Network Working Group S. Legg

Network Working Group S. Legg
Request for Comments: 3642 Adacel Technologies
Category: Informational October 2003

Common Elements of Generic String Encoding Rules (GSER) Encodings

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

The Generic String Encoding Rules (GSER) describe a human readable
text encoding for an Abstract Syntax Notation One (ASN.1) value of
any ASN.1 type. Specifications making use of GSER may wish to
provide an equivalent Augmented Backus-Naur Form (ABNF) description
of the GSER encoding for a particular ASN.1 type as a convenience for
implementors. This document supports such specifications by
providing equivalent ABNF for the GSER encodings for ASN.1 types that
commonly occur in Lightweight Directory Access Protocol (LDAP)
syntaxes.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions. . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Separators . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. ASN.1 Built-in Types . . . . . . . . . . . . . . . . . . . . . 2
5. ASN.1 Restricted String Types. . . . . . . . . . . . . . . . . 7
6. Directory ASN.1 Types. . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
9. Intellectual Property Notice . . . . . . . . . . . . . . . . . 12
10. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

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1. Introduction

The Generic String Encoding Rules (GSER) [7] define a human readable
text encoding, based on ASN.1 [8] value notation, for an ASN.1 value
of any ASN.1 type. Specifications making use of GSER may wish to
provide a non-normative equivalent ABNF [3] description of the GSER
encoding for a particular ASN.1 type as a convenience for
implementors unfamiliar with ASN.1. This document supports such
specifications by providing equivalent ABNF for the GSER encodings
for ASN.1 types that commonly occur in LDAP [10] or X.500 [11]
attribute and assertion syntaxes, as well as equivalent ABNF for the
GSER encodings for the ASN.1 built-in types.

The ABNF given in this document does not replace or alter GSER in any
way. If there is a discrepancy between the ABNF specified here and
the encoding defined by GSER [7], then GSER is to be taken as
definitive.

2. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document are
to be interpreted as described in BCP 14, RFC 2119 [1]. The key word
"OPTIONAL" is exclusively used with its ASN.1 meaning.

3. Separators

Certain separators are commonly used in constructing equivalent ABNF
for SET and SEQUENCE types.

sp = *%x20 ; zero, one or more space characters
msp = 1*%x20 ; one or more space characters

sep = [ "," ]

The <sep> rule is used in the ABNF description of the encoding for
ASN.1 SET or SEQUENCE types where all the components are either
OPTIONAL or DEFAULT. It encodes to an empty string if and only if
the immediately preceding character in the encoding is "{", i.e., it
is only empty for the first optional component actually present in
the SET or SEQUENCE value being encoded.

4. ASN.1 Built-in Types

This section describes the GSER encoding of values of the ASN.1
built-in types, except for the restricted character string types.

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The <BIT-STRING> rule describes the GSER encoding of values of the
BIT STRING type without a named bit list.

BIT-STRING = bstring / hstring

If the number of bits in a BIT STRING value is a multiple of four the
<hstring> form of <BIT-STRING> MAY be used. Otherwise, the <bstring>
form of <BIT-STRING> is used. The <bstring> rule encodes each bit as
the character "0" or "1" in order from the first bit to the last bit.
The <hstring> rule encodes each group of four bits as a hexadecimal
number where the first bit is the most significant. An odd number of
hexadecimal digits is permitted.

hstring = squote *hexadecimal-digit squote %x48 ; '...'H
hexadecimal-digit = %x30-39 / ; "0" to "9"
%x41-46 ; "A" to "F"

bstring = squote *binary-digit squote %x42 ; '...'B
binary-digit = "0" / "1"

squote = %x27 ; ' (single quote)

The <BOOLEAN> rule describes the GSER encoding of values of the
BOOLEAN type.

BOOLEAN = %x54.52.55.45 / ; "TRUE"
%x46.41.4C.53.45 ; "FALSE"

The <CHARACTER-STRING> rule describes the GSER encoding of values of
the associated type for the unrestricted CHARACTER STRING type.

CHARACTER-STRING = "{" sp id-identification msp Identification ","
sp id-data-value msp OCTET-STRING
sp "}"

id-identification = %x69.64.65.6E.74.69.66.69.63.61.74.69.6F.6E
; "identification"
id-data-value = %x64.61.74.61.2D.76.61.6C.75.65 ; "data-value"

Identification = ( id-syntaxes ":" Syntaxes ) /
( id-syntax ":" OBJECT-IDENTIFIER ) /
( id-presentation-context-id ":" INTEGER ) /
( id-context-negotiation ":"
ContextNegotiation ) /
( id-transfer-syntax ":" OBJECT-IDENTIFIER ) /
( id-fixed ":" NULL )

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id-syntaxes = %x73.79.6E.74.61.78.65.73
; "syntaxes"
id-syntax = %x73.79.6E.74.61.78 ; "syntax"
id-presentation-context-id = %x70.72.65.73.65.6E.74.61.74.69.6F.6E
%x2D.63.6F.6E.74.65.78.74.2D.69.64
; "presentation-context-id"
id-context-negotiation = %x63.6F.6E.74.65.78.74.2D.6E.65.67.6F
%x74.69.61.74.69.6F.6E
; "context-negotiation"
id-transfer-syntax = %x74.72.61.6E.73.66.65.72.2D.73.79.6E
%x74.61.78 ; "transfer-syntax"
id-fixed = %x66.69.78.65.64 ; "fixed"

Syntaxes = "{" sp id-abstract msp OBJECT-IDENTIFIER ","
sp id-transfer msp OBJECT-IDENTIFIER
sp "}"
id-abstract = %x61.62.73.74.72.61.63.74 ; "abstract"
id-transfer = %x74.72.61.6E.73.66.65.72 ; "transfer"

ContextNegotiation = "{" sp id-presentation-context-id msp
INTEGER ","
sp id-transfer-syntax msp
OBJECT-IDENTIFIER
sp "}"

The <INTEGER> rule describes the GSER encoding of values of the
INTEGER type without a named number list. The <INTEGER-0-MAX> rule
describes the GSER encoding of values of the constrained type INTEGER
(0..MAX). The <INTEGER-1-MAX> rule describes the GSER encoding of
values of the constrained type INTEGER (1..MAX).

INTEGER = "0" / positive-number / ("-" positive-number)
INTEGER-0-MAX = "0" / positive-number
INTEGER-1-MAX = positive-number
positive-number = non-zero-digit *decimal-digit
decimal-digit = %x30-39 ; "0" to "9"
non-zero-digit = %x31-39 ; "1" to "9"

The <EMBEDDED-PDV> rule describes the GSER encoding of values of the
associated type for the EMBEDDED PDV type.

EMBEDDED-PDV = "{" sp id-identification msp Identification ","
sp id-data-value msp OCTET-STRING
sp "}"

The <EXTERNAL> rule describes the GSER encoding of values of the
associated type for the EXTERNAL type.

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EXTERNAL = "{" [ sp id-direct-reference msp
OBJECT-IDENTIFIER "," ]
[ sp id-indirect-reference msp INTEGER "," ]
[ sp id-data-value-descriptor msp
ObjectDescriptor "," ]
sp id-encoding msp Encoding
sp "}"

id-direct-reference = %x64.69.72.65.63.74.2D.72.65.66.65.72
%x65.6E.63.65
; "direct-reference"
id-indirect-reference = %x69.6E.64.69.72.65.63.74.2D.72.65.66
%x65.72.65.6E.63.65
; "indirect-reference"
id-data-value-descriptor = %x64.61.74.61.2D.76.61.6C.75.65.2D.64
%x65.73.63.72.69.70.74.6F.72
; "data-value-descriptor"
id-encoding = %x65.6E.63.6F.64.69.6E.67
; "encoding"

Encoding = ( id-single-ASN1-type ":" Value ) /
( id-octet-aligned ":" OCTET-STRING ) /
( id-arbitrary ":" BIT-STRING )

id-single-ASN1-type = %x73.69.6E.67.6C.65.2D.41.53.4E.31.2D.74.79
%x70.65
; "single-ASN1-type"
id-octet-aligned = %x6F.63.74.65.74.2D.61.6C.69.67.6E.65.64
; "octet-aligned"

id-arbitrary = %x61.72.62.69.74.72.61.72.79
; "arbitrary"

The <Value> rule is defined by GSER [7]. It represents the GSER
encoding of a single value of the ASN.1 type identified by the
direct-reference and/or indirect-reference components.

The <NULL> rule describes the GSER encoding of values of the NULL
type.

NULL = %x4E.55.4C.4C ; "NULL"

The <OBJECT-IDENTIFIER> rule describes the GSER encoding of values of
the OBJECT IDENTIFIER type.

OBJECT-IDENTIFIER = numeric-oid / descr
numeric-oid = oid-component 1*( "." oid-component )
oid-component = "0" / positive-number

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An OBJECT IDENTIFIER value is encoded using either the dotted decimal
representation or an object descriptor name, i.e., <descr>. The
<descr> rule is described in RFC 2252 [4]. An object descriptor name
is potentially ambiguous and should be used with care.

The <OCTET-STRING> rule describes the GSER encoding of values of the
OCTET STRING type.

OCTET-STRING = hstring

The octets are encoded in order from the first octet to the last
octet. Each octet is encoded as a pair of hexadecimal digits where
the first digit corresponds to the four most significant bits of the
octet. If the hexadecimal string does not have an even number of
digits, the four least significant bits in the last octet are assumed
to be zero.

The <REAL> rule describes the GSER encoding of values of the REAL
type.

REAL = "0" ; zero
/ PLUS-INFINITY ; positive infinity
/ MINUS-INFINITY ; negative infinity
/ realnumber ; positive base 10 REAL value
/ ( "-" realnumber ) ; negative base 10 REAL value
/ real-sequence-value ; non-zero base 2 or 10 REAL value

PLUS-INFINITY = %x50.4C.55.53.2D.49.4E.46.49.4E.49.54.59
; "PLUS-INFINITY"

MINUS-INFINITY = %x4D.49.4E.55.53.2D.49.4E.46.49.4E.49.54.59
; "MINUS-INFINITY"

realnumber = mantissa exponent
mantissa = (positive-number [ "." *decimal-digit ])
/ ( "0." *("0") positive-number )
exponent = "E" ( "0" / ([ "-" ] positive-number))

real-sequence-value = "{" sp id-mantissa msp INTEGER ","
sp id-base msp ( "2" / "10" ) ","
sp id-exponent msp INTEGER sp "}"
id-mantissa = %x6D.61.6E.74.69.73.73.61 ; "mantissa"
id-base = %x62.61.73.65 ; "base"
id-exponent = %x65.78.70.6F.6E.65.6E.74 ; "exponent"

A value of the REAL type MUST be encoded as "0" if it is zero.

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The <RELATIVE-OID> rule describes the GSER encoding of values of the
RELATIVE-OID type.

RELATIVE-OID = oid-component *( "." oid-component )

5. ASN.1 Restricted String Types

This section describes the GSER encoding of values of the ASN.1
restricted character string types. The characters of a value of a
restricted character string type are always encoded as a UTF-8
character string between double quotes. For some of the ASN.1 string
types, this requires a translation to or from the UTF-8 encoding.
Some of the ASN.1 string types permit only a subset of the characters
representable in UTF-8. Any double quote characters in the character
string, where allowed by the character set, are escaped by being
repeated.

The <UTF8String> rule describes the GSER encoding of values of the
UTF8String type. The characters of this string type do not require
any translation before being encoded.

UTF8String = StringValue
StringValue = dquote *SafeUTF8Character dquote

dquote = %x22 ; " (double quote)

SafeUTF8Character = %x00-21 / %x23-7F / ; ASCII minus dquote
dquote dquote / ; escaped double quote
%xC0-DF %x80-BF / ; 2 byte UTF-8 character
%xE0-EF 2(%x80-BF) / ; 3 byte UTF-8 character
%xF0-F7 3(%x80-BF) ; 4 byte UTF-8 character

The <NumericString>, <PrintableString>, <VisibleString>,
<ISO646String>, <IA5String>, <GeneralizedTime> and <UTCTime> rules
describe the GSER encoding of values of the correspondingly named
ASN.1 types. The characters of these string types are compatible
with UTF-8 and do not require any translation before being encoded.
The GeneralizedTime and UTCTime types use the VisibleString character
set, but have a strictly defined format.

NumericString = dquote *(decimal-digit / space) dquote
space = %x20

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PrintableString = dquote *PrintableCharacter dquote
PrintableCharacter = decimal-digit / space
/ %x41-5A ; A to Z
/ %x61-7A ; a to z
/ %x27-29 ; ' ( )
/ %x2B-2F ; + , - . /
/ %x3A ; :
/ %x3D ; =
/ %x3F ; ?

ISO646String = VisibleString
VisibleString = dquote *SafeVisibleCharacter dquote
SafeVisibleCharacter = %x20-21
/ %x23-7E ; printable ASCII minus dquote
/ dquote dquote ; escaped double quote

IA5String = dquote *SafeIA5Character dquote
SafeIA5Character = %x00-21 / %x23-7F ; ASCII minus dquote
/ dquote dquote ; escaped double quote

century = 2(%x30-39) ; "00" to "99"
year = 2(%x30-39) ; "00" to "99"
month = ( %x30 %x31-39 ) ; "01" (January) to "09"
/ ( %x31 %x30-32 ) ; "10" to "12"
day = ( %x30 %x31-39 ) ; "01" to "09"
/ ( %x31-32 %x30-39 ) ; "10" to "29"
/ ( %x32 %x30-31 ) ; "30" to "31"
hour = ( %x30-31 %x30-39 ) / ( %x32 %x30-33 ) ; "00" to "23"
minute = %x30-35 %x30-39 ; "00" to "59"
second = ( %x30-35 %x30-39 ) ; "00" to "59"
/ ( %x36 %x30 ) ; "60" (a leap second)

UTCTime = dquote year month day hour minute [ second ]
[ %x5A / u-differential ] dquote
u-differential = ( "-" / "+" ) hour minute

GeneralizedTime = dquote century year month day hour
[ minute [ second ] ] [ fraction ]
[ %x5A / g-differential ] dquote
fraction = ( "." / "," ) 1*(%x30-39)
g-differential = ( "-" / "+" ) hour [ minute ]

The <BMPString> and <UniversalString> rules describe the GSER
encoding of values of the BMPString and UniversalString types
respectively. BMPString (UCS-2) and UniversalString (UCS-4) values
are translated into UTF-8 [6] character strings before being encoded
according to <StringValue>.

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BMPString = StringValue
UniversalString = StringValue

The <TeletexString>, <T61String>, <VideotexString>, <GraphicString>,
<GeneralString> and <ObjectDescriptor> rules describe the GSER
encoding of values of the correspondingly named ASN.1 types. Values
of these string types are translated into UTF-8 character strings
before being encoded according to <StringValue>. The
ObjectDescriptor type uses the GraphicString character set.

TeletexString = StringValue
T61String = StringValue
VideotexString = StringValue
GraphicString = StringValue
GeneralString = StringValue
ObjectDescriptor = GraphicString

6. Directory ASN.1 Types

This section describes the GSER encoding of values of selected ASN.1
types defined for LDAP and X.500. The ABNF rule names beginning with
uppercase letters describe the GSER encoding of values of the ASN.1
type with the same name.

AttributeType = OBJECT-IDENTIFIER

The characters of a DirectoryString are translated into UTF-8
characters as required before being encoded between double quotes
with any embedded double quotes escaped by being repeated.

DirectoryString = StringValue /
( id-teletexString ":" TeletexString ) /
( id-printableString ":" PrintableString ) /
( id-bmpString ":" BMPString ) /
( id-universalString ":" UniversalString ) /
( id-uTF8String ":" UTF8String )

id-teletexString = %x74.65.6C.65.74.65.78.53.74.72.69.6E.67
; "teletexString"
id-printableString = %x70.72.69.6E.74.61.62.6C.65
%x53.74.72.69.6E.67 ; "printableString"
id-bmpString = %x62.6D.70.53.74.72.69.6E.67 ; "bmpString"
id-universalString = %x75.6E.69.76.65.72.73.61.6C
%x53.74.72.69.6E.67 ; "universalString"
id-uTF8String = %x75.54.46.38.53.74.72.69.6E.67
; "uTF8String"

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The <RDNSequence> rule describes the GSER encoding of values of the
RDNSequence type, which is syntactically equivalent to the
DistinguishedName and LocalName types. The <RDNSequence> rule
encodes a name as an LDAPDN character string between double quotes.
The character string is first derived according to the
<distinguishedName> rule in Section 3 of RFC 2253 [5], and then it is
encoded between double quotes with any embedded double quotes escaped
by being repeated.

DistinguishedName = RDNSequence
LocalName = RDNSequence
RDNSequence = dquote *SafeUTF8Character dquote

The <RelativeDistinguishedName> rule describes the GSER encoding of
values of the RelativeDistinguishedName type that are not part of an
RDNSequence value. The <RelativeDistinguishedName> rule encodes an
RDN as a double quoted string containing the RDN as it would appear
in an LDAPDN character string. The character string is first derived
according to the <name-component> rule in Section 3 of RFC 2253 [5],
and then any embedded double quote characters are escaped by being
repeated. This resulting string is output between double quotes.

RelativeDistinguishedName = dquote *SafeUTF8Character dquote

The <ORAddress> rule encodes an X.400 address as an IA5 character
string between double quotes. The character string is first derived
according to Section 4.1 of RFC 2156 [2], and then any embedded
double quotes are escaped by being repeated. This resulting string
is output between double quotes.

ORAddress = dquote *SafeIA5Character dquote

7. Security Considerations

This document contains an alternative description of parts of the
Generic String Encoding Rules, but does not replace or alter GSER in
any way. For the full security implications of using GSER, see the
Security Considerations section for GSER [7].

8. References

8.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
between X.400 and RFC 822/MIME", RFC 2156, January 1998.

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[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[4] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions",
RFC 2252, December 1997.

[5] Wahl, M., Kille, S. and T. Howes, "Lightweight Directory Access
Protocol (v3): UTF-8 String Representation of Distinguished
Names", RFC 2253, December 1997.

[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.

[7] Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
Types", RFC 3641, October 2003.

[8] ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1:2002
Information technology - Abstract Syntax Notation One (ASN.1):
Specification of basic notation

8.2. Informative References

[9] Hovey, R. and S. Bradner, "The Organizations Involved in the
IETF Standards Process", BCP 11, RFC 2028, October 1996.

[10] Hodges, J. and R. Morgan, "Lightweight Directory Access Protocol
(v3): Technical Specification", RFC 3377, September 2002.

[11] ITU-T Recommendation X.500 (1993) | ISO/IEC 9594-1:1994,
Information Technology - Open Systems Interconnection - The
Directory: Overview of concepts, models and services

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9. Intellectual Property Notice

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

10. Author's Address

Steven Legg
Adacel Technologies Ltd.
250 Bay Street
Brighton, Victoria 3186
AUSTRALIA

Phone: +61 3 8530 7710
Fax: +61 3 8530 7888
EMail: [email protected]

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11. Full Copyright Statement

Copyright (C) The Internet Society (2003). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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