Generic String Encoding Rules (GSER) for ASN.1 Types
RFC 3641

Network Working Group                                            S. Legg
Request for Comments: 3641                           Adacel Technologies
Category: Standards Track                                   October 2003

          Generic String Encoding Rules (GSER) for ASN.1 Types

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 Internet Society (2003).  All Rights Reserved.

Abstract

   This document defines a set of Abstract Syntax Notation One (ASN.1)
   encoding rules, called the Generic String Encoding Rules (GSER), that
   produce a human readable text encoding for values of any given ASN.1
   data type.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Conventions. . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Generic String Encoding Rules. . . . . . . . . . . . . . . . .  3
       3.1.  Type Referencing Notations . . . . . . . . . . . . . . .  3
       3.2.  Restricted Character String Types. . . . . . . . . . . .  4
       3.3.  ChoiceOfStrings Types. . . . . . . . . . . . . . . . . .  5
       3.4.  Identifiers. . . . . . . . . . . . . . . . . . . . . . .  6
       3.5.  BIT STRING . . . . . . . . . . . . . . . . . . . . . . .  7
       3.6.  BOOLEAN. . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.7.  ENUMERATED . . . . . . . . . . . . . . . . . . . . . . .  8
       3.8.  INTEGER. . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.9.  NULL . . . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.10. OBJECT IDENTIFIER and RELATIVE-OID . . . . . . . . . . .  8
       3.11. OCTET STRING . . . . . . . . . . . . . . . . . . . . . .  9
       3.12. CHOICE . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.13. SEQUENCE and SET . . . . . . . . . . . . . . . . . . . . 10
       3.14. SEQUENCE OF and SET OF . . . . . . . . . . . . . . . . . 10
       3.15. CHARACTER STRING . . . . . . . . . . . . . . . . . . . . 11
       3.16. EMBEDDED PDV . . . . . . . . . . . . . . . . . . . . . . 11
       3.17. EXTERNAL . . . . . . . . . . . . . . . . . . . . . . . . 11

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       3.18. INSTANCE OF. . . . . . . . . . . . . . . . . . . . . . . 11
       3.19. REAL . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       3.20. Variant Encodings. . . . . . . . . . . . . . . . . . . . 12
   4.  GSER Transfer Syntax . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 13
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       6.1.  Normative References . . . . . . . . . . . . . . . . . . 13
       6.2.  Informative References . . . . . . . . . . . . . . . . . 14
   7.  Intellectual Property Notice . . . . . . . . . . . . . . . . . 15
   8.  Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   This document defines a set of ASN.1 [8] encoding rules, called the
   Generic String Encoding Rules or GSER, that produce a human readable
   UTF-8 [6] character string encoding of ASN.1 values of any given
   arbitrary ASN.1 type.

   Note that "ASN.1 value" does not mean a Basic Encoding Rules (BER)
   [12] encoded value.  The ASN.1 value is an abstract concept that is
   independent of any particular encoding.  BER is just one possible
   encoding of an ASN.1 value.

   GSER is based on ASN.1 value notation [8], with changes to
   accommodate the notation's use as a transfer syntax, and to support
   well established ad-hoc string encodings for Lightweight Directory
   Access Protocol (LDAP) [14] directory data types.

   Though primarily intended for defining the LDAP-specific encoding of
   new LDAP attribute syntaxes and assertion syntaxes, these encoding
   rules could also be used in other domains where human readable
   renderings of ASN.1 values would be useful.

   Referencing GSER is sufficient to define a human readable text
   encoding for values of a specific ASN.1 type, however other
   specifications may wish to provide a customized Augmented Backus-Naur
   Form (ABNF) [3] description, independent of the ASN.1, as a
   convenience for the implementor (equivalent ABNF for the GSER
   encodings for ASN.1 types commonly occurring in LDAP syntaxes is
   provided in a separate document [15]).  Such a specification SHOULD
   state that if there is a discrepancy between the customized ABNF and
   the GSER encoding defined by this document, that the GSER encoding
   takes precedence.

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2.  Conventions

   Throughout this document, "type" shall be taken to mean an ASN.1
   type, and "value" shall be taken to mean an ASN.1 value.

   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 [1].

3.  Generic String Encoding Rules

   The GSER encoding of a value of any ASN.1 type is described by the
   following ABNF [3]:

      Value = BitStringValue /
              BooleanValue /
              CharacterStringValue /
              ChoiceValue /
              EmbeddedPDVValue /
              EnumeratedValue /
              ExternalValue /
              GeneralizedTimeValue /
              IntegerValue /
              InstanceOfValue /
              NullValue /
              ObjectDescriptorValue /
              ObjectIdentifierValue /
              OctetStringValue /
              RealValue /
              RelativeOIDValue /
              SequenceOfValue /
              SequenceValue /
              SetOfValue /
              SetValue /
              StringValue /
              UTCTimeValue /
              VariantEncoding

   The ABNF for each of the above rules is given in the following
   sections.

3.1 Type Referencing Notations

   A value of a type with a defined type name is encoded according to
   the type definition on the right hand side of the type assignment for
   the type name.

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   A value of a type denoted by the use of a parameterized type with
   actual parameters is encoded according to the parameterized type with
   the DummyReferences [11] substituted with the actual parameters.

   A value of a tagged or constrained type is encoded as a value of the
   type without the tag or constraint, respectively.  Tags do not appear
   in the string encodings defined by this document.  See X.680 [8] and
   X.682 [10] for the details of ASN.1 constraint notation.

   A value of an open type denoted by an ObjectClassFieldType (Clause 14
   of X.681 [9]) is encoded according to the specific type of the value.

   A value of a fixed type denoted by an ObjectClassFieldType is encoded
   according to that fixed type.

   A value of a selection type is encoded according to the type
   referenced by the selection type.

   A value of a type described by TypeFromObject notation (Clause 15 of
   X.681 [9]) is encoded according to the denoted type.

   A value of a type described by ValueSetFromObjects notation (Clause
   15 of X.681 [9]) is encoded according to the governing type.

3.2.  Restricted Character String Types

   The contents of a string value are encoded as a UTF-8 character
   string between double quotes, regardless of the ASN.1 string type.
   Depending on the ASN.1 string type and an application's internal
   representation of that string type, a translation to or from the
   UTF-8 character encoding may be required.  NumericString,
   PrintableString, IA5String, and VisibleString (ISO646String) are
   compatible with UTF-8 and do not require any translation.  BMPString
   (UCS-2) and UniversalString (UCS-4) have a direct mapping to and from
   UTF-8 [6].  For the remaining string types see X.680 [8].  Any
   embedded double quotes in the resulting UTF-8 character string are
   escaped by repeating the double quote characters.

   A value of the NumericString, PrintableString, TeletexString
   (T61String), VideotexString, IA5String, GraphicString, VisibleString
   (ISO646String), GeneralString, BMPString, UniversalString or
   UTF8String type is encoded according to the <StringValue> rule.

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      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

   A value of the GeneralizedTime type, UTCTime type or ObjectDescriptor
   type is encoded as a string value.  GeneralizedTime and UTCTime use
   the VisibleString character set so the conversion to UTF-8 is
   trivial.  ObjectDescriptor uses the GraphicString type.

      GeneralizedTimeValue  = StringValue
      UTCTimeValue          = StringValue
      ObjectDescriptorValue = StringValue

3.3.  ChoiceOfStrings Types

   It is not uncommon for ASN.1 specifications to define types that
   offer a CHOICE between two or more alternative ASN.1 string types,
   where the particular alternative chosen carries no semantic
   significance (DirectoryString [7] being a prime example).  Such types
   are defined to avoid having to use a complicated character encoding
   for all values when most values could use a simpler string type, or
   to deal with evolving requirements that compel the use of a broader
   character set while still maintaining backward compatibility.

   GSER encodes values of all the ASN.1 string types as UTF-8 character
   strings so the particular alternative that is chosen from a purely
   syntactic CHOICE of string types makes no material difference to the
   final encoding of the string value.

   While there are certain ASN.1 constructs that betray the semantic
   significance of the alternatives within a CHOICE type, the absence of
   those constructs does not necessarily mean that a CHOICE type is
   purely syntactic.  Therefore, it is necessary for specifications to
   declare the purely syntactic CHOICE types so that they may be more
   compactly encoded (see Section 3.12).  These declared CHOICE types
   are referred to as ChoiceOfStrings types.

   To be eligible to be declared a ChoiceOfStrings type, an ASN.1 type
   MUST satisfy the following conditions.

   a) The type is a CHOICE type.

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   b) The component type of each alternative is one of the following
      ASN.1 restricted string types: NumericString, PrintableString,
      TeletexString (T61String), VideotexString, IA5String,
      GraphicString, VisibleString (ISO646String), GeneralString,
      BMPString, UniversalString or UTF8String.

   c) All the alternatives are of different restricted string types,
      i.e., no two alternatives have the same ASN.1 restricted string
      type.

   d) Either none of the alternatives has a constraint, or all of the
      alternatives have exactly the same constraint.

   Tagging on the alternative types is ignored.

   Consider the ASN.1 parameterized type definition of DirectoryString.

      DirectoryString { INTEGER : maxSize } ::= CHOICE {
          teletexString     TeletexString (SIZE (1..maxSize)),
          printableString   PrintableString (SIZE (1..maxSize)),
          bmpString         BMPString (SIZE (1..maxSize)),
          universalString   UniversalString (SIZE (1..maxSize)),
          uTF8String        UTF8String (SIZE (1..maxSize)) }

   Any use of the DirectoryString parameterized type with an actual
   parameter defines an ASN.1 type that satisfies the above conditions.
   Recognising that the alternative within a DirectoryString carries no
   semantic significance, this document declares (each and every use of)
   DirectoryString{} to be a ChoiceOfStrings type.

   Other specifications MAY declare other types satisfying the above
   conditions to be ChoiceOfStrings types.  The declaration SHOULD be
   made at the point where the ASN.1 type is defined, otherwise it
   SHOULD be made at the point where it is introduced as, or in, an LDAP
   attribute or assertion syntax.

3.4.  Identifiers

   An <identifier> conforms to the definition of an identifier in ASN.1
   notation (Clause 11.3 of X.680 [8]).  It begins with a lowercase
   letter and is followed by zero or more letters, digits, and hyphens.
   A hyphen is not permitted to be the last character, nor is it to be
   followed by another hyphen.  The case of letters in an identifier is
   always significant.

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      identifier    = lowercase *alphanumeric *(hyphen 1*alphanumeric)
      alphanumeric  = uppercase / lowercase / decimal-digit
      uppercase     = %x41-5A  ; "A" to "Z"
      lowercase     = %x61-7A  ; "a" to "z"
      decimal-digit = %x30-39  ; "0" to "9"
      hyphen        = "-"

3.5.  BIT STRING

   A value of the BIT STRING type is encoded according to the
   <BitStringValue> rule.  If the definition of the BIT STRING type
   includes a named bit list, the <bit-list> form of <BitStringValue>
   MAY be used.  If the number of bits in a BIT STRING value is a
   multiple of four, the <hstring> form of <BitStringValue> MAY be used.
   Otherwise, the <bstring> form of <BitStringValue> is used.

      BitStringValue = bstring / hstring / bit-list

   The <bit-list> rule encodes the one bits in the bit string value as a
   comma separated list of identifiers.  Each <identifier> MUST be one
   of the identifiers in the named bit list, and MUST NOT appear more
   than once in the same <bit-list>.  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.

      bit-list          = "{" [ sp identifier
                             *( "," sp identifier ) ] sp "}"

      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"

      sp                = *%x20  ; zero, one or more space characters
      squote            =  %x27  ; ' (single quote)

3.6.  BOOLEAN

   A value of the BOOLEAN type is encoded according to the
   <BooleanValue> rule.

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

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3.7.  ENUMERATED

   A value of the ENUMERATED type is encoded according to the
   <EnumeratedValue> rule.  The <identifier> MUST be one of those in the
   list of enumerations in the definition of the ENUMERATED type.

      EnumeratedValue = identifier

3.8.  INTEGER

   A value of the INTEGER type is encoded according to the
   <IntegerValue> rule.  If the definition of the INTEGER type includes
   a named number list, the <identifier> form of <IntegerValue> MAY be
   used, in which case the <identifier> MUST be one of the identifiers
   in the named number list.

      IntegerValue    = "0" /
                        positive-number /
                        ("-" positive-number) /
                        identifier

      positive-number = non-zero-digit *decimal-digit
      non-zero-digit  = %x31-39  ; "1" to "9"

3.9.  NULL

   A value of the NULL type is encoded according to the <NullValue>
   rule.

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

3.10.  OBJECT IDENTIFIER and RELATIVE-OID

   A value of the OBJECT IDENTIFIER type is encoded according to the
   <ObjectIdentifierValue> rule.  The <ObjectIdentifierValue> rule
   allows either a dotted decimal representation of the OBJECT
   IDENTIFIER value 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.

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

   A value of the RELATIVE-OID type is encoded according to the
   <RelativeOIDValue> rule.

      RelativeOIDValue = oid-component *( "." oid-component )

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3.11.  OCTET STRING

   A value of the OCTET STRING type is encoded according to the
   <OctetStringValue> rule.  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.

      OctetStringValue = hstring

3.12.  CHOICE

   A value of a CHOICE type is encoded according to the <ChoiceValue>
   rule.  The <ChoiceOfStringsValue> encoding MAY be used if the
   corresponding CHOICE type has been declared a ChoiceOfStrings type.
   This document declares DirectoryString to be a ChoiceOfStrings type
   (see Section 3.3).  Otherwise, the <IdentifiedChoiceValue> form of
   <ChoiceValue> is used.

      ChoiceValue           = IdentifiedChoiceValue /
                              ChoiceOfStringsValue
      IdentifiedChoiceValue = identifier ":" Value
      ChoiceOfStringsValue  = StringValue

   For implementations that recognise the internal structure of the
   DirectoryString CHOICE type (e.g., X.500 directories [16]), if the
   character string between the quotes in a <StringValue> contains only
   characters that are permitted in a PrintableString, the
   DirectoryString is assumed to use the printableString alternative,
   otherwise it is assumed to use the uTF8String alternative.  The
   <IdentifiedChoiceValue> rule MAY be used for a value of type
   DirectoryString to indicate an alternative other than the one that
   would be assumed from the string contents.  No matter what
   alternative is chosen, the <Value> will still be a UTF-8 encoded
   character string.  However, it is a syntax error if the characters in
   the UTF-8 string cannot be represented in the string type of the
   chosen alternative.

   Implementations that do not care about the internal structure of a
   DirectoryString value MUST be able to parse the
   <IdentifiedChoiceValue> form for a DirectoryString value, though the
   particular identifier found will be of no interest.

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3.13.  SEQUENCE and SET

   A value of a SEQUENCE type is encoded according to the
   <SequenceValue> rule.  The <ComponentList> rule encodes a comma
   separated list of the particular component values present in the
   SEQUENCE value, where each component value is preceded by the
   corresponding identifier from the SEQUENCE type definition.  The
   components are encoded in the order of their definition in the
   SEQUENCE type.

      SequenceValue = ComponentList

      ComponentList = "{" [ sp NamedValue *( "," sp NamedValue) ] sp "}"
      NamedValue    = identifier msp Value
      msp           = 1*%x20  ; one or more space characters

   A value of a SET type is encoded according to the <SetValue> rule.
   The components are encoded in the order of their definition in the
   SET type (i.e., just like a SEQUENCE value).  This is a deliberate
   departure from ASN.1 value notation where the components of a SET can
   be written in any order.

      SetValue = ComponentList

   SEQUENCE and SET type definitions are sometimes extended by the
   inclusion of additional component types, so an implementation SHOULD
   be capable of skipping over any <NamedValue> encoding with an
   identifier that is not recognised, on the assumption that the sender
   is using a more recent definition of the SEQUENCE or SET type.

3.14.  SEQUENCE OF and SET OF

   A value of a SEQUENCE OF type is encoded according to the
   <SequenceOfValue> rule, as a comma separated list of the instances in
   the value.  Each instance is encoded according to the component type
   of the SEQUENCE OF type.

      SequenceOfValue = "{" [ sp Value *( "," sp Value) ] sp "}"

   A value of a SET OF type is encoded according to the <SetOfValue>
   rule, as a list of the instances in the value.  Each instance is
   encoded according to the component type of the SET OF type.

      SetOfValue      = "{" [ sp Value *( "," sp Value) ] sp "}"

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3.15.  CHARACTER STRING

   A value of the unrestricted CHARACTER STRING type is encoded
   according to the corresponding SEQUENCE type defined in Clause 40.5
   of X.680 [8] (see [15] for equivalent ABNF).

      CharacterStringValue = SequenceValue

3.16.  EMBEDDED PDV

   A value of the EMBEDDED PDV type is encoded according to the
   corresponding SEQUENCE type defined in Clause 33.5 of X.680 [8] (see
   [15] for equivalent ABNF).

      EmbeddedPDVValue = SequenceValue

3.17.  EXTERNAL

   A value of the EXTERNAL type is encoded according to the
   corresponding SEQUENCE type defined in Clause 8.18.1 of X.690 [12]
   (see [15] for equivalent ABNF).

      ExternalValue = SequenceValue

3.18.  INSTANCE OF

   A value of the INSTANCE OF type is encoded according to the
   corresponding SEQUENCE type defined in Annex C of X.681 [9].

      InstanceOfValue = SequenceValue

3.19.  REAL

   A value of the REAL type MUST be encoded as "0" if it is zero,
   otherwise it is encoded as the special value <PLUS-INFINITY>, the
   special value <MINUS-INFINITY>, an optionally signed <realnumber>, or
   as a value of the corresponding SEQUENCE type for REAL defined in
   Clause 20.5 of X.680 [8] (see [15] for equivalent ABNF).

      RealValue  = "0"               ; zero REAL value
                   / PLUS-INFINITY   ; positive infinity
                   / MINUS-INFINITY  ; negative infinity
                   / realnumber      ; positive base 10 REAL value
                   / "-" realnumber  ; negative base 10 REAL value
                   / SequenceValue   ; non-zero REAL value, base 2 or 10

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

      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"

3.20.  Variant Encodings

   The values of some named complex ASN.1 types have special string
   encodings.  These special encodings are always used instead of the
   encoding that would otherwise apply based on the ASN.1 type
   definition.

      VariantEncoding = RDNSequenceValue /
                        RelativeDistinguishedNameValue /
                        ORAddressValue

   A value of the RDNSequence type, i.e., a distinguished name, is
   encoded according to the <RDNSequenceValue> rule, as a quoted LDAPDN
   character string.  The character string is first derived according to
   the <distinguishedName> rule in Section 3 of RFC 2253 [5], and then
   encoded as if it were a UTF8String value, i.e., between double quotes
   with any embedded double quotes escaped by being repeated.

      RDNSequenceValue = StringValue

   A RelativeDistinguishedName value that is not part of an RDNSequence
   value is encoded according to the <RelativeDistinguishedNameValue>
   rule as a quoted character string.  The character string is first
   derived according to the <name-component> rule in Section 3 of RFC
   2253 [5], and then encoded as if it were a UTF8String value.

      RelativeDistinguishedNameValue = StringValue

   A value of the ORAddress type is encoded according to the
   <ORAddressValue> rule as a quoted character string.  The character
   string is first derived according to the textual representation of
   MTS.ORAddress from RFC 2156 [2], and then encoded as if it were an
   IA5String value.

      ORAddressValue = StringValue

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4.  GSER Transfer Syntax

   The following OBJECT IDENTIFIER has been assigned by Adacel
   Technologies, under an arc assigned to Adacel by Standards Australia,
   to identify the Generic String Encoding Rules:

      { 1 2 36 79672281 0 0 }

   This OBJECT IDENTIFIER would be used, for example, to describe the
   transfer syntax for a GSER encoded data-value in an EMBEDDED PDV
   value.

5.  Security Considerations

   The Generic String Encoding Rules do not define a canonical encoding.
   That is, a transformation from a GSER encoding into some other
   encoding (e.g., BER) and back into GSER will not necessarily
   reproduce the original GSER octet encoding.  Therefore, GSER MUST NOT
   be used where a canonical encoding is needed.

   Furthermore, GSER does not necessarily enable the exact octet
   encoding of values of the TeletexString, VideotexString,
   GraphicString or GeneralString types to be reconstructed, so a
   transformation from a Distinguished Encoding Rules (DER) [12]
   encoding to GSER and back to DER may not reproduce the original DER
   encoding.  Therefore, GSER MUST NOT be used to re-encode, whether for
   storage or transmission, ASN.1 abstract values whose original binary
   encoding must be recoverable.  Such recovery is needed for the
   verification of digital signatures.  In such cases, protocols ought
   to use DER or a DER-reversible encoding.

   When interpreting security-sensitive fields, and in particular fields
   used to grant or deny access, implementations MUST ensure that any
   comparisons are done on the underlying abstract value, regardless of
   the particular encoding used.

6.  References

6.1.  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.

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

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   [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]  ITU-T Recommendation X.520 (1993) | ISO/IEC 9594-6:1994,
        Information Technology - Open Systems Interconnection - The
        Directory: Selected attribute types

   [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

   [9]  ITU-T Recommendation X.681 (07/02) | ISO/IEC 8824-2:2002
        Information technology - Abstract Syntax Notation One (ASN.1):
        Information object specification

   [10] ITU-T Recommendation X.682 (07/02) | ISO/IEC 8824-3:2002
        Information technology - Abstract Syntax Notation One (ASN.1):
        Constraint specification

   [11] ITU-T Recommendation X.683 (07/02) | ISO/IEC 8824-4:2002
        Information technology - Abstract Syntax Notation One (ASN.1):
        Parameterization of ASN.1 specifications

   [12] ITU-T Recommendation X.690 (07/02) | ISO/IEC 8825-1:2002
        Information technology - ASN.1 encoding rules: Specification of
        Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and
        Distinguished Encoding Rules (DER)

6.2.  Informative References

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

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

   [15] Legg, S., "Common Elements of Generic String Encoding Rules
        (GSER) Encodings", RFC 3642, October 2003.

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

7.  Intellectual Property Notice

   The IETF takes no position regarding the validity or scope of any
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   pertain to the implementation or use of the technology described in
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   The IETF invites any interested party to bring to its attention any
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8.  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: steven.legg@adacel.com.au

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9.  Full Copyright Statement

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Acknowledgement

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

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