ISO/IEC 18033-7:2022

Information security — Encryption algorithms — Part 7: Tweakable block ciphers

This document specifies tweakable block ciphers. A tweakable block cipher is a family of n-bit permutations parametrized by a secret key value and a public tweak value. Such primitives are generic tools that can be used as building blocks to construct cryptographic schemes such as encryption, Message Authentication Codes, authenticated encryption, etc. A total of five different tweakable block ciphers are defined. They are categorized in Table 1.

Sécurité de l'information — Algorithmes de chiffrement — Partie 7: Chiffrements par blocs paramétrables

General Information

• Status: Published
• Publication Date: 12-Apr-2022
• 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 2 - Cryptography and security mechanisms
• Current Stage: 6060 - International Standard published
• Start Date: 13-Apr-2022
• Due Date: 15-Jun-2023
• Completion Date: 13-Apr-2022

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 18033-7
First edition
2022-04
Information security — Encryption
algorithms —
Part 7:
Tweakable block ciphers
Sécurité de l'information — Algorithmes de chiffrement —
Partie 7: Chiffrements par blocs paramétrables
Reference number
© ISO/IEC 2022
© ISO/IEC 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ii
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Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 Requirements on the usage of tweakable block ciphers . 3
6 Deoxys-TBC . 3
6.1 Deoxys-TBC versions . 3
6.2 Deoxys-TBC encryption . 4
6.3 Deoxys-TBC decryption . 5
6.4 Deoxys-TBC tweakey schedule . 6
7 Skinny . 7
7.1 Skinny versions . 7
7.2 Skinny encryption . 8
7.3 Skinny decryption . 10
7.4 Skinny tweakey schedule . 11
Annex A (informative) Numerical examples .14
Annex B (normative) Object identifiers .16
Bibliography .18
iii
© ISO/IEC 2022 – All rights reserved

Foreword
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Commission) form the specialized system for worldwide standardization. National bodies that are
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accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives or
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www.iso.org/iso/foreword.html. In the IEC, see www.iec.ch/understanding-standards.
This document was prepared by 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 18033 series can be found on the ISO and IEC websites.
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 2022 – All rights reserved

Introduction
This document specifies tweakable block ciphers. A tweakable block cipher is a family of permutations
parametrized by a secret key value and a public tweak value.
v
© ISO/IEC 2022 – All rights reserved

INTERNATIONAL STANDARD ISO/IEC 18033-7:2022(E)
Information security — Encryption algorithms —
Part 7:
Tweakable block ciphers
1 Scope
This document specifies tweakable block ciphers. A tweakable block cipher is a family of n-bit
permutations parametrized by a secret key value and a public tweak value. Such primitives are generic
tools that can be used as building blocks to construct cryptographic schemes such as encryption,
Message Authentication Codes, authenticated encryption, etc.
A total of five different tweakable block ciphers are defined. They are categorized in Table 1.
Table 1 — Tweakable block ciphers specified
Block length Tweakey length Algorithm name
128 bits 256 bits Deoxys-TBC-256
128 bits 384 bits Deoxys-TBC-384
64 bits 192 bits Skinny-64/192
128 bits 256 bits Skinny-128/256
128 bits 384 bits Skinny-128/384
2 Normative references
There are no normative references in this document.
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 https:// www .electropedia .org/
3.1
block
string of bits of a defined length
[SOURCE: ISO/IEC 18033-1:2021 3.5]
3.2
ciphertext
data which has been transformed to hide its information content
[SOURCE: ISO/IEC 18033-1:2021, 3.7]
© ISO/IEC 2022 – All rights reserved

3.3
encryption algorithm
process which transforms plaintext into ciphertext
[SOURCE: ISO/IEC 18033-1:2021, 3.12]
3.4
key
sequence of symbols that controls the operation of a cryptographic transformation (e.g. encryption,
decryption)
[SOURCE: ISO/IEC 11770-1:2010, 2.12, modified – the list of cryptographic mechanisms is removed]
3.5
plaintext
unencrypted information
[SOURCE: ISO/IEC 18033-1:2021, 3.20]
3.6
tweak
non-secret sequence of symbols that controls the operation of a cryptographic transformation (e.g.
encryption, decryption)
3.7
tweakable block cipher
symmetric encryption system with the property that the encryption algorithm operates on a block of
plaintext, i.e. a string of bits of a defined length, and a tweakey (3.8) to yield a block of ciphertext
3.8
tweakey
sequence of symbols that controls the operation of a cryptographic transformation (e.g. encryption,
decryption)
Note 1 to entry: The tweakey is the concatenation of the key and the tweak inputs.
4 Symbols
k key bit-length for a tweakable block cipher
Nr the number of rounds of the tweakable block cipher
n plaintext/ciphertext bit-length for a tweakable block cipher
t tweak bit-length for a tweakable block cipher
a ← b replaces the value of the variable a with the value of the variable b
|| concatenation of bit-strings
⊕ bitwise exclusive-OR operation
M diffusion matrix of the tweakable block cipher
X n-bit internal state of the tweakable block cipher
GF(i) finite field of i elements
8 8 4 3

base field as GF(2 ), defined by the irreducible polynomial x + x + x + x + 1
© ISO/IEC 2022 – All rights reserved

λ sub-tweakey value
ρ table of rotation values for the ShiftRows / ShiftRowsInv functions of Deoxys-TBC and for
the ShiftRowsRight / ShiftRowsRightInv of Skinny
h byte permutation in the tweakey schedule algorithm of Deoxys-TBC
P cell permutation in the tweakey schedule algorithm of Skinny
T
[i, … , j] sequence of integers starting from i included, ending at j included, with a step of 1
5 Requirements on the usage of tweakable block ciphers
Both Deoxys-TBC and Skinny ciphers propose a tweakey input that can be utilized as key and/or tweak
material, up to the user needs. Therefore, the user can freely choose which part of the tweakey is
dedicated to key and/or tweak material. However, whatever the combination of key/tweak size chosen
by the user, it shall be such that the key size is at least 128 bits.
In general, the tweak may be made public and a user can repeat the same (tweak,key) combination
without causing a security degradation. Some use-cases may require stricter conditions to meet the
user's security requirements, and these additional conditions shall always be satisfied.
NOTE Modes of operation offering beyond-birthday security are an example for requiring stricter conditions
as they often fail if the same tweak is repeated under the same key.
Skinny-64/192 version shall only be used to instantiate security algorithms guaranteeing an upper
bound on the adversarial advantage that remains meaningful as long as the adversary processes less
than 2 data blocks.
This document describes the Skinny and Deoxys-TBC configuration where the least-significant portion
of the tweakey input is loaded with the tweak and the most-significant portion of the tweakey input is
loaded with the key material, i.e. tweakey = key || tweak. Keying material shall never be reused across
instances with differing tweak sizes.
Annex A provides numerical examples of Deoxys-TBC-256, Deoxys-TBC-384, Skinny-64/192,
Skinny-128/256 and Skinny-128/384. Annex B defines the object identifiers which shall be used to
identify the algorithms specified in this document.
6 Deoxys-TBC
6.1 Deoxys-TBC versions
The Deoxys-TBC algorithm (originally published in Reference [4], slightly modified for improved
performances in Reference [5], the latter being the version described in this document) is a tweakable
block cipher. Deoxys-TBC operates on a plaintext block of 16 bytes numbered from most-significant to
least-significant byte [0,…,15]. The internal state X of the cipher is a (4×4) matrix of bytes, initialized
from the plaintext block of 16 bytes as follows:
 04 812 
 
15 913
 
X = .
 
26 10 14
 
 
37 11 15
 
This document defines two versions of Deoxys-TBC. For Deoxys-TBC-256 the tweakey is of size 256 bits
and consists of a key of size k ≥ 128 and a tweak of size t = 256-k. For Deoxys-TBC-384 the tweakey is of
size 384 bits and consists of a key of size k ≥ 128 and a tweak of size t = 384-k.
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6.2 Deoxys-TBC encryption
The number of rounds Nr is 14 for Deoxys-TBC-256 and 16 for Deoxys-TBC-384. One round of Deoxys-
[3]
TBC encryption, similar to a round in the Advanced Encryption Standard (AES) , has the following
four transformations applied to the internal state in the order specified below:
— AddSubTweakey(X, λ): bitwise exclusive-or (XOR)
...