ISO/IEC 8825-4:2021

Information technology — ASN.1 encoding rules — Part 4: XML Encoding Rules (XER)

This document specifies a set of basic XML Encoding Rules (BASIC-XER) that may be used to derive a transfer syntax for values of types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1 and Rec. ITU-T X.681 | ISO/IEC 8824-2. This document also specifies a set of Canonical XML Encoding Rules (CXER) which provide constraints on the basic XML Encoding Rules and produce a unique encoding for any given ASN.1 value. This document further specifies a set of extended XML Encoding Rules (EXTENDED-XER) which adds further encoders options, and also allows the ASN.1 specifier to vary the encoding that would be produced by BASIC-XER. It is implicit in the specification of these encoding rules that they are also used for decoding. The encoding rules specified in this document : – are used at the time of communication; – are intended for use in circumstances where displaying of values and/or processing them using commonly available XML tools (such as browsers) is the major concern in the choice of encoding rules; – allow the extension of an abstract syntax by addition of extra values for all forms of extensibility described in Rec. ITU-T X.680 | ISO/IEC 8824‑1. This document also specifies the syntax and semantics of XER encoding instructions, and the rules for their assignment and combination. XER encoding instructions can be used to control the EXTENDED-XER encoding for specific ASN.1 types.

Technologies de l'information — Règles de codage ASN.1 — Partie 4: Règles de codage XML (XER)

General Information

• Status: Published
• Publication Date: 29-Jun-2021
• ICS: 35.100.60 - Presentation layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6/WG 10 - Directory, ASN.1 and Registration
• Current Stage: 6060 - International Standard published
• Start Date: 30-Jun-2021
• Due Date: 02-Jul-2023
• Completion Date: 30-Jun-2021

Relations

• Revises: ISO/IEC 8825-4:2015 - Information technology — ASN.1 encoding rules: XML Encoding Rules (XER) — Part 4:
• Effective Date: 15-Aug-2020

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 8825-4
Fourth edition
2021-06
Information technology — ASN.1
encoding rules —
Part 4:
XML Encoding Rules (XER)
Technologies de l'information — Règles de codage ASN.1 —
Partie 4: Règles de codage XML (XER)
Reference number
ISO/IEC 8825-4:2021(E)
©
ISO/IEC 2021

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ISO/IEC 8825-4:2021(E)

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ii © ISO/IEC 2021 – All rights reserved

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ISO/IEC 8825-4:2021(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
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the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives or
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Attention is drawn to the possibility that some of the elements of this document may be the subject of
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of any patent rights identified during the development of the document will be in the Introduction and/or
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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 6, Telecommunications and information exchange between systems, in collaboration
with ITU-T. The identical text is published as ITU-T X.693 (02/2021).
This fourth edition cancels and replaces the third edition (ISO/IEC 8825-4:2015), which has been
technically revised. It also incorporates ISO/IEC 8825-4:2015/Cor 1:2018.
A list of all parts in the ISO/IEC 8825 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.

© ISO/IEC 2021 – All rights reserved iii

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ISO/IEC 8825-4:2021(E)
CONTENTS
Page
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards . 1
2.2 Additional references . 2
3 Definitions . 2
3.1 ASN.1 Basic Encoding Rules (BER) . 2
3.2 Additional definitions . 2
4 Abbreviations . 4
5 Encodings specified by this Recommendation | International Standard . 4
6 Encoding instructions specified by this Recommendation | International Standard . 5
7 Conformance . 5
8 Basic XML encoding rules . 5
8.1 Production of a complete BASIC-XER encoding . 5
8.2 The XML prolog . 6
8.3 The XML document element . 6
8.4 Encoding of the EXTERNAL type . 7
8.5 Encoding of the open type . 7
8.6 Decoding of types with extension markers . 7
9 Canonical XML encoding rules . 7
9.1 General rules for canonical XER. 7
9.2 Real values . 8
9.3 Bitstring value . 8
9.4 Octetstring value . 8
9.5 Sequence value . 8
9.6 Set value . 8
9.7 Set-of value . 8
9.8 Object identifier value . 9
9.9 Relative object identifier value . 9
9.10 GeneralizedTime . 9
9.11 UTCTime . 9
9.12 Open type value. 10
9.13 The TIME type and the useful time types . 10
10 Extended XML encoding rules . 10
10.1 General . 10
10.2 EXTENDED-XER conformance . 11
10.3 Structure of an EXTENDED-XER encoding . 13
11 Notation, character set and lexical items used in XER encoding instructions . 13
12 Keywords . 14
13 Assigning an XER encoding instruction to an ASN.1 type using a type prefix . 15
14 Assigning an XER encoding instruction using an XER encoding control section . 17
14.1 The encoding instruction assignment list . 17
© ISO/IEC 2021 – All rights reserved
Rec. ITU-T X.693 (02/2021) iii

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ISO/IEC 8825-4:2021(E)
Page
14.2 Identification of the targets for an XER encoding instruction using a target list . 18
15 Multiple assignment of XER encoding instructions . 23
15.1 Order in which multiple assignments are considered . 23
15.2 Effect of assigning a negating encoding instruction . 24
15.3 Multiple assignment of encoding instructions with multiple categories . 24
15.4 Multiple assignment of XER encoding instructions of the same category . 24
15.5 Permitted combinations of final encoding instructions . 25
16 XER encoding instruction support for XML namespaces and qualified names . 26
17 Specification of EXTENDED-XER encodings . 27
17.1 The XML document element . 28
17.2 The "TypeNameOrModifiedTypeName" production . 28
17.3 The "AttributeList" production . 28
17.4 The "ExtendedXMLValue" production . 28
17.5 The "ExtendedXMLChoiceValue" production . 30
17.6 The "ExtendedXMLSequenceValue" and "ExtendedXMLSetValue" productions . 30
17.7 The "ExtendedXMLSequenceOfValue" and "ExtendedXMLSetOfValue" productions . 31
17.8 The "ModifiedXMLIntegerValue" production . 32
17.9 The "ModifiedXMLRealValue" production . 32
18 The ANY-ATTRIBUTES encoding instruction . 33
18.1 General . 33
18.2 Restrictions . 34
18.3 Effect on encodings . 34
19 The ANY-ELEMENT encoding instruction . 35
19.1 General . 35
19.2 Restrictions . 35
19.3 Effect on encodings . 36
20 The ATTRIBUTE encoding instruction . 37
20.1 General . 37
20.2 Restrictions . 37
20.3 Effect on encodings . 37
21 The BASE64 encoding instruction . 39
21.1 General . 39
21.2 Restrictions . 39
21.3 Effect on encodings . 39
22 The DECIMAL encoding instruction . 40
22.1 General . 40
22.2 Restrictions . 40
22.3 Effect on encodings . 40
23 The DEFAULT-FOR-EMPTY encoding instruction . 41
23.1 General . 41
© ISO/IEC 2021 – All rights reserved
iv Rec. ITU-T X.693 (02/2021)

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ISO/IEC 8825-4:2021(E)
Page
23.2 Restrictions . 41
23.3 Effect on encodings . 42
24 The ELEMENT encoding instruction . 43
24.1 General . 43
24.2 Restrictions . 43
24.3 Effect on encodings . 43
25 The EMBED-VALUES encoding instruction . 43
25.1 General . 43
25.2 Restrictions . 43
25.3 Effect on encodings . 44
26 The GLOBAL-DEFAULTS encoding instruction . 44
26.1 General . 44
26.2 Restrictions . 45
26.3 Effect on encodings . 45
27 The LIST encoding instruction . 45
27.1 General . 45
27.2 Restrictions . 45
27.3 Effect on encodings . 46
28 The NAME encoding instruction . 46
28.1 General . 46
28.2 Restrictions . 47
28.3 Effect on encodings . 48
29 The NAMESPACE encoding instruction . 48
29.1 General . 48
29.2 Restrictions . 49
29.3 Effect on encodings . 49
30 The PI-OR-COMMENT encoding instruction . 50
30.1 General . 50
30.2 Restrictions . 50
30.3 Effect on the encodings . 50
31 The TEXT encoding instruction . 51
31.1 General . 51
31.2 Restrictions . 51
31.3 Effect on encodings . 52
32 The UNTAGGED encoding instruction . 52
32.1 General . 52
32.2 Restrictions . 53
32.3 Effect on encodings . 53
33 The USE-NIL encoding instruction . 54
33.1 General . 54
33.2 Restrictions . 54
33.3 Effect on encodings . 55
34 The USE-NUMBER encoding instruction . 55
34.1 General . 55
34.2 Restrictions . 55
© ISO/IEC 2021 – All rights reserved
 Rec. ITU-T X.693 (02/2021) v

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ISO/IEC 8825-4:2021(E)
Page
34.3 Effect on encodings . 55
35 The USE-ORDER encoding instruction . 56
35.1 General . 56
35.2 Restrictions . 56
35.3 Effect on encodings . 57
36 The USE-QNAME encoding instruction . 57
36.1 General . 57
36.2 Restrictions . 58
36.3 Effect on encodings . 58
37 The USE-TYPE encoding instruction . 58
37.1 General . 58
37.2 Restrictions . 58
37.3 Effect on encodings . 59
38 The USE-UNION encoding instruction . 59
38.1 General . 59
38.2 Restrictions . 59
38.3 Effect on encodings . 60
39 The WHITESPACE encoding instruction . 61
39.1 General . 61
39.2 Restrictions . 61
39.3 Effect on encodings . 61
40 Identification of the encoding rules . 62
Annex A Examples of BASIC-XER and CXER encodings . 63
A.1 ASN.1 description of the record structure . 63
A.2 ASN.1 description of a record value . 63
A.3 Basic XML representation of this record value . 63
A.4 Canonical XML representation of this record value . 64
Annex B – Partial XML content and deterministic encodings . 65
B.1 Partial XML content . 65
B.2 Recommended restrictions on encodings producing partial XML element content . 65
Annex C – Examples of extended-XER encodings using XER encoding instructions . 68
C.1 Introduction . 68
C.2 Simple examples . 68
C.3 More complex examples .
...