ISO/IEC 8825-1:2021

INTERNATIONAL ISO/IEC
STANDARD 8825-1
Sixth edition
2021-06
Information technology — ASN.1
encoding rules —
Part 1:
Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules
(CER) and Distinguished Encoding
Rules (DER)
Technologies de l'information — Règles de codage ASN.1 —
Partie 1: Spécification des règles de codage de base (BER), des règles
de codage canoniques (CER) et des règles de codage distinctives
(DER)
Reference number
©
ISO/IEC 2021
© ISO/IEC 2021
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ii © ISO/IEC 2021 – All rights reserved

Foreword
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This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration
with ITU-T. The identical text is published as ITU-T X.690 (02/2021).
This sixth edition cancels and replaces the fifth edition (ISO/IEC 8825-1:2015), which has been
technically revised.
A list of all parts in the ISO/IEC 8825 series can be found on the ISO and IEC websites.
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© ISO/IEC 2021 – All rights reserved iii

CONTENTS
Page
Introduction . v
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards. 1
2.2 Additional references . 1
3 Definitions . 2
4 Abbreviations . 2
5 Notation . 3
6 Convention . 3
7 Conformance . 3
8 Basic encoding rules . 3
8.1 General rules for encoding . 3
8.1.1 Structure of an encoding . 3
8.1.2 Identifier octets . 4
8.1.3 Length octets . 5
8.1.4 Contents octets . 6
8.1.5 End-of-contents octets . 6
8.2 Encoding of a boolean value . 7
8.3 Encoding of an integer value. 7
8.4 Encoding of an enumerated value . 7
8.5 Encoding of a real value . 7
8.6 Encoding of a bitstring value . 9
8.7 Encoding of an octetstring value . 10
8.8 Encoding of a null value . 10
8.9 Encoding of a sequence value . 10
8.10 Encoding of a sequence-of value . 11
8.11 Encoding of a set value . 11
8.12 Encoding of a set-of value . 11
8.13 Encoding of a choice value . 11
8.14 Encoding of a value of a prefixed type . 11
8.15 Encoding of an open type . 12
8.16 Encoding of an instance-of value . 12
8.17 Encoding of a value of the embedded-pdv type . 13
8.18 Encoding of a value of the external type . 13
8.19 Encoding of an object identifier value . 14
8.20 Encoding of a relative object identifier value . 15
8.21 Encoding of an OID internationalized resource identifier value. 15
8.22 Encoding of a relative OID internationalized resource identifier value . 15
8.23 Encoding for values of the restricted character string types . 15
8.24 Encoding for values of the unrestricted character string type . 18
8.25 Encoding for values of the Useful Types . 18
8.26 Encoding for values of the TIME type and the useful time types . 18
8.26.1 Encoding for values of the TIME type . 18
8.26.2 Encoding for values of the DATE type . 18
8.26.3 Encoding for values of the TIME-OF-DAY type . 18
8.26.4 Encoding for values of the DATE-TIME type . 18
8.26.5 Encoding for values of the DURATION type . 18
Rec. ITU-T X.690 (02/2021) iii
© ISO/IEC 2021 – All rights reserved

9 Canonical encoding rules . 19
9.1 Length forms . 19
9.2 String encoding forms . 19
9.3 Set components. 19
10 Distinguished encoding rules . 19
10.1 Length forms . 19
10.2 String encoding forms . 20
10.3 Set components. 20
11 Restrictions on BER employed by both CER and DER . 20
11.1 Boolean values . 20
11.2 Unused bits . 20
11.3 Real values . 20
11.4 GeneralString values . 21
11.5 Set and sequence components with default value . 21
11.6 Set-of components . 21
11.7 GeneralizedTime . 21
11.8 UTCTime . 21
11.8.4 Examples of valid representations . 22
11.8.5 Examples of invalid representations . 22
11.9 The TIME type and the useful time types . 22
12 Use of BER, CER and DER in transfer syntax definition . 22
Annex A – Example of encodings. 24
A.1 ASN.1 description of the record structure . 24
A.2 ASN.1 description of a record value . 24
A.3 Representation of this record value . 24
Annex B – Identification of Encoding Rules . 26
Annex C – Illustration of real value encoding .
...