ETSI TS 104 053-2 V1.2.1 (2025-02)

TECHNICAL SPECIFICATION
TETRA Air Interface Security, Algorithms Specifications;
Part 2: TETRA Encryption Algorithms, TEA Set B

2 ETSI TS 104 053-2 V1.2.1 (2025-02)

Reference
RTS/TCCE-06225
Keywords
air interface, algorithm, DMO, security, TETRA,
V+D
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis Cedex - FRANCE

Tel.: +33 4 92 94 42 00  Fax: +33 4 93 65 47 16

Siret N° 348 623 562 00017 - APE 7112B
Association à but non lucratif enregistrée à la
Sous-Préfecture de Grasse (06) N° w061004871

Important notice
The present document can be downloaded from the
ETSI Search & Browse Standards application.
The present document may be made available in electronic versions and/or in print. The content of any electronic and/or
print versions of the present document shall not be modified without the prior written authorization of ETSI. In case of any
existing or perceived difference in contents between such versions and/or in print, the prevailing version of an ETSI
deliverable is the one made publicly available in PDF format on ETSI deliver repository.
Users should be aware that the present document may be revised or have its status changed,
this information is available in the Milestones listing.
If you find errors in the present document, please send your comments to
the relevant service listed under Committee Support Staff.
If you find a security vulnerability in the present document, please report it through our
Coordinated Vulnerability Disclosure (CVD) program.
Notice of disclaimer & limitation of liability
The information provided in the present deliverable is directed solely to professionals who have the appropriate degree of
experience to understand and interpret its content in accordance with generally accepted engineering or
other professional standard and applicable regulations.
No recommendation as to products and services or vendors is made or should be implied.
No representation or warranty is made that this deliverable is technically accurate or sufficient or conforms to any law
and/or governmental rule and/or regulation and further, no representation or warranty is made of merchantability or fitness
for any particular purpose or against infringement of intellectual property rights.
In no event shall ETSI be held liable for loss of profits or any other incidental or consequential damages.

Any software contained in this deliverable is provided "AS IS" with no warranties, express or implied, including but not
limited to, the warranties of merchantability, fitness for a particular purpose and non-infringement of intellectual property
rights and ETSI shall not be held liable in any event for any damages whatsoever (including, without limitation, damages
for loss of profits, business interruption, loss of information, or any other pecuniary loss) arising out of or related to the use
of or inability to use the software.
Copyright Notification
No part may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm except as authorized by written permission of ETSI.
The content of the PDF version shall not be modified without the written authorization of ETSI.
The copyright and the foregoing restriction extend to reproduction in all media.

© ETSI 2025.
All rights reserved.
ETSI
3 ETSI TS 104 053-2 V1.2.1 (2025-02)
Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
1 Scope . 5
2 References . 5
2.1 Normative references . 5
2.2 Informative references . 5
3 Definition of terms, symbols and abbreviations . 6
3.1 Terms . 6
3.2 Symbols . 6
3.3 Abbreviations . 6
4 TEA encryption Set B Algorithm specifications . 6
4.1 Input and Output Parameters . 6
5 TEA5 - Specification of the Algorithm . 7
5.1 Introduction . 7
5.2 TEA5 IV Expansion . 7
5.3 TEA5 Derivation of the Mode Key and Mode IV . 8
5.4 TEA5 Derivation of the Keystream Bits . 9
5.5 TEA5 - Lookup Table for IV Mixing . 10
5.6 TEA5 - Definition of the Combining Function f . 10
6 TEA6 - Specification of the Algorithm . 11
6.1 Introduction . 11
6.2 TEA6 - IV Expansion . 11
6.3 TEA6 - Derivation of the Mode Key and Mode IV . 12
6.4 TEA6 - Derivation of the Keystream Bits . 13
6.5 TEA6 - Lookup Table for IV Mixing . 14
6.6 TEA6 - Definition of the Combining Function f . 14
7 TEA7-Specification of the Algorithm . 15
7.1 Introduction . 15
7.2 TEA7- IV Expansion . 15
7.3 TEA7- Derivation of the Mode Key and Mode IV . 16
7.4 TEA7- Derivation of the Keystream Bits . 17
7.5 TEA7 - Lookup Table for IV Mixing . 18
7.6 TEA7 - Definition of the Combining Function f . 18
Annex A (informative): Bibliography . 20
History . 21

ETSI
4 ETSI TS 104 053-2 V1.2.1 (2025-02)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
ETSI IPR online database.
Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not
referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
DECT™, PLUGTESTS™, UMTS™ and the ETSI logo are trademarks of ETSI registered for the benefit of its
Members. 3GPP™, LTE™ and 5G™ logo are trademarks of ETSI registered for the benefit of its Members and of the
3GPP Organizational Partners. oneM2M™ logo is a trademark of ETSI registered for the benefit of its Members and of ®
the oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association.
Foreword
This Technical Specification (TS) has been produced by ETSI Technical Committee TETRA and Critical
Communications Evolution (TCCE).
The present document is part 2 of a multi-part deliverable covering the specifications of the TETRA standard
encryption, authentication and key management algorithms, as identified below:
Part 1: "TETRA Encryption Algorithms, TEA Set A";
Part 2: "TETRA Encryption Algorithms, TEA Set B";
Part 3: "TETRA Authentication and Key Management Algorithms TAA1";
Part 4: "TETRA Authentication and Key Management Algorithms TAA2".
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.

ETSI
5 ETSI TS 104 053-2 V1.2.1 (2025-02)
1 Scope
The present document specifies the Terrestrial Trunked Radio system (TETRA) set B encryption algorithms TEA 5, 6
and 7. These algorithms are designed to meet the requirements set out in the requirements specification for the
Additional TETRA Encryption Algorithm Suite [i.2].
The TETRA Air interface security function provides mechanisms for confidentiality of control signalling and user
speech and data at the air interface, authentication and key management mechanisms for the air interface and for the
Inter-System Interface (ISI). TETRA Air Interface security mechanisms are described in the TETRA V+D security
specification [1] and the TETRA Direct Mode security specification [2].
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
ETSI docbox.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents are necessary for the application of the present document:
[1] ETSI TS 100 392-7: "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D);
Part 7: Security".
[2] ETSI TS 100 396-6: "Terrestrial Trunked Radio (TETRA); Direct Mode Operation (DMO);
Part 6: Security".
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] Daemen, J. and Rijmen, V. (1999): "AES proposal: Rijndael', document version 2". Submission to
NIST AES competition (1999).
[i.2] ETSI TCCE(21)000002r2: "Requirements Specification for the Additional TETRA Encryption
Algorithm Suite".
ETSI
6 ETSI TS 104 053-2 V1.2.1 (2025-02)
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
Cipher Key (CK): value that is used to determine the transformation of plain text to cipher text in a cryptographic
algorithm
Initialization Vector (IV): sequence of symbols that randomize the KSG inside the encryption unit
key stream: pseudo random stream of symbols that is generated by a KSG for encipherment and decipherment
Key Stream Generator (KSG): cryptographic algorithm which produces a stream of binary digits, which can be used
for encipherment and decipherment
NOTE: The initial state of the KSG is determined by the IV value.
Key Stream Segment (KSS): key stream of arbitrary length
LENGTH: required length of the key stream in bits
TEA set A: set of air interface encryption algorithms comprising TEA1, TEA2, TEA3 and TEA4
TEA set B: set of air interface encryption algorithms comprising TEA5, TEA6 and TEA7
TETRA algorithm: mathematical description of a cryptographic process used for either of the security processes
authentication or encryption
3.2 Symbols
Void.
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
CK Cipher Key
CKM Mode Key
GF Galois Field
IV Initialization Vector
IVM Mode IV
IVX eXpanded IV
KSS Key Stream Segment
ISI Inter System Interface
KSG Key Stream Generator
4 TEA encryption Set B Algorithm specifications
4.1 Input and Output Parameters
As specified in clause 8.3 of [i.2], the input parameters to the algorithm are:
• an initialization vector IV consisting of 80 bits IV[0], …, IV[79];
• a cipher key CK consisting of 192 bits CK[0], …, CK[191];
ETSI
7 ETSI TS 104 053-2 V1.2.1 (2025-02)
• the required length LENGTH of the key stream in bits. This can, according to [i.2], take any value from 1 up
to 8 288. However, the design of the algorithm allows it to deliver, securely, a length of keystream up to
2 bits, making it potentially suitable for future applications where an increased length of KSS output is
required.
The corresponding output from the algorithm is then a key stream segment KSS consisting of LENGTH bits KSS[0],
…, KSS[LENGTH-1].
5 TEA5 - Specification of the Algorithm
5.1 Introduction
In outline, the algorithm operates as follows:
• the 80 bits of initialization vector IV, considered as 10 elements of the Galois field GF (2 ), are mixed using a
10-stage linear recursion over GF(2 ) to give 24 bytes which form a 192-bit mixed initialization vector IVX;
• the cipher key CK and mixed initialization vector IVX are combined to produce a 192-bit Mode Key, CKM,
and a 192-bit Mode IV, IVM;
• successive 256-bit blocks are formed as a concatenation Mode IV || 'T', 'E', 'A', '5' || counter, where the byte
values 'T', 'E', 'A', '5' code the name of the algorithm in ASCII, and the 32-bit counter takes successive values
0, 1, …;
• these successive 256-blocks are encrypted using the variant of Rijndael [i.1] with parameters giving a block
length of 256 bits and key length 192 bits. The Mode Key is used as the Rijndael key. The 256-bit blocks
obtained as a result of these Rijndael encryptions are concatenated to form KSS; some bits will be discarded
from the final ciphertext block if LENGTH is not exactly divisible by 256.
The algorithm is specified precisely in clauses 5.2 to 5.6.
5.2 TEA5 IV Expansion
The 80-bit IV is expanded to a 192-bit mixed IV, IVX, as follows:
7 6
• from the initialization vector bits IV[0], …, IV[79], form 10 bytes b[0], …, b[9], where b[i] = 2 IV[8×i] + 2
IV[8×i+1] + … + IV[8×i+7], for i = 0, …, 9;
7 6 7 6
B[0] + 2 B[1] + … + B[7] with the element z B[0] + z
• for any bits B[0], …, B[7], the byte is identified as 2
8 8
B[1] + … + B[7] of GF(2 ), where z is a generator of GF(2 ) satisfying the Rijndael polynomial
8 4 3
x + x + x + x + 1 in GF(2)[x];
• for i = 10, …, 43, a byte b[i] is obtained from bytes b[i-1], …, b[i-10] according to the rule b[i] = b[i-10] ⊕
7 6 4 2 7 6 4 2
b[i-9] ⊕ (z + z + z + z + z + 1) b[i-1]. The byte (z + z + z + z + z + 1) b[i-1] can be obtained from b[i-1]
using the lookup table defined in clause 5.5;
7 6
• the 24 bytes b[20], b[21], …, b[43] contain the bits IVX[0], …, IVX[191], where b[20+i] = 2 IVX[8i] + 2
IVX[8i + 1] + … + IVX[8i + 7] for i = 0, …, 23.
This process is illustrated in figures 1 and 2 below.
ETSI
8 ETSI TS 104 053-2 V1.2.1 (2025-02)

Figure 1: IV expansion
Figure 2: IVX extraction
5.3 TEA5 Derivation of the Mode Key and Mode IV
The Mode Key consists of 192 bits CKM[0], …, CKM[191] and the Mode IV consists of 192 bits IVM[0], …,
IVM[191]. An 8-bit to 8-bit combining function f is applied to successive 8-bit inputs formed from 4 bits of the cipher
key CK and 4 bits from the mixed initialization vector IVX, and the result is taken to be a further 4 bits of Mode Key
and 4 bits of Mode IV. More precisely, for each i in the range 0, …, 47,
7 6 4 3 2
2 CKM[4i] + 2 CKM[4i+1] + … + 2 CKM[4i+3] + 2 IVM[4i] + 2 IVM[4i+1] + … + IVM[4i+3] =
7 6 4 3 2
f(2 CK[4i] + 2 CK[4i+1] + … + 2 CK[4i+3] + 2 IVX[4i] + 2 IVX[4i+1] + … + IVX[4i+3])
The combining function f is defined in clause 5.6.
The process for deriving the Mode Key and Mode IV from the cipher key CK and mixed initialization vector IVX is
illustrated in figure 3 below.

Figure 3: Mode Key and Mode IV derivation
ETSI
9 ETSI TS 104 053-2 V1.2.1 (2025-02)
5.4 TEA5 Derivation of the Keystream Bits
The keystream bits KSS are obtained using Rijndael [i.1] with key length 192 bits and block size 256 bits, used in a
7 6
counter mode. The key used is the Mode Key CKM, arranged into bytes 2 CKM[8i] + 2 CKM[8i+1] + … +
CKM[8i+7] for i = 0, …, 23. The Rijndael algorithm is run in encryption mode to encrypt LENGTH/256 successive
plaintext blocks, where the notation LENGTH/256 denotes the least integer ≥ the floating-point quotient
LENGTH/256. The plaintext for encryption j, for j = 0, …, LENGTH/256 -1, is, informally, IVM || 'T', 'E', 'A', '5' || j,
where the byte values 'T', 'E', 'A', '5' code the name of the algorithm in ASCII, and j is coded as 4 bytes; more precisely,
it is the 32-byte sequence p , …, p , where:
0 31
7 6
• p = 2 IVM[8i] + 2 IVM[8i+1] + … + IVM[8i+7] for i = 0, …, 23;
i
• p = 84, p = 69, p = 65, p = 53 (those four values being in decimal);
24 25 26 27
24 16 8
• 2 p + 2 p + 2 p + p = j.
28 29 30 31
The keystream bit KSS[i] is the bit C [t], which is written as:
s
• i = 256r + 8s + t, for 0 ≤ s ≤ 31 and 0 ≤ t ≤ 7;
• c , …, c are the ciphertext bytes obtained from the encryption where j = r;
0 31
7 6
• c = 2 C [0] + 2 C [1] + … + C [7], for bits C [0], …, C [7].
s s s s s s
Note that if (LENGTH mod 256) ≤ 248 then one or more higher numbered ciphertext bytes from the last block will be
discarded. If (LENGTH mod 8) > 0 then one or more less significant bits from the last used ciphertext byte will be
discarded.
Note that the maximum value of LENGTH, the number of bits of required keystream, is 8 288, according to the
specification [i.2]. Since this maximum number of required keystream bits ≤ 2 , the 32-bit counter j can be
implemented as an 8-bit counter with the other three bytes fixed to zero. If, in a future application, the maximum
number of bits of required keystream is no more than 2 bits, then the 32-bit counter can be implemented as a 16-bit
counter with the other two bytes fixed to zero. If a full range of values for the 32-bit counter is implemented, keystream
sequences of length up to 2 can be generated.
The use of Rijndael in counter mode to produce keystream bits is shown in figure 4 below.

Figure 4: Keystream generation
ETSI
10 ETSI TS 104 053-2 V1.2.1 (2025-02)
5.5 TEA5 - Lookup Table for IV Mixing
7 6 4 2
Table 1 implements Galois Field multiplication by z + z + z + z + z + 1, as discussed in clause 5.2. Different rows
correspond to different values of the most significant 4 bits of the input, and columns to the least significant 4 bits. For
example, the value corresponding to 0x12 is found in the row labelled 0x1? and column labelled 0x?2, and is the byte
value 0x6a.
Table 1
0x?0 0x?1 0x?2 0x?3 0x?4 0x?5 0x?6 0x?7 0x?8 0x?9 0x?a 0x?b 0x?c 0x?d 0x?e 0x?f
0x0?
0x00 0xd7 0xb5 0x62 0x71 0xa6 0xc4 0x13 0xe2
...