ISO/IEC 8825-1:1995

INTERNATIONAL ISO/IEC
STANDARD 8825-l
First edition
1995-l O-l 5
Information technology - ASN.1 encoding
rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)
Technologies de /‘information - R&g/es de codage ASN. I: Spkifica tions
pour /es r&g/es de base de codage (BER), /es r&g/es canoniques de codage
(CER) et /es r&g/es de distinction de codage (DER)
Reference number
lSO/IEC 8825-l :I 995(E)
ISO/IEC 8825-l: 1995(E)
CONTENTS
Page
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Identical Recommendations I International Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1
2.2 Additional references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~.
4 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Convention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Conformance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Basic encoding rules .
8.1 General rules for encoding .
Encoding of a boolean value .
8.2
..............................................................................................................
8.3 Encoding of an integer value
Encoding of an enumerated value .
8.4
8.5 Encoding of a real value .
..............................................................................................................
8.6 Encoding of a bitstring value
8.7 Encoding of an octetstring value .
.....................................................................................................................
8.8 Encoding of a null value
Encoding of a sequence value .
8.9
........................................................................................................
8.10 Encoding of a sequence-of value
Encoding of a set value .
8.11
8.12 Encoding of a set-of value .
8.13 Encoding of a choice value .
8.14 Encoding of a tagged value .
Encoding of an open type .
8.15
Encoding of an instance-of value .
8.16
..................................................................................
8.17 Encoding of a value of the embedded-pdv type
8.18 Encoding of a value of the external type .
................................................................................................
8.19 Encoding of an object identifier value
Encoding for values of the restricted character string types .
8.20
.............................................................
8.21 Encoding for values of the unrestricted character string type
o ISO/IEC 1995
All rights reserved. Unless otherwise specified, no part of this publication may be
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ISO/IEC Copyright Office l Case postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
ii
ISO/IEC 8825-l: 1995(E)
0 1s0/lEc
9 Canonical encoding rules . 19
91 Length forms .
String encoding forms . 19
9:2
93 . Set components .
......................................................................................................................... 19
10 Distinguished encoding rules
10.1 Length forms .
......................................................................................................................... 20
10.2 String encoding forms
10.3 Set components .
Restrictions on BER employed by both CER and DER . 20
11.1 Boolean values .
11.2 Unused bits .
11.3 Real values . 20
11.4 GeneralString values . 21
11.5 Set and sequence components with default value . 21
11.6 Set-of components .
11.7 GeneralizedTime .
12 Use of BER, CER and DER in transfer syntax definition .
Annex A - Example of encodings . 23
A. 1 ASN. 1 description of the record structure .
A.2 ASN. 1 description of a record value . 23
A.3 Representation of this record value . 23
.................................................................................................... 25
Annex B - Assignment of object identifier values
........................................................................................................... 26
Annex C - Illustration of real value encoding
.......................................................................... 28
Annex D - Use of the DER and CER in data origin authentication
D. 1 The problem to be solved .
D.2 The approach to a solution .
D.3 The implementation optimization . 29
. . .
ISO/IEC 8825-l: 1995(E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized system for worldwide
standardization. National bodies that are members of IS0 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.
IS0 and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with
IS0 and IEC, also take part in the work.
In the field of information technology, IS0 and IEC have established a joint
technical committee, ISO/IEC JTC 1. 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.
International Standard ISOKIEC 8825-l was prepared by Joint Technical
Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 21, Open
systems interconnection, data management and open distributed processing, in
collaboration with ITU-T. The identical text is published as ITU-T
Recommendation X.690.
This is a revision of ISO/IEC 8825:1990.
ISO/IEC 8825:1995 consists of the following parts, under the general title
Information technology - ASN. I encoding rules:
- Part I: Specification of Basic Encoding Rules (BER), Canonical Encoding
Rules (CER) and Distinguished Encoding Rules (DER)
- Part 2: Specification of Packed Encoding Rules
Annexes A to D of this part of ISO/IEC 8825: 1995 are for information only.
IV
ISO/IEC 8825=1:1995(E)
0 ISO/IEC
Introduction
\
ITU-T Rec. X.680 I ISO/IEC 8824-1, ITU-T Rec. X.681 I ISO/IEC 8824-2, ITU-T Rec. X.682 I ISOLIEC 8824-3,
ITU-T Rec. X.683 I ISO/IEC 8824-4 (Abstract Syntax Notation One or ASN. 1) together specify a notation for the
definition of abstract syntaxes, enabling application layer standards to define the types of information they need to
transfer using the presentation service. It also specifies a notation for the specification of values of a defined type.
This Recommendation I 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.l
notation. This Recommendation I International Standard defines three sets of encoding rules, called basic encoding
rules, canonical encoding rules and distinguished encoding rules. Whereas the basic encoding rules gives 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.
A gi ves a n example of encoding rules. It does not form an integral
Annex the application of the basic part of this
Recom .mend .ation I International Standard.
Annex B summarizes the assignment of object identifier values made in this Recommendation International Standard. It
does not form an integral part of this Recommendation I International Standard.
encoding reals. It does not form an
Annex C gives ex .amples of applying the bas ic encoding rules for integral part of this
Recom mendation I International Standard.
Annex D provides a tutorial on the distinguished encoding rules to provide an integrity service for OS1
the use of
communications. It does not form an integral of this Recommendation I International Standard.
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ISO/IEC 8825-l : 1995 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
INFORMATION TECHNOLOGY - ASN.l ENCODING RULES:
SPECIFICATION OF BASIC ENCODING RULES (BER),
CANONICAL ENCODING RULES (CER)
AND DISTINGUISHED ENCODING RULES (DER)
1 Scope
This Recommendation I 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 ITU-T Rec. X.680 (1994) I
ISOLIEC 8824-1:1995, ITU-T Rec. X.681 (1994) I ISO/IEC 8824-2:1995, ITU-T Rec. X.682 (1994) I
ISOLIEC 8824 3: 1995, and ITU-T Rec. X.683 (1994) I ISO/IEC 8824-4: 1995, 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.
These encoding rules are used at the time of communication (by the presentation service provider when required by a
presentation context).
Normative references
The following Recommendations and International Standards contain provisions which, through reference in this text,
constitute provisions of this Recommendation I 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 I International Standard are encouraged to investigate the possibility of applying the most recent
editions of the Recommendations and Standards listed below. Members of IEC and IS0 maintain registers of currently
valid International Standards. The Telecommunications Standardization Bureau of the ITU maintains a list of currently
valid ITU-T Recommendations.
Identical Recommendations I International Standards
21 .
-
ITU-T Recommendation X.200 (1994) I ISO/IEC 7498-l : 1994, Information technology - Open Systems
Interconnection - Basic Reference Model: The basic model.
-
ITU-T Recommendation X.226 (1994) I ISO/IEC 8823-1:1994, Information technology - Open Systems
- Connection-oriented presentation protocol: Protocol specification.
Interconnection
-
ITU-T Recommendation X.680 (1994) I ISO/IEC 8824-l : 1995, Information technology - Abstract Syntax
Notation One (ASN.1): Specification of basic notation.
-
ITU-T Recommendation X.68 1 (1994) I ISO/IEC 8824-2: 1995, Information technology - Abstract Syntax
Notation One (ASN. 1): Information object specification.
-
ITU-T Recommendation X.682 (1994) I ISO/IEC 8824-3: 1995, Information technology - Abstract Syntax
Notation One (ASN. 1): Constraint specification.
-
ITU-T Recommendation X.683 (1994) I ISO/IEC 8824-4: 1995, Information technology - Abstract Syntax
Notation One (ASN. 1): Parameterization of ASN. 1 specifications.
22 0 Additional references
-
IS0 International Register of Coded Character Sets to be used with Escape Sequence.
-
ISO/IEC 2022: 1994, Information technology - Character code structure and extension techniques.
-
IS0 6093: 1985, Information processing - Representation of numerical values in character strings for
information interchange.
ITU-T Rec. X.690 (1994 E) 1
ISO/IEC 8825-l : 1995 (E)
-
ISO/IEC 6429: 1992, Information technology - Control functions for coded character sets.
-
CCITT Recommendation X.208 (1988), Specification of Abstract Syntax Notation One (ASN. 1).
-
ISOIIEC 8824-l to 8824: 1990, Information technology - Open Systems Interconnection - Specification of
Abstract Syntax Notation One (ASN.1).
-
ISOIIEC 10646-l: 1993, Information technology - Universal Multiple-Octet Coded Character Set (KS) -
Part 1: Architecture and Basic Multilingual Plane.
Definitions
For the purposes of this Recommendation I International Standard the definitions of ISO/IEC 7498-l and ITU-T Rec. X.680 I
ISO/IEC 8824-l and the following definitions apply.
31 dynamic conformance: A statement of the requirem Lent for an implementation to adhere to the behavior
communication.
piescri bed by this Recommendation I Internation al Standard in an instance of
static conformance: A statement of the requirement for support by an implementation of a valid set of
features from among those defined by this Recommendation I International Standard.
33 . data value: Information specified as the value of a type; the type and the value are defined using ASN. 1.
. encoding (of a data value): The complete sequence of octets used to represent the data value.
35 . 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 I International Standard, as other Recommendations I International Standards use it to mean “data value”.
36 . length octets: Part of a data value encoding following the identifier octets which is used to determine the end
of the encoding.
37 . contents octets: That part of a data value encoding which represents a particular value, to distinguish it from
other values of the same type.
38 . 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.
39 . primitive encoding: A data value encoding in which the contents octets directly represent the value.
encoding in which the contents octets are the complete encoding of one
3.10 constructed encoding: A data value
or more data values.
3.11 receiver: An implementation decoding the octets produced sender, in order to identify the data value
by a
which was encoded.
3.12 sender: An implementation encoding a data value for transfer.
3.13 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
ASN.l Abstract Syntax Notation One
Basic Encoding Rules of ASN.l
BER
CER Canonical Encoding Rules of ASN.l
DER Distinguished Encoding Rules of ASN. 1
ULA Upper Layer Architecture
Notation
This Recommendation I International Standard references the notation defined by ITU-T Rec. X.680 I ISO/IEC 8824-l.
2 IT&T Rec. X.690 (1994 E)
ISO/IEC 8825-l : 1995 (E)
6 Convention
61 This Recommendation I International Standard specifies the value of each octet in an encoding by use of the
tkms “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.
62 For the purposes of this Recommendation I 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”.
63 . For the purpose of this Recommendation I 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, Sl, is
greater than another, S2, if and only if either:
a) S1 and S2 have identical octets in every position up to and including the final octet in S2, but S1 is longer;
or
S1 and S2 have different octets in one or more positions, and in the first such position, the octet in Sl is
b)
greater than that in S2, considering the octets as unsigned binary numbers whose bit n has weight 2”-
Conformance
7.1 Dynamic conformance is specified by clause to clause inclusive.
Static conformance is specified by those standards which specify the application of one or more of these
7.2
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.
No alternative encodings are permitted by the Canonical Encoding Rules or Distinguished Encoding Rules.
7.4
8 Basic encoding rules
General rules for encoding
81 .
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);
contents octets (see 8.1.4);
C>
end-of-contents octets (see 8.1.5).
d)
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).
Figure 1 illustrates the structure of an encoding (primitive or constructed). Figure 2 illustrates an alternative
8.1.1.3
constructed encoding.
Identifier octets
8.1.2
The identifier octets shall encode the ASN.l tag (class and number) of the type of the data value.
8.1.2.1
ITU-T Rec. X.690 (1994 E) 3
ISO/IEC 8825-l : 1995 (E)
Identifier octets Length octets Contents octets
I
The number of octets
in the contents octets
(see 8.1.3.2)
TIS04830-94/dO 1
Figure 1 - Structure of an encoding
End of contents octets
ldentif ier octets Length octets 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)
TIS0484@94/dO2
Figure 2 - An alternative constructed encoding
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
Bit 7
Class
Universal
Application
Context-specific
Private
Figure 3 illustrates the form of an identifier octet for a type with a tag whose number is in the range zero to
8.1.2.3
30 (inclusive).
ITU-T Rec. X.690 (1994 E)
ISO/IEC 8825-l : 1995 (E)
ldentif ier octet
8 7 6 5 4 3 2 1
Bits
Class P/C Number of tag
- 0 = Primitive
- 1 = Constructed
TlSO4850-94/dO3
Figure 3 - Identifier octet (low tag number)
8.1.2.4 For tags with a number greater than or equal to 3 1, 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 111112.
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;
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
Class P/C 1 1 1 1 1
= Number of tag
*
A
TIS04860-94/dO4
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 clauses specify whether the encoding is primitive or constructed for each type.
8.1.2.6 ITU-T Rec. X.680 I ISOLIEC 8824-l 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.
ITU-T Rec. X.690 (1994 E) 5
ISO/IEC 8825-l : 1995 (E)
8.1.2.7 ITU-T Rec. X.681 I ISO/IEC 8824-2, subclauses 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.l 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:
use the definite form (see 8.1.3.3) if the encoding is primitive;
a)
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;
use the indefinite form (see 8.1.3.6) if the encoding is constructed and is not all immediately available.
Cl
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.
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;
the value 11111111~ shall not be used.
C>
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.
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 S), 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 S) 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.l.
6 ITU-T Rec. X.690 (1994 E)
ISO/IEC 8825-l : 1995 (E)
End-of-contents octets
8.1.5
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
I
I
Absent
O”16 046
82 . Encoding of a boolean value
The encoding of a boolean value shall be primitive. The contents octets shall consist of a single octet.
8.2.1
If the boolean value is
8.2.2
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
Oh6 0116 FF16
. Encoding of an integer value
The encoding of an integer value shall be primitive. The contents octets shall consist of one or more octets.
8.3.1
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
shall not all be ones; and
a>
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 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 2N, 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.
84 . 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.
ITU-T Rec. X.690 (1994 E) 7
ISO/IEC 8825-l : 1995 (E)
85 . 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 zero, there shall be no contents octets in the encoding.
8.5.3 If the real value is non-zero, then the base used for the encoding shall be B’, chosen by the sender. If B’ is 2, 8
or 16, a binary encoding, specified in 8.5.5, shall be used. If B’ is 10, a character encoding, specified in 8.5.6, shall be
used.
NOTE - The form of storage, generation, or processing by senders and receivers, and the form used in the ASN. 1 value
notation are all independent of the base used for transfer.
8.5.4 Bit 8 of the first contents octet shall be set as follows:
if bit 8 = 1, then the binary encoding specified in 8.5.5 applies;
a>
b) if bit 8 = 0 and bit 7 = 0, then the decimal encoding specified in 8.5.6 applies;
if bit 8 = 0 and bit 7 = 1, then a “SpecialRealValue” (see ITU-T Rec. X.680 I ISO/IEC 8824-l) is encoded
Cl
as specified in 8.5.7.
8.5.5 When binary encoding is used (bit 8 = l), then if the mantissa, M is non-zero, it shall be represented by a
sign S, a non-negative integer value N and a binary scaling factor F, such that
M=SXNX~~
OIF<4
S=+l or-l
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 clause. This alignment can not 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.
8.5.5.1 Bit 7 of the first contents octets shall be 1 if S is -1 and 0 otherwise.
8.5.5.2 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 I International Standard.
8.5.5.3 Bits 4 to 3 of the first contents octet shall encode the value of the binary scaling factor F as an unsigned binary
integer.
8.5.5.4 Bits 2 to 1 of the first contents octet shall encode the format of the exponent as follows:
contents octet encodes the value of the exponent as a two’s
if bits 2 to 1 are 00, then the second
a)
complement binary number;
b) if bits 2 to 1 are 01, then the second and third contents octets encode the value of the exponent as a two’s
complement binary number;
exponent as a
if bits 2 to 1 are 10, then the second, third and fourth contents octets encode the value of the
d
two’ s complement binary number;
d) if bits 2 to 1 are 11, then the second contents octet encodes the number of octets, X say, (as an unsigned
binary number) used to encode the value of the exponent, and the third up to the (X plus 3)th (inclusive)
contents octets encode the value of the exponent as a two’s complement binary number; the value of X
shall be at least one; the first nine bits of the transmitted exponent shall not be all zeros or all ones.
8.5.5.5 The remaining contents octets encode the value of the integer N (see 8.5.5) as an unsigned binary number.
NOTES
1 For non-canonical BER there is no requirement for floating point normalization of the mantissa. This allows an
implementor to transmit octets containing the mantissa without performing shift functions on the mantissa in memory. In the
Canonical Encoding Rules and the Distinguished Encoding Rules normalization is specified and the mantissa (unless it is 0) needs to
be repeatedly shifted until the least significant bit is a 1.
This representation of real is very different from the formats ly used in floating point hardware, but
2 numbers
be easily converted to and from such formats (see annex C).
has been designed to
8 ITU-T Rec. X.690 (1994 E)
ISO/IEC 8825-l : 1995 (E)
When decimal encoding is used (bits 8 to 7 = 00), all the contents octets following the first contents octet form
8.5.6
a field, as the term is used in IS0 6093, of a length chosen by the sender, and encoded according to IS0 6093. The
choice of IS0 6093 number representation is specified by bits 6 to 1 of the first contents octet as follows:
Number representation
Bits 6 to I
00 0001 IS0 6093 NRl form
00 0010 IS0 6093 NR2 form
000011 IS0 6093 NR3 form
The remaining values of bits 6 to 1 are reserved for to this Recommendation I International Standard.
There shall be no use of scaling factors specified in accompanying documentation (see IS0 6093).
NOTES
1 The recommendations in IS0 6093 concerning the use of at least one digit to the left of the decimal mark are also
recommended in this Recommendation I International Standard, but are not mandatory.
2 Use of the normalized form (see IS0 6093) is a sender’s option, and has no significance.
8.5.7 When “SpecialRealValues” are to be encoded (bits 8 to 7 = 01), there shall be only one contents octet, with
values as follows:
01000000 Value is PLUS-INFINITY
01000001 Value is MINUS-INFINITY
All other values having bits 8 and 7 equal to 0 and 1 respectively are reserved for addenda to this Recommendation I
International Standard.
86 . Encoding of a bitstring value
8.6.1 The encoding of a bitstring value shall be either primitive or constructed at the option of the sender.
NOTE - Where it is necessary to transfer part of a bit string before the entire bitstring is available, the constructed
encoding is used.
The contents octets for the primitive encoding shall contain an initial octet followed by zero, one or more
8.6.2
subsequent octets.
8.6.2.1 The bits in the bitstring, commencing with the first bit and proceeding to the last bit, shall be placed in bits 8
to 1 of the first subsequent octet, followed by bits 8 to 1 of the second subsequent octet, followed by bits 8 to 1 of each
octet in turn, followed by as many bits as are needed of the final subsequent octet, commencing with bit 8.
NOTE - The terms “first bit” and “trailing bit” are defined in ITU-T Rec. X.680 I ISO/IEC 8824-l.
8.6.2.2 The initial octet shall encode, as an unsigned binary integer with bit 1 as the least significant bit, the number of
unused bits in the final subsequent octet. The number shall be in the range zero to seven.
8.6.2.3 If the bitstring is empty, there shall be no subsequent octets, and the initial octet shall be zero.
8.6.2.4 Where ITU-T Rec. X.680 I ISO/IEC 8824-1, subclause 19.7 applies, a BER encoder/decoder can add or
remove trailing 0 bits from the value.
NOTE - If a bitstring value has no 1 bits then an encoder (as a sender’s option) may encode the value with an initial octet
set to 0 or may encode it as a bit string with one or more 0 bits following the initial octet.
8.6.3 The contents octets for the constructed encoding shall consist of zero, one, or more nested encodings.
NOTE - Each such encoding includes identifier, length, and contents octets, and may include end-of-contents octets if it is
constructed.
8.6.4 To encode a bitstring value in this way, it is segmented. Each segment shall consist of a series of consecutive
bits of the value, and with the possible exception of the last, shall contain a number of bits which is a multiple of eight.
Each bit in the overall value shall be in precisely one segment, but there shall be no significance placed on the segment
boundaries.
NOTE - A segment may be of size zero, i.e. contain no bits.
8.6.4.1 Each encoding in the contents octets shall represent a segment of the overall bitstring, the encoding arising
from a recursive application of this clause. In this recursive application, each segment is treated as if it were a bitstring
value. The encodings of the segments shall appear in the contents octets in the order in which their bits appear in the
overall value.
ITU-T Rec. X.690 (1994 E) 9
ISO/IEC 8825-l : 1995 (E)
NOTES
1 As a consequence of this recursion, each encoding in the contents octets may itself be primitive or constructed.
However, such encodings will usually be primitive.
2 In particular, the tags in the contents octets are always universal class, number 3.
- If of type BIT STRING, the value ‘OA3B5F291CD’H can be encoded as shown below. In this
8.6.4.2 Example
example, the Bit String is represented as a primitive:
BitString Length Contents
o~0A3B5F~~KDo16
0316 0716
The value shown above can also be encoded as shown below. In this example, the Bit String is represented as a
constructor:
BitString Length Contents
2316 8o16
BitString Length Contents
oooA3B16
0316 0316
045F29lcDo16
0316 0516
EOC Length
O”16 O”16
87 . Encoding of an octetstring value
The encoding of an octetstring value shall be either primitive or constructed at the option of the sender.
8.7.1
NOTE - Where it is necessary to transfer part of an octet string before the entire octetstring is available, the constructed
encoding is used.
8.7.2 The primitive encoding contains zero, one or more contents octets equal in value to the octets in the data value,
in the order they appear in the data value, and with the most significant bit of an octet of the data value aligned with the
most significant bit of an octet of the contents octets.
8.7.3 The contents octets for the constructed encoding shall consist of zero, one, or more encodings.
NOTE - Each such encoding includes identifier, length, and contents octets, and may include end-of-contents octets if it is
constructed.
8.7.3.1 To encode an octetstring value in this way, it is segmented. Each segment shall consist of a series of
consecutive octets of the value. There shall be no significance placed on the segment boundaries.
NOTE - A segment may be of size zero, i.e. contain no octets.
8.7.3.2 Each encoding in the contents octets shall represent a segment of the overall octetstring, the encoding arising
from a recursive application of this subclause. In this recursive application, each segment is treated as if it were a
octetstring value. The encodings of the segments shall appear in the contents octets in
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