ISO/IEC 8825-2:2021

INTERNATIONAL ISO/IEC
STANDARD 8825-2
Sixth edition
2021-06
Information technology — ASN.1
encoding rules —
Part 2:
Specification of Packed Encoding
Rules (PER)
Technologies de l'information — Règles de codage ASN.1 —
Partie 2: Spécification des règles de codage compact (PER)
Reference number
ISO/IEC 8825-2:2021(E)
©
ISO/IEC 2021

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ISO/IEC 8825-2:2021(E)

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ISO/IEC 8825-2:2021(E)
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.691 (02/2021).
This sixth edition cancels and replaces the fifth edition (ISO/IEC 8825-2:2015), which has been
technically revised. It also incorporates ISO/IEC 8825-2:2015/Cor 1:2017.
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.

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ISO/IEC 8825-2:2021(E)
CONTENTS
Page
Introduction . vi
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards . 1
2.2 Additional references . 1
3 Definitions . 2
3.1 Specification of Basic Notation . 2
3.2 Information Object Specification . 2
3.3 Constraint Specification . 2
3.4 Parameterization of ASN.1 Specification . 2
3.5 Basic Encoding Rules . 2
3.6 PER Encoding Instructions . 2
3.7 Additional definitions . 2
4 Abbreviations . 5
5 Notation . 5
6 Convention . 5
7 Encoding rules defined in this Recommendation | International Standard . 5
8 Conformance . 6
9 PER encoding instructions . 6
10 The approach to encoding used for PER . 7
10.1 Use of the type notation . 7
10.2 Use of tags to provide a canonical order . 7
10.3 PER-visible constraints . 7
10.4 Type and value model used for encoding . 9
10.5 Structure of an encoding . 9
10.6 Types to be encoded . 10
11 Encoding procedures . 10
11.1 Production of the complete encoding . 10
11.2 Open type fields . 11
11.3 Encoding as a non-negative-binary-integer . 11
11.4 Encoding as a 2's-complement-binary-integer . 12
11.5 Encoding of a constrained whole number . 12
11.6 Encoding of a normally small non-negative whole number . 13
11.7 Encoding of a semi-constrained whole number . 13
11.8 Encoding of an unconstrained whole number . 13
11.9 General rules for encoding a length determinant . 14
12 Encoding the boolean type . 16
13 Encoding the integer type . 16
14 Encoding the enumerated type . 17
15 Encoding the real type . 18
16 Encoding the bitstring type . 18
17 Encoding the octetstring type . 19
18 Encoding the null type . 19
19 Encoding the sequence type . 19
20 Encoding the sequence-of type . 20
21 Encoding the set type . 21
22 Encoding the set-of type . 21
23 Encoding the choice type. 21
Rec. ITU-T X.691 (02/2021) iii
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ISO/IEC 8825-2:2021(E)
24 Encoding the object identifier type . 22
25 Encoding the relative object identifier type . 22
26 Encoding the internationalized resource reference type . 22
27 Encoding the relative internationalized resource reference type . 23
28 Encoding the embedded-pdv type . 23
29 Encoding of a value of the external type . 23
30 Encoding the restricted character string types . 24
31 Encoding the unrestricted character string type . 26
32 Encoding the time type, the useful time types, the defined time types and the additional time types . 26
32.1 General . 26
32.2 Encoding subtypes with the "Basic=Date" property setting . 30
32.3 Encoding subtypes with the "Basic=Time" property setting . 32
32.4 Encoding subtypes with the "Basic=Date-Time" property setting . 35
32.5 Encoding subtypes with the "Basic=Interval Interval-type=SE" property setting . 35
32.6 Encoding subtypes with the "Basic=Interval Interval-type=D" property setting . 36
32.7 Encoding subtypes with the "Basic=Interval Interval-type=SD" or
"Basic=Interval Interval-type=DE" property setting . 37
32.8 Encoding subtypes with the "Basic=Rec-Interval Interval-type=SE"
property setting. 38
32.9 Encoding subtypes with the "Basic=Rec-Interval Interval-type=D" property setting . 38
32.10 Encoding subtypes with the "Basic=Rec-Interval Interval-type=SD" or
"Basic=Rec-Interval Interval-type=DE" property setting . 39
32.11 Encoding subtypes with mixed settings of the Basic property . 40
33 Object identifiers for transfer syntaxes . 42
Annex A – Example of encodings . 43
A.1 Record that does not use subtype constraints . 43
A.1.1 ASN.1 description of the record structure . 43
A.1.2 ASN.1 description of a record value . 43
A.1.3 ALIGNED PER representation of this record value . 43
A.1.4 UNALIGNED PER representation of this record value . 44
A.2 Record that uses subtype constraints . 46
A.2.1 ASN.1 description of the record structure . 46
A.2.2 ASN.1 description of a record value . 46
A.2.3 ALIGNED PER representation of this record value . 46
A.2.4 UNALIGNED PER representation of this record value . 47
A.3 Record that uses extension markers . 48
A.3.1 ASN.1 description of the record structure . 48
A.3.2 ASN.1 description of a record value . 49
A.3.3 ALIGNED PER representation of this record value . 49
A.3.4 UNALIGNED PER representation of this record value . 50
A.4 Record that uses extension addition groups . 52
A.4.1 ASN.1 description of the record structure . 52
A.4.2 ASN.1 description of a record value . 52
A.4.3 ALIGNED PER representation of this record value . 52
A.4.4 UNALIGNED PER representation of this record value . 53
Annex B – Combining PER-visible and non-PER-visible constraints . 54
B.1 General . 54
B.2 Extensibility and visibility of constraints in PER . 54
B.2.1 General . 54
B.2.2 PER-visibility of constraints . 55
B.2.3 Effective constraints . 56
B.3 Examples . 57
iv Rec. ITU-T X.691 (02/2021)
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ISO/IEC 8825-2:2021(E)
Annex C – Support for the PER algorithms . 59
Annex D – Support for the ASN.1 rules of extensibility . 60
Annex E – Tutorial annex on concatenation of PER encodings . 61
Annex F – Identification of Encoding Rules . 62

 Rec. ITU-T X.691 (02/2021) v
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ISO/IEC 8825-2:2021(E)
Introduction
Specifications 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 together describe Abstract Syntax Notation One (ASN.1),
a notation for the definition of messages to be exchanged between peer applications.
This Recommendation | International Standard defines encoding rules that may be applied to values of types defined using
the notation specified in Rec. ITU-T X.680 | ISO/IEC 8824-1. 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 are more than one set of encoding rules that can be applied to values of ASN.1 types. This Recommendation |
International Standard defines a set of Packed Encoding Rules (PER), so called because they achieve a much more
compact representation than that achieved by the Basic Encoding Rules (BER) and its derivatives described in Rec.
ITU-T X.690 | ISO/IEC 8825-1 which is referenced for some parts of the specification of these Packed Encoding Rules.

vi Rec. ITU-T X.691 (02/2021)
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ISO/IEC 8825-2:2021 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology –
ASN.1 encoding rules:
Specification of Packed Encoding Rules (PER)
1 Scope
This Recommendation | International Standard specifies a set of Packed Encoding Rules that may be used to derive a
transfer syntax for values of types defined in Rec. ITU-T X.680 | ISO/IEC 8824-1. These Packed Encoding Rules are also
to be applied for decoding such a transfer syntax in order to identify the data values being transferred.
The encoding rules specified in this Recommendation | International Standard:
– are used at the time of communication;
– are intended for use in circumstances where minimizing the size of the representation of values is the major
concern in the choice of encoding rules;
– allow the extension of an abstract syntax by addition of extra values, preserving the encodings
of the existing values, for all forms of extension described in Rec. ITU-T X.680 | ISO/IEC 8824-1;
– can be modified in accordance with the provisions of Rec. ITU-T X.695 | ISO/IEC 8825-6.
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 (2021) | ISO/IEC 8824-1:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Specification of basic notation.
– Recommendation ITU-T X.681 (2021) | ISO/IEC 8824-2:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Information object specification.
– Recommendation ITU-T X.682 (2021) | ISO/IEC 8824-3:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Constraint specification.
– Recommendation ITU-T X.683 (2021) | ISO/IEC 8824-4:2021, Information technology – Abstract Syntax
Notation One (ASN.1): Parameterization of ASN.1 specifications.
– Recommendation ITU-T X.690 (2021) | ISO/IEC 8825-1:2021, 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.695 (2021) | ISO/IEC 8825-6:2021, Information technology – ASN.1 encoding
rules: Registration and application of PER encoding instructions.
NOTE – The references above shall be interpreted as references to the identified Recommendations | International Standards
together with all their published amendments and technical corrigenda.
2.2 Additional references
– ISO/IEC 646:1991, Information technology – ISO 7-bit coded character set for information interchange.
– 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.
 Rec. ITU-T X.691 (02/2021) 1
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ISO/IEC 8825-2:2021 (E)
– ISO 6093:1985, Information processing – Representation of numerical values in character strings for
information interchange.
– ISO International Register of Coded Character Sets to be Used with Escape Sequences.
– ISO/IEC 10646:2003, Information technology – Universal Multiple-Octet Coded Character Set (UCS).
3 Definitions
For the purposes of this Recommendation | International Standard, the following definitions apply.
3.1 Specification of Basic Notation
For the purposes of this Recommendation | International Standard, all the definitions in Rec. ITU-T X.680 |
ISO/IEC 8824-1 apply.
3.2 Information Object Specification
For the purposes of this Recommendation | International Standard, all the definitions in Rec. ITU-T X.681 |
ISO/IEC 8824-2 apply.
3.3 Constraint Specification
This Recommendation | International Standard makes use of the following terms defined in Rec. ITU-T X.682 |
ISO/IEC 8824-3:
a) component relation constraint;
b) table constraint.
3.4 Parameterization of ASN.1 Specification
This Recommendation | International Standard makes use of the following term defined in Rec. ITU-T X.683 |
ISO/IEC 8824-4:
– variable constraint.
3.5 Basic Encoding Rules
This Recommendation | International Standard makes use of the following terms defined in Rec. ITU-T X.690 |
ISO/IEC 8825-1:
a) data value;
b) dynamic conformance;
c) encoding (of a data value);
d) receiver;
e) sender;
f) static conformance.
3.6 PER Encoding Instructions
This Recommendation | International Standard makes use of the following term defined in Rec. ITU-T X.695 |
ISO/IEC 8825-6:
– identifying keyword.
3.7 Additional definitions
For the purposes of this Recommendation | International Standard, the following definitions apply.
3.7.1 2's-complement-binary-integer encoding: The encoding of a whole number into a bit-field (octet-aligned in
the ALIGNED variant) of a specified length, or into the minimum number of octets that will accommodate that whole
number encoded as a 2's-complement-integer, which provides representations for whole numbers that are equal to, greater
than, or less than zero, as specified in 11.4.
2 Rec. ITU-T X.691 (02/2021)
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ISO/IEC 8825-2:2021 (E)
NOTE 1 – The value of a two's complement binary number is derived by numbering the bits in the contents octets, starting with
N
bit 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.
NOTE 2 – Whole number is a synonym for the mathematical term integer. It is used here to avoid confusion with the ASN.1 type
integer.
3.7.2 abstract syntax value: A value of an abstract syntax (defined as the set of values of a single ASN.1 type),
which is to be encoded by PER, or which is to be generated by PER decoding.
NOTE – The single ASN.1 type associated with an abstract syntax is formally identified by an object of class ABSTRACT-SYNTAX.
3.7.3 bit-field: The product of some part of the encoding mechanism that consists of an ordered set of bits that are
not necessarily a multiple of eight.
NOTE – If the use of this term is followed by "octet-aligned in the ALIGNED variant", this means that the bit-field is required to
begin on an octet boundary in the complete encoding for the aligned variant of PER.
3.7.4 canonical encoding: A complete encoding of an abstract syntax 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 bitstrings in the transfer syntax and values in the abstract syntax.
3.7.5 composite type: A set, sequence, set-of, sequence-of, choice, embedded-pdv, external or unrestricted character
string type.
3.7.6 composite value: The value of a composite type.
3.7.7 constrained whole number: A whole number which is constrained by PER-visible constraints to lie within a
range from "lb" to "ub" with the value "lb" less than or equal to "ub", and the values of "lb" and "ub" as permitted values.
NOTE – Constrained whole numbers occur in the encoding which identifies the chosen alternative of a choice type, the length of
character, octet and bit string types whose length has been restricted by PER-visible constraints to a maximum length, the count of
the number of components in a sequence-of or set-of type that has been restricted by PER-visible constraints to a maximum number
of components, the value of an integer type that has been constrained by PER-visible constraints to lie within finite minimum and
maximum values, and the value that denotes an enumeration in an enumerated type.
3.7.8 effective size constraint (for a constrained string type): A single finite size constraint that could be applied
to a built-in string type and whose effect would be to permit all and only those lengths that can be present in the constrained
string type.
NOTE 1 – For example, the following has an effective size constraint:
A ::= IA5String (SIZE(1.4) | SIZE(10.15))
since it can be rewritten with a single size constraint that applies to all values:
A ::= IA5String (SIZE(1.4 | 10.15))
whereas the following has no effective size constraint since the string
...