ISO/IEC 8825-1:2015

Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) - Part 1:

ISO/IEC 8825-1:2015 specifies a set of basic encoding rules that may be used to derive the specification of a transfer syntax for values of types defined using the notation specified in Rec. ITU-T X.680 | ISO/IEC 8824-1, Rec. ITU-T X.681 | ISO/IEC 8824-2, Rec. ITU-T X.682 | ISO/IEC 8824-3, and Rec. ITU-T X.683 | ISO/IEC 8824-4, collectively referred to as Abstract Syntax Notation One or ASN.1. These basic encoding rules are also to be applied for decoding such a transfer syntax in order to identify the data values being transferred. It also specifies a set of canonical and distinguished encoding rules that restrict the encoding of values to just one of the alternatives provided by the basic encoding rules.

Technologies de l'information — Règles de codage ASN.1: Spécification des règles de codage de base (BER), des règles de codage canoniques (CER) et des règles de codage distinctives (DER) — Partie 1:

General Information

• Status: Withdrawn
• Publication Date: 11-Nov-2015
• Withdrawal Date: 11-Nov-2015
• 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 - Telecommunications and information exchange between systems
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 30-Jun-2021
• Completion Date: 30-Oct-2025

Relations

• Consolidated By: ISO 22418:2020 - Intelligent transport systems - Fast service announcement protocol (FSAP) for general purposes in ITS
• Effective Date: 17-Dec-2022
• Revised: ISO/IEC 8825-1:2021 - Information technology - ASN.1 encoding rules - Part 1: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)
• Effective Date: 14-Oct-2020
• Revises: ISO/IEC 8825-1:2008 - Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) - Part 1:
• Effective Date: 05-Nov-2015

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 8825-1
Fifth edition
2015-11-15
Information technology — ASN.1
encoding rules: Specification of Basic
Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished
Encoding Rules (DER)
Technologies de l'information — Règles de codage ASN.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 2015
©  ISO/IEC 2015
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ii © ISO/IEC 2015 – All rights reserved

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. In the field of information
technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.
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.
This fifth edition cancels and replaces the fourth edition of ISO/IEC 8825-1:2008 which has been technically
revised. It also incorporates ISO/IEC 8825-1:2008/Cor.1:2012 and ISO/IEC 8825-5:2008.Cor.2:2014.
ISO/IEC 8825-1 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 (08/2015).
© ISO/IEC 2015 – All rights reserved iii

I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n

ITU-T X.690
(08/2015)
TELECOMMUNICATION
STANDARDIZATION SECTOR
OF ITU
SERIES X: DATA NETWORKS, OPEN SYSTEM
COMMUNICATIONS AND SECURITY
OSI networking and system aspects – Abstract Syntax
Notation One (ASN.1)
Information technology – ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)

Recommendation ITU-T X.690
ITU-T X-SERIES RECOMMENDATIONS
DATA NETWORKS, OPEN SYSTEM COMMUNICATIONS AND SECURITY

PUBLIC DATA NETWORKS
Services and facilities X.1–X.19
Interfaces X.20–X.49
Transmission, signalling and switching X.50–X.89
Network aspects X.90–X.149
Maintenance X.150–X.179
Administrative arrangements X.180–X.199
OPEN SYSTEMS INTERCONNECTION
Model and notation X.200–X.209
Service definitions X.210–X.219
Connection-mode protocol specifications X.220–X.229
Connectionless-mode protocol specifications X.230–X.239
PICS proformas X.240–X.259
Protocol Identification X.260–X.269
Security Protocols X.270–X.279
Layer Managed Objects X.280–X.289
Conformance testing X.290–X.299
INTERWORKING BETWEEN NETWORKS
General X.300–X.349
Satellite data transmission systems X.350–X.369
IP-based networks X.370–X.379
MESSAGE HANDLING SYSTEMS X.400–X.499
DIRECTORY X.500–X.599
OSI NETWORKING AND SYSTEM ASPECTS
Networking X.600–X.629
Efficiency X.630–X.639
Quality of service X.640–X.649
Naming, Addressing and Registration X.650–X.679
Abstract Syntax Notation One (ASN.1) X.680–X.699
OSI MANAGEMENT
Systems management framework and architecture X.700–X.709
Management communication service and protocol X.710–X.719
Structure of management information X.720–X.729
Management functions and ODMA functions X.730–X.799
SECURITY X.800–X.849
OSI APPLICATIONS
Commitment, concurrency and recovery X.850–X.859
Transaction processing X.860–X.879
Remote operations X.880–X.889
Generic applications of ASN.1 X.890–X.899
OPEN DISTRIBUTED PROCESSING X.900–X.999
INFORMATION AND NETWORK SECURITY X.1000–X.1099
SECURE APPLICATIONS AND SERVICES X.1100–X.1199
CYBERSPACE SECURITY X.1200–X.1299
SECURE APPLICATIONS AND SERVICES X.1300–X.1399
CYBERSECURITY INFORMATION EXCHANGE X.1500–X.1599
CLOUD COMPUTING SECURITY X.1600–X.1699

For further details, please refer to the list of ITU-T Recommendations.

INTERNATIONAL STANDARD ISO/IEC 8825-1
RECOMMENDATION ITU-T X.690
Information technology – ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)

Summary
Recommendation ITU-T X.690 | ISO/IEC 8825-1 defines a set of Basic Encoding Rules (BER) that may be applied to
values of types defined using the ASN.1 notation. Application of these encoding rules produces a transfer syntax for such
values. It is implicit in the specification of these encoding rules that they are also used for decoding. This
Recommendation | International Standard defines also a set of Distinguished Encoding Rules (DER) and a set of
Canonical Encoding Rules (CER) both of which provide constraints on the Basic Encoding Rules (BER). The key
difference between them is that DER uses the definite length form of encoding while CER uses the indefinite length
form. DER is more suitable for the small encoded values, while CER is more suitable for the large ones. It is implicit in
the specification of these encoding rules that they are also used for decoding.

History
*
Edition Recommendation Approval Study Group Unique ID
1.0 ITU-T X.690 1994-07-01 7 11.1002/1000/3046
1.1 ITU-T X.690 (1994) Technical Cor. 1 1995-11-21 7 11.1002/1000/3283
1.2 ITU-T X.690 (1994) Technical Cor. 2 1997-12-12 7 11.1002/1000/4182
1.3 ITU-T X.690 (1994) Technical Cor. 3 1997-12-12 7 11.1002/1000/4183
2.0 ITU-T X.690 1997-12-12 7 11.1002/1000/4447
2.1 ITU-T X.690 (1997) Technical Cor. 1 1999-06-18 7 11.1002/1000/4705
2.2 ITU-T X.690 (1997) Amd. 1 1999-06-18 7 11.1002/1000/4704
2.3 ITU-T X.690 (1997) Technical Cor. 2 2001-02-02 7 11.1002/1000/5335
3.0 ITU-T X.690 2002-07-14 17 11.1002/1000/6089
3.1 ITU-T X.690 (2002) Amd. 1 2003-10-29 17 11.1002/1000/7021
3.2 ITU-T X.690 (2002) Amd. 2 2006-06-13 17 11.1002/1000/8838
3.3 ITU-T X.690 (2002) Technical Cor. 1 2007-05-29 17 11.1002/1000/9108
4.0 ITU-T X.690 2008-11-13 17 11.1002/1000/9608
4.1 ITU-T X.690 (2008) Cor. 1 2011-10-14 17 11.1002/1000/11378
4.2 ITU-T X.690 (2008) Cor. 2 2014-03-01 17 11.1002/1000/12147
5.0 ITU-T X.690 2015-08-13 17 11.1002/1000/12483

*
To access the Recommendation, type the URL http://handle.itu.int/ in the address field of your web browser, followed by the
Recommendation's unique ID. For example, http://handle.itu.int/11.1002/1000/11830-en.
Rec. ITU-T X.690 (08/2015) i
FOREWORD
The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of
telecommunications, information and communication technologies (ICTs). The ITU Telecommunication
Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical,
operating and tariff questions and issuing Recommendations on them with a view to standardizing
telecommunications on a worldwide basis.
The World Telecommunication Standardization Assembly (WTSA), which meets every four years,
establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on
these topics.
The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
In some areas of information technology which fall within ITU-T's purview, the necessary standards are
prepared on a collaborative basis with ISO and IEC.

NOTE
In this Recommendation, the expression "Administration" is used for conciseness to indicate both a
telecommunication administration and a recognized operating agency.
Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain
mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the
Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some
other obligatory language such as "must" and the negative equivalents are used to express requirements. The
use of such words does not suggest that compliance with the Recommendation is required of any party.

INTELLECTUAL PROPERTY RIGHTS
ITU draws attention to the possibility that the practice or implementation of this Recommendation may
involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence,
validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others
outside of the Recommendation development process.
As of the date of approval of this Recommendation, ITU had not received notice of intellectual property,
protected by patents, which may be required to implement this Recommendation. However, implementers
are cautioned that this may not represent the latest information and are therefore strongly urged to consult the
TSB patent database at http://www.itu.int/ITU-T/ipr/.

 ITU 2015
All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the
prior written permission of ITU.
ii Rec. ITU-T X.690 (11/2015)
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 . 1
4 Abbreviations . 2
5 Notation . 2
6 Convention . 2
7 Conformance . 3
8 Basic encoding rules . 3
8.1 General rules for encoding . 3
8.2 Encoding of a boolean value . 6
8.3 Encoding of an integer value . 6
8.4 Encoding of an enumerated value . 7
8.5 Encoding of a real value . 7
8.6 Encoding of a bitstring value. 8
8.7 Encoding of an octetstring value . 9
8.8 Encoding of a null value . 10
8.9 Encoding of a sequence value . 10
8.10 Encoding of a sequence-of value. 10
8.11 Encoding of a set value . 10
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 . 12
8.18 Encoding of a value of the external type . 12
8.19 Encoding of an object identifier value. 13
8.20 Encoding of a relative object identifier value . 14
8.21 Encoding of an OID internationalized resource identifier value . 14
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 . 17
8.25 Encoding for values of the useful types . 17
8.26 Encoding for values of the TIME type and the useful time types . 17
9 Canonical encoding rules . 17
9.1 Length forms . 18
9.2 String encoding forms . 18
9.3 Set components . 18
10 Distinguished encoding rules. 18
10.1 Length forms . 18
10.2 String encoding forms . 18
10.3 Set components . 19
11 Restrictions on BER employed by both CER and DER . 19
11.1 Boolean values . 19
11.2 Unused bits . 19
11.3 Real values . 19
Rec. ITU-T X.690 (08/2015) iii

11.4 GeneralString values . 19
11.5 Set and sequence components with default value . 20
11.6 Set-of components . 20
11.7 GeneralizedTime . 20
11.8 UTCTime . 20
11.9 The TIME type and the useful time types. 21
12 Use of BER, CER and DER in transfer syntax definition . 21
Annex A – Example of encodings . 23
A.1 ASN.1 description of the record structure . 23
A.2 ASN.1 description of a record value . 23
A.3 Representation of this record value . 23
Annex B – Identification of Encoding Rules . 25
Annex C – Illustration of real value encoding . 26

iv Rec. ITU-T X.690 (11/2015)
Introduction
Rec. ITU-T X.680 | ISO/IEC 8824-1, Rec. ITU-T X.681 | ISO/IEC 8824-2, Rec. ITU-T X.682 | ISO/IEC 8824-3,
Rec. ITU-T X.683 | ISO/IEC 8824-4 (Abstract Syntax Notation One or ASN.1) together specify a notation for the
definition of abstract syntaxes, enabling application standards to define the types of information they need to transfer. It
also specifies a notation for the specification of values of a defined type.
This Recommendation | International Standard defines encoding rules that may be applied to values of types defined
using the ASN.1 notation. Application of these encoding rules produces a transfer syntax for such values. It is implicit
in the specification of these encoding rules that they are also to be used for decoding.
There may be more than one set of encoding rules that can be applied to values of types that are defined using the
ASN.1 notation. This Recommendation | International Standard defines three sets of encoding rules, called basic
encoding rules, canonical encoding rules and distinguished encoding rules. Whereas the basic encoding rules give the
sender of an encoding various choices as to how data values may be encoded, the canonical and distinguished encoding
rules select just one encoding from those allowed by the basic encoding rules, eliminating all of the sender's options.
The canonical and distinguished encoding rules differ from each other in the set of restrictions that they place on the
basic encoding rules.
The distinguished encoding rules is more suitable than the canonical encoding rules if the encoded value is small
enough to fit into the available memory and there is a need to rapidly skip over some nested values. The canonical
encoding rules is more suitable than the distinguished encoding rules if there is a need to encode values that are so large
that they cannot readily fit into the available memory or it is necessary to encode and transmit a part of a value before
the entire value is available. The basic encoding rules is more suitable than the canonical or distinguished encoding
rules if the encoding contains a set value or set-of value and there is no need for the restrictions that the canonical and
distinguished encoding rules impose. This is due to the memory and CPU overhead that the latter encoding rules exact
in order to guarantee that set values and set-of values have just one possible encoding.
Annex A gives an example of the application of the basic encoding rules. It does not form an integral part of this
Recommendation | International Standard.
Annex B summarizes the assignment of object identifier and OID internationalized resource identifier values made in
this Recommendation | International Standard. It does not form an integral part of this Recommendation | International
Standard.
Annex C gives examples of applying the basic encoding rules for encoding reals. It does not form an integral part of this
Recommendation | International Standard.

Rec. ITU-T X.690 (08/2015) v
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology – ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER)
and Distinguished Encoding Rules (DER)
1 Scope
This Recommendation | International Standard specifies a set of basic encoding rules that may be used to derive the
specification of a transfer syntax for values of types defined using the notation specified in Rec. ITU-T X.680 |
ISO/IEC 8824-1, Rec. ITU-T X.681 | ISO/IEC 8824-2, Rec. ITU-T X.682 | ISO/IEC 8824-3, and Rec. ITU-T X.683 |
ISO/IEC 8824-4, collectively referred to as Abstract Syntax Notation One or ASN.1. These basic encoding rules are
also to be applied for decoding such a transfer syntax in order to identify the data values being transferred. It also
specifies a set of canonical and distinguished encoding rules that restrict the encoding of values to just one of the
alternatives provided by the basic encoding rules.
2 Normative references
The following Recommendations and International Standards contain provisions which, through reference in this text,
constitute provisions of this Recommendation | International Standard. At the time of publication, the editions indicated
were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on this
Recommendation | International Standard are encouraged to investigate the possibility of applying the most recent
edition of the Recommendations and Standards listed below. Members of IEC and ISO maintain registers of currently
valid International Standards. The Telecommunication Standardization Bureau of the ITU maintains a list of currently
valid ITU-T Recommendations.
NOTE – This Recommendation | International Standard is based on ISO/IEC 10646:2003. It cannot be applied using later
versions of this standard.
2.1 Identical Recommendations | International Standards
– Recommendation ITU-T X.680 (2015) | ISO/IEC 8824-1:2015, Information technology – Abstract
Syntax Notation One (ASN.1): Specification of basic notation.
– Recommendation ITU-T X.681 (2015) | ISO/IEC 8824-2:2015, Information technology – Abstract
Syntax Notation One (ASN.1): Information object specification.
– Recommendation ITU-T X.682 (2015) | ISO/IEC 8824-3:2015, Information technology – Abstract
Syntax Notation One (ASN.1): Constraint specification.
– Recommendation ITU-T X.683 (2015) | ISO/IEC 8824-4:2015, Information technology – Abstract
Syntax Notation One (ASN.1): Parameterization of ASN.1 specifications.
2.2 Additional references
– ISO International Register of Coded Character Sets to be used with Escape Sequences.
– ISO/IEC 2022:1994, Information technology – Character code structure and extension techniques.
– ISO/IEC 2375:2003, Information technology – Procedure for registration of escape sequences and
coded character sets.
– ISO 6093:1985, Information processing – Representation of numerical values in character strings for
information interchange.
– ISO/IEC 6429:1992, Information technology – Control functions for coded character sets.
– ISO/IEC 10646:2003, Information technology – Universal Multiple-Octet Coded Character Set (UCS).
3 Definitions
For the purposes of this Recommendation | International Standard, the definitions of Rec. ITU-T X.200 |
ISO/IEC 7498-1 and Rec. ITU-T X.680 | ISO/IEC 8824-1 and the following definitions apply.
Rec. ITU-T X.690 (08/2015) 1
3.1 canonical encoding: A complete encoding of an abstract value obtained by the application of encoding rules
that have no implementation-dependent options. Such rules result in the definition of a 1-1 mapping between
unambiguous and unique encodings and values in the abstract syntax.
3.2 constructed encoding: A data value encoding in which the contents octets are the complete encoding of
one or more data values.
3.3 contents octets: That part of a data value encoding which represents a particular value, to distinguish it
from other values of the same type.
3.4 data value: Information specified as the value of a type; the type and the value are defined using ASN.1.
3.5 dynamic conformance: A statement of the requirement for an implementation to adhere to the
prescribed behaviour in an instance of communication.
3.6 encoding (of a data value): The complete sequence of octets used to represent the data value.
3.7 end-of-contents octets: Part of a data value encoding, occurring at its end, which is used to determine
the end of the encoding.
NOTE – Not all encodings require end-of-contents octets.
3.8 identifier octets: Part of a data value encoding which is used to identify the type of the value.
NOTE – Some ITU-T Recommendations use the term "data element" for this sequence of octets, but the term is not used in this
Recommendation | International Standard, as other Recommendations | International Standards use it to mean "data value".
3.9 length octets: Part of a data value encoding following the identifier octets which is used to determine the
end of the encoding.
3.10 primitive encoding: A data value encoding in which the contents octets directly represent the value.
3.11 receiver: An implementation decoding the octets produced by a sender, in order to identify the data value
which was encoded.
3.12 sender: An implementation encoding a data value for transfer.
3.13 static conformance: A statement of the requirement for support by an implementation of a valid set of
features from among the defined features.
3.14 trailing 0 bit: A 0 in the last position of a bitstring value.
NOTE – The 0 in a bitstring value consisting of a single 0 bit is a trailing 0 bit. Its removal produces an empty bitstring.
4 Abbreviations
For the purposes of this Recommendation | International Standard, the following abbreviations apply:
ASN.1 Abstract Syntax Notation One
BER Basic Encoding Rules of ASN.1
CER Canonical Encoding Rules of ASN.1
DER Distinguished Encoding Rules of ASN.1
ULA Upper Layer Architecture
UTF8 Universal Transformation Function 8-bit (see ISO/IEC 10646, Annex D)
5 Notation
This Recommendation | International Standard references the notation defined by Rec. ITU-T X.680 | ISO/IEC 8824-1.
6 Convention
6.1 This Recommendation | International Standard specifies the value of each octet in an encoding by use of
the terms "most significant bit" and "least significant bit".
NOTE – Lower layer specifications use the same notation to define the order of bit transmission on a serial line, or the
assignment of bits to parallel channels.
6.2 For the purposes of this Recommendation | International Standard only, the bits of an octet are numbered
from 8 to 1, where bit 8 is the "most significant bit", and bit 1 is the "least significant bit".
2 Rec. ITU-T X.690 (08/2015)
6.3 For the purpose of this Recommendation | International Standard, two octet strings can be compared. One
octet string is equal to another if they are of the same length and are the same at each octet position. An octet string, S ,
is greater than another, S , if and only if either:
a) S and S have identical octets in every position up to and including the final octet in S , but S is longer;
1 2 2 1
or
b) S and S have different octets in one or more positions, and in the first such position, the octet in S is
1 2 1
n–1
greater than that in S , considering the octets as unsigned binary numbers whose bit n has weight 2 .
7 Conformance
7.1 Dynamic conformance is specified by clauses 8 to 12 inclusive.
7.2 Static conformance is specified by those standards which specify the application of one or more of these
encoding rules.
7.3 Alternative encodings are permitted by the basic encoding rules as a sender's option. Receivers who
claim conformance to the basic encoding rules shall support all alternatives.
NOTE – Examples of such alternative encodings appear in 8.1.3.2 b) and Table 3.
7.4 No alternative encodings are permitted by the Canonical Encoding Rules or Distinguished Encoding
Rules.
8 Basic encoding rules
8.1 General rules for encoding
8.1.1 Structure of an encoding
8.1.1.1 The encoding of a data value shall consist of four components which shall appear in the following order:
a) identifier octets (see 8.1.2);
b) length octets (see 8.1.3);
c) contents octets (see 8.1.4);
d) end-of-contents octets (see 8.1.5).
8.1.1.2 The end-of-contents octets shall not be present unless the value of the length octets requires them to be
present (see 8.1.3).
8.1.1.3 Figure 1 illustrates the structure of an encoding (primitive or constructed). Figure 2 illustrates an
alternative constructed encoding.
Identifier octets Length octets Contents octets
The number of octets
in the contents octets
(see 8.1.3.2) X.690_F1
Figure 1 – Structure of an encoding
Rec. ITU-T X.690 (08/2015) 3
Identifier octets Length octets Contents octets End-of-contents octets
Indicates that the contents Indicates that there are
octets are terminated by no further encodings
end of contents octets in the contents octets
(see 8.1.3.6)
X.690_F2
Figure 2 – An alternative constructed encoding
8.1.1.4 Encodings specified in this Recommendation | International Standard are not affected by either the ASN.1
subtype notation or the ASN.1 type extensibility notation.
NOTE – This means that all constraint notation is ignored when determining encodings, and all extensibility markers in CHOICE,
SEQUENCE and SET are ignored, with the extensions treated as if they were in the extension root of the type.
8.1.1.5 There are no encoding instructions (see Rec. ITU-T X.680 | ISO/IEC 8824-1, 3.8.27) defined for the encoding
rules specified in this Recommendation | International Standard.
8.1.2 Identifier octets
8.1.2.1 The identifier octets shall encode the ASN.1 tag (class and number) of the type of the data value.
8.1.2.2 For tags with a number ranging from zero to 30 (inclusive), the identifier octets shall comprise a single
octet encoded as follows:
a) bits 8 and 7 shall be encoded to represent the class of the tag as specified in Table 1;
b) bit 6 shall be a zero or a one according to the rules of 8.1.2.5;
c) bits 5 to 1 shall encode the number of the tag as a binary integer with bit 5 as the most significant bit.
Table 1 – Encoding of class of tag
Class Bit 8 Bit 7
Universal 0 0
Application 0 1
Context-specific 1 0
Private 1 1
8.1.2.3 Figure 3 illustrates the form of an identifier octet for a type with a tag whose number is in the range zero
to 30 (inclusive).
Identifier octet
8 7 6 5 4 3 2 1
Bits
Tag number
Class P/C
0 = Primitive
1 = Constructed
X.690_F3
Figure 3 – Identifier octet (low tag number)
8.1.2.4 For tags with a number greater than or equal to 31, the identifier shall comprise a leading octet followed
by one or more subsequent octets.
8.1.2.4.1 The leading octet shall be encoded as follows:
a) bits 8 and 7 shall be encoded to represent the class of the tag as listed in Table 1;
b) bit 6 shall be a zero or a one according to the rules of 8.1.2.5;
c) bits 5 to 1 shall be encoded as 11111 .
8.1.2.4.2 The subsequent octets shall encode the number of the tag as follows:
a) bit 8 of each octet shall be set to one unless it is the last octet of the identifier octets;
4 Rec. ITU-T X.690 (08/2015)
b) bits 7 to 1 of the first subsequent octet, followed by bits 7 to 1 of the second subsequent octet, followed
in turn by bits 7 to 1 of each further octet, up to and including the last subsequent octet in the identifier
octets shall be the encoding of an unsigned binary integer equal to the tag number, with bit 7 of the first
subsequent octet as the most significant bit;
c) bits 7 to 1 of the first subsequent octet shall not all be zero.
8.1.2.4.3 Figure 4 illustrates the form of the identifier octets for a type with a tag whose number is greater than 30.
Subsequent octets
Leading octet 2nd octet Last octet
1 1 1 1 1 1 1 1 0
Class P/C
+ + + +
X.690_F4
= Number of tag
Figure 4 – Identifier octets (high tag number)
8.1.2.5 Bit 6 shall be set to zero if the encoding is primitive, and shall be set to one if the encoding is
constructed.
NOTE – Subsequent subclauses specify whether the encoding is primitive or constructed for each type.
8.1.2.6 Rec. ITU-T X.680 | ISO/IEC 8824-1 specifies that the tag of a type defined using the CHOICE keyword
takes the value of the tag of the type from which the chosen data value is taken.
8.1.2.7 Rec. ITU-T X.681 | ISO/IEC 8824-2, 14.2 and 14.4, specifies that the tag of a type defined using
"ObjectClassFieldType" is indeterminate if it is a type field, a variable-type value field, or a variable-type value set
field. This type is subsequently defined to be an ASN.1 type, and the complete encoding is then identical to that of a
value of the assigned type (including the identifier octets).
8.1.3 Length octets
8.1.3.1 Two forms of length octets are specified. These are:
a) the definite form (see 8.1.3.3); and
b) the indefinite form (see 8.1.3.6).
8.1.3.2 A sender shall:
a) use the definite form (see 8.1.3.3) if the encoding is primitive;
b) use either the definite form (see 8.1.3.3) or the indefinite form (see 8.1.3.6), a sender's option, if the
encoding is constructed and all immediately available;
c) use the indefinite form (see 8.1.3.6) if the encoding is constructed and is not all immediately available.
8.1.3.3 For the definite form, the length octets shall consist of one or more octets, and shall represent the number
of octets in the contents octets using either the short form (see 8.1.3.4) or the long form (see 8.1.3.5) as a sender's
option.
NOTE – The short form can only be used if the number of octets in the contents octets is less than or equal to 127.
8.1.3.4 In the short form, the length octets shall consist of a single octet in which bit 8 is zero and bits 7 to 1
encode the number of octets in the contents octets (which may be zero), as an unsigned binary integer with bit 7 as the
most significant bit.
EXAMPLE
L = 38 can be encoded as 00100110
8.1.3.5 In the long form, the length octets shall consist of an initial octet and one or more subsequent octets. The
initial octet shall be encoded as follows:
a) bit 8 shall be one;
b) bits 7 to 1 shall encode the number of subsequent octets in the length octets, as an unsigned binary
integer with bit 7 as the most significant bit;
c) the value 11111111 shall not be used.
Rec. ITU-T X.690 (08/2015) 5
NOTE 1 – This restriction is introduced for possible future extension.
Bits 8 to 1 of the first subsequent octet, followed by bits 8 to 1 of the second subsequent octet, followed in turn by bits 8
to 1 of each further octet up to and including the last subsequent octet, shall be the encoding of an unsigned binary
integer equal to the number of octets in the contents octets, with bit 8 of the first subsequent octet as the most
significant bit.
EXAMPLE
L = 201 can be encoded as:
NOTE 2 – In the long form, it is a sender's option whether to use more length octets than the minimum necessary.
8.1.3.6 For the indefinite form, the length octets indicate that the contents octets are terminated by end-of-
contents octets (see 8.1.5), and shall consist of a single octet.
8.1.3.6.1 The single octet shall have bit 8 set to one, and bits 7 to 1 set to zero.
8.1.3.6.2 If this form of length is used, then end-of-contents octets (see 8.1.5) shall be present in the encoding
following the contents octets.
8.1.4 Contents octets
The contents octets shall consist of zero, one or more octets, and shall encode the data value as specified in subsequent
clauses.
NOTE – The contents octets depend on the type of the data value; subsequent clauses follow the same sequence as the definition
of types in ASN.1.
8.1.5 End-of-contents octets
The end-of-contents octets shall be present if the length is encoded as specified in 8.1.3.6, otherwise they shall not be
present.
The end-of-contents octets shall consist of two zero octets.
NOTE – The end-of-contents octets can be considered as the encoding of a value whose tag is universal class, whose form is
primitive, whose number of the tag is zero, and whose contents are absent, thus:
End-of-contents Length Contents
00 00 Absent
16 16
8.2 Encoding of a boolean value
8.2.1 The encoding of a boolean value shall be primitive. The contents octets shall consist of a single octet.
8.2.2 If the boolean value is:
FALSE
the octet shall be zero.
If the boolean value is
TRUE
the octet shall have any non-zero value, as a sender's option.
EXAMPLE
If of type BOOLEAN, the value TRUE can be encoded as:
Boolean Length Contents
01 01 FF
16 16 16
8.3 Encoding of an integer value
8.3.1 The encoding of an integer value shall be primitive. The contents octets shall consist of one or more
octets.
6 Rec. ITU-T X.690 (08/2015)
8.3.2 If the contents octets of an integer value encoding consist of more than one octet, then the bits of the first
octet and bit 8 of the second octet:
a) shall not all be ones; and
b) shall not all be zero.
NOTE – These rules ensure that an integer value is always encoded in the smallest possible number of octets.
8.3.3 The contents octets shall be a two's complement binary number equal to the integer value, and consisting
of bits 8 to 1 of the first octet, followed by bits 8 to 1 of the second octet, followed by bits 8 to 1 of each octet in turn up
to and including the last octet of the contents octets.
NOTE – The value of a two's complement binary number is derived by numbering the bits in the contents octets, starting with bit
N
1 of the last octet as bit zero and ending the numbering with bit 8 of the first octet. Each bit is assigned a numerical value of 2 ,
where N is its position in the above numbering sequence. The value of the two's complement binary number is obtained by
summing the numerical values assigned to each bit for those bits which are set to one, excluding bit 8 of the first octet, and then
reducing this value by the numerical value assigned to bit 8 of the first octet if that bit is set to one.
8.4 Encoding of an enumerated value
The encoding of an enumerated value shall be that of the integer value with which it is associated.
NOTE – It is primitive.
8.5 Encoding of a real value
8.5.1 The encoding of a real value shall be primitive.
8.5.2 If the real value is the value plus zero, there shall be no contents octets in the encoding.
8.5.3 If the real value is the value minus zero, then it shall be encoded as specified in 8.5.9.
8.5.4 For a non-zero real value, if the base of the abstract value is 10, then the base of the encoded value shall be
10, and if the base of the abstract value is 2 the base of the encoded value shall be 2, 8 or 16 as a sender's option.
8.5.5 If the real value is non-zero, then the base used for the encoding shall be B' as specified in 8.5.4. If B' is
2, 8 or 16, a binary encoding, specified in 8.5.7, shall be used. If B' is 10, a character encoding, specified in 8.5.8, shall
be used.
8.5.6 Bit 8 of the first contents octet shall be set as follows:
a) if bit 8 = 1, then the binary encoding specified in 8.5.7 applies;
b) if bit 8 = 0 and bit 7 = 0, then the decimal encoding specified in 8.5.8 applies;
c) if bit 8 = 0 and bit 7 = 1, then either a "SpecialRealValue" (see Rec. ITU-T X.680 | ISO/IEC 8824-1) or
the value minus zero is encoded as specified in 8.5.9.
8.5.7 When binary encoding is used (bit 8 = 1), then if the mantissa M is non-zero, it shall be represented by a
sign S, a positive integer value N and a binary scaling factor F, such that:
F
M = S  N  2
0  F < 4
S = +1 or –1
NOTE – The binary scaling factor F is required under certain circumstances in order to align the implied point of the mantissa to
the position required by the encoding rules of this subclause. This alignment cannot always be achieved by modification of the
exponent E. If the base B' used for encoding is 8 or 16, the implied point can only be moved in steps of 3 or 4 bits, respectively,
by changing the component E. Therefore, values of the binary scaling factor F other than zero may be required in order to move
the implied point to the required position.
8.5.7.1 Bit 7 of the first contents octets shall be 1 if S is –1 and 0 otherwise.
8.5.7.2 Bits 6 to 5 of the first contents octets shall encode the value of the base B' as follows:
Bits 6 to 5 Base
00 base 2
01 base 8
10 base 16
11 Reserved for further editions of this Recommendation | International Standard.
8.5.7.3 Bits 4 to 3 of the first contents octet shall en
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