National foreword

This British Standard is the UK implementation of ISO/IEC 18031:2025. It supersedes BS ISO/IEC 18031:2011+A1:2017, which is withdrawn.

The UK participation in its preparation was entrusted to Technical Committee IST/33/2, Cryptography and Security Mechanisms.

A list of organizations represented on this committee can be obtained on request to its committee manager.

Contractual and legal considerations

This publication has been prepared in good faith, however no representation, warranty, assurance or undertaking (express or implied) is or will be made, and no responsibility or liability is or will be accepted by BSI in relation to the adequacy, accuracy, completeness or reasonableness of this publication. All and any such responsibility and liability is expressly disclaimed to the full extent permitted by the law.

This publication is provided as is, and is to be used at the recipient’s own risk.

The recipient is advised to consider seeking professional guidance with respect to its use of this publication.

This publication is not intended to constitute a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

This British Standard was published under the authority of the Standards Policy and Strategy Committee on 28 February 2025.

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 or www.iec.ch/members_experts/refdocs).

ISO and IEC draw attention to the possibility that the implementation of this document may involve the use of (a) patent(s). ISO and IEC take no position concerning the evidence, validity or applicability of any claimed patent rights in respect thereof. As of the date of publication of this document, ISO and IEC had not received notice of (a) patent(s) which may be required to implement this document. However, implementers are cautioned that this may not represent the latest information, which may be obtained from the patent database available at www.iso.org/patents and https://patents.iec.ch. ISO and IEC shall not be held responsible for identifying any or all such patent rights.

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. In the IEC, see www.iec.ch/understanding-standards.

This document was prepared by Joint Technical Committee ISO/IEC/JTC 1 Information Technology, Subcommittee SC 27, Information security, cybersecurity and privacy protection.

This third edition cancels and replaces the second edition (ISO/IEC 18031:2011), which has been technically revised. It also incorporates the Amendment ISO/IEC 18031:2011/Amd 1:2017 and the Technical Corrigendum ISO/IEC 18031:2011/Cor 1:2014.

The main changes are as follows:

removal of the MQ_DRBG, Micali-Schnorr DRBG, Dual_EC_DRBG and SHA-1;
addition and harmonization of the terms and definitions in Clause 3;
addition of conversion methods for random number generation;
update of the requirements for DRBGs and NRBGs.

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 and www.iec.ch/national-committees.

Introduction

This document sets out specific requirements that, when met, will result in the development of a random bit generator that can be applicable to cryptographic applications.

Numerous cryptographic applications involve the use of random bits. These cryptographic applications include the following:

random keys and initialization values (IVs) for encryption,
random keys for keyed MAC algorithms,
random private keys for digital signature algorithms,
random values to be used in entity authentication mechanisms,
random values to be used in key-establishment protocols,
random PINs and passwords,
nonces.

The purpose of this document is to establish a conceptual model, terminology and requirements related to the building blocks and properties of systems used for random bit generation in or for cryptographic applications.

It is possible to categorize random bit generators into two types, namely, non-deterministic and deterministic random bit generators.

A non-deterministic random bit generator can be defined as a random bit generating mechanism that continuously uses a source of entropy to generate a random bit stream.

A deterministic random bit generator can be defined as a bit generating mechanism that uses deterministic mechanisms such as cryptographic algorithms to generate a random bit stream. In this type of bit stream generation, there is a specific input (normally called a seed) and perhaps some optional input, which, depending on its application, can either be publicly available or not. The seed is processed by a function which provides an output.

NOTE This document also discusses hybrid random bit generators, which incorporate elements of both non-deterministic and deterministic generators.

In this document, variable symbols and variable descriptive terms are given in italic font.

1 Scope

This document specifies a conceptual model for a random bit generator for cryptographic purposes, together with the elements of this model.

This document specifies the characteristics of the main elements required for both non-deterministic and deterministic random bit generators. It also establishes the security requirements for both non-deterministic and deterministic random bit generators.

Techniques for statistical testing of random bit generators for the purposes of independent verification or validation and detailed designs for such generators are outside the scope of this document.

2 Normative references

3 Terms and definitions

4 Symbols

5 Properties and requirements of a random bit generator

5.1 Properties of arandom bit generator

5.2 Requirements of an RBG

5.3 Additional information for an RBG

6 RBG model

6.1 Conceptual functional model for random bit generation

6.2 RBG basic components

7 Types of RBGs

7.1 Introduction to the types of RBGs

7.2 Non-deterministic random bit generators

7.3 Deterministic random bit generators

7.4 The RBG spectrum

8 Overview and requirements for an NRBG

8.1 NRBG overview

8.2 Functional model of an NRBG

8.3 NRBG entropy sources

8.4 NRBG additional inputs

8.5 NRBG internal state

8.6 NRBG internal state transition functions

8.7 NRBG output generation function

8.8 NRBG health tests

8.9 NRBG component interaction

9 Overview and requirements for a DRBG

9.1 DRBG overview

9.2 Functional model of a DRBG

9.3 DRBG randomness source

9.4 Additional inputs for a DRBG

9.5 Internal state for a DRBG

9.6 Internal state transition function for a DRBG

9.7 Output generation function for a DRBG

9.8 Health tests for a DRBG

9.9 Additional requirements for DRBG keys

Bibliography

BS ISO/IEC 18031:2025

Information technology. Security techniques. Random bit generation

Current

Published:

28 Feb 2025