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 2021
© ISO/IEC 2021
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ii © ISO/IEC 2021 – All rights reserved

Foreword
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This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration
with ITU-T. The identical text is published as ITU-T X.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.
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© ISO/IEC 2021 – All rights reserved iii

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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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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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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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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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 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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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 can be arbitrarily long if it does not contain any characters
other than 'a', 'b' and 'c':
B ::= IA5String (SIZE(1.4) | FROM("abc"))
NOTE 2 – The effective size constraint is used only to determine the encoding of lengths.
3.7.9 effective permitted-alphabet constraint (for a constrained restricted character string type): A single
permitted-alphabet constraint that could be applied to a built-in known-multiplier character string type and whose effect
would be to permit all and only those characters that can be present in at least one character position of any one of the
values in the constrained restricted character string type.
NOTE 1 – For example, in:
Ax ::= IA5String (FROM("AB") | FROM("CD"))
Bx ::= IA5String (SIZE(1.4) | FROM("abc"))
Ax has an effective permitted-alphabet constraint of "ABCD". Bx has an effective permitted-alphabet constraint that consists
of the entire IA5String alphabet since there is no smaller permitted-alphabet constraint that applies to all values of Bx.
NOTE 2 – The effective permitted-alphabet constraint is used only to determine the encoding of characters.
3.7.10 enumeration index: The non-negative whole number associated with an "EnumerationItem" in an enumerated
type. The enumeration indices are determined by sorting the "EnumerationItem"s into ascending order by their
enumeration value, then by assigning an enumeration index starting with zero for the first "EnumerationItem", one for
the second, and so on up to the last "EnumerationItem" in the sorted list.
NOTE – "EnumerationItem"s in the "RootEnumeration" are sorted separately from those in the "AdditionalEnumeration".
3.7.11 extensible for PER encoding: A property of a type which requires that PER identifies an encoding of a value
as that of a root value or as that of an extension addition.
NOTE – Root values are normally encoded more efficiently than extension additions.
Rec. ITU-T X.691 (02/2021) 3
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3.7.12 field-list: An ordered set of bit-fields that is produced as a result of applying these encoding rules to components
of an abstract value.
3.7.13 indefinite-length: An encoding whose length is greater than 64K-1 or whose maximum length cannot be
determined from the ASN.1 notation.
3.7.14 fixed-length type: A type such that the value of the outermost length determinant in an encoding of this type
can be determined (using the mechanisms specified in this Recommendation | International Standard) from the type
notation (after the application of PER-visible constraints only) and is the same for all possible values of the type.
3.7.15 fixed value: A value such that it can be determined (using the mechanisms specified in this Recommendation |
International Standard) that this is the only permitted value (after the application of PER-visible constraints only) of the
type governing it.
3.7.16 known-multiplier character string type: A restricted character string type where the number of octets in the
encoding is a known fixed multiple of the number of characters in the character string for all permitted character string
values. The known-multiplier character string types are IA5String, PrintableString, VisibleString,
NumericString, UniversalString and BMPString.
3.7.17 length determinant: A count (of bits, octets, characters, or components) determining the length of part or all
of a PER encoding.
3.7.18 normally small non-negative whole number: A part of an encoding which represents values of an unbounded
non-negative integer, but where small values are more likely to occur than large ones.
3.7.19 normally small length: A length encoding which represents values of an unbounded length, but where small
lengths are more likely to occur than large ones.
3.7.20 non-negative-binary-integer encoding: The encoding of a constrained or semi-constrained whole number into
either a bit-field of a specified length, or 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 non-negative-binary-integer
which provides representations for whole numbers greater than or equal to zero, as specified in 11.3.
NOTE – The value of a non-negative-binary-number is derived by numbering the bits in the contents octets, starting with bit 1 of
N
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 non-negative-binary-number is obtained by summing the
numerical values assigned to each bit for those bits which are set to one.
3.7.21 outermost type: An ASN.1 type whose encoding is included in a non-ASN.1 carrier or as the value of other
ASN.1 constructs (see 11.1.1).
NOTE – PER encodings of an outermost type are always an integral multiple of eight bits.
3.7.22 PER-visible constraint: An instance of use of the ASN.1 constraint notation which affects the PER encoding
of a value.
3.7.23 relay-safe encoding: A complete encoding of an abstract syntax value which can be decoded (including any
embedded encodings) without knowledge of the environment in which the encoding was performed.
3.7.24 semi-constrained whole number: A whole number which is constrained by PER-visible constraints to exceed
or equal some value "lb" with the value "lb" as a permitted value, and which is not a constrained whole number.
NOTE – Semi-constrained whole numbers occur in the encoding of the length of unconstrained (and in some cases constrained)
character, octet and bit string types, the count of the number of components in unconstrained (and in some cases constrained)
sequence-of and set-of types, and the value of an integer type that has been constrained to exceed some minimum value.
3.7.25 simple type: A type that is not a composite type.
3.7.26 textually dependent: A term used to identify the case where if some reference name is used in evaluating an
element set, the value of the element set is considered to be dependent on that reference name, regardless of whether the
actual set arithmetic being performed is such that the final value of the element set is independent of the actual element
set value assigned to the reference name.
NOTE – For example, the following definition of Foo is textually dependent on Bar even though Bar has no effect on Foos set of
values (thus, according to 10.3.6 the constraint on Foo is not PER-visible since Bar is constrained by a table constraint and Foo is
textually dependent on Bar).
MY-CLASS ::= CLASS { &name PrintableString, &age INTEGER } WITH SYNTAX{&name , &age}
MyObjectSet MY-CLASS ::= { {"Jack", 7} | {"Jill", 5} }
Bar ::= MY-CLASS.&age ({MyObjectSet})
Foo ::= INTEGER (Bar | 1.100)
3.7.27 unconstrained whole number: A whole number which is not constrained by PER-visible constraints.
NOTE – Unconstrained whole numbers occur only in the encoding of a value of the integer type.
4 Rec. ITU-T X.691 (02/2021)
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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
PER Packed Encoding Rules of ASN.1
16K 16384
32K 32768
48K 49152
64K 65536
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 defines 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, the bits of an octet are numbered from 8
to 1, where bit 8 is the "most significant bit" and bit 1 the "least significant bit".
6.3 The term "octet" is frequently used in this Recommendation | International Standard to stand for "eight bits".
The use of this term in place of "eight bits" does not carry any implications of alignment. Where alignment is intended, it
is explicitly stated in this Recommendation | International Standard.
7 Encoding rules defined in this Recommendation | International Standard
7.1 This Recommendation | International Standard specifies four encoding rules (together with their associated
object identifiers) which can be used to encode and decode the values of an abstract syntax defined as the values of a
single (known) ASN.1 type. This clause describes their applicability and properties.
7.2 Without knowledge of the type of the value encoded, it is not possible to determine the structure of the encoding
(under any of the PER encoding rule algorithms). In particular, the end of the encoding cannot be determined from the
encoding itself without knowledge of the type being encoded.
7.3 PER encodings are always relay-safe provided the abstract values of the types EXTERNAL, EMBEDDED PDV and
CHARACTER STRING are constrained to prevent the carriage of OSI presentation context identifiers.
7.4 The most general encoding rule algorithm specified in this Recommendation | International Standard is
BASIC-PER, which does not in general produce a canonical encoding.
7.5 A second encoding rule algorithm specified in this Recommendation | International Standard is
CANONICAL-PER, which produces encodings that are canonical. This is defined as a restriction of implementation-
dependent choices in the BASIC-PER encoding.
NOTE 1 – CANONICAL-PER produces canonical encodings that have applications when authenticators need to be applied to
abstract values.
NOTE 2 – Any implementation conforming to CANONICAL-PER for encoding is conformant to BASIC-PER for encoding. Any
implementation conforming to BASIC-PER for decoding is conformant to CANONICAL-PER for decoding. Thus, encodings
made according to CANONICAL-PER are encodings that are permitted by BASIC-PER.
7.6 If a type encoded with BASIC-PER or CANONICAL-PER contains EMBEDDED PDV, CHARACTER STRING or
EXTERNAL types, then the outer encoding ceases to be relay-safe unless the transfer syntax used for all the EMBEDDED
PDV, CHARACTER STRING and EXTERNAL types is relay safe. If a type encoded with CANONICAL-PER contains
Rec. ITU-T X.691 (02/2021) 5
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EMBEDDED PDV, EXTERNAL or CHARACTER STRING types, then the outer encoding ceases to be canonical unless the
transfer syntax used for all the EMBEDDED PDV, EXTERNAL and CHARACTER STRING types is canonical.
NOTE – The character transfer syntaxes supporting all character abstract syntaxes of the form {iso standard 10646
level-1(1) .} are canonical. Those supporting {iso standard 10646 level-2(2) .} and {iso
standard 10646 level-3(3) .} are not always canonical. All the above character transfer syntaxes are relay-safe.
7.7 Both BASIC-PER and CANONICAL-PER come in two variants, the ALIGNED variant, and the UNALIGNED
variant. In the ALIGNED variant, padding bits are inserted from time to time to restore octet alignment. In the
UNALIGNED variant, no padding bits are ever inserted.
7.8 There are no interworking possibilities between the ALIGNED variant and the UNALIGNED variant.
7.9 PER encodings are self-delimiting only with knowledge of the type of the encoded value. Encodings are always
a multiple of eight bits. When carried in an EXTERNAL type they shall be carried in the OCTET STRING choice alternative,
unless the EXTERNAL type itself is encoded in PER, in which case the value may be encoded as a single ASN.1 type
(i.e., an open type). When carried in OSI presentation protocol, the "full encoding" (as defined in Rec. ITU-T X.226 |
ISO/IEC 8823-1) with the OCTET STRING choice alternative shall be used.
7.10 The rules of this Recommendation | International Standard apply to both algorithms and to both variants unless
otherwise stated (but see 9.2 and 9.3).
7.11 Annex C is informative, and gives recommendations on which combinations of PER to implement in order to
maximize the chances of interworking.
8 Conformance
8.1 Dynamic conformance is specified by clause 9 onwards.
8.2 Static conformance is specified by those standards which specify the application of these Packed Encoding
Rules.
NOTE – Annex C provides guidance on static conformance in relation to support for the two variants of the two encoding rule
algorithms. This guidance is designed to ensure interworking, while recognizing the benefits to some applications of encodings
that are neither relay-safe nor canonical.
8.3 The rules in this Recommendation | International Standard are specified in terms of an encoding procedure.
Implementations are not required to mirror the procedure specified, provided the bit string produced as the complete
encoding of an abstract syntax value is identical to one of those specified in this Recommendation | International Standard
for the applicable transfer syntax.
8.4 Implementations performing decoding are required to produce the abstract syntax value corresponding to any
received bit string which could be produced by a sender conforming to the encoding rules identified in the transfer syntax
associated with the material being decoded.
NOTE 1 – In general there are no alternative encodings defined for the BASIC-PER explicitly stated in this Recommendation |
International Standard. The BASIC-PER becomes canonical by specifying relay-safe operation and by restricting some of the
encoding options of other ISO/IEC Standards that are referenced. CANONICAL-PER provides an alternative to both the
Distinguished Encoding Rules and Canonical Encoding Rules (see Rec. ITU-T X.690 | ISO/IEC 8825-1) where a canonical and
relay-safe encoding is required.
NOTE 2 – When CANONICAL-PER is used to provide a canonical encoding, it is recommended that any resulting encrypted hash
value that is derived from it should have associated with it an algorithm identifier that identifies CANONICAL-PER as the
transformation from the abstract syntax value to an initial bitstring (which is then hashed).
9 PER encoding instructions
9.1 PER encoding instructions can be associated with a type in accordance with the provisions of Rec. ITU-T X.680
| ISO/IEC 8824-1 and Rec. ITU-T X.695 | ISO/IEC 8825-6.
NOTE 1 – The application of some PER encoding instructions can make it impossible to encode all the abstract values of the type.
Where this can arise, the
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