Information security — Encryption algorithms — Part 1: General

This document is general in nature and provides definitions that apply in subsequent parts of the ISO/IEC 18033 series. It introduces the nature of encryption and describes certain general aspects of its use and properties.

Sécurité de l'information — Algorithmes de chiffrement — Partie 1: Généralités

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Published
Publication Date
09-Sep-2021
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6060 - International Standard published
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10-Sep-2021
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10-Sep-2021
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INTERNATIONAL ISO/IEC
STANDARD 18033-1
Third edition
2021-09
Information security — Encryption
algorithms —
Part 1:
General
Sécurité de l'information — Algorithmes de chiffrement —
Partie 1: Généralités
Reference number
ISO/IEC 18033-1:2021(E)
ISO/IEC 2021
---------------------- Page: 1 ----------------------
ISO/IEC 18033-1:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO/IEC 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 18033-1:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 5

5 Nature of encryption ........................................................................................................................................................................................ 5

5.1 Purpose of encryption ...................................................................................................................................................................... 5

5.2 Symmetric and asymmetric encryption systems ...................................................................................................... 6

5.3 Key management ................................................................................................................................................................................... 6

6 Use and properties of encryption ....................................................................................................................................................... 6

6.1 General ........................................................................................................................................................................................................... 6

6.2 Asymmetric encryption systems ............................................................................................................................................. 7

6.3 Block ciphers ............................................................................................................................................................................................ 7

6.3.1 General...................................................................................................................................................................................... 7

6.3.2 Modes of operation ........................................................................................................................................................ 7

6.3.3 Message authentication codes (MACs) ......................................................................................................... 7

6.4 Stream ciphers ........................................................................................................................................................................................ 8

6.5 Identity-based encryption systems ....................................................................................................................................... 8

6.6 Homomorphic encryption systems ....................................................................................................................................... 8

7 Object identifiers ................................................................................................................................................................................................. 8

Annex A (informative) Criteria for submission of encryption systems for possible inclusion

in the ISO/IEC 18033 series ...................................................................................................................................................................... 9

Annex B (informative) Criteria for the deletion of encryption systems from

the ISO/IEC 18033 series ..........................................................................................................................................................................14

Annex C (informative) Attacks on encryption algorithms ...........................................................................................................15

Bibliography .............................................................................................................................................................................................................................18

© ISO/IEC 2021 – All rights reserved iii
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ISO/IEC 18033-1:2021(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 or www .iec .ch/ members

_experts/ refdocs).

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 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. 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 18033-1:2015), which has been

technically revised. The main changes compared with the previous edition are as follows:

— Clause 3 has been refined;

— criteria for submission of encryption systems have been refined for possible inclusion in the

ISO/IEC 18033 series; and
— the use and security properties of encryption algorithms have been clarified.
A list of all parts in the ISO/IEC 18033 series can be found on the ISO website.

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.
iv © ISO/IEC 2021 – All rights reserved
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ISO/IEC 18033-1:2021(E)
Introduction

The ISO/IEC 18033 series specifies encryption systems for the purpose of data confidentiality. The

inclusion of encryption systems in this document is intended to promote their use as reflecting the

current state of the art in encryption systems.

The primary purpose of encryption systems is to protect the confidentiality of stored or transmitted

data. An encryption algorithm is applied to data (often called plaintext) to yield encrypted data (or

ciphertext). This process is known as encryption. The encryption algorithm should be designed so that

the ciphertext yields no information about the plaintext except, perhaps, its length. Associated with

every encryption algorithm is a corresponding decryption algorithm, which transforms ciphertext

back into its original plaintext.

Encryption systems work in association with a key. In a symmetric encryption system, the same key is

used in both the encryption and decryption algorithms. In an asymmetric encryption system, different

but related keys are used for encryption and decryption. ISO/IEC 18033-2 and ISO/IEC 18033-5 focus

on two different classes of asymmetric encryption systems, known as conventional asymmetric

encryption systems (or just asymmetric encryption systems), and identity-based encryption systems.

ISO/IEC 18033-3 and ISO/IEC 18033-4 focus on two different classes of symmetric encryption systems,

known as block ciphers and stream ciphers. ISO/IEC 18033-6 focuses on a specific class of encryption

systems called homomorphic.
© ISO/IEC 2021 – All rights reserved v
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INTERNATIONAL STANDARD ISO/IEC 18033-1:2021(E)
Information security — Encryption algorithms —
Part 1:
General
1 Scope

This document is general in nature and provides definitions that apply in subsequent parts of the

ISO/IEC 18033 series.

It introduces the nature of encryption and describes certain general aspects of its use and properties.

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 18033-2, Information technology — Security techniques — Encryption algorithms — Part 2:

Asymmetric ciphers

ISO/IEC 18033-3, Information technology — Security techniques — Encryption algorithms — Part 3: Block

ciphers

ISO/IEC 18033-4, Information technology — Security techniques — Encryption algorithms — Part 4:

Stream ciphers

ISO/IEC 18033-5, Information technology — Security techniques — Encryption algorithms — Part 5:

Identity-based ciphers

ISO/IEC 18033-6, IT Security techniques — Encryption algorithms — Part 6: Homomorphic encryption

ISO/IEC 18033-7, Information technology — Security techniques — Encryption algorithms — Part 7:

Tweakable block ciphers
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology 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
asymmetric cryptographic technique

cryptographic technique that uses two related transformations, a public transformation [defined by

the public key (3.22)] and a private transformation [defined by the private key (3.21)]

Note 1 to entry: The two transformations have the property that, given the public transformation, it is

computationally infeasible to derive the private transformation. Computational feasibility depends on the

specific security requirements and environment.

[SOURCE: ISO/IEC 11770-1:2010, 2.1, modified — The last sentence in note 1 to entry has been added.]

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ISO/IEC 18033-1:2021(E)
3.2
asymmetric encryption system
asymmetric cipher
asymmetric encipherment system

system based on asymmetric cryptographic techniques (3.1) whose public transformation is used for

encryption (3.11) and whose private transformation is used for decryption (3.9)
Note 1 to entry: A method for key (3.17) pair generation is assumed.

[SOURCE: ISO/IEC 9798-1:2010, 3.2, modified — The admitted terms "asymmetric cipher" and

"asymmetric encipherment system" and note 1 to entry have been added.]
3.3
attack

algorithm that performs computations and that can request the encryption (3.11) and/or decryption

(3.9) of adaptively chosen texts under a single secret key (3.25)/private key (3.21), with the purpose of

recovering either the unknown plaintext (3.20) for a given ciphertext (3.7), which may be adaptively

chosen but for which a request to decrypt the ciphertext is not issued, or a secret key/private key

Note 1 to entry: Attacks are discussed in detail in Annex C.
3.4
attack cost

ratio of the average workload of the attack (3.3) to an equivalent number of calls to the encryption

algorithm (3.12) under attack multiplied by the success probability of the attack

Note 1 to entry: Using the notation defined in Clause 4, the attack cost is equal to the ratio W/P.

Note 2 to entry: Other attack cost metrics and properties, such as memory complexity, data complexity, the

ability to be accelerated by specialized hardware or parallelizability may also be important in judging the impact

of a cryptographic attack.
3.5
block
string of bits of a defined length
3.6
block cipher

symmetric encryption system (3.29) with the property that the encryption algorithm (3.12) operates on a

block (3.5) of plaintext (3.20) to yield a block of ciphertext (3.7)

Note 1 to entry: The block ciphers standardized in ISO/IEC 18033-3 have the property that plaintext and

ciphertext blocks are of the same length.
3.7
ciphertext
data which has been transformed to hide its information content
3.8
cryptanalytic attack
attack (3.3) against a cipher (3.13) that makes use of properties of the cipher

Note 1 to entry: Every cryptanalytic attack has its own attack model, some of which may or may not be applicable

to specific implementations. Since the application of a cipher is generally unknown to the cipher designer, all

possible models in the single key (3.17) setting need to be considered when assessing the security of an algorithm.

Several existing application examples also show the need to consider multi-key settings.

Note 2 to entry: Cryptanalytic attacks do not include implementation-specific attacks, e.g. involving side channel

analysis.
2 © ISO/IEC 2021 – All rights reserved
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ISO/IEC 18033-1:2021(E)
3.9
decryption
decipherment
reversal of a corresponding encryption (3.11)

[SOURCE: ISO/IEC 11770-1:2010, 2.6, modified — The admitted term "decipherment" has been added;

note 1 to entry has been removed.]
3.10
decryption algorithm
decipherment algorithm
process which transforms ciphertext (3.7) into plaintext (3.20)
3.11
encryption
encipherment

(reversible) transformation of data by an encryption algorithm (3.12) to produce ciphertext (3.7), i.e. to

hide the information content of the data
3.12
encryption algorithm
encipherment algorithm
process which transforms plaintext (3.20) into ciphertext (3.7)
3.13
encryption system
encipherment system
cipher

cryptographic technique used to protect the confidentiality of data, and which consists of three

component processes: an encryption algorithm (3.12), a decryption algorithm (3.10), and a method for

generating keys (3.17)
3.14
generic attack

attack (3.3) against an encryption system (3.13) which does not rely on the encryption system design

and can be used to recover a secret key (3.25)/private key (3.21) or plaintext (3.20)

Note 1 to entry: Generic attacks depend on models and goals, see Clause A.2 for details.

3.15
homomorphic encryption system
homomorphic cipher
homomorphic encipherment system

encryption system (3.13) with the property that if certain computations are performed on the ciphertext

(3.7), the plaintext (3.20) obtained after decryption (3.9) will have had the same computations applied

to it
3.16
identity-based encryption system
identity-based cipher

asymmetric encryption system (3.2) in which the encryption algorithm (3.12) takes an arbitrary string as

a public key (3.22)

[SOURCE: ISO/IEC 18033-5:2015, 3.6, modified — The preferred term has been changed to "identity-

based encryption system"; "identity-based cipher" has been changed to an admitted term; "asymmetric

cipher" has been changed to "asymmetric encryption system".]
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ISO/IEC 18033-1:2021(E)
3.17
key

sequence of symbols that controls the operation of a cryptographic transformation [e.g. encryption

(3.11), decryption (3.9)]

[SOURCE: ISO/IEC 11770-1:2010, 2.12, modified — The list of cryptographic mechanisms has been

removed.]
3.18
keystream

pseudorandom sequence of symbols, intended to be secret, used by the encryption (3.11) and decryption

algorithms (3.10) of a stream cipher (3.27)

Note 1 to entry: If a portion of the keystream is known by an attacker, then it shall be computationally infeasible

for the attacker to deduce more than a negligible amount of information about the remainder of the keystream.

Computational feasibility depends on the specific security requirements and environment.

3.19
n-bit block cipher

block cipher (3.6) with the property that plaintext (3.20) blocks (3.5) and ciphertext (3.7) blocks are n

bits in length
3.20
plaintext
cleartext
unencrypted information
3.21
private key
key (3.17) of an entity’s key pair which is known only by that entity

[SOURCE: ISO/IEC 9594-8:2020, 3.5.50, modified — The parenthesis "(In a public-key cryptosystem)",

"That" at the beginning of the definition and the period at the end have been deleted.]

3.22
public key
key (3.17) of an entity’s key pair which is publicly known

[SOURCE: ISO/IEC 9594-8:2020, 3.5.57, modified — “That” at the beginning of the definition and the

period at the end have been deleted.]
3.23
public-key certificate

public key (3.22) of an entity, together with some other information, rendered unforgeable by digital

signature with the private key (3.21) of the certification authority that issued it

[SOURCE: ISO/IEC 9594-8:2020, 3.5.58, modified — “The” at the beginning of the definition, the

parenthesis “(CA)” and the period at the end have been deleted.]
3.24
public-key infrastructure
PKI

infrastructure able to support the management of public keys (3.22) able to support authentication,

encryption (3.11), integrity or non-repudiation services

[SOURCE: ISO/IEC 9594-8:2020, 3.5.60, modified — “The” at the beginning of the definition and the

period at the end have been deleted.]
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ISO/IEC 18033-1:2021(E)
3.25
secret key

key (3.17) used with symmetric cryptographic techniques (3.28) by a specified set of entities

[SOURCE: ISO/IEC 11770-3:2015, 3.36]
3.26
security strength

number associated with the amount of work (e.g. the number of operations) that is required to break a

cryptographic algorithm

Note 1 to entry: For key (3.17) recovery, a security strength of k bits implies that the workload required to break

the cipher (3.13) is equivalent to 2 executions of the cipher. For further information on the application of security

strength to selecting cryptographic algorithms for this document, see C.1.4.
3.27
stream cipher

symmetric encryption system (3.29) with the property that the encryption algorithm (3.12) involves

combining a sequence of plaintext (3.20) symbols with a sequence of keystream (3.18) symbols one

symbol at a time, using an invertible function

Note 1 to entry: Two types of stream cipher can be identified: synchronous stream ciphers and self-synchronous

stream ciphers, distinguished by the method used to obtain the keystream.
3.28
symmetric cryptographic technique
cryptographic technique for which all transformations use the same key (3.17)
3.29
symmetric encryption system
symmetric encipherment system
symmetric cipher
encryption system (3.13) based on symmetric cryptographic techniques (3.28)
4 Symbols and abbreviated terms
ECB electronic code book
k key length
MAC message authentication code
n plaintext/ciphertext block length for a block cipher
P probability that a cryptanalytic attack will succeed

W workload or complexity of an attack, measured in terms of the number of calls to the cryp-

tographic algorithm
5 Nature of encryption
5.1 Purpose of encryption

The primary purpose of encryption systems is to protect the confidentiality of stored or transmitted

data. Encryption algorithms achieve this by transforming plaintext into ciphertext, from which it is

computationally infeasible to find any information about the content of the plaintext unless the secret/

private key is also known. However, in many cases, the length of the ciphertext is not concealed by

encryption, since the length of the ciphertext is typically the same as, or a little larger than, the length

of the corresponding plaintext.
© ISO/IEC 2021 – All rights reserved 5
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ISO/IEC 18033-1:2021(E)

It is important to note that encryption does not always, by itself, protect the integrity or the origin

of data. In many cases, it is possible, without knowledge of the key, to modify encrypted text with

predictable effects on the recovered plaintext. In order to ensure integrity and origin of data it is

often necessary to use additional techniques, such as those described in ISO/IEC 9796 (all parts),

ISO/IEC 9797 (all parts), ISO/IEC 14888 (all parts), ISO/IEC 19772, ISO/IEC 29192-2, ISO/IEC 29192-3

and ISO/IEC 29192-4.
5.2 Symmetric and asymmetric encryption systems

Symmetric and asymmetric encryption systems differ in their method of key generation.

— In a symmetric encryption system, the same secret key is used with both the encryption and

decryption algorithms. Knowledge of this key is required to perform both encryption and decryption,

and knowledge of the secret key therefore needs to be restricted to those parties authorized to

access the data which the key is used to encrypt.

— In an asymmetric encryption system, different but related keys are used for encryption and

decryption. Hence, keys are generated in matching pairs, where one key of the pair is the encryption

key and the other is the decryption key. Even with knowledge of the encryption key, it is assumed

to be computationally infeasible to find any information about the content of a plaintext from its

corresponding ciphertext. In many situations, it is possible to make the encryption key public.

Hence, this key is often referred to as the public key. The corresponding decryption key typically

has only one owner and remains confidential. Hence, it is referred to as the private key. Anyone who

knows the public encryption key is able to encrypt data intended for the holder of the corresponding

private key, while only the private decryption key holder is able to decrypt it.
5.3 Key management

The use of all types of cryptography relies on the management of cryptographic keys. All encryption

systems, both symmetric and asymmetric, require all the parties using the cipher to have access

to the necessary keys. This gives rise to the need for key management, involving the generation,

distribution, and ongoing management of keys. An overall framework for key management is given in

ISO/IEC 11770-1.

The problem of key management is rather different depending on whether the keys are for symmetric

or asymmetric encryption systems. For symmetric encryption systems, it is necessary to arrange

for secret keys to be generated and shared by pairs (or larger groups) of entities. For asymmetric

encryption systems, it is necessary for key pairs to be generated and for public keys to be distributed in

such a way that their authenticity is guaranteed. In an identity-based encryption system, the public key

is an arbitrary data string, which is usually chosen from some public information associated with the

entity which decrypts ciphertexts.

Methods to establish shared secret keys using symmetric cryptographic techniques are specified in

ISO/IEC 11770-2. Methods to establish shared secret keys using asymmetric cryptographic techniques

are specified in ISO/IEC 11770-3. ISO/IEC 11770-3 also specifies techniques for the reliable distribution

of public keys for asymmetric cryptographic techniques. Methods to establish shared secret keys using

weak secrets are specified in ISO/IEC 11770-4.
6 Use and properties of encryption
6.1 General

The criteria used for submission of encryption systems for possible inclusion in, and for their deletion

from, the ISO/IEC 18033 series are defined in Annexes A and B.
6 © ISO/IEC 2021 – All rights reserved
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ISO/IEC 18033-1:2021(E)
6.2 Asymmetric encryption systems

The encryption algorithm for an asymmetric encryption system defines a mapping from the set of

permissible plaintext messages (typically a set of bit strings) to the set of ciphertext messages (typically

also a set of bit strings). The set of permissible messages and the set of ciphertexts depends on both the

choice of encryption system and the key pair.

For an asymmetric encryption system, the encryption algorithm depends on a public key, whereas

decryption depends on a private key. Hence, while the ciphertext block corresponding to a chosen

plaintext block can be readily computed, it shall be infeasible for anyone, other than the holder of the

private key, to deduce the plaintext block corresponding to a chosen ciphertext block. However, if an

interceptor of ciphertext knows the public key used to produce it, and also knows that the plaintext

has been chosen from a small set of possibilities, it can become possible to deduce the plaintext by an

exhaustive search through all possible plaintexts.

As a result, and in order to achieve a satisfactory level of security, it is necessary to incorporate

random data in the encryption process so that the ciphertext block corresponding to a given plaintext

block cannot be predicted. Detailed techniques for incorporating random data are described in

ISO/IEC 18033-2.

Authenticity of public keys is of great importance when using asymmetric encryption algorithms.

Assurance in the authenticity of a public key can, for example, be provided using a PKI.

6.3 Block ciphers
6.3.1 General

A block cipher is a symmetric encryption system with the property that the encryption algorithm

operates on blocks of plaintext to yield ciphertext blocks. Each key for a block cipher defines a particular

invertible mapping of plaintext blocks to ciphertext blocks (and a corresponding inverse mapping used

for decryption). If, as is typically the case, the plaintext blocks and ciphertext blocks are all blocks of n

binary digits, then each key simply defines a permutation on the set of all n-bit blocks.

Block ciphers can be used in a wide variety of ways. Two of the most important applications are the

modes of operation described in 6.3.2 (modes that provide confidentiality) and 6.3.3 (modes that provide

integrity control), but there are many other uses such as in hash-functions (see ISO/IEC 10118-2) and

random-number generators (see ISO/IEC 18031).

The criteria used for submission of encryption systems for possible inclusion in, and for their deletion

from, the ISO/IEC 18033 series are defined in Annexes A and B.
6.3.2 Modes of operation

There are many ways in which an n-bit block cipher can be used to encipher plaintext. Such methods are

known as modes of operation for block ciphers. Modes of operation are defined in ISO/IEC 10116. If the

number of bits in the plaintext happens to be n, then encryption can be achieved by simply applying the

encryption process to this block, an encryption mode known as electronic code book (ECB). However,

for arbitrary length plaintext, it is necessary to employ a more sophisticated approach. For this and

other reasons, it is often necessary to use one of the other modes of operation defined in ISO/IEC 10116.

6.3.3 Message authentication
...

INTERNATIONAL ISO/IEC
STANDARD 18033-1
Third edition
Information security — Encryption
algorithms —
Part 1:
General
Partie 1: Généralités
PROOF/ÉPREUVE
Reference number
ISO/IEC 18033-1:2021(E)
ISO/IEC 2021
---------------------- Page: 1 ----------------------
ISO/IEC 18033-1:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO/IEC 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 18033-1:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 5

5 Nature of encryption ........................................................................................................................................................................................ 5

5.1 Purpose of encryption ...................................................................................................................................................................... 5

5.2 Symmetric and asymmetric encryption systems ...................................................................................................... 6

5.3 Key management ................................................................................................................................................................................... 6

6 Use and properties of encryption ....................................................................................................................................................... 6

6.1 General ........................................................................................................................................................................................................... 6

6.2 Asymmetric encryption systems ............................................................................................................................................. 7

6.3 Block ciphers ............................................................................................................................................................................................ 7

6.3.1 General...................................................................................................................................................................................... 7

6.3.2 Modes of operation ........................................................................................................................................................ 7

6.3.3 Message authentication codes (MACs) ......................................................................................................... 7

6.4 Stream ciphers ........................................................................................................................................................................................ 8

6.5 Identity-based ciphers ......... ............................................................................................................................................................. 8

6.6 Homomorphic encryption systems ....................................................................................................................................... 8

7 Object identifiers ................................................................................................................................................................................................. 8

Annex A (informative) Criteria for submission of encryption systems for possible inclusion

in the ISO/IEC 18033 series ...................................................................................................................................................................... 9

Annex B (informative) Criteria for the deletion of encryption systems from

the ISO/IEC 18033 series ..........................................................................................................................................................................14

Annex C (informative) Attacks on encryption algorithms ...........................................................................................................15

Bibliography .............................................................................................................................................................................................................................18

© ISO/IEC 2021 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO/IEC 18033-1:2021(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 or www .iec .ch/ members

_experts/ refdocs).

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 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. 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 18033-1:2015), which has been

technically revised. The main changes compared with the previous edition are as follows:

— Clause 3 has been refined;

— criteria for submission of ciphers have been refined for possible inclusion in the ISO/IEC 18033

series; and
— the use and security properties of encryption algorithms have been clarified.
A list of all parts in the ISO/IEC 18033 series can be found on the ISO website.

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.
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ISO/IEC 18033-1:2021(E)
Introduction

The ISO/IEC 18033 series specifies ciphers for the purpose of data confidentiality. The inclusion of

ciphers in this document is intended to promote their use as reflecting the current state of the art in

encryption techniques.

The primary purpose of encryption techniques is to protect the confidentiality of stored or transmitted

data. An encryption algorithm is applied to data (often called plaintext) to yield encrypted data (or

ciphertext). This process is known as encryption. The encryption algorithm should be designed so that

the ciphertext yields no information about the plaintext except, perhaps, its length. Associated with

every encryption algorithm is a corresponding decryption algorithm, which transforms ciphertext

back into its original plaintext.

Ciphers work in association with a key. In a symmetric encryption system, the same key is used in both

the encryption and decryption algorithms. In an asymmetric encryption system, different but related

keys are used for encryption and decryption. ISO/IEC 18033-2 and ISO/IEC 18033-5 focus on two

different classes of asymmetric encryption systems, known as conventional asymmetric encryption

systems (or just asymmetric encryption systems), and identity-based ciphers. ISO/IEC 18033-3 and

ISO/IEC 18033-4 focus on two different classes of symmetric encryption systems, known as block

ciphers and stream ciphers. ISO/IEC 18033-6 focuses on a specific class of encryption algorithms called

homomorphic.
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INTERNATIONAL STANDARD ISO/IEC 18033-1:2021(E)
Information security — Encryption algorithms —
Part 1:
General
1 Scope

This document is general in nature and provides definitions that apply in subsequent parts of the

ISO/IEC 18033 series.

It introduces the nature of encryption and describes certain general aspects of its use and properties.

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 18033-2, Information technology — Security techniques — Encryption algorithms — Part 2:

Asymmetric ciphers

ISO/IEC 18033-3, Information technology — Security techniques — Encryption algorithms — Part 3: Block

ciphers

ISO/IEC 18033-4, Information technology — Security techniques — Encryption algorithms — Part 4:

Stream ciphers

ISO/IEC 18033-5, Information technology — Security techniques — Encryption algorithms — Part 5:

Identity-based ciphers

ISO/IEC 18033-6, IT Security techniques — Encryption algorithms — Part 6: Homomorphic encryption

ISO/IEC 18033-7, Information technology — Security techniques — Encryption algorithms — Part 7:

Tweakable block ciphers
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology 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
asymmetric cryptographic technique

cryptographic technique that uses two related transformations, a public transformation [defined by

the public key (3.22)] and a private transformation [defined by the private key (3.21)]

Note 1 to entry: The two transformations have the property that, given the public transformation, it is

computationally infeasible to derive the private transformation. Computational feasibility depends on the

specific security requirements and environment.

[SOURCE: ISO/IEC 11770-1:2010, 2.1, modified — The last sentence in note 1 to entry has been added.]]

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ISO/IEC 18033-1:2021(E)
3.2
asymmetric encryption system
asymmetric cipher
asymmetric encipherment system

system based on asymmetric cryptographic techniques (3.1) whose public transformation is used for

encryption (3.11) and whose private transformation is used for decryption (3.9)
Note 1 to entry: A method for key (3.17) pair generation is assumed.

[SOURCE: ISO/IEC 9798-1:2010, 3.2, modified — The admitted terms "asymmetric cipher" and

"asymmetric encipherment system" and note 1 to entry have been added.]
3.3
attack

algorithm that performs computations and that can request the encryption (3.11) and/or decryption

(3.9) of adaptively chosen texts under a single secret key (3.25)/private key (3.21), with the purpose of

recovering either the unknown plaintext (3.20) for a given ciphertext (3.7), which may be adaptively

chosen but for which a request to decrypt the ciphertext is not issued, or a secret key/private key

Note 1 to entry: Attacks are discussed in detail in Annex C.
3.4
attack cost

ratio of the average workload of the attack (3.3) to an equivalent number of calls to the encryption

algorithm (3.12) under attack multiplied by the success probability of the attack

Note 1 to entry: Using the notation defined in Clause 4, the attack cost is equal to the ratio W/P.

Note 2 to entry: Other attack cost metrics and properties, such as memory complexity, data complexity, the

ability to be accelerated by specialized hardware or parallelizability may also be important in judging the impact

of a cryptographic attack.
3.5
block
string of bits of a defined length
3.6
block cipher

symmetric encryption (3.11) system with the property that the encryption algorithm (3.12) operates on

a block (3.5) of plaintext (3.20) to yield a block of ciphertext (3.7)

Note 1 to entry: The block ciphers standardized in ISO/IEC 18033-3 have the property that plaintext and

ciphertext blocks are of the same length.
3.7
ciphertext
data which has been transformed to hide its information content
3.8
cryptanalytic attack
attack (3.3) against a cipher (3.13) that makes use of properties of the cipher

Note 1 to entry: Every cryptanalytic attack has its own attack model, some of which may or may not be applicable

to specific implementations. Since the application of a cipher is generally unknown to the cipher designer, all

possible models in the single key (3.17) setting need to be considered when assessing the security of an algorithm.

Several existing application examples also show the need to consider multi-key settings.

Note 2 to entry: Cryptanalytic attacks do not include implementation-specific attacks, e.g. involving side channel

analysis.
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ISO/IEC 18033-1:2021(E)
3.9
decryption
decipherment
reversal of a corresponding encryption (3.11)

[SOURCE: ISO/IEC 11770-1:2010, 2.6, modified — The admitted term "decipherment" has been added;

note 1 to entry has been removed.]
3.10
decryption algorithm
decipherment algorithm
process which transforms ciphertext (3.7) into plaintext (3.20)
3.11
encryption
encipherment

(reversible) transformation of data by an encryption algorithm (3.12) to produce ciphertext (3.7), i.e. to

hide the information content of the data
3.12
encryption algorithm
encipherment algorithm
process which transforms plaintext (3.20) into ciphertext (3.7)
3.13
encryption system
encipherment system
cipher

cryptographic technique used to protect the confidentiality of data, and which consists of three

component processes: an encryption algorithm (3.12), a decryption algorithm (3.10), and a method for

generating keys (3.17)
3.14
generic attack

attack (3.3) against a encryption system (3.13) which does not rely on the encryption system design and

can be used to recover a secret key (3.25)/private key (3.21) or plaintext (3.20)

Note 1 to entry: Generic attacks depend on models and goals, see A.2 for details.

3.15
homomorphic encryption system
homomorphic cipher
homomorphic encipherment system

encryption system (3.13) with the property that if certain computations are performed on the ciphertext

(3.7), the plaintext (3.20) obtained after decryption (3.9) will have had the same computations applied

to it
3.16
identity-based cipher
identity-based encryption system

asymmetric encryption system (3.2) in which the encryption algorithm (3.12) takes an arbitrary string as

a public key (3.22)

[SOURCE: ISO/IEC 18033-5:2015, 3.6, modified — The admitted term "identity-based encryption

system" has been added.]
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ISO/IEC 18033-1:2021(E)
3.17
key

sequence of symbols that controls the operation of a cryptographic transformation [e.g. encryption

(3.11), decryption (3.9)]

[SOURCE: ISO/IEC 11770-1:2010, 2.12, modified — The list of cryptographic mechanisms has been

removed.]
3.18
keystream

pseudorandom sequence of symbols, intended to be secret, used by the encryption (3.11) and decryption

algorithm (3.10) of a stream cipher (3.27)

Note 1 to entry: If a portion of the keystream is known by an attacker, then it shall be computationally infeasible

for the attacker to deduce more than a negligible amount of information about the remainder of the keystream.

Computational feasibility depends on the specific security requirements and environment.

3.19
n-bit block cipher

block cipher (3.6) with the property that plaintext (3.20) blocks (3.5) and ciphertext (3.7) blocks are n

bits in length
3.20
plaintext
cleartext
unencrypted information
3.21
private key
key (3.17) of an entity’s key pair which is known only by that entity

[SOURCE: ISO/IEC 9594-8:2020, 3.5.50, modified — The parenthesis "(In a public-key cryptosystem)",

"That" at the beginning of the definition and the period at the end have been deleted.]

3.22
public key
key (3.17) of an entity’s key pair which is publicly known

[SOURCE: ISO/IEC 9594-8:2020, 3.5.57, modified — “That” at the beginning of the definition and the

period at the end have been deleted.]
3.23
public-key certificate

public key (3.22) of an entity, together with some other information, rendered unforgeable by digital

signature with the private key (3.21) of the certification authority that issued it

[SOURCE: ISO/IEC 9594-8:2020, 3.5.58, modified — “The” at the beginning of the definition, the

parenthesis “(CA)” and the period at the end have been deleted.]
3.24
public-key infrastructure
PKI

infrastructure able to support the management of public keys (3.22) able to support authentication,

encryption (3.11), integrity or non-repudiation services

[SOURCE: ISO/IEC 9594-8:2020, 3.5.60, modified — “The” at the beginning of the definition and the

period at the end have been deleted.]
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ISO/IEC 18033-1:2021(E)
3.25
secret key

key (3.17) used with symmetric cryptographic techniques (3.28) by a specified set of entities

[SOURCE: ISO/IEC 11770-3:2015, 3.36]
3.26
security strength

number associated with the amount of work (e.g. the number of operations) that is required to break a

cryptographic algorithm

Note 1 to entry: For key (3.17) recovery, a security strength of k bits implies that the workload required to break

the cipher (3.13) is equivalent to 2 executions of the cipher. For further information on the application of security

strength to selecting cryptographic algorithms for this document, see C.1.4.
3.27
stream cipher

symmetric encryption system (3.29) with the property that the encryption algorithm (3.12) involves

combining a sequence of plaintext (3.20) symbols with a sequence of keystream (3.18) symbols one

symbol at a time, using an invertible function

Note 1 to entry: Two types of stream cipher can be identified: synchronous stream ciphers and self-synchronous

stream ciphers, distinguished by the method used to obtain the keystream.
3.28
symmetric cryptographic technique
cryptographic technique for which all transformations use the same key (3.17)
3.29
symmetric encryption system
symmetric encipherment system
symmetric cipher
encryption system (3.13) based on symmetric cryptographic techniques (3.28)
4 Symbols and abbreviated terms
ECB electronic code book
k key length
MAC Message Authentication Code
n plaintext/ciphertext block length for a block cipher
P probability that a cryptanalytic attack will succeed

W workload or complexity of an attack, measured in terms of the number of calls to the cryp-

tographic algorithm
5 Nature of encryption
5.1 Purpose of encryption

The primary purpose of encryption systems is to protect the confidentiality of stored or transmitted

data. Encryption algorithms achieve this by transforming plaintext into ciphertext, from which it is

computationally infeasible to find any information about the content of the plaintext unless the secret/

private key is also known. However, in many cases, the length of the ciphertext is not concealed by

encryption, since the length of the ciphertext is typically the same as, or a little larger than, the length

of the corresponding plaintext.
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ISO/IEC 18033-1:2021(E)

It is important to note that encryption does not always, by itself, protect the integrity or the origin

of data. In many cases, it is possible, without knowledge of the key, to modify encrypted text with

predictable effects on the recovered plaintext. In order to ensure integrity and origin of data it is

often necessary to use additional techniques, such as those described in ISO/IEC 9796 (all parts),

ISO/IEC 9797 (all parts), ISO/IEC 14888 (all parts), ISO/IEC 19772, ISO/IEC 29192-2, ISO/IEC 29192-3

and ISO/IEC 29192-4.
5.2 Symmetric and asymmetric encryption systems

Symmetric and asymmetric encryption systems differ in their method of key generation.

— In a symmetric encryption system, the same secret key is used with both the encryption and

decryption algorithms. Knowledge of this key is required to perform both encryption and decryption,

and knowledge of the secret key therefore needs to be restricted to those parties authorized to

access the data which the key is used to encrypt.

— In an asymmetric encryption system, different but related keys are used for encryption and

decryption. Hence, keys are generated in matching pairs, where one key of the pair is the encryption

key and the other is the decryption key. Even with knowledge of the encryption key, it is assumed

to be computationally infeasible to find any information about the content of a plaintext from its

corresponding ciphertext. In many situations, it is possible to make the encryption key public.

Hence, this key is often referred to as the public key. The corresponding decryption key typically

has only one owner and remains confidential. Hence, it is referred to as the private key. Anyone who

knows the public encryption key is able to encrypt data intended for the holder of the corresponding

private key, while only the private decryption key holder is able to decrypt it.
5.3 Key management

The use of all types of cryptography relies on the management of cryptographic keys. All encryption

systems, both symmetric and asymmetric, require all the parties using the cipher to have access

to the necessary keys. This gives rise to the need for key management, involving the generation,

distribution, and ongoing management of keys. An overall framework for key management is given in

ISO/IEC 11770-1.

The problem of key management is rather different depending on whether the keys are for symmetric

or asymmetric encryption systems. For symmetric encryption systems, it is necessary to arrange

for secret keys to be generated and shared by pairs (or larger groups) of entities. For asymmetric

encryption systems, it is necessary for key pairs to be generated and for public keys to be distributed

in such a way that their authenticity is guaranteed. In an identity-based cipher, the public key is an

arbitrary data string, which is usually chosen from some public information associated with the entity

which decrypts ciphertexts.

Methods to establish shared secret keys using symmetric cryptographic techniques are specified in

ISO/IEC 11770-2. Methods to establish shared secret keys using asymmetric cryptographic techniques

are specified in ISO/IEC 11770-3. ISO/IEC 11770-3 also specifies techniques for the reliable distribution

of public keys for asymmetric cryptographic techniques. Methods to establish shared secret keys using

weak secrets are specified in ISO/IEC 11770-4.
6 Use and properties of encryption
6.1 General

The criteria used for submission of encryption systems for possible inclusion in, and for their deletion

from, the ISO/IEC 18033 series are defined in Annexes A and B.
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ISO/IEC 18033-1:2021(E)
6.2 Asymmetric encryption systems

The encryption algorithm for an asymmetric encryption system defines a mapping from the set of

permissible plaintext messages (typically a set of bit strings) to the set of ciphertext messages (typically

also a set of bit strings). The set of permissible messages and the set of ciphertexts depends on both the

choice of encryption system and the key pair.

For an asymmetric encryption system, the encryption algorithm depends on a public key, whereas

decryption depends on a private key. Hence, while the ciphertext block corresponding to a chosen

plaintext block can be readily computed, it shall be infeasible for anyone, other than the holder of the

private key, to deduce the plaintext block corresponding to a chosen ciphertext block. However, if an

interceptor of ciphertext knows the public key used to produce it, and also knows that the plaintext

has been chosen from a small set of possibilities, it can become possible to deduce the plaintext by an

exhaustive search through all possible plaintexts.

As a result, and in order to achieve a satisfactory level of security, it is necessary to incorporate

random data in the encryption process so that the ciphertext block corresponding to a given plaintext

block cannot be predicted. Detailed techniques for incorporating random data are described in

ISO/IEC 18033-2.

Authenticity of public keys is of great importance when using asymmetric encryption algorithms.

Assurance in the authenticity of a public key can, for example, be provided using a PKI.

6.3 Block ciphers
6.3.1 General

A block cipher is a symmetric encryption system with the property that the encryption algorithm

operates on blocks of plaintext, i.e. strings of bits of a defined length, to yield ciphertext blocks. Each key

for a block cipher defines a particular invertible mapping of plaintext blocks to ciphertext blocks (and a

corresponding inverse mapping used for decryption). If, as is typically the case, the plaintext blocks and

ciphertext blocks are all blocks of n binary digits, then each key simply defines a permutation on the set

of all n-bit blocks.

Block ciphers can be used in a wide variety of ways. Two of the most important applications are the

modes of operation described in 6.3.2 (modes that provide confidentiality) and 6.3.3 (modes that provide

integrity control), but there are many other uses such as in hash-functions (see ISO/IEC 10118-2) and

random-number generators (see ISO/IEC 18031).

The criteria used for submission of encryption systems for possible inclusion in, and for their deletion

from, the ISO/IEC 18033 series are defined in Annexes A and B.
6.3.2 Modes of operation

There are many ways in which an n-bit block cipher can be used to encipher plaintext. Such methods are

known as modes of operation for block ciphers. Modes of operation are defined in ISO/IEC 10116. If the

number of bits in the plaintext happens to be n, then encryption can be achieved by simply applying the

encryption process to this block, an encryption mode known as electronic code book (ECB). However,

for arbitrary length plaintext, it is necessary to employ a more sophisticated approach. For this and

other reasons, it is often necessary to use one of the other modes of operation defined in ISO/IEC 10116.

6.3.3 Message authentication codes (MACs)
Although encryption does not provide data integri
...

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