ISO/IEC DIS 8825-1

Information technology -- ASN.1 encoding rules

Technologies de l'information -- Règles de codage ASN.1

Standards Content (sample)

DRAFT INTERNATIONAL STANDARD
ISO/IEC DIS 8825-1
ISO/IEC JTC 1/SC 6 Secretariat: KATS
Voting begins on: Voting terminates on:
2020-09-28 2020-12-21
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)
ICS: 35.100.60
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
This document is circulated as received from the committee secretariat.
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STANDARDS MAY ON OCCASION HAVE TO
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WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/IEC DIS 8825-1:2020(E)
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ISO/IEC DIS 8825-1:2020(E)
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ISO/IEC DIS 8825-1:2020(E)
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 ........................................................................................................................................................ 2

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 ......................................................... 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 .................................................... 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

9 Canonical encoding rules ........................................................................................................................... 18

9.1 Length forms ................................................................................................................................... 18

9.2 String encoding forms ..................................................................................................................... 18

9.3 Set components ............................................................................................................................... 18

Rec. ITU-T X.690 (08/2015) iii
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ISO/IEC DIS 8825-1:2020(E)

10 Distinguished encoding rules...................................................................................................................... 19

10.1 Length forms ................................................................................................................................... 19

10.2 String encoding forms ..................................................................................................................... 19

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 ...................................................................................................................................... 20

11.4 GeneralString values ....................................................................................................................... 20

11.5 Set and sequence components with default value ........................................................................... 20

11.6 Set-of components ........................................................................................................................... 20

11.7 GeneralizedTime ............................................................................................................................. 20

11.8 UTCTime ........................................................................................................................................ 21

11.8.4 Examples of valid representations ................................................................................. 21

11.8.5 Examples of invalid representations .............................................................................. 21

11.9 The TIME type and the useful time types........................................................................................ 21

12 Use of BER, CER and DER in transfer syntax definition .......................................................................... 22

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

Rec. ITU-T X.690 (08/2015) iv
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ISO/IEC DIS 8825-1:2020(E)
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
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ISO/IEC DIS 8825-1:2020(E)
ISO/IEC 8825-1:2015 (E)
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 (2020) | ISO/IEC 8824-1:2020, Information technology – Abstract

Syntax Notation One (ASN.1): Specification of basic notation.

– Recommendation ITU-T X.681 (2020) | ISO/IEC 8824-2:2020, Information technology – Abstract

Syntax Notation One (ASN.1): Information object specification.

– Recommendation ITU-T X.682 (2020) | ISO/IEC 8824-3:2020, Information technology – Abstract

Syntax Notation One (ASN.1): Constraint specification.

– Recommendation ITU-T X.683 (2020) | ISO/IEC 8824-4:2020, 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).

Rec. ITU-T X.690 (08/2015) 1
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ISO/IEC DIS 8825-1:2020(E)
ISO/IEC 8825-1:2015 (E)
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.

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.
Rec. ITU-T X.690 (08/2015) 2
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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".

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:

1 2

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

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
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ISO/IEC DIS 8825-1:2020(E)
ISO/IEC 8825-1:2015 (E)
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;

Rec. ITU-T X.690 (08/2015) 4
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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 (s
...