Encoding Instructions for the Generic String Encoding Rules (GSER)
A small notation that lets an ASN.1 CHOICE of strings be written as a bare string in GSER text.
Legg, S., "Encoding Instructions for the Generic String Encoding Rules (GSER)", RFC 4792, DOI 10.17487/RFC4792, January 2007, <https://www.rfc-editor.org/info/rfc4792>.
@misc{rfc4792,
series = {Request for Comments},
number = 4792,
howpublished = {RFC 4792},
publisher = {RFC Editor},
doi = {10.17487/RFC4792},
url = {https://www.rfc-editor.org/info/rfc4792},
author = {S. Legg},
title = {{Encoding Instructions for the Generic String Encoding Rules (GSER)}},
pagetotal = 9,
year = 2007,
month = jan,
} RFC text
Contents
Network Working Group S. Legg
Request for Comments: 4792 eB2Bcom
Updates: 3641 January 2007
Category: Standards Track
Encoding Instructions for the
Generic String Encoding Rules (GSER)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
Abstract Syntax Notation One (ASN.1) defines a general framework for
annotating types in an ASN.1 specification with encoding instructions
that alter how values of those types are encoded according to ASN.1
encoding rules. This document defines the supporting notation for
encoding instructions that apply to the Generic String Encoding Rules
(GSER) and, in particular, defines an encoding instruction to provide
a machine-processable representation for the declaration of a GSER
ChoiceOfStrings type.
Table of Contents
1. Introduction ....................................................2
2. Conventions .....................................................2
3. Notation for GSER Encoding Instructions .........................2
4. The CHOICE-OF-STRINGS Encoding Instruction ......................3
4.1. Effect on GSER Encodings ...................................5
4.2. Replacement of Existing ChoiceOfStrings Declarations .......6
5. Security Considerations .........................................7
6. Normative References ............................................7
1. Introduction
Abstract Syntax Notation One (ASN.1) [X.680] defines a general
framework for annotating types in an ASN.1 specification with
encoding instructions [X.680-1] that alter how values of those types
are encoded according to ASN.1 encoding rules. This document defines
the supporting notation for encoding instructions that apply to the
Generic String Encoding Rules (GSER) [GSER], and in particular
defines an encoding instruction, the CHOICE-OF-STRINGS encoding
instruction, to provide a machine-processable representation for the
declaration of a GSER ChoiceOfStrings type.
The CHOICE-OF-STRINGS encoding instruction SHOULD be used instead of
simply declaring a ChoiceOfStrings type.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[BCP14].
Throughout this document, "type" shall be taken to mean an ASN.1
type, and "value" shall be taken to mean an ASN.1 abstract value,
unless qualified otherwise.
A reference to an ASN.1 production [X.680] (e.g., Type, NamedType) is
a reference to text in an ASN.1 specification corresponding to that
production.
3. Notation for GSER Encoding Instructions
The grammar of ASN.1 permits the application of encoding instructions
[X.680-1], through type prefixes and encoding control sections, that
modify how abstract values are encoded by nominated encoding rules.
The generic notation for type prefixes and encoding control sections
is defined by the ASN.1 basic notation [X.680] [X.680-1], and
includes an encoding reference to identify the specific encoding
rules that are affected by the encoding instruction.
The encoding reference that identifies the Generic String Encoding
Rules is literally GSER.
The specific notation for an encoding instruction for a particular
set of encoding rules is left to the specification of those encoding
rules. Consequently, this companion document to the GSER
specification [GSER] defines the notation for GSER encoding
instructions. Specifically, it elaborates the EncodingInstruction
and EncodingInstructionAssignmentList placeholder productions of the
ASN.1 basic notation.
In the context of the GSER encoding reference the EncodingInstruction
production is defined as follows, using the conventions of the ASN.1
basic notation:
EncodingInstruction ::=
ChoiceOfStringsInstruction
In the context of the GSER encoding reference the
EncodingInstructionAssignmentList production (which only appears in
an encoding control section) is empty:
EncodingInstructionAssignmentList ::= empty
4. The CHOICE-OF-STRINGS Encoding Instruction
The CHOICE-OF-STRINGS encoding instruction allows a GSER encoder to
encode the alternative of a CHOICE (of restricted string types)
without the leading identifier. The optional PrecedenceList also
allows a specification writer to alter the order in which a GSER
decoder will consider the alternatives of the CHOICE as it determines
which alternative has been encoded when the identifier is absent.
The notation for a CHOICE-OF-STRINGS encoding instruction is defined
as follows:
UnionInstruction ::= "CHOICE-OF-STRINGS" AlternativesPrecedence ?
AlternativesPrecedence ::= "PRECEDENCE" PrecedenceList
PrecedenceList ::= identifier PrecedenceList ?
The Type in the EncodingPrefixedType for a CHOICE-OF-STRINGS encoding
instruction SHALL be:
(a) a BuiltinType that is a ChoiceType, or
(b) a ConstrainedType that is not a TypeWithConstraint where the Type
in the ConstrainedType is one of (a) to (d), or
(c) a BuiltinType that is a PrefixedType that is a TaggedType where
the Type in the TaggedType is one of (a) to (d), or
(d) a BuiltinType that is a PrefixedType that is an
EncodingPrefixedType where the Type in the EncodingPrefixedType
is one of (a) to (d).
The effect of this condition is to force the CHOICE-OF-STRINGS
encoding instruction to be textually co-located with the CHOICE type
definition to which it applies. This makes it clear to a reader that
the encoding instruction applies to every use of the CHOICE type no
matter how it might be referenced.
The ChoiceType in case (a) is said to be "subject to" the CHOICE-OF-
STRINGS encoding instruction.
The Type of each NamedType of the ChoiceType in case (a) MUST be:
(1) the NumericString, PrintableString, TeletexString (T61String),
VideotexString, IA5String, GraphicString, VisibleString
(ISO646String), GeneralString, BMPString, UniversalString, or
UTF8String type, or
(2) a type notation that references a type that is one of (1) to (4),
or
(3) a constrained type where the type that is constrained is one of
(1) to (4), or
(4) a prefixed type where the type that is prefixed is one of (1) to
(4).
ASIDE: A tagged type is a special case of a prefixed type. An
effect of case (4) is that tagging is not significant.
The ASN.1 restricted string type in case (1) MUST be different for
each NamedType in the ChoiceType, i.e., no two alternatives have the
same restricted string type.
If case (3) applies to any NamedType, then the constraint in case (3)
MUST be the same for each NamedType, i.e., either none of the
alternatives has a constraint, or all of the alternatives have
exactly the same constraint.
Each identifier in the PrecedenceList MUST be the identifier of a
NamedType of the ChoiceType.
A particular identifier SHALL NOT appear more than once in the same
PrecedenceList.
4.1. Effect on GSER Encodings
A value of a CHOICE type is encoded according to the <ChoiceValue>
[GSER] Augmented Backus-Naur Form [ABNF] rule. The ABNF for
<ChoiceValue> is reproduced here for convenience:
ChoiceValue = IdentifiedChoiceValue /
ChoiceOfStringsValue
IdentifiedChoiceValue = identifier ":" Value
ChoiceOfStringsValue = StringValue
The <IdentifiedChoiceValue> rule MUST be used to encode values of a
CHOICE type where the ChoiceType is not subject to a CHOICE-OF-
STRINGS encoding instruction.
The chosen alternative of a value of a CHOICE type corresponds to
some NamedType in the definition of the type. The <identifier> in
the <IdentifiedChoiceValue> is the identifier of this NamedType.
Either the <IdentifiedChoiceValue> rule or the <ChoiceOfStringsValue>
rule is used to encode values of a CHOICE type where the ChoiceType
is subject to a CHOICE-OF-STRINGS encoding instruction.
If <ChoiceOfStringsValue> has been used, then a GSER decoder MUST
determine the chosen alternative by considering the alternatives of
the CHOICE in the order prescribed below and accepting the first
alternative that allows all of the characters in the <StringValue>.
If the CHOICE-OF-STRINGS encoding instruction has a PrecedenceList,
then the alternatives of the ChoiceType referenced by the
PrecedenceList are considered in the order identified by that
PrecedenceList, and then the remaining alternatives are considered in
the order of their definition in the ChoiceType. If the CHOICE-OF-
STRINGS encoding instruction does not have a PrecedenceList, then all
the alternatives of the ChoiceType are considered in the order of
their definition in the ChoiceType.
A GSER encoder MUST use <IdentifiedChoiceValue> if a GSER decoder
would determine the chosen alternative to be something other than the
chosen alternative of the CHOICE value being encoded; otherwise,
<ChoiceOfStringsValue> MAY be used.
Example
Consider this type definition:
[GSER:CHOICE-OF-STRINGS PRECEDENCE basicName] CHOICE {
extendedName UTF8String,
basicName PrintableString
}
If a <ChoiceOfStringsValue> has been used, then a GSER decoder
would first consider whether the <StringValue> was a valid
basicName (a PrintableString) before considering whether it was a
valid extendedName (a UTF8String).
4.2. Replacement of Existing ChoiceOfStrings Declarations
In line with the previous declaration [GSER] of the DirectoryString
type as a ChoiceOfStrings type, applications using GSER MUST add this
encoding instruction:
[GSER:CHOICE-OF-STRINGS PRECEDENCE printableString uTF8String]
immediately before the "CHOICE" keyword in the definition of the
DirectoryString type in the third and every subsequent edition of the
SelectedAttributeTypes ASN.1 module of X.520 [X.520-3] [X.520-4]
[X.520-5].
For example, this is how the DirectoryString definition would appear
in the third, fourth and fifth editions:
DirectoryString{INTEGER:maxSize} ::=
[GSER:CHOICE-OF-STRINGS PRECEDENCE printableString uTF8String]
CHOICE {
teletexString TeletexString(SIZE (1..maxSize)),
printableString PrintableString(SIZE (1..maxSize)),
universalString UniversalString(SIZE (1..maxSize)),
bmpString BMPString(SIZE (1..maxSize)),
uTF8String UTF8String(SIZE (1..maxSize))
}
The uTF8String alternative did not appear in the second edition of
the SelectedAttributeTypes ASN.1 module of X.520 [X.520-2]. For
compatibility, applications using GSER with the second edition of
X.520 MUST add this encoding instruction:
[GSER:CHOICE-OF-STRINGS PRECEDENCE printableString]
immediately before the "CHOICE" keyword in the definition of the
DirectoryString type.
For example, this is how the DirectoryString definition would appear
in the second edition:
DirectoryString{INTEGER:maxSize} ::=
[GSER:CHOICE-OF-STRINGS PRECEDENCE printableString]
CHOICE {
teletexString TeletexString(SIZE (1..maxSize)),
printableString PrintableString(SIZE (1..maxSize)),
universalString UniversalString(SIZE (1..maxSize))
}
5. Security Considerations
This specification changes the manner in which ChoiceOfStrings types
are declared but does not alter the existing behaviour of GSER
implementations. The security considerations for GSER are unchanged
(see [GSER]).
6. Normative References
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[GSER] Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
Types", RFC 3641, October 2003.
[ABNF] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
[X.520-2] ITU-T Recommendation X.520 (1993) | ISO/IEC 9594-6:1994,
Information Technology - Open Systems Interconnection -
The Directory: Selected attribute types
[X.520-3] ITU-T Recommendation X.520 (08/97) | ISO/IEC 9594-6:1998,
Information Technology - Open Systems Interconnection -
The Directory: Selected attribute types
[X.520-4] ITU-T Recommendation X.520 (02/01) | ISO/IEC 9594-6:2001,
Information technology - Open Systems Interconnection -
The Directory: Selected attribute types
[X.520-5] ITU-T Recommendation X.520 (08/05) | ISO/IEC 9594-6:2005,
Information technology - Open Systems Interconnection -
The Directory: Selected attribute types
[X.680] ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X.680-1] ITU-T Recommendation X.680 (2002) Amendment 1 (10/03) |
ISO/IEC 8824-1:2002/Amd 1:2004, Support for EXTENDED-XER.
Author's Address
Dr. Steven Legg
eB2Bcom
Suite 3, Woodhouse Corporate Centre
935 Station Street
Box Hill North, Victoria 3129
AUSTRALIA
Phone: +61 3 9896 7830
Fax: +61 3 9896 7801
EMail: steven.legg@eb2bcom.com
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AI ELI5
On this page
§1Why Encoding Instructions Exist
Abstract Syntax Notation One (ASN.1) lets a specification annotate a type with encoding instructions, which change how values of that type are written by a named set of encoding rules. RFC 4792 defines the notation of those instructions for the Generic String Encoding Rules (GSER), the text encoding of RFC 3641.
It also defines one instruction, CHOICE-OF-STRINGS, to give a machine-processable way to declare a GSER ChoiceOfStrings type. The RFC says this instruction SHOULD be used instead of simply declaring a ChoiceOfStrings type.
ASN.1 describes the shape of data, and GSER writes it as text. This note adds a way to put a sticky note on a shape that says how to write it.
§3Notation for GSER Encoding Instructions
The generic notation for type prefixes and encoding control sections comes from the ASN.1 basic notation. It includes an encoding reference that names which encoding rules are affected, and for GSER that reference is literally GSER.
The notation for a specific set of rules is left to the specification of those rules, so this document fills in the EncodingInstruction and EncodingInstructionAssignmentList placeholders. For GSER, an EncodingInstruction is a ChoiceOfStringsInstruction, and the assignment list, which only appears in an encoding control section, is empty.
The big rulebook leaves a blank for each encoding to fill in. This note fills in the blank for GSER, and GSER only has one kind of sticky note.
§4The CHOICE-OF-STRINGS Instruction
The instruction lets a GSER encoder write the chosen alternative of a CHOICE of restricted string types without the leading identifier. An optional PRECEDENCE list lets the specification writer change the order in which a decoder considers the alternatives when the identifier is absent. The notation is "CHOICE-OF-STRINGS" optionally followed by PRECEDENCE and a list of identifiers.
The instruction must sit textually next to the CHOICE definition it applies to. That makes it clear to a reader that it applies to every use of the type, however the type is referenced.
Each alternative of the CHOICE MUST be a restricted string type such as PrintableString, IA5String, BMPString or UTF8String, possibly through a reference, a constraint or a prefix. Tagging is not significant. No two alternatives may have the same restricted string type, and if any alternative has a constraint, all must have exactly the same one.
Each identifier in the PrecedenceList MUST name an alternative of the CHOICE, and none may appear more than once.
The rules say each choice must be a different kind of text, so the reader can tell them apart. The optional list says which kind to try first.
§4.1Effect on GSER Encodings
A CHOICE value is encoded with the ChoiceValue rule, which is either IdentifiedChoiceValue (an identifier, a colon and the value) or ChoiceOfStringsValue (just a StringValue). If the CHOICE is not subject to a CHOICE-OF-STRINGS instruction, the identified form MUST be used.
If the bare string form is used, a decoder MUST work out the alternative by trying the alternatives in order and accepting the first one that allows all of the characters in the string. With a PrecedenceList, the listed alternatives come first in list order, then the rest in definition order. Without one, all are tried in definition order.
An encoder MUST use the identified form whenever a decoder would pick a different alternative from the one being encoded. Otherwise it MAY use the bare string.
In the RFC's example, with PRECEDENCE basicName, a decoder checks whether the string is a valid basicName (a PrintableString) before checking extendedName (a UTF8String).
When the label is missing, the reader tries each kind of text in order and picks the first one that fits. If the real answer is not what the reader would guess, the writer has to include the label.
[GSER:CHOICE-OF-STRINGS PRECEDENCE basicName] CHOICE {
extendedName UTF8String,
basicName PrintableString
}§4.2Replacing Existing DirectoryString Declarations
The earlier GSER document declared the X.520 DirectoryString type as a ChoiceOfStrings type. Applications using GSER MUST now add [GSER:CHOICE-OF-STRINGS PRECEDENCE printableString uTF8String] just before the CHOICE keyword in the third and every later edition of the SelectedAttributeTypes ASN.1 module of X.520.
The second edition of X.520 has no uTF8String alternative. For compatibility, applications using GSER with that edition MUST instead add [GSER:CHOICE-OF-STRINGS PRECEDENCE printableString]. The RFC shows both resulting DirectoryString definitions.
There is one well-known box of this kind, called DirectoryString. This section says exactly which sticky note to put on it, with a slightly shorter note for the older version of the box.
§5Security Considerations
The specification changes the manner in which ChoiceOfStrings types are declared but does not alter the existing behaviour of GSER implementations. The security considerations for GSER are unchanged and are those of RFC 3641.
Nothing about how the text is read or written changes, only how the rule is written down, so nothing new is risky.