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INFORMATIONAL

Internet Engineering Task Force (IETF)                        C. Wallace
Request for Comments: 6025                            Cygnacom Solutions
Category: Informational                                      C. Gardiner
ISSN: 2070-1721                                         BBN Technologies
                                                            October 2010


                           ASN.1 Translation

Abstract

   Abstract Syntax Notation One (ASN.1) is widely used throughout the
   IETF Security Area and has been for many years.  Some specifications
   were written using a now deprecated version of ASN.1 and some were
   written using the current version of ASN.1.  Not all ASN.1 compilers
   support both older and current syntax.  This document is intended to
   provide guidance to specification authors and to implementers
   converting ASN.1 modules from one version of ASN.1 to another version
   without causing changes to the "bits on the wire".  This document
   does not provide a comprehensive tutorial of any version of ASN.1.
   Instead, it addresses ASN.1 features that are used in IETF Security
   Area specifications with a focus on items that vary with the ASN.1
   version.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6025.












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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  ASN.1 Design Elements  . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Open Types . . . . . . . . . . . . . . . . . . . . . . . .  3
       2.1.1.  ANY DEFINED BY . . . . . . . . . . . . . . . . . . . .  4
       2.1.2.  OCTET STRINGs and BIT STRINGs  . . . . . . . . . . . .  5
       2.1.3.  Information Object Classes . . . . . . . . . . . . . .  5
     2.2.  Constraints  . . . . . . . . . . . . . . . . . . . . . . .  8
       2.2.1.  Simple Table Constraints . . . . . . . . . . . . . . .  8
       2.2.2.  Component Relation Constraints . . . . . . . . . . . .  9
       2.2.3.  Content Constraints  . . . . . . . . . . . . . . . . . 11
     2.3.  Parameterization . . . . . . . . . . . . . . . . . . . . . 12
     2.4.  Versioning and Extensibility . . . . . . . . . . . . . . . 13
       2.4.1.  Extension Markers  . . . . . . . . . . . . . . . . . . 14
       2.4.2.  Version Brackets . . . . . . . . . . . . . . . . . . . 14
   3.  Character Set Differences  . . . . . . . . . . . . . . . . . . 15
   4.  ASN.1 Translation  . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Downgrading from X.68x to X.208  . . . . . . . . . . . . . 16
     4.2.  Upgrading from X.208 to X.68x  . . . . . . . . . . . . . . 16
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 18











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1.  Introduction

   This document is intended to serve as a tutorial for converting ASN.1
   modules written using [CCITT.X208.1988] to [CCITT.X680.2002], or vice
   versa.  Preparation of this document was motivated by [RFC5911] and
   [RFC5912], which provide updated ASN.1 modules for a number of RFCs.

   The intent of this specification is to assist with translation of
   ASN.1 from one version to another without resulting in any changes to
   the encoded results when using the Basic Encoding Rules or
   Distinguished Encoding Rules [CCITT.X209.1988] [CCITT.X690.2002].
   Other encoding rules were not considered.

   Transforming a new ASN.1 module to an older ASN.1 module can be
   performed in a fairly mechanical manner; much of the transformation
   consists of deleting new constructs.  Transforming an older ASN.1
   module to a newer ASN.1 module can also be done fairly mechanically,
   if one does not wish to move many of the constraints that are
   contained in the prose into the ASN.1 module.  If the constraints are
   to be added, then the conversion can be a complex process.

1.1.  Terminology

   This document addresses two different versions of ASN.1.  The old
   (1988) version was defined in a single document (X.208) and the newer
   (1998, 2002) version is defined in a series of documents (X.680,
   X.681, X.682, and X.683).  For convenience, the series of documents
   is henceforth referred to as X.68x.  Specific documents from the
   series are referenced by name where appropriate.

2.  ASN.1 Design Elements

   When translating an ASN.1 module from X.208 syntax to X.68x syntax,
   or vice versa, many definitions do not require or benefit from
   change.  Review of the original ASN.1 modules updated by [RFC5911]
   and [RFC5912] and the revised modules included in those documents
   indicates that most changes can be sorted into one of a few
   categories.  This section describes these categories.

2.1.  Open Types

   Protocols often feature flexible designs that enable other (later)
   specifications to define the syntax and semantics of some features.
   For example, [RFC5280] includes the definition of the Extension
   structure.  There are many instances of extensions defined in other
   specifications.  Several mechanisms to accommodate this practice are
   available in X.208, X.68x, or both, as described below.




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2.1.1.  ANY DEFINED BY

   X.208 defines the ANY DEFINED BY production for specifying open
   types.  This typically appears in a SEQUENCE along with an OBJECT
   IDENTIFIER that indicates the type of object that is encoded.  The
   ContentInfo structure, shown below from [RFC5652], uses ANY DEFINED
   BY along with an OBJECT IDENTIFIER field to identify and convey
   arbitrary types of data.  Each content type to be wrapped in a
   ContentInfo is assigned a unique OBJECT IDENTIFIER, such as the
   id-signedData shown below.  However, X.208 does not provide a formal
   means for establishing a relationship between a type and the type
   identifier.  Any associations are done in the comments of the module
   and/or the text of the associated document.

   -- from RFC 5652
   ContentInfo ::= SEQUENCE {
       contentType ContentType,
       content [0] EXPLICIT ANY DEFINED BY contentType }

   ContentType ::= OBJECT IDENTIFIER

   id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
      us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }

   ANY DEFINED BY may also appear using an INTEGER to indicate the type
   of object that is encoded, as shown in the following example from
   [RFC5280].

   -- from RFC 5280
   ExtensionAttribute ::=  SEQUENCE {
       extension-attribute-type [0] IMPLICIT INTEGER
           (0..ub-extension-attributes),
       extension-attribute-value [1]
           ANY DEFINED BY extension-attribute-type }

   Though the usage of ANY DEFINED BY was deprecated in 1994, it appears
   in some active specifications.  The AttributeValue definition in
   [RFC5280] uses ANY with a DEFINED BY comment to bind the value to a
   type identifier field.

   -- from RFC 5280
   AttributeTypeAndValue ::= SEQUENCE {
       type     AttributeType,
       value    AttributeValue }

   AttributeType ::= OBJECT IDENTIFIER

   AttributeValue ::= ANY -- DEFINED BY AttributeType



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2.1.2.  OCTET STRINGs and BIT STRINGs

   Both X.208 and X.68x allow open types to be implemented using OCTET
   STRINGs and BIT STRINGs as containers.  The definitions of Extension
   and SubjectPublicKeyInfo in [RFC5280] demonstrate the usage of OCTET
   STRING and BIT STRING, respectively, to convey information that is
   further defined using ASN.1.

   -- from RFC 5280
   Extension  ::=  SEQUENCE  {
       extnID      OBJECT IDENTIFIER,
       critical    BOOLEAN DEFAULT FALSE,
       extnValue   OCTET STRING
       -- contains the DER encoding of an ASN.1 value
       -- corresponding to the extension type identified
       -- by extnID
   }

   SubjectPublicKeyInfo  ::=  SEQUENCE  {
        algorithm            AlgorithmIdentifier,
        subjectPublicKey     BIT STRING  }

   In both cases, the prose of the specification describes that the
   adjacent OBJECT IDENTIFIER value indicates the type of structure
   within the value of the primitive OCTET STRING or BIT STRING type.
   For example, where an extnID field contains the value
   id-ce-basicConstraints, the extnValue field contains an encoded
   BasicConstraints as the value of the OCTET STRING, as shown in the
   dump of an encoded extension below.

   Tag Length      Value
   30   15:         SEQUENCE {
   06    3:           OBJECT IDENTIFIER basicConstraints (2 5 29 19)
   01    1:           BOOLEAN TRUE
   04    5:           OCTET STRING, encapsulates {
   30    3:               SEQUENCE {
   01    1:                 BOOLEAN TRUE
          :                 }
          :               }
          :           }

2.1.3.  Information Object Classes

   Information object classes are defined in [CCITT.X681.2002].  Object
   classes allow protocol designers to relate pieces of data that are in
   some way associated.  In the most generic of terms, an Information
   Object class can be thought of as a database schema, with Information
   Object Sets being instances of the databases.



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   Unlike type definitions, object classes with the same structure are
   not equivalent.  Thus, if you have:

      FOO ::= TYPE-IDENTIFIER

      BAR ::= TYPE-IDENTIFIER

   FOO and BAR are not interchangeable.

   TYPE-IDENTIFIER is one of the predefined information object classes
   in Annex A of [CCITT.X681.2002].  This provides for a simple mapping
   from an OBJECT IDENTIFIER to a data type.  The tag UNIQUE on &id
   means that this value may appear only once in an Information Object
   Set; however, multiple objects can be defined with the same &id
   value.

   [RFC5911] uses the TYPE-IDENTIFIER construction to update the
   definition of ContentInfo, as shown below.

   -- TYPE-IDENTIFIER definition from X.681
   TYPE-IDENTIFIER ::= CLASS
   {
       &id OBJECT IDENTIFIER UNIQUE,
       &Type
   }
   WITH SYNTAX {&Type IDENTIFIED BY &id}

   -- from updated RFC 5652 module in [RFC5911]
   CONTENT-TYPE ::= TYPE-IDENTIFIER
   ContentType ::= CONTENT-TYPE.&id

   ContentInfo ::= SEQUENCE {
       contentType        CONTENT-TYPE.
                       &id({ContentSet}),
       content            [0] EXPLICIT CONTENT-TYPE.
                       &Type({ContentSet}{@contentType})}

   ContentSet CONTENT-TYPE ::= {
       --  Define the set of content types to be recognized.
       ct-Data | ct-SignedData | ct-EncryptedData | ct-EnvelopedData |
       ct-AuthenticatedData | ct-DigestedData, ... }

   -- other CONTENT-TYPE instances not shown for brevity
   ct-SignedData CONTENT-TYPE ::=
        { SignedData IDENTIFIED BY id-signedData}






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   This example illustrates the following:

   o  Definition of an information object class: TYPE-IDENTIFIER and
      CONTENT-TYPE are information object classes.

   o  Definition of an information object, or an instance of an
      information object class: ct-SignedData is an information object.

   o  Definition of an information object set: ContentSet is an
      information object set.

   o  Usage of an information object: The definition of ContentInfo uses
      information from the CONTENT-TYPE information object class.

   o  Defining constraints using an object set: Both the contentType and
      content fields are constrained by ContentSet.

   As noted above, TYPE-IDENTIFIER simply associates an OBJECT
   IDENTIFIER with an arbitrary data type.  CONTENT-TYPE is a TYPE-
   IDENTIFIER.  The WITH SYNTAX component allows for a more natural
   language expression of information object definitions.

   ct-SignedData is the name of an information object that associated
   the identifier id-signedData with the data type SignedData.  It is an
   instance of the CONTENT-TYPE information object class.  The &Type
   field is assigned the value SignedData, and the &id field is assigned
   the value id-signedData.  The example above uses the syntax provided
   by the WITH SYNTAX component of the TYPE-IDENTIFIER definition.  An
   equivalent definition that does not use the provided syntax is as
   follows:

   ct-SignedData CONTENT-TYPE ::=
   {
       &id id-signedData,
       &Type SignedData
   }

   ContentSet is the name of a set of information objects derived from
   the CONTENT-TYPE information object class.  The set contains six
   information objects and is extensible, as indicated by the ellipsis
   (see Section 2.4, "Versioning and Extensibility").

   ContentInfo is defined using type information from an information
   object, i.e., the type of the contentType field is that of the &id
   field from CONTENT-TYPE.  An equivalent definition is as follows:

   ContentType ::= OBJECT IDENTIFIER




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   Both fields in ContentInfo are constrained.  The contentType field is
   constrained using a simple table constraint that restricts the values
   to those from the corresponding field of the objects in ContentSet.
   The content field is constrained using a component relationship
   constraint.  Constraints are discussed in the next section.

2.2.  Constraints

   The X.68x versions of the ASN.1 specifications introduced the ability
   to use the object information sets as part of the constraint on the
   values that a field can take.  Simple table constraints are used to
   restrict the set of values that can occur in a field.  Component
   relation constraints allow for the restriction of a field based on
   contents of other fields in the type.

2.2.1.  Simple Table Constraints

   Simple table constraints are widely used in [RFC5911] and [RFC5912]
   to limit implementer options (although the constraints are almost
   always followed by or include extensibility markers, which make the
   parameters serve as information not as limitations).  Table
   constraints are defined in [CCITT.X682.2002].

   Some ASN.1 compilers have the ability to use the simple table
   constraint to check that a field contains one of the legal values.

   The following example from [RFC5911] demonstrates using table
   constraints to clarify the intended usage of a particular field.  The
   parameters indicate the types of attributes that are typically found
   in the signedAttrs and unsignedAttrs fields.  In this example, the
   object sets are disjoint but this is not required.  For example, in
   [RFC5912], there is some overlap between the CertExtensions and
   CrlExtensions sets.

   -- from updated RFC 5652 module in [RFC5911]
   SignerInfo ::= SEQUENCE {
       version CMSVersion,
       sid SignerIdentifier,
       digestAlgorithm DigestAlgorithmIdentifier,
       signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
       signatureAlgorithm SignatureAlgorithmIdentifier,
       signature SignatureValue,
       unsignedAttrs [1] IMPLICIT Attributes
            {{UnsignedAttributes}} OPTIONAL }

   SignedAttributes ::= Attributes {{ SignedAttributesSet }}





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   SignedAttributesSet ATTRIBUTE ::=
          { aa-signingTime | aa-messageDigest | aa-contentType, ... }

   UnsignedAttributes ATTRIBUTE ::= { aa-countersignature, ... }

2.2.2.  Component Relation Constraints

   Component relation constraints are often used to bind the type field
   of an open type to the identifier field.  Using the binding in this
   way allows a compiler to immediately decode the associated type when
   the containing structure is defined.  The following example from
   [RFC2560] as updated in [RFC5912] demonstrates this usage.

   -- from updated RFC 2560 module in [RFC5912]
   RESPONSE ::= TYPE-IDENTIFIER

   ResponseSet RESPONSE ::= {basicResponse, ...}

   ResponseBytes ::=       SEQUENCE {
       responseType        RESPONSE.
                               &id ({ResponseSet}),
       response            OCTET STRING (CONTAINING RESPONSE.
                               &Type({ResponseSet}{@responseType}))}

   In this example, the response field is constrained to contain a type
   identified by the responseType field.  The controlling field is
   identified using atNotation, e.g., "@responseType". atNotation can be
   defined relative to the outermost SEQUENCE, SET, or CHOICE or
   relative to the field with which the atNotation is associated.  When
   there is no '.' immediately after the '@', the field appears as a
   member of the outermost SEQUENCE, SET, or CHOICE.  When there is a
   '.' immediately after the '@', each '.' represents a SEQUENCE, SET,
   or CHOICE starting with the SEQUENCE, SET, or CHOICE that contains
   the field with which the atNotation is associated.  For example,
   ResponseBytes could have been written as shown below.  In this case,
   the syntax is very similar since the innermost and outermost
   SEQUENCE, SET, or CHOICE are the same.

   ResponseBytes ::=       SEQUENCE {
       responseType        RESPONSE.
                               &id ({ResponseSet}),
       response            OCTET STRING (CONTAINING RESPONSE.
                               &Type({ResponseSet}{@.responseType}))}

   The TaggedRequest example from [RFC5912] provides an example where
   the outermost and innermost SEQUENCE, SET, or CHOICE are different.
   Relative to the atNotation included in the definition of the




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   requestMessageValue field, the outermost SEQUENCE, SET, or CHOICE is
   TaggedRequest, and the innermost is the SEQUENCE used to define the
   orm field.

   TaggedRequest ::= CHOICE {
      tcr               [0] TaggedCertificationRequest,
      crm               [1] CertReqMsg,
      orm               [2] SEQUENCE {
          bodyPartID            BodyPartID,
          requestMessageType    OTHER-REQUEST.&id({OtherRequests}),
          requestMessageValue   OTHER-REQUEST.&Type({OtherRequests}
                                    {@.requestMessageType})
      }
   }

   When referencing a field using atNotation, the definition of the
   field must be included within the outermost SEQUENCE, SET, or CHOICE.
   References to fields within structures that are defined separately
   are not allowed.  For example, the following example includes invalid
   atNotation in the definition of the signature field within the SIGNED
   parameterized type.

   AlgorithmIdentifier{ALGORITHM-TYPE, ALGORITHM-TYPE:AlgorithmSet} ::=
             SEQUENCE {
                 algorithm   ALGORITHM-TYPE.&id({AlgorithmSet}),
                 parameters  ALGORITHM-TYPE.
                        &Params({AlgorithmSet}{@algorithm}) OPTIONAL
             }

   -- example containing invalid atNotation
   SIGNED{ToBeSigned} ::= SEQUENCE {
      toBeSigned           ToBeSigned,
      algorithmIdentifier  AlgorithmIdentifier
                               { SIGNATURE-ALGORITHM, {...}}
      },
      signature BIT STRING (CONTAINING SIGNATURE-ALGORITHM.&Value(
                               {SignatureAlgorithms}
                               {@algorithmIdentifier.algorithm}))
   }

   Alternatively, the above example could be written with correct
   atNotation as follows, with the definition of the algorithm field
   included within ToBeSigned.








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     SIGNED{ToBeSigned} ::= SEQUENCE {
        toBeSigned           ToBeSigned,
        algorithmIdentifier  SEQUENCE {
            algorithm        SIGNATURE-ALGORITHM.
                                 &id({SignatureAlgorithms}),
            parameters       SIGNATURE-ALGORITHM.
                                 &Params({SignatureAlgorithms}
                                     {@algorithmIdentifier.algorithm})
        },
        signature BIT STRING (CONTAINING SIGNATURE-ALGORITHM.&Value(
                                 {SignatureAlgorithms}
                                 {@algorithmIdentifier.algorithm}))
     }

   In the above example, the outermost SEQUENCE, SET, or CHOICE relative
   to the parameters field is the SIGNED parameterized type.  The
   innermost structure is the SEQUENCE used as the type for the
   algorithmIdentifier field.  The atNotation for the parameters field
   could be expressed using any of the following representations:

      @algorithmIdentifier.algorithm

      @.algorithm

   The atNotation for the signature field has only one representation.

2.2.3.  Content Constraints

   Open types implemented as OCTET STRINGs or BIT STRINGs can be
   constrained using the contents constraints syntax defined in
   [CCITT.X682.2002].  Below are the revised definitions from [RFC5911]
   and [RFC5912].  These show usage of OCTET STRING and BIT STRING along
   with constrained sets of identifiers.  The Extension definition uses
   a content constraint that requires the value of the OCTET STRING to
   be an encoding of the type associated with the information object
   selected from the ExtensionSet object set using the value of the
   extnID field.  For reasons described in Section 2.2.2, "Component
   Relation Constraints", the SubjectPublicKeyInfo definition relies on
   prose to bind the BIT STRING to the type identifier because it is not
   possible to express a content constraint that includes a component
   relationship constraint to bind the type value within the algorithm
   field to the subjectPublicKey field.









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   -- from updated RFC 5280 module in [RFC5912]
   Extension{EXTENSION:ExtensionSet} ::= SEQUENCE {
       extnID      EXTENSION.&id({ExtensionSet}),
       critical    BOOLEAN
       -- (EXTENSION.&Critical({ExtensionSet}{@extnID}))
                          DEFAULT FALSE,
       extnValue   OCTET STRING (CONTAINING
                     EXTENSION.&ExtnType({ExtensionSet}{@extnID}))
                     --  contains the DER encoding of the ASN.1 value
                     --  corresponding to the extension type identified
                     --  by extnID
   }

   SubjectPublicKeyInfo  ::=  SEQUENCE  {
       algorithm            AlgorithmIdentifier{PUBLIC-KEY,
                                {PublicKeyAlgorithms}},
       subjectPublicKey     BIT STRING
   }

2.3.  Parameterization

   Parameterization is defined in [CCITT.X683.2002] and can also be used
   to define new types in a way similar to the macro notation described
   in Annex A of X.208.  The following example from [RFC5912] shows this
   usage.  The toBeSigned field takes the type passed as a parameter.

   -- from [RFC5912]
   SIGNED{ToBeSigned} ::= SEQUENCE {
       toBeSigned  ToBeSigned,
       algorithm   AlgorithmIdentifier{SIGNATURE-ALGORITHM,
                       {SignatureAlgorithms}},
       signature   BIT STRING
   }

   -- from updated RFC5280 module in [RFC5912]
   Certificate  ::=  SIGNED{TBSCertificate}

   Parameters need not be simple types.  The following example
   demonstrates the usage of an information object class and an
   information object set as parameters.  The first parameter in the
   definition of AlgorithmIdentifier is an information object class.
   Information object classes used for this parameter must have &id and
   &Params fields, which determine the type of the algorithm and
   parameters fields.  Other fields may be present in the information
   object class, but they are not used by the definition of
   AlgorithmIdentifier, as demonstrated by the SIGNATURE-ALGORITHM class





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   shown below.  The second parameter is an information object set that
   is used to constrain the values that appear in the algorithm and
   parameters fields.

   -- from [RFC5912]
   AlgorithmIdentifier{ALGORITHM-TYPE, ALGORITHM-TYPE:AlgorithmSet}
       ::= SEQUENCE
   {
       algorithm   ALGORITHM-TYPE.&id({AlgorithmSet}),
       parameters  ALGORITHM-TYPE.&Params
                     ({AlgorithmSet}{@algorithm}) OPTIONAL
   }

   SIGNATURE-ALGORITHM ::= CLASS {
       &id             OBJECT IDENTIFIER,
       &Params         OPTIONAL,
       &Value          OPTIONAL,
       &paramPresence  ParamOptions DEFAULT absent,
       &HashSet        DIGEST-ALGORITHM OPTIONAL,
       &PublicKeySet   PUBLIC-KEY OPTIONAL,
       &smimeCaps      SMIME-CAPS OPTIONAL
   } WITH SYNTAX {
       IDENTIFIER &id
       [VALUE &Value]
       [PARAMS [TYPE &Params] ARE &paramPresence ]
       [HASHES &HashSet]
       [PUBLIC KEYS &PublicKeySet]
       [SMIME CAPS &smimeCaps]
   }

   -- from updated RFC 2560 module in [RFC5912]
   BasicOCSPResponse       ::= SEQUENCE {
       tbsResponseData      ResponseData,
       signatureAlgorithm   AlgorithmIdentifier{SIGNATURE-ALGORITHM,
                             {sa-dsaWithSHA1 | sa-rsaWithSHA1 |
                                  sa-rsaWithMD5 | sa-rsaWithMD2, ...}},
       signature            BIT STRING,
       certs            [0] EXPLICIT SEQUENCE OF Certificate OPTIONAL
   }

2.4.  Versioning and Extensibility

   Specifications are often revised and ASN.1 modules updated to include
   new components.  [CCITT.X681.2002] provides two mechanisms useful in
   supporting extensibility: extension markers and version brackets.






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2.4.1.  Extension Markers

   An extension marker is represented by an ellipsis (i.e., three
   adjacent periods).  Extension markers are included in specifications
   at points where the protocol designer anticipates future changes.
   This can also be achieved by including EXTENSIBILITY IMPLIED in the
   ASN.1 module definition.  EXTENSIBILITY IMPLIED is the equivalent to
   including an extension marker in each type defined in the ASN.1
   module.  Extensibility markers are used throughout [RFC5911] and
   [RFC5912] where object sets are defined.  In other instances, the
   updated modules retroactively added extension markers where fields
   were added to an earlier version by an update, as shown in the
   CertificateChoices example below.

   Examples:

   -- from updated RFC 3370 module in [RFC5911]
   KeyAgreementAlgs KEY-AGREE ::= { kaa-esdh | kaa-ssdh, ...}

   -- from updated RFC 5652 module in [RFC5911]
   CertificateChoices ::= CHOICE {
       certificate Certificate,
       extendedCertificate [0] IMPLICIT ExtendedCertificate,
            -- Obsolete
       ...,
       [[3: v1AttrCert [1] IMPLICIT AttributeCertificateV1]],
            -- Obsolete
       [[4: v2AttrCert [2] IMPLICIT AttributeCertificateV2]],
       [[5: other      [3] IMPLICIT OtherCertificateFormat]]
   }

   Protocol designers should use extension markers within definitions
   that are likely to change.  For example, extensibility markers should
   be used when enumerating error values.

2.4.2.  Version Brackets

   Version brackets can be used to indicate features that are available
   in later versions of an ASN.1 module but not in earlier versions.
   [RFC5912] added version brackets to the definition of TBSCertificate
   to illustrate the addition of the issuerUniqueID, subjectUniqueID,
   and extensions fields, as shown below.









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   -- from updated RFC 5280 module in [RFC5912]
   TBSCertificate  ::=  SEQUENCE  {
       version         [0]  Version DEFAULT v1,
       serialNumber         CertificateSerialNumber,
       signature            AlgorithmIdentifier{SIGNATURE-ALGORITHM,
                                 {SignatureAlgorithms}},
       issuer               Name,
       validity             Validity,
       subject              Name,
       subjectPublicKeyInfo SubjectPublicKeyInfo,
       ... ,
       [[2:               -- If present, version MUST be v2
       issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,
       subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL
       ]],
       [[3:               -- If present, version MUST be v3 --
       extensions      [3]  ExtensionSet{{CertExtensions}} OPTIONAL
       ]], ... }

3.  Character Set Differences

   X.68s uses a character set that is a superset of the character set
   defined in X.208.  The character set defined in X.208 includes the
   following:

      A to Z

      a to z

      0 to 9

      :=,{}<.

      ()[]-'"

   The character set in X.68x additionally includes the following:

      !&*/;>@^_|

   The > and | characters can also be used in X.208 syntax in macro
   definitions.










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4.  ASN.1 Translation

4.1.  Downgrading from X.68x to X.208

   At a minimum, downgrading an ASN.1 module from X.68x syntax to X.208
   requires the removal of features not supported by X.208.  As
   indicated above, the features most commonly used in IETF Security
   Area ASN.1 modules are information object classes (and object sets),
   content constraints, parameterization, extension markers, and version
   brackets.  Extension markers and version brackets can simply be
   deleted (or commented out).  The definitions for information object
   classes and object sets can also be deleted or commented out, as
   these will not be used.  The following checklist can be used in most
   cases:

   o  Remove all Information Set Class, Information Set Object, and
      Information Set Object Set definitions and imports from the file.

   o  Replace all fixed Type Information Set Class element references
      with the fixed type.  (That is, replace FOO.&id with OBJECT
      IDENTIFIER.)

   o  Delete all simple constraints.

   o  Delete all CONTAINING statements.

   o  Replace all variable Type Information Set Class element references
      with either ANY or ANY DEFINED BY statements.

   o  Remove version and extension markers.

   o  Manually enforce all instances of parameterized types.

4.2.  Upgrading from X.208 to X.68x

   The amount of change associated with upgrading from X.208 syntax to
   X.68x syntax is dependent on the reasons for changing and personal
   style.  A minimalist approach could consist of altering any
   deprecated features, most commonly ANY DEFINED BY, and adding any
   necessary extensibility markers.  A more comprehensive approach may
   include the introduction of constraints, parameterization,
   versioning, extensibility, etc.









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   The following checklist can be used when upgrading a module without
   introducing constraints:

      Use TYPE-IDENTIFIER.&Type for "ANY".

      Use TYPE-IDENTIFIER.&Type for "ANY DEFINED BY ...".

   When constraints are introduced during an upgrade, additional steps
   are necessary:

   1.  Identify each unique class that should be defined based on what
       types of things exist.

   2.  Define an Information Object Class for each of the classes above
       with the appropriate elements.

   3.  Define all of the appropriate Information Object Sets based on
       the classes defined in step 2 along with the different places
       that they should be used.

   4.  Replace ANY by the appropriate class and variable type element.

   5.  Replace ANY DEFINED BY with the appropriate variable type element
       and the components constraint.  Replace the element used in the
       constraint with the appropriate fixed type element and simple
       constraint.

   6.  Add any simple constraints as appropriate.

   7.  Define any objects and fill in elements for object sets as
       appropriate.

5.  Security Considerations

   Where a module is downgraded from X.68x syntax to X.208 there is loss
   of potential automated enforcement of constraints expressed by the
   author of the module being downgraded.  These constraints should be
   captured in prose or ASN.1 comments and enforced through other means,
   as necessary.

   Depending on the feature set of the ASN.1 compiler being used, the
   code to enforce and use constraints may be generated automatically or
   may require the programmer to do this independently.  It is the
   responsibility of the programmer to ensure that the constraints on
   the ASN.1 expressed either in prose or in the ASN.1 module are
   actually enforced.





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6.  References

6.1.  Normative References

   [CCITT.X208.1988]  International Telephone and Telegraph Consultative
                      Committee, "Specification of Abstract Syntax
                      Notation One (ASN.1)", CCITT Recommendation X.208,
                      November 1988.

   [CCITT.X680.2002]  International Telephone and Telegraph Consultative
                      Committee, "Abstract Syntax Notation One (ASN.1):
                      Specification of basic notation",
                      CCITT Recommendation X.680, July 2002.

   [CCITT.X681.2002]  International Telephone and Telegraph Consultative
                      Committee, "Abstract Syntax Notation One (ASN.1):
                      Information object specification",
                      CCITT Recommendation X.681, July 2002.

   [CCITT.X682.2002]  International Telephone and Telegraph Consultative
                      Committee, "Abstract Syntax Notation One (ASN.1):
                      Constraint specification", CCITT Recommendation
                      X.682, July 2002.

   [CCITT.X683.2002]  International Telephone and Telegraph Consultative
                      Committee, "Abstract Syntax Notation One (ASN.1):
                      Parameterization of ASN.1 specifications",
                      CCITT Recommendation X.683, July 2002.

6.2.  Informative References

   [CCITT.X209.1988]  International Telephone and Telegraph Consultative
                      Committee, "Specification of Basic Encoding Rules
                      for Abstract Syntax Notation One (ASN.1)",
                      CCITT Recommendation X.209, 1988.

   [CCITT.X690.2002]  International Telephone and Telegraph Consultative
                      Committee, "ASN.1 encoding rules: Specification of
                      basic encoding Rules (BER), Canonical encoding
                      rules (CER) and Distinguished encoding rules
                      (DER)", CCITT Recommendation X.690, July 2002.

   [RFC2560]          Myers, M., Ankney, R., Malpani, A., Galperin, S.,
                      and C. Adams, "X.509 Internet Public Key
                      Infrastructure Online Certificate Status Protocol
                      - OCSP", RFC 2560, June 1999.





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   [RFC5280]          Cooper, D., Santesson, S., Farrell, S., Boeyen,
                      S., Housley, R., and W. Polk, "Internet X.509
                      Public Key Infrastructure Certificate and
                      Certificate Revocation List (CRL) Profile",
                      RFC 5280, May 2008.

   [RFC5652]          Housley, R., "Cryptographic Message Syntax (CMS)",
                      STD 70, RFC 5652, September 2009.

   [RFC5911]          Hoffman, P. and J. Schaad, "New ASN.1 Modules for
                      Cryptographic Message Syntax (CMS) and S/MIME",
                      RFC 5911, June 2010.

   [RFC5912]          Hoffman, P. and J. Schaad, "New ASN.1 Modules for
                      the Public Key Infrastructure Using X.509 (PKIX)",
                      RFC 5912, June 2010.

Authors' Addresses

   Carl Wallace
   Cygnacom Solutions
   Suite 5400
   7925 Jones Branch Drive
   McLean, VA  22102

   EMail: cwallace@cygnacom.com


   Charles Gardiner
   BBN Technologies
   10 Moulton Street
   Cambridge, MA  02138

   EMail: gardiner@bbn.com

















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