ISO/IEC 20085-2:2020

IT Security techniques — Test tool requirements and test tool calibration methods for use in testing non-invasive attack mitigation techniques in cryptographic modules — Part 2: Test calibration methods and apparatus

This document specifies the test calibration methods and apparatus used when calibrating test tools for cryptographic modules under ISO/IEC 19790 and ISO/IEC 24759 against the test metrics defined in ISO/IEC 17825 for mitigation of non-invasive attack classes.

Techniques de sécurité IT — Exigences de l'outil de test et méthodes d'étalonnage de l'outil de test utilisées pour tester les techniques d'atténuation des attaques non invasives dans les modules cryptographiques — Partie 2: Méthodes et appareillage d'étalonnage et d'essai

General Information

• Status: Published
• Publication Date: 04-Mar-2020
• ICS: 35.030 - IT Security
• Technical Committee: ISO/IEC JTC 1/SC 27 - Information security, cybersecurity and privacy protection
• Drafting Committee: ISO/IEC JTC 1/SC 27/WG 3 - Security evaluation, testing and specification
• Current Stage: 6060 - International Standard published
• Start Date: 05-Mar-2020
• Due Date: 23-Oct-2019
• Completion Date: 05-Mar-2020

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 20085-2
First edition
2020-03
IT Security techniques — Test tool
requirements and test tool calibration
methods for use in testing non-
invasive attack mitigation techniques
in cryptographic modules —
Part 2:
Test calibration methods and
apparatus
Techniques de sécurité IT — Exigences de l'outil de test et méthodes
d'étalonnage de l'outil de test utilisées pour tester les techniques
d'atténuation des attaques non invasives dans les modules
cryptographiques —
Partie 2: Méthodes et appareillage d'étalonnage et d'essai
Reference number
ISO/IEC 20085-2:2020(E)
©
ISO/IEC 2020

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ISO/IEC 20085-2:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2020
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Published in Switzerland
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ISO/IEC 20085-2:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 2
5 Test tools . 2
5.1 Tools and analysis . 2
5.2 Determining the test result . 2
5.3 Measurement tool . 2
5.4 Analysis tool . 2
6 Calibration methods . 3
6.1 Aspects . 3
6.2 Introduction to calibration procedure . 3
6.2.1 General knowledge of calibration procedure . 3
6.2.2 Accuracy of test tools . 3
6.2.3 Measurement tool . 4
6.2.4 Calibration principle . 4
6.3 Calibration procedure . 4
6.3.1 General. 4
6.3.2 Point of measurement . 5
6.3.3 Parameter adjustment . 5
6.4 Calibration metrics . 5
7 Artefact . 6
7.1 General . 6
7.2 Side-channel analysis . 6
7.3 Open target . 6
7.3.1 General. 6
7.3.2 General specification . 6
7.3.3 Example specification . 6
7.4 Closed target . 6
Annex A (informative) Cryptographic algorithms and calibration metrics .7
Annex B (informative) Countermeasures to tune the security strength .9
Annex C (informative) An example artefact implementation — A hardware security module
emulated with an FPGA .11
Annex D (informative) An example artefact implementation — A microcontroller .13
Annex E (informative) An example artefact implementation — Signal generator .15
Bibliography .16
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ISO/IEC 20085-2:2020(E)

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that
are members of ISO or IEC participate in the development of International Standards through
technical committees established by the respective organization to deal with particular fields of
technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with ISO and IEC, also
take part in the work.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for
the different types of document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject
of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent
rights. Details of any patent rights identified during the development of the document will be in the
Introduction and/or on the ISO list of patent declarations received (see www .iso .org/ patents) or the IEC
list of patent declarations received (see http:// patents .iec .ch).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 27, Information security, cybersecurity and privacy protection.
A list of all parts in the ISO/IEC 20085 series can be found on the ISO website.
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ISO/IEC 20085-2:2020(E)

Introduction
Cryptographic modules provide cryptographic services and protect critical security parameters.
Protection of critical security parameters can either be logical, physical, or both. Information such as
knowledge of critical security parameters can leak out of the cryptographic module during operation,
if the module is not designed to mitigate such leakage. Without mitigation, a malevolent attacker can
record available side-channel leakage. This leakage is a physical quantity related to the critical security
parameters and can be analysed in a manner to extract information about those parameters. Such
analysis is passive, in that it simply collects the side-channel leakage measurements which can be freely
acquired with an apparatus. Notice that the measurement tool can, as well, be adaptively controlled.
This kind of extraction and analysis is referred to as non-invasive. Techniques that allow the extraction
of critical security parameters out of this non-invasive leakage is termed an attack on the module.
Non-invasive attack testing is a method to determine whether the leakage of a cryptographic module
can be exploited to extract critical security parameters. A non-invasive attack test tool returns a pass
status if the cryptographic module leakage is determined to be of a minimal amount which may prevent
disclosure of critical security parameters. Otherwise, it returns a fail status.
This document focuses on the calibration of the side-channel measurement tool. This calibration
process enables two measurement tools to record measurements equally usable in terms of side
channel analysis. Calibration is presented as the combination of two techniques:
a) definition of a method for calibration;
b) requirement of a reference cryptographic module (called an artefact) to define a clear threshold
between test results, in terms of fail or pass.
Both aspects are covered in this document.
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INTERNATIONAL STANDARD ISO/IEC 20085-2:2020(E)
IT Security techniques — Test tool requirements and test
tool calibration methods for use in testing non-invasive
attack mitigation techniques in cryptographic modules —
Part 2:
Test calibration methods and apparatus
1 Scope
This document specifies the test calibration methods and apparatus used when calibrating test tools
for cryptographic modules under ISO/IEC 19790 and ISO/IEC 24759 against the test metrics defined in
ISO/IEC 17825 for mitigation of non-invasive attack classes.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 17825, Information technology — Security techniques — Testing methods for the mitigation of
non-invasive attack classes against cryptographic modules
ISO/IEC 19790, Information technology — Security techniques — Security requirements for
cryptographic modules
ISO/IEC 20085-1, IT Security techniques — Test tool requirements and test tool calibration methods for
use in testing non-invasive attack mitigation techniques in cryptographic modules — Part 1: Test tools and
techniques
3 Terms and definitions
For the purposes of this document, the terms, definitions given in ISO/IEC 19790, ISO/IEC 17825,
ISO/IEC 20085-1, and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
artefact
signal source which is representative of a leaking device for use when conducting calibration (3.2)
Note 1 to entry: Its purpose is to generate information from sensitive data. It mimics the leakage from an actual
cryptographic module when operated.
3.2
calibration
process of setting threshold to adequate values, such that the border between pass and fail can be
reproduced identically between different non-invasive test tools
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3.3
countermeasure
design method aiming at reducing the information leakage
3.4
security strength
number of traces to detect a leakage using ISO/IEC 17825 metrics
4 Abbreviated terms
A/D Analog/Digital
[14]
AES Advanced Encryption Standard
[15]
RSA Rivest–Shamir–Adleman
5 Test tools
5.1 Tools and analysis
A test tool for non-invasive attack mitigation for cryptographic modules consists of collecting side-
channel information in a non-invasive manner, reprocessing it (e.g. aligning and filtering), and applying
corresponding analysis methods to determine whether the target module leaks critical information.
A test tool, conforming to ISO/IEC 20085-1, consists of a data measurement tool and an analysis tool.
A single test tool can test different analysis types of attack including power analysis, electromagnetic
analysis, and timing analysis as shown in ISO/IEC 17825. The measurement tool contains a probe
that converts a particular physical quantity, such as electric current, into an electric voltage level at a
certain conversion ratio (or a more complex relationship in case the relationship is not linear), where
the measurement tool captures the voltage level with an A/D converter and stores the digital data into
internal storage at a specified capturing rate. A triggering mechanism is necessary to determine the
capture timings. Triggering can require a dedicated probe connected to a specified part of the IUT in
order to provide accurate capture timings. The analysis tool is a computer that controls the test process
as well as carries out analysis computations.
5.2 Determining the test result
The specification and operation of the test tool shall meet the requirements described in ISO/IEC 20085-1.
The purpose of the test tool is to determine whether the amount of leaked information as the result of
the side-channel analysis is below or above a given threshold M as specified in ISO/IEC 17825.
5.3 Measurement tool
The measurement tool in the non-invasive attack test can have some variations in its physical
characteristics that can influence the test results. If two different test tools use the same analysis
tool and, thus, the same test method and use different components in their measurement tools, the
test results can be different. The calibration is conducted to compensate for this difference, which is
attributed to the measurement tools.
5.4 Analysis tool
The analysis tool in the non-invasive attack test controls the measurement process and conducts the
data-processing. These can be implemented deterministically, i.e. without the influence of any variance.
There can be differences in the numeric precisions of computation, or the analysis algorithms adopted.
Therefore, the tool calibration covers the complete test tool, including the measurement tool and
analysis tool.
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ISO/IEC 20085-2:2020(E)

6 Calibration methods
6.1 Aspects
This clause discusses calibration methods.
The importance of calibration is to avoid the situation where an IUT is considered secure but in actual
operation it is not due to the insufficient quality of the measurement tools. Such a situation would be
detrimental to the trust in non-invasive attack evaluation and test methods. Calibration is a process
related to the analysed cryptographic algorithm and IUT, its countermeasures (if any), the test
equipment, and thereby provide mutual comparison and traceability.
The relationship between the calibration method and non-invasive test tool is depicted in Figure 1. The
constant M is the threshold in terms of number of measurements to extract critical security parameters
(e.g. a secret key), according to ISO/IEC 17825, and m is the number of traces required to extract critical
security parameters of the IUT.
Figure 1 — Block diagram illustrating the relationship between calibration method and non-
invasive test tool
6.2 Introduction to calibration procedure
6.2.1 General knowledge of calibration procedure
The non-invasive attack test tool calibration method specified in this clause is to enable different non-
invasive test tools acquired from different sources to have a uniform test quality and, thus, the same
test results. The calibration method shall be implemented by using two elements: the target test tool to
be calibrated and the target IUT, which is also referred to as a test target apparatus or an artefact.
NOTE This document does not preclude from using multiple artefacts. A module (Device Under Test) passes
or fails when tested by a test tool, calibrated according to one artefact. If multiple artefacts are used, they behave
identically.
Such an artefact is designed to perform a certain mitigation level against particular non-invasive
attacks. Tool developers may develop their own tools at the uniform quality achieved by calibrating
their tools with a uniformly adjusted artefact.
6.2.2 Accuracy of test tools
Each part of the test tool shall provide appropriate accuracies that meet the requirements specified in
this document. These accuracies have some intrinsic connection; for example, the changes in sampling
rate or the amount of jitter in the trigger affects the signal-to-noise ratio (S/N) in a similar manner to
additional noise in the measurement circuitry. Sampling clocks which are synchronized to the IUT clock
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[1]
can result in a considerably better S/N compared to unsynchronized sampling clocks . Unsynchronized
sampling clocks require careful consideration especially if a calibration artefact provides a clock for the
sampling device to synchronize, but the IUT does not.
The degree of precision is impacted by:
— quantization level (the number of bits that the A/D converter outputs);
— sampling rate (the frequency of data capturing), and source of sampling clock (synchronous to IUT
clock or asynchronous);
— frequency bandwidth.
The degree of accuracy is impacted by:
— error in the voltage level (including the converted figure from the electric-current);
— error in timing;
— value of the S/N (estimated as per section 5.1 of Reference [16]).
6.2.3 Measurement tool
The measurement tool is an active device, which converts some physical quantity (time, power,
electromagnetic radiation, etc.) into an electric quantity (e.g. voltage). It is characterized by its effect
on the signal: ideally, it has little distortion, high bandwidth, and low noise figure (i.e. it does reduce the
S/N in a small proportion).
6.2.4 Calibration principle
Calibration of a non-invasive attack test tool is carried out to check whether the tool determines the test
result of an IUT correctly at the given Security Level as prescribed in ISO/IEC 17825. Adjustments of the
internal parameters of the tool can be necessary so that the tool correctly determines the test result.
Calibration can be used to achieve two goals.
Goal 1: To check whether
a) an IUT which is built to pass, shall pass; and
b) an IUT which is built to fail, shall fail.
Goal 2: To enable comparative tests with different test tools
A calibration process shall use a known target artefact whose behaviour and degree of mitigation
against the subject attack methods are predetermined. The test tool to be calibrated shall be operated
and adjusted in accordance with the calibration procedure so it shows test results within the
predetermined range of the degree of mitigation against the target artefact. Thus, a single calibration
procedure deals with two different degrees of mitigation that can be provided with a single artefact
with a variable mitigation capability, or two different artefacts with slightly different degrees of
mitigation that define the acceptable range.
6.3 Calibration procedure
6.3.1 General
Calibration procedures shall be performed using defined known types of artefacts. The calibration shall
follow the test methods specified in ISO/IEC 17825 and ISO/IEC 20085-1 by testing the given artefact
as an IUT. The calibration operator can adjust the test tool parameters by the specified procedures if
necessary, so that the tool indicates the correct test results with regards to the defined mitigation level
of the artefact.
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ISO/IEC 20085-2:2020(E)

A calibration process shall repeat the test procedure twice as specified in ISO/IEC 20085-1 for the
subject Security Level by slightly changing the test parameters. The first test procedure shall repeat
cryptographic operations as needed and the test tool shall gather the necessary information, shall
analyse the data, and shall determine whether the target artefact passes or fails the test. The second
test shall be performed with a little higher or lower test parameter and indicates the test result likewise.
If the test tool shows that its determination criteria is lower (tends to indicate passes, or overlooks the
leakage) than a specified threshold, the calibration procedure of the test tool shall terminate indicating
a failure of calibration. If the determination is higher (tends to indicate fails), the test tool conditioning
parameters shall be adjusted accordingly.
The process is given in Algorithm 1 below.
Let M be the threshold trace number upon which an attack is termed successful according to
ISO/IEC 17825, that is M = 10 000 (Security Level 3) or M = 100 000 (Security Level 4).
Algorithm 1: Calibration process.
1.  Input: test tool
2.  Output: accept or reject
3.  {
4.    The security strength is incrementally increased before the
     threshold is reached;
5.    Get the number of traces m0 to detect a leakage;
6.    The security strength is incrementally decreased before the
     threshold is reached;
7.    Get the number of traces m1 to recover the secret concealed
     by the artefact;
8.    if m0 < M and m1 > M:
9.     return accept;
10.   else
11.     return reject;
12.  }

The “accept” return value indicates that the non-invasive test tool is calibrated, whereas the “reject”
return value indicates on the contrary that the non-invasive test tool is not calibrated.
The calibration illustrated in the flow chart of Algorithm 1 may be used repeatedly, until the non-
invasive test tool is calibrated.
6.3.2 Point of measurement
If probes are used, whether electrical contacts or EM probes, they shall be placed on the specified points
on the IUT as described in ISO/IEC 17825 and ISO/IEC 20085-1.
6.3.3 Parameter adjustment
Necessary adjustments may be conducted to maximize the S/N of the measurements.
6.4 Calibration metrics
Calibration metrics for particular cryptographic algorithms are specified in Annex A. These metrics
concern first-order attacks (as opposed to high-order attacks, for which there is no consensual modus
operandi). The important parameter in the measurements is the S/N of the artefact and its operating
environment.
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ISO/IEC 20085-2:2020(E)

7 Artefact
7.1 General
A non-invasive test target artefact is a known cryptographic module. Several different non-invasive
attack mitigation levels are desired. Due to the calibration principle described in 6.2.4, a non-invasive
test target artefact shall be used for the calibration purposes.
7.2 Side-channel analysis
The target artefact shall perform cryptographic operations repeatedly as needed. All the necessary
information such as cipher texts shall be provided for the non-invasive side-channel attack method
concerned. Likewise, all the necessary physical quantities shall be measured.
7.3 Open target
7.3.1 General
An open target artefact may be used for calibration as described in Clause 6. A cryptographic module
with all the design information required to determine the strength level of security against particular
attack methods is considered an open target artefact.
7.3.2 General specification
An open target artefact shall have the following properties:
— S/N properties at electrical contacts: and
— S/N properties of magnetic field at specified locations.
Guidance can be found in References [5] and/or [6].
7.3.3 Example specification
Annexes B, C, and D show example specifications that meet the general specification described in 7.3.1.
7.4 Closed target
A closed target artefact may be used for calibration as described in Clause 6. Cryptographic modules
with known strength levels of security against particular attack methods may be used as calibration
artefacts. However, the countermeasures and other implementation information may not be available.
In this case, they are called closed target artefacts.
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Annex A
(informative)

Cryptographic algorithms and calibration metrics
A.1 Overview
This annex defines the calibration metrics for specific cryptographic algorithms, Security Levels, and
attack methods.
A.2 Calibration metrics for selected cryptographic algorithms
A.2.1 AES
A.2.1.1 General
AES is a block cipher which can be typically executed in constant time. Therefore, it is particularly
vulnerable to side-channel attacks. Sub-clauses A.2.1.2 and A.2.1.3 reference how the security of an
implementation can be measured objectively.
A.2.1.2 DPA
A.2.1.2.1 General
It is possible to measure the S/N for DPA attacks. From this value, one can derive whether the device is
secure or not. According to Equation (1) in B.2., the number of measurements required to break AES is
obtained from an algorithmic constant and the given S/N.
A.2.1.2.2 Security Level 3
Security Level 3 should be tested by the application of Formula (B.3) with M = 10 000.
A.2.1.2.3 Security Level 4
Security Level 4 should be tested by the application of Formula (B.3) with M = 100 000.
A.2.1.3 SPA
A.2.1.3.1 General
For software implementations of AES (which generally ex
...