ASN.1 Translation
RFC 6025
| Document | Type | RFC - Informational (October 2010; No errata) | |
|---|---|---|---|
| Authors | Charles Gardiner , Carl Wallace | ||
| Last updated | 2019-08-24 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | WG Document | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 6025 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Tim Polk | ||
| Send notices to | (None) | ||
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.
Wallace & Gardiner Informational [Page 1]
RFC 6025 ASN.1 Translation October 2010
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
Wallace & Gardiner Informational [Page 2]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 3]
RFC 6025 ASN.1 Translation October 2010
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
Wallace & Gardiner Informational [Page 4]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 5]
RFC 6025 ASN.1 Translation October 2010
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}
Wallace & Gardiner Informational [Page 6]
RFC 6025 ASN.1 Translation October 2010
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
Wallace & Gardiner Informational [Page 7]
RFC 6025 ASN.1 Translation October 2010
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 }}
Wallace & Gardiner Informational [Page 8]
RFC 6025 ASN.1 Translation October 2010
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
Wallace & Gardiner Informational [Page 9]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 10]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 11]
RFC 6025 ASN.1 Translation October 2010
-- 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
Wallace & Gardiner Informational [Page 12]
RFC 6025 ASN.1 Translation October 2010
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,
¶mPresence 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 ¶mPresence ]
[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.
Wallace & Gardiner Informational [Page 13]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 14]
RFC 6025 ASN.1 Translation October 2010
-- 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.
Wallace & Gardiner Informational [Page 15]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 16]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 17]
RFC 6025 ASN.1 Translation October 2010
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.
Wallace & Gardiner Informational [Page 18]
RFC 6025 ASN.1 Translation October 2010
[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
Wallace & Gardiner Informational [Page 19]