communicating parties.
In a "Black-box model" (see figure 1), the attacker is assumed to have access to inputs and outputs, to be able to observe
and modify the communication (read, intercept and/or substitute). He has no access to the encryption keys, or more
generally to the system performing cryptographic operations. He has no access to the software binary or to the
execution environment internals, and may abuse the software functionality.
The implementation stays opaque ("black").
Figure 1: Black-box threat model
In a "Grey-box threat model" (see figure 2), the attacker has the same power as in the black-box model, but he also
partially knows and/or have access to the internal structure of the system performing cryptographic operations
(e.g. access to the documentation of the internal data structure, the cryptographic algorithms used). He has an indirect
access, through side channel analysis, to execution environment internals and may disrupt the software execution
through faults. In this model, the attacker does no
...