Network Working Group S. Farrell
Request for Comments: 3281 Baltimore Technologies
Category: Standards Track R. Housley
RSA Laboratories
April 2002
An Internet Attribute Certificate
Profile for Authorization
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 (2002). All Rights Reserved.
Abstract
This specification defines a profile for the use of X.509 Attribute
Certificates in Internet Protocols. Attribute certificates may be
used in a wide range of applications and environments covering a
broad spectrum of interoperability goals and a broader spectrum of
operational and assurance requirements. The goal of this document is
to establish a common baseline for generic applications requiring
broad interoperability as well as limited special purpose
requirements. The profile places emphasis on attribute certificate
support for Internet electronic mail, IPSec, and WWW security
applications.
Table of Contents
1. Introduction................................................. 2
1.1 Delegation and AC chains............................... 4
1.2 Attribute Certificate Distribution ("push" vs. "pull"). 4
1.3 Document Structure..................................... 6
2. Terminology.................................................. 6
3. Requirements................................................. 7
4. Attribute Certificate Profile................................ 7
4.1 X.509 Attribute Certificate Definition................. 8
4.2 Profile of Standard Fields............................. 10
4.2.1 Version.......................................... 10
4.2.2 Holder........................................... 11
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4.2.3 Issuer........................................... 12
4.2.4 Signature........................................ 12
4.2.5 Serial Number.................................... 12
4.2.6 Validity Period.................................. 13
4.2.7 Attributes....................................... 13
4.2.8 Issuer Unique Identifier......................... 14
4.2.9 Extensions....................................... 14
4.3 Extensions............................................. 14
4.3.1 Audit Identity................................... 14
4.3.2 AC Targeting..................................... 15
4.3.3 Authority Key Identifier......................... 17
4.3.4 Authority Information Access..................... 17
4.3.5 CRL Distribution Points.......................... 17
4.3.6 No Revocation Available.......................... 18
4.4 Attribute Types........................................ 18
4.4.1 Service Authentication Information............... 19
4.4.2 Access Identity.................................. 19
4.4.3 Charging Identity................................ 20
4.4.4 Group............................................ 20
4.4.5 Role............................................. 20
4.4.6 Clearance........................................ 21
4.5 Profile of AC issuer's PKC............................. 22
5. Attribute Certificate Validation............................. 23
6. Revocation................................................... 24
7. Optional Features............................................ 25
7.1 Attribute Encryption................................... 25
7.2 Proxying............................................... 27
7.3 Use of ObjectDigestInfo................................ 28
7.4 AA Controls............................................ 29
8. Security Considerations...................................... 30
9. IANA Considerations.......................................... 32
10. References.................................................. 32
Appendix A: Object Identifiers.................................. 34
Appendix B: ASN.1 Module........................................ 35
Author's Addresses.............................................. 39
Acknowledgements................................................ 39
Full Copyright Statement........................................ 40
1. Introduction
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.
X.509 public key certificates (PKCs) [X.509-1997, X.509-2000,
PKIXPROF] bind an identity and a public key. An attribute
certificate (AC) is a structure similar to a PKC; the main difference
being that the AC contains no public key. An AC may contain
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attributes that specify group membership, role, security clearance,
or other authorization information associated with the AC holder.
The syntax for the AC is defined in Recommendation X.509, making the
term "X.509 certificate" ambiguous.
Some people constantly confuse PKCs and ACs. An analogy may make the
distinction clear. A PKC can be considered to be like a passport: it
identifies the holder, tends to last for a long time, and should not
be trivial to obtain. An AC is more like an entry visa: it is
typically issued by a different authority and does not last for as
long a time. As acquiring an entry visa typically requires
presenting a passport, getting a visa can be a simpler process.
Authorization information may be placed in a PKC extension or placed
in a separate attribute certificate (AC). The placement of
authorization information in PKCs is usually undesirable for two
reasons. First, authorization information often does not have the
same lifetime as the binding of the identity and the public key.
When authorization information is placed in a PKC extension, the
general result is the shortening of the PKC useful lifetime. Second,
the PKC issuer is not usually authoritative for the authorization
information. This results in additional steps for the PKC issuer to
obtain authorization information from the authoritative source.
For these reasons, it is often better to separate authorization
information from the PKC. Yet, authorization information also needs
to be bound to an identity. An AC provides this binding; it is
simply a digitally signed (or certified) identity and set of
attributes.
An AC may be used with various security services, including access
control, data origin authentication, and non-repudiation.
PKCs can provide an identity to access control decision functions.
However, in many contexts the identity is not the criterion that is
used for access control decisions, rather the role or group-
membership of the accessor is the criterion used. Such access
control schemes are called role-based access control.
When making an access control decision based on an AC, an access
control decision function may need to ensure that the appropriate AC
holder is the entity that has requested access. One way in which the
linkage between the request or identity and the AC can be achieved is
the inclusion of a reference to a PKC within the AC and the use of
the private key corresponding to the PKC for authentication within
the access request.
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ACs may also be used in the context of a data origin authentication
service and a non-repudiation service. In these contexts, the
attributes contained in the AC provide additional information about
the signing entity. This information can be used to make sure that
the entity is authorized to sign the data. This kind of checking
depends either on the context in which the data is exchanged or on
the data that has been digitally signed.
1.1 Delegation and AC chains
The X.509 standard [X.509-2000] defines authorization as the
"conveyance of privilege from one entity that holds such privilege,
to another entity". An AC is one authorization mechanism.
An ordered sequence of ACs could be used to verify the authenticity
of a privilege asserter's privilege. In this way, chains or paths of
ACs could be employed to delegate authorization.
Since the administration and processing associated with such AC
chains is complex and the use of ACs in the Internet today is quite
limited, this specification does NOT RECOMMEND the use of AC chains.
Other (future) specifications may address the use of AC chains. This
specification deals with the simple cases, where one authority issues
all of the ACs for a particular set of attributes. However, this
simplification does not preclude the use of several different
authorities, each of which manages a different set of attributes.
For example, group membership may be included in one AC issued by one
authority, and security clearance may be included in another AC
issued by another authority.
This means that conformant implementations are only REQUIRED to be
able to process a single AC at a time. Processing of more than one
AC, one after another, may be necessary. Note however, that
validation of an AC MAY require validation of a chain of PKCs, as
specified in [PKIXPROF].
1.2 Attribute Certificate Distribution ("push" vs. "pull")
As discussed above, ACs provide a mechanism to securely provide
authorization information to, for example, access control decision
functions. However, there are a number of possible communication
paths for ACs.
In some environments, it is suitable for a client to "push" an AC to
a server. This means that no new connections between the client and
server are required. It also means that no search burden is imposed
on servers, which improves performance and that the AC verifier is
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only presented with what it "needs to know." The "push" model is
especially suitable in inter-domain cases where the client's rights
should be assigned within the client's "home" domain.
In other cases, it is more suitable for a client to simply
authenticate to the server and for the server to request or "pull"
the client's AC from an AC issuer or a repository. A major benefit
of the "pull" model is that it can be implemented without changes to
the client or to the client-server protocol. The "pull" model is
especially suitable for inter-domain cases where the client's rights
should be assigned within the server's domain, rather than within the
client's domain.
There are a number of possible exchanges involving three entities:
the client, the server, and the AC issuer. In addition, a directory
service or other repository for AC retrieval MAY be supported.
Figure 1 shows an abstract view of the exchanges that may involve
ACs. This profile does not specify a protocol for these exchanges.
+--------------+
| | Server Acquisition
| AC issuer +----------------------------+
| | |
+--+-----------+ |
| |
| Client |
| Acquisition |
| |
+--+-----------+ +--+------------+
| | AC "push" | |
| Client +-------------------------+ Server |
| | (part of app. protocol) | |
+--+-----------+ +--+------------+
| |
| Client | Server
| Lookup +--------------+ | Lookup
| | | |
+---------------+ Repository +---------+
| |
+--------------+
Figure 1: AC Exchanges
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1.3 Document Structure
Section 2 defines some terminology. Section 3 specifies the
requirements that this profile is intended to meet. Section 4
contains the profile of the X.509 AC. Section 5 specifies rules for
AC validation. Section 6 specifies rules for AC revocation checks.
Section 7 specifies optional features which MAY be supported;
however, support for these features is not required for conformance
to this profile. Finally, appendices contain the list of OIDs
required to support this specification and an ASN.1 module.
2. Terminology
For simplicity, we use the terms client and server in this
specification. This is not intended to indicate that ACs are only to
be used in client-server environments. For example, ACs may be used
in the S/MIME v3 context, where the mail user agent would be both a
"client" and a "server" in the sense the terms are used here.
Term Meaning
AA Attribute Authority, the entity that issues the
AC, synonymous in this specification with "AC
issuer"
AC Attribute Certificate
AC user any entity that parses or processes an AC
AC verifier any entity that checks the validity of an AC and
then makes use of the result
AC issuer the entity which signs the AC, synonymous in this
specification with "AA"
AC holder the entity indicated (perhaps indirectly) in the
holder field of the AC
Client the entity which is requesting the action for
which authorization checks are to be made
Proxying In this specification, Proxying is used to mean
the situation where an application server acts as
an application client on behalf of a user.
Proxying here does not mean granting of authority.
PKC Public Key Certificate - uses the type ASN.1
Certificate defined in X.509 and profiled in RFC
2459. This (non-standard) acronym is used in order
to avoid confusion about the term "X.509
certificate".
Server the entity which requires that the authorization
checks are made
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3. Requirements
This AC profile meets the following requirements.
Time/Validity requirements:
1. Support for short-lived as well as long-lived ACs. Typical
short-lived validity periods might be measured in hours, as
opposed to months for PKCs. Short validity periods allow ACs to
be useful without a revocation mechanism.
Attribute Types:
2. Issuers of ACs should be able to define their own attribute types
for use within closed domains.
3. Some standard attribute types, which can be contained within ACs,
should be defined. Examples include "access identity," "group,"
"role," "clearance," "audit identity," and "charging identity."
4. Standard attribute types should be defined in a manner that
permits an AC verifier to distinguish between uses of the same
attribute in different domains. For example, the "Administrators
group" as defined by Baltimore and the "Administrators group" as
defined by SPYRUS should be easily distinguished.
Targeting of ACs:
5. It should be possible to "target" an AC at one, or a small number
of, servers. This means that a trustworthy non-target server will
reject the AC for authorization decisions.
Push vs. Pull
6. ACs should be defined so that they can either be "pushed" by the
client to the server, or "pulled" by the server from a repository
or other network service, including an online AC issuer.
4. Attribute Certificate Profile
ACs may be used in a wide range of applications and environments
covering a broad spectrum of interoperability goals and a broader
spectrum of operational and assurance requirements. The goal of this
document is to establish a common baseline for generic applications
requiring broad interoperability and limited special purpose
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requirements. In particular, the emphasis will be on supporting the
use of attribute certificates for informal Internet electronic mail,
IPSec, and WWW applications.
This section presents a profile for ACs that will foster
interoperability. This section also defines some private extensions
for the Internet community.
While the ISO/IEC/ITU documents use the 1993 (or later) version of
ASN.1, this document uses the 1988 ASN.1 syntax, as has been done for
PKCs [PKIXPROF]. The encoded certificates and extensions from either
ASN.1 version are bit-wise identical.
Where maximum lengths for fields are specified, these lengths refer
to the DER encoding and do not include the ASN.1 tag or length
fields.
Conforming implementations MUST support the profile specified in this
section.
4.1 X.509 Attribute Certificate Definition
X.509 contains the definition of an AC given below. All types that
are not defined in this document can be found in [PKIXPROF].
AttributeCertificate ::= SEQUENCE {
acinfo AttributeCertificateInfo,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING
}
AttributeCertificateInfo ::= SEQUENCE {
version AttCertVersion -- version is v2,
holder Holder,
issuer AttCertIssuer,
signature AlgorithmIdentifier,
serialNumber CertificateSerialNumber,
attrCertValidityPeriod AttCertValidityPeriod,
attributes SEQUENCE OF Attribute,
issuerUniqueID UniqueIdentifier OPTIONAL,
extensions Extensions OPTIONAL
}
AttCertVersion ::= INTEGER { v2(1) }
Holder ::= SEQUENCE {
baseCertificateID [0] IssuerSerial OPTIONAL,
-- the issuer and serial number of
-- the holder's Public Key Certificate
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entityName [1] GeneralNames OPTIONAL,
-- the name of the claimant or role
objectDigestInfo [2] ObjectDigestInfo OPTIONAL
-- used to directly authenticate the holder,
-- for example, an executable
}
ObjectDigestInfo ::= SEQUENCE {
digestedObjectType ENUMERATED {
publicKey (0),
publicKeyCert (1),
otherObjectTypes (2) },
-- otherObjectTypes MUST NOT
-- be used in this profile
otherObjectTypeID OBJECT IDENTIFIER OPTIONAL,
digestAlgorithm AlgorithmIdentifier,
objectDigest BIT STRING
}
AttCertIssuer ::= CHOICE {
v1Form GeneralNames, -- MUST NOT be used in this
-- profile
v2Form [0] V2Form -- v2 only
}
V2Form ::= SEQUENCE {
issuerName GeneralNames OPTIONAL,
baseCertificateID [0] IssuerSerial OPTIONAL,
objectDigestInfo [1] ObjectDigestInfo OPTIONAL
-- issuerName MUST be present in this profile
-- baseCertificateID and objectDigestInfo MUST NOT
-- be present in this profile
}
IssuerSerial ::= SEQUENCE {
issuer GeneralNames,
serial CertificateSerialNumber,
issuerUID UniqueIdentifier OPTIONAL
}
AttCertValidityPeriod ::= SEQUENCE {
notBeforeTime GeneralizedTime,
notAfterTime GeneralizedTime
}
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Although the Attribute syntax is defined in [PKIXPROF], we repeat
the definition here for convenience.
Attribute ::= SEQUENCE {
type AttributeType,
values SET OF AttributeValue
-- at least one value is required
}
AttributeType ::= OBJECT IDENTIFIER
AttributeValue ::= ANY DEFINED BY AttributeType
Implementers should note that the DER encoding (see [X.509-
1988],[X.208-1988]) of the SET OF values requires ordering of the
encodings of the values. Though this issue arises with respect to
distinguished names, and has to be handled by [PKIXPROF]
implementations, it is much more significant in this context, since
the inclusion of multiple values is much more common in ACs.
4.2 Profile of Standard Fields
GeneralName offers great flexibility. To achieve interoperability,
in spite of this flexibility, this profile imposes constraints on the
use of GeneralName.
Conforming implementations MUST be able to support the dNSName,
directoryName, uniformResourceIdentifier, and iPAddress options.
This is compatible with the GeneralName requirements in [PKIXPROF]
(mainly in section 4.2.1.7).
Conforming implementations MUST NOT use the x400Address,
ediPartyName, or registeredID options.
Conforming implementations MAY use the otherName option to convey
name forms defined in Internet Standards. For example, Kerberos
[KRB] format names can be encoded into the otherName, using a
Kerberos 5 principal name OID and a SEQUENCE of the Realm and the
PrincipalName.
4.2.1 Version
The version field MUST have the value of v2. That is, the version
field is present in the DER encoding.
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Note: This version (v2) is not backwards compatible with the previous
attribute certificate definition (v1) from the 1997 X.509 standard
[X.509-1997], but is compatible with the v2 definition from X.509
(2000) [X.509-2000].
4.2.2 Holder
The Holder field is a SEQUENCE allowing three different (optional)
syntaxes: baseCertificateID, entityName and objectDigestInfo. Where
only one option is present, the meaning of the Holder field is clear.
However, where more than one option is used, there is a potential for
confusion as to which option is "normative", which is a "hint" etc.
Since the correct position is not clear from [X.509-2000], this
specification RECOMMENDS that only one of the options be used in any
given AC.
For any environment where the AC is passed in an authenticated
message or session and where the authentication is based on the use
of an X.509 PKC, the holder field SHOULD use the baseCertificateID.
With the baseCertificateID option, the holder's PKC serialNumber and
issuer MUST be identical to the AC holder field. The PKC issuer MUST
have a non-empty distinguished name which is to be present as the
single value of the holder.baseCertificateID.issuer construct in the
directoryName field. The AC holder.baseCertificateID.issuerUID field
MUST only be used if the holder's PKC contains an issuerUniqueID
field. If both the AC holder.baseCertificateID.issuerUID and the PKC
issuerUniqueID fields are present, the same value MUST be present in
both fields. Thus, the baseCertificateID is only usable with PKC
profiles (like [PKIXPROF]) which mandate that the PKC issuer field
contain a non-empty distinguished name value.
Note: An empty distinguished name is a distinguished name where the
SEQUENCE OF relative distinguished names is of zero length. In a DER
encoding, this has the value '3000'H.
If the holder field uses the entityName option and the underlying
authentication is based on a PKC, the entityName MUST be the same as
the PKC subject field or one of the values of the PKC subjectAltName
field extension (if present). Note that [PKIXPROF] mandates that the
subjectAltName extension be present if the PKC subject is an empty
distinguished name. See the security considerations section which
mentions some name collision problems that may arise when using the
entityName option.
In any other case where the holder field uses the entityName option,
only one name SHOULD be present.
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Implementations conforming to this profile are not required to
support the use of the objectDigest field. However, section 7.3
specifies how this optional feature MAY be used.
Any protocol conforming to this profile SHOULD specify which AC
holder option is to be used and how this fits with the supported
authentication schemes defined in that protocol.
4.2.3 Issuer
ACs conforming to this profile MUST use the v2Form choice, which MUST
contain one and only one GeneralName in the issuerName, which MUST
contain a non-empty distinguished name in the directoryName field.
This means that all AC issuers MUST have non-empty distinguished
names. ACs conforming to this profile MUST omit the
baseCertificateID and objectDigestInfo fields.
Part of the reason for the use of the v2Form containing only an
issuerName is that it means that the AC issuer does not have to know
which PKC the AC verifier will use for it (the AC issuer). Using the
baseCertificateID field to reference the AC issuer would mean that
the AC verifier would have to trust the PKC that the AC issuer chose
(for itself) at AC creation time.
4.2.4 Signature
Contains the algorithm identifier used to validate the AC signature.
This MUST be one of the signing algorithms defined in [PKIXALGS].
Conforming implementations MUST honor all MUST/SHOULD/MAY signing
algorithm statements specified in [PKIXALGS].
4.2.5 Serial Number
For any conforming AC, the issuer/serialNumber pair MUST form a
unique combination, even if ACs are very short-lived.
AC issuers MUST force the serialNumber to be a positive integer, that
is, the sign bit in the DER encoding of the INTEGER value MUST be
zero - this can be done by adding a leading (leftmost) '00'H octet if
necessary. This removes a potential ambiguity in mapping between a
string of octets and an integer value.
Given the uniqueness and timing requirements above, serial numbers
can be expected to contain long integers. AC users MUST be able to
handle serialNumber values longer than 4 octets. Conformant ACs MUST
NOT contain serialNumber values longer than 20 octets.
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There is no requirement that the serial numbers used by any AC issuer
follow any particular ordering. In particular, they need not be
monotonically increasing with time. Each AC issuer MUST ensure that
each AC that it issues contains a unique serial number.
4.2.6 Validity Period
The attrCertValidityPeriod (a.k.a. validity) field specifies the
period for which the AC issuer certifies that the binding between the
holder and the attributes fields will be valid.
The generalized time type, GeneralizedTime, is a standard ASN.1 type
for variable precision representation of time. The GeneralizedTime
field can optionally include a representation of the time
differential between the local time zone and Greenwich Mean Time.
For the purposes of this profile, GeneralizedTime values MUST be
expressed in Coordinated universal time (UTC) (also known as
Greenwich Mean Time or Zulu)) and MUST include seconds (i.e., times
are YYYYMMDDHHMMSSZ), even when the number of seconds is zero.
GeneralizedTime values MUST NOT include fractional seconds.
(Note: this is the same as specified in [PKIXPROF], section
4.1.2.5.2.)
AC users MUST be able to handle an AC which, at the time of
processing, has parts of its validity period or all its validity
period in the past or in the future (a post-dated AC). This is valid
for some applications, such as backup.
4.2.7 Attributes
The attributes field gives information about the AC holder. When the
AC is used for authorization, this will often contain a set of
privileges.
The attributes field contains a SEQUENCE OF Attribute. Each
Attribute MAY contain a SET OF values. For a given AC, each
AttributeType OBJECT IDENTIFIER in the sequence MUST be unique. That
is, only one instance of each attribute can occur in a single AC, but
each instance can be multi-valued.
AC users MUST be able to handle multiple values for all attribute
types.
An AC MUST contain at least one attribute. That is, the SEQUENCE OF
Attributes MUST NOT be of zero length.
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Some standard attribute types are defined in section 4.4.
4.2.8 Issuer Unique Identifier
This field MUST NOT be used unless it is also used in the AC issuer's
PKC, in which case it MUST be used. Note that [PKIXPROF] states that
this field SHOULD NOT be used by conforming CAs, but that
applications SHOULD be able to parse PKCs containing the field.
4.2.9 Extensions
The extensions field generally gives information about the AC as
opposed to information about the AC holder.
An AC that has no extensions conforms to the profile; however,
section 4.3 defines the extensions that MAY be used with this
profile, and whether or not they may be marked critical. If any
other critical extension is used, the AC does not conform to this
profile. However, if any other non-critical extension is used, the
AC does conform to this profile.
The extensions defined for ACs provide methods for associating
additional attributes with holders. This profile also allows
communities to define private extensions to carry information unique
to those communities. Each extension in an AC may be designated as
critical or non-critical. An AC using system MUST reject an AC if it
encounters a critical extension it does not recognize; however, a
non-critical extension may be ignored if it is not recognized.
Section 4.3 presents recommended extensions used within Internet ACs
and standard locations for information. Communities may elect to use
additional extensions; however, caution should be exercised in
adopting any critical extensions in ACs which might prevent use in a
general context.
4.3 Extensions
4.3.1 Audit Identity
In some circumstances, it is required (e.g. by data protection/data
privacy legislation) that audit trails not contain records which
directly identify individuals. This circumstance may make the use of
the AC holder field unsuitable for use in audit trails.
To allow for such cases, an AC MAY contain an audit identity
extension. Ideally it SHOULD be infeasible to derive the AC holder's
identity from the audit identity value without the cooperation of the
AC issuer.
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RFC 3281 An Internet Attribute Certificate April 2002
The value of the audit identity, along with the AC issuer/serial,
SHOULD then be used for audit/logging purposes. If the value of the
audit identity is suitably chosen, a server/service administrator can
use audit trails to track the behavior of an AC holder without being
able to identify the AC holder.
The server/service administrator in combination with the AC issuer
MUST be able to identify the AC holder in cases where misbehavior is
detected. This means that the AC issuer MUST be able to determine
the actual identity of the AC holder from the audit identity.
Of course, auditing could be based on the AC issuer/serial pair;
however, this method does not allow tracking of the same AC holder
with multiple ACs. Thus, an audit identity is only useful if it
lasts for longer than the typical AC lifetime. Auditing could also
be based on the AC holder's PKC issuer/serial; however, this will
often allow the server/service administrator to identify the AC
holder.
As the AC verifier might otherwise use the AC holder or some other
identifying value for audit purposes, this extension MUST be critical
when used.
Protocols that use ACs will often expose the identity of the AC
holder in the bits on-the-wire. In such cases, an opaque audit
identity does not make use of the AC anonymous; it simply ensures
that the ensuing audit trails do not contain identifying information.
The value of an audit identity MUST be longer than zero octets. The
value of an audit identity MUST NOT be longer than 20 octets.
name id-pe-ac-auditIdentity
OID { id-pe 4 }
syntax OCTET STRING
criticality MUST be TRUE
4.3.2 AC Targeting
To target an AC, the target information extension, imported from
[X.509-2000], MAY be used to specify a number of servers/services.
The intent is that the AC SHOULD only be usable at the specified
servers/services. An (honest) AC verifier who is not amongst the
named servers/services MUST reject the AC.
If this extension is not present, the AC is not targeted and may be
accepted by any server.
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In this profile, the targeting information simply consists of a list
of named targets or groups.
The following syntax is used to represent the targeting information:
Targets ::= SEQUENCE OF Target
Target ::= CHOICE {
targetName [0] GeneralName,
targetGroup [1] GeneralName,
targetCert [2] TargetCert
}
TargetCert ::= SEQUENCE {
targetCertificate IssuerSerial,
targetName GeneralName OPTIONAL,
certDigestInfo ObjectDigestInfo OPTIONAL
}
The targetCert CHOICE within the Target structure is only present to
allow future compatibility with [X.509-2000] and MUST NOT be used.
The targets check passes if the current server (recipient) is one of
the targetName fields in the Targets SEQUENCE, or if the current
server is a member of one of the targetGroup fields in the Targets
SEQUENCE. In this case, the current server is said to "match" the
targeting extension.
How the membership of a target within a targetGroup is determined is
not defined here. It is assumed that any given target "knows" the
names of the targetGroups to which it belongs or can otherwise
determine its membership. For example, the targetGroup specifies a
DNS domain, and the AC verifier knows the DNS domain to which it
belongs. For another example, the targetGroup specifies "PRINTERS,"
and the AC verifier knows whether or not it is a printer or print
server.
Note: [X.509-2000] defines the extension syntax as a "SEQUENCE OF
Targets". Conforming AC issuer implementations MUST only produce one
"Targets" element. Confirming AC users MUST be able to accept a
"SEQUENCE OF Targets". If more than one Targets element is found in
an AC, the extension MUST be treated as if all Target elements had
been found within one Targets element.
name id-ce-targetInformation
OID { id-ce 55 }
syntax SEQUENCE OF Targets
criticality MUST be TRUE
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4.3.3 Authority Key Identifier
The authorityKeyIdentifier extension, as profiled in [PKIXPROF], MAY
be used to assist the AC verifier in checking the signature of the
AC. The [PKIXPROF] description should be read as if "CA" meant "AC
issuer." As with PKCs, this extension SHOULD be included in ACs.
Note: An AC, where the issuer field used the baseCertificateID
CHOICE, would not need an authorityKeyIdentifier extension, as it is
explicitly linked to the key in the referred certificate. However,
as this profile states (in section 4.2.3), ACs MUST use the v2Form
with issuerName CHOICE, this duplication does not arise.
name id-ce-authorityKeyIdentifier
OID { id-ce 35 }
syntax AuthorityKeyIdentifier
criticality MUST be FALSE
4.3.4 Authority Information Access
The authorityInformationAccess extension, as defined in [PKIXPROF],
MAY be used to assist the AC verifier in checking the revocation
status of the AC. Support for the id-ad-caIssuers accessMethod is
NOT REQUIRED by this profile since AC chains are not expected.
The following accessMethod is used to indicate that revocation status
checking is provided for this AC, using the OCSP protocol defined in
[OCSP]:
id-ad-ocsp OBJECT IDENTIFIER ::= { id-ad 1 }
The accessLocation MUST contain a URI, and the URI MUST contain an
HTTP URL [URL] that specifies the location of an OCSP responder. The
AC issuer MUST, of course, maintain an OCSP responder at this
location.
name id-ce-authorityInfoAccess
OID { id-pe 1 }
syntax AuthorityInfoAccessSyntax
criticality MUST be FALSE
4.3.5 CRL Distribution Points
The crlDistributionPoints extension, as profiled in [PKIXPROF], MAY
be used to assist the AC verifier in checking the revocation status
of the AC. See section 6 for details on revocation.
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If the crlDistributionPoints extension is present, then exactly one
distribution point MUST be present. The crlDistributionPoints
extension MUST use the DistributionPointName option, which MUST
contain a fullName, which MUST contain a single name form. That name
MUST contain either a distinguished name or a URI. The URI MUST be
either an HTTP URL or an LDAP URL [URL].
name id-ce-cRLDistributionPoints
OID { id-ce 31 }
syntax CRLDistPointsSyntax
criticality MUST be FALSE
4.3.6 No Revocation Available
The noRevAvail extension, defined in [X.509-2000], allows an AC
issuer to indicate that no revocation information will be made
available for this AC.
This extension MUST be non-critical. An AC verifier that does not
understand this extension might be able to find a revocation list
from the AC issuer, but the revocation list will never include an
entry for the AC.
name id-ce-noRevAvail
OID { id-ce 56 }
syntax NULL (i.e. '0500'H is the DER encoding)
criticality MUST be FALSE
4.4 Attribute Types
Some of the attribute types defined below make use of the
IetfAttrSyntax type, also defined below. The reasons for using this
type are:
1. It allows a separation between the AC issuer and the attribute
policy authority. This is useful for situations where a single
policy authority (e.g. an organization) allocates attribute
values, but where multiple AC issuers are deployed for performance
or other reasons.
2. The syntaxes allowed for values are restricted to OCTET STRING,
OBJECT IDENTIFIER, and UTF8String, which significantly reduces the
complexity associated with matching more general syntaxes. All
multi-valued attributes using this syntax are restricted so that
each value MUST use the same choice of value syntax. For example,
AC issuers must not use one value with an oid and a second value
with a string.
Farrell & Housley Standards Track [Page 18]RFC 3281 An Internet Attribute Certificate April 2002
IetfAttrSyntax ::= SEQUENCE {
policyAuthority [0] GeneralNames OPTIONAL,
values SEQUENCE OF CHOICE {
octets OCTET STRING,
oid OBJECT IDENTIFIER,
string UTF8String
}
}
In the descriptions below, each attribute type is either tagged
"Multiple Allowed" or "One Attribute value only; multiple values
within the IetfAttrSyntax". This refers to the SET OF
AttributeValues; the AttributeType still only occurs once, as
specified in section 4.2.7.
4.4.1 Service Authentication Information
The SvceAuthInfo attribute identifies the AC holder to the
server/service by a name, and the attribute MAY include optional
service specific authentication information. Typically this will
contain a username/password pair for a "legacy" application.
This attribute provides information that can be presented by the AC
verifier to be interpreted and authenticated by a separate
application within the target system. Note that this is a different
use to that intended for the accessIdentity attribute in 4.4.2 below.
This attribute type will typically be encrypted when the authInfo
field contains sensitive information, such as a password.
name id-aca-authenticationInfo
OID { id-aca 1 }
Syntax SvceAuthInfo
values: Multiple allowed
SvceAuthInfo ::= SEQUENCE {
service GeneralName,
ident GeneralName,
authInfo OCTET STRING OPTIONAL
}
4.4.2 Access Identity
The accessIdentity attribute identifies the AC holder to the
server/service. For this attribute the authInfo field MUST NOT be
present.
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This attribute is intended to be used to provide information about
the AC holder, that can be used by the AC verifier (or a larger
system of which the AC verifier is a component) to authorize the
actions of the AC holder within the AC verifier's system. Note that
this is a different use to that intended for the svceAuthInfo
attribute described in 4.4.1 above.
name id-aca-accessIdentity
OID { id-aca 2 }
syntax SvceAuthInfo
values: Multiple allowed
4.4.3 Charging Identity
The chargingIdentity attribute identifies the AC holder for charging
purposes. In general, the charging identity will be different from
other identities of the holder. For example, the holder's company
may be charged for service.
name id-aca-chargingIdentity
OID { id-aca 3 }
syntax IetfAttrSyntax
values: One Attribute value only; multiple values within the
IetfAttrSyntax
4.4.4 Group
The group attribute carries information about group memberships of
the AC holder.
name id-aca-group
OID { id-aca 4 }
syntax IetfAttrSyntax
values: One Attribute value only; multiple values within the
IetfAttrSyntax
4.4.5 Role
The role attribute, specified in [X.509-2000], carries information
about role allocations of the AC holder.
The syntax used for this attribute is:
RoleSyntax ::= SEQUENCE {
roleAuthority [0] GeneralNames OPTIONAL,
roleName [1] GeneralName
}
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The roleAuthority field MAY be used to specify the issuing authority
for the role specification certificate. There is no requirement that
a role specification certificate necessarily exists for the
roleAuthority. This differs from [X.500-2000], where the
roleAuthority field is assumed to name the issuer of a role
specification certificate. For example, to distinguish the
administrator role as defined by "Baltimore" from that defined by
"SPYRUS", one could put the value "urn:administrator" in the roleName
field and the value "Baltimore" or "SPYRUS" in the roleAuthority
field.
The roleName field MUST be present, and roleName MUST use the
uniformResourceIdentifier CHOICE of the GeneralName.
name id-at-role
OID { id-at 72 }
syntax RoleSyntax
values: Multiple allowed
4.4.6 Clearance
The clearance attribute, specified in [X.501-1993], carries clearance
(associated with security labeling) information about the AC holder.
The policyId field is used to identify the security policy to which
the clearance relates. The policyId indicates the semantics of the
classList and securityCategories fields.
This specification includes the classList field exactly as it is
specified in [X.501-1993]. Additional security classification
values, and their position in the classification hierarchy, may be
defined by a security policy as a local matter or by bilateral
agreement. The basic security classification hierarchy is, in
ascending order: unmarked, unclassified, restricted, confidential,
secret, and top-secret.
An organization can develop its own security policy that defines
security classification values and their meanings. However, the BIT
STRING positions 0 through 5 are reserved for the basic security
classification hierarchy.
If present, the SecurityCategory field provides further authorization
information. The security policy identified by the policyId field
indicates the syntaxes that are allowed to be present in the
securityCategories SET. An OBJECT IDENTIFIER identifies each of the
allowed syntaxes. When one of these syntaxes is present in the
securityCategories SET, the OBJECT IDENTIFIER associated with that
syntax is carried in the SecurityCategory.type field.
Farrell & Housley Standards Track [Page 21]RFC 3281 An Internet Attribute Certificate April 2002
Clearance ::= SEQUENCE {
policyId [0] OBJECT IDENTIFIER,
classList [1] ClassList DEFAULT {unclassified},
securityCategories
[2] SET OF SecurityCategory OPTIONAL
}
ClassList ::= BIT STRING {
unmarked (0),
unclassified (1),
restricted (2)
confidential (3),
secret (4),
topSecret (5)
}
SecurityCategory ::= SEQUENCE {
type [0] IMPLICIT OBJECT IDENTIFIER,
value [1] ANY DEFINED BY type
}
-- This is the same as the original syntax which was defined
-- using the MACRO construct, as follows:
-- SecurityCategory ::= SEQUENCE {
-- type [0] IMPLICIT SECURITY-CATEGORY,
-- value [1] ANY DEFINED BY type
-- }
--
-- SECURITY-CATEGORY MACRO ::=
-- BEGIN
-- TYPE NOTATION ::= type | empty
-- VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
-- END
name { id-at-clearance }
OID { joint-iso-ccitt(2) ds(5) module(1)
selected-attribute-types(5) clearance (55) }
syntax Clearance - imported from [X.501-1993]
values Multiple allowed
4.5 Profile of AC issuer's PKC
The AC issuer's PKC MUST conform to [PKIXPROF], and the keyUsage
extension in the PKC MUST NOT explicitly indicate that the AC
issuer's public key cannot be used to validate a digital signature.
In order to avoid confusion regarding serial numbers and revocations,
Farrell & Housley Standards Track [Page 22]RFC 3281 An Internet Attribute Certificate April 2002
an AC issuer MUST NOT also be a PKC Issuer. That is, an AC issuer
cannot be a CA as well. So, the AC issuer's PKC MUST NOT have a
basicConstraints extension with the cA BOOLEAN set to TRUE.
5. Attribute Certificate Validation
This section describes a basic set of rules that all valid ACs MUST
satisfy. Some additional checks are also described which AC
verifiers MAY choose to implement.
To be valid an AC MUST satisfy all of the following:
1. Where the holder uses a PKC to authenticate to the AC verifier,
the AC holder's PKC MUST be found, and the entire certification
path of that PKC MUST be verified in accordance with [PKIXPROF].
As noted in the security considerations section, if some other
authentication scheme is used, AC verifiers need to be very
careful mapping the identities (authenticated identity, holder
field) involved.
2. The AC signature must be cryptographically correct, and the AC
issuer's entire PKC certification path MUST be verified in
accordance with [PKIXPROF].
3. The AC issuer's PKC MUST also conform to the profile specified in
section 4.5 above.
4. The AC issuer MUST be directly trusted as an AC issuer (by
configuration or otherwise).
5. The time for which the AC is being evaluated MUST be within the AC
validity. If the evaluation time is equal to either notBeforeTime
or notAfterTime, then the AC is timely and this check succeeds.
Note that in some applications, the evaluation time MAY not be the
same as the current time.
6. The AC targeting check MUST pass as specified in section 4.3.2.
7. If the AC contains an unsupported critical extension, the AC MUST
be rejected.
Support for an extension in this context means:
1. The AC verifier MUST be able to parse the extension value.
2. Where the extension value SHOULD cause the AC to be rejected, the
AC verifier MUST reject the AC.
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Additional Checks:
1. The AC MAY be rejected on the basis of further AC verifier
configuration. For example, an AC verifier may be configured to
reject ACs which contain or lack certain attributes.
2. If the AC verifier provides an interface that allows applications
to query the contents of the AC, then the AC verifier MAY filter
the attributes from the AC on the basis of configured information.
For example, an AC verifier might be configured not to return
certain attributes to certain servers.
6. Revocation
In many environments, the validity period of an AC is less than the
time required to issue and distribute revocation information.
Therefore, short-lived ACs typically do not require revocation
support. However, long-lived ACs and environments where ACs enable
high value transactions MAY require revocation support.
Two revocation schemes are defined, and the AC issuer should elect
the one that is best suited to the environment in which the AC will
be employed.
"Never revoke" scheme:
ACs may be marked so that the relying party understands that no
revocation status information will be made available. The
noRevAvail extension is defined in section 4.3.6, and the
noRevAvail extension MUST be present in the AC to indicate use of
this scheme.
Where no noRevAvail is present, the AC issuer is implicitly
stating that revocation status checks are supported, and some
revocation method MUST be provided to allow AC verifiers to
establish the revocation status of the AC.
"Pointer in AC" scheme:
ACs may "point" to sources of revocation status information, using
either an authorityInfoAccess extension or a crlDistributionPoints
extension within the AC.
For AC users, the "never revoke" scheme MUST be supported, and the
"pointer in AC" scheme SHOULD be supported. If only the "never
revoke" scheme is supported, then all ACs that do not contain a
noRevAvail extension, MUST be rejected.
Farrell & Housley Standards Track [Page 24]RFC 3281 An Internet Attribute Certificate April 2002
For AC issuers, the "never revoke" scheme MUST be supported. If all
ACs that will ever be issued by that AC issuer, contains a noRevAvail
extension, the "pointer in AC" scheme need not be supported. If any
AC can be issued that does not contain the noRevAvail extension, the
"pointer in AC" scheme MUST be supported.
An AC MUST NOT contain both a noRevAvail and a "pointer in AC".
An AC verifier MAY use any source for AC revocation status
information.
7. Optional Features
This section specifies features that MAY be implemented. Conformance
to this profile does NOT require support for these features; however,
if these features are offered, they MUST be offered as described
below.
7.1 Attribute Encryption
Where an AC will be carried in clear within an application protocol
or where an AC contains some sensitive information like a legacy
application username/password, then encryption of AC attributes MAY
be needed.
When a set of attributes are to be encrypted within an AC, the
Cryptographic Message Syntax, EnvelopedData structure [CMS] is used
to carry the ciphertext and associated per-recipient keying
information.
This type of attribute encryption is targeted. Before the AC is
signed, the attributes are encrypted for a set of predetermined
recipients.
The AC then contains the ciphertext inside its signed data. The
EnvelopedData (id-envelopedData) ContentType is used, and the content
field will contain the EnvelopedData type.
The ciphertext is included in the AC as the value of an encAttrs
attribute. Only one encAttrs attribute can be present in an AC;
however, the encAttrs attribute MAY be multi-valued, and each of its
values will contain an independent EnvelopedData.
Each value can contain a set of attributes (each possibly a multi-
valued attribute) encrypted for a set of predetermined recipients.
Farrell & Housley Standards Track [Page 25]
RFC 3281 An Internet Attribute Certificate April 2002
The cleartext that is encrypted has the type:
ACClearAttrs ::= SEQUENCE {
acIssuer GeneralName,
acSerial INTEGER,
attrs SEQUENCE OF Attribute
}
The DER encoding of the ACClearAttrs structure is used as the
encryptedContent field of the EnvelopedData. The DER encoding MUST
be embedded in an OCTET STRING.
The acIssuer and acSerial fields are present to prevent ciphertext
stealing. When an AC verifier has successfully decrypted an
encrypted attribute, it MUST then check that the AC issuer and
serialNumber fields contain the same values. This prevents a
malicious AC issuer from copying ciphertext from another AC (without
knowing its corresponding plaintext).
The procedure for an AC issuer when encrypting attributes is
illustrated by the following (any other procedure that gives the same
result MAY be used):
1. Identify the sets of attributes that are to be encrypted for
each set of recipients.
2. For each attribute set which is to be encrypted:
2.1. Create an EnvelopedData structure for the data for this
set of recipients.
2.2. Encode the ContentInfo containing the EnvelopedData as a
value of the encAttrs attribute.
2.3. Ensure the cleartext attributes are not present in the
to-be-signed AC.
3. Add the encAttrs (with its multiple values) to the AC.
Note that there may be more than one attribute of the same type (the
same OBJECT IDENTIFIER) after decryption. That is, an AC MAY contain
the same attribute type both in clear and in encrypted form (and
indeed several times if the same recipient is associated with more
than one EnvelopedData). One approach implementers may choose, would
be to merge attribute values following decryption in order to re-
establish the "once only" constraint.
name id-aca-encAttrs
OID { id-aca 6}
Syntax ContentInfo
values Multiple Allowed
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If an AC contains attributes, apparently encrypted for the AC
verifier, the decryption process MUST not fail. If decryption does
fail, the AC MUST be rejected.
7.2 Proxying
When a server acts as a client for another server on behalf of the AC
holder, the server MAY need to proxy an AC. Such proxying MAY have
to be done under the AC issuer's control, so that not every AC is
proxiable and so that a given proxiable AC can be proxied in a
targeted fashion. Support for chains of proxies (with more than one
intermediate server) MAY also be required. Note that this does not
involve a chain of ACs.
In order to meet this requirement we define another extension,
ProxyInfo, similar to the targeting extension.
When this extension is present, the AC verifier must check that the
entity from which the AC was received was allowed to send it and that
the AC is allowed to be used by this verifier.
The proxying information consists of a set of proxy information, each
of which is a set of targeting information. If the verifier and the
sender of the AC are both named in the same proxy set, the AC can
then be accepted (the exact rule is given below).
The effect is that the AC holder can send the AC to any valid target
which can then only proxy to targets which are in one of the same
proxy sets as itself.
The following data structure is used to represent the
targeting/proxying information.
ProxyInfo ::= SEQUENCE OF Targets
As in the case of targeting, the targetCert CHOICE MUST NOT be used.
A proxy check succeeds if either one of the conditions below is met:
1. The identity of the sender, as established by the underlying
authentication service, matches the holder field of the AC, and
the current server "matches" any one of the proxy sets. Recall
that "matches" is as defined section 4.3.2.
Farrell & Housley Standards Track [Page 27]RFC 3281 An Internet Attribute Certificate April 2002
2. The identity of the sender, as established by the underlying
authentication service, "matches" one of the proxy sets (call it
set "A"), and the current server is one of the targetName fields
in the set "A", or the current server is a member of one of the
targetGroup fields in set "A".
When an AC is proxied more than once, a number of targets will be on
the path from the original client, which is normally, but not always,
the AC holder. In such cases, prevention of AC "stealing" requires
that the AC verifier MUST check that all targets on the path are
members of the same proxy set. It is the responsibility of the AC-
using protocol to ensure that a trustworthy list of targets on the
path is available to the AC verifier.
name id-pe-ac-proxying
OID { id-pe 10 }
syntax ProxyInfo
criticality MUST be TRUE
7.3 Use of ObjectDigestInfo
In some environments, it may be required that the AC is not linked
either to an identity (via entityName) or to a PKC (via
baseCertificateID). The objectDigestInfo CHOICE in the holder field
allows support for this requirement.
If the holder is identified with the objectDigestInfo field, then the
AC version field MUST contain v2 (the integer 1).
The idea is to link the AC to an object by placing a hash of that
object into the holder field of the AC. For example, this allows
production of ACs that are linked to public keys rather than names.
It also allows production of ACs which contain privileges associated
with an executable object such as a Java class. However, this
profile only specifies how to use a hash over a public key or PKC.
That is, conformant ACs MUST NOT use the otherObjectTypes value for
the digestedObjectType.
To link an AC to a public key, the hash must be calculated over the
representation of that public key which would be present in a PKC,
specifically, the input for the hash algorithm MUST be the DER
encoding of a SubjectPublicKeyInfo representation of the key. Note:
This includes the AlgorithmIdentifier as well as the BIT STRING. The
rules given in [PKIXPROF] for encoding keys MUST be followed. In
this case, the digestedObjectType MUST be publicKey and the
otherObjectTypeID field MUST NOT be present.
Farrell & Housley Standards Track [Page 28]RFC 3281 An Internet Attribute Certificate April 2002
Note that if the public key value used as input to the hash function
has been extracted from a PKC, it is possible that the
SubjectPublicKeyInfo from that PKC is NOT the value which should be
hashed. This can occur if DSA Dss-parms are inherited as described
in section 7.3.3 of [PKIXPROF]. The correct input for hashing in
this context will include the value of the parameters inherited from
the CA's PKC, and thus may differ from the SubjectPublicKeyInfo
present in the PKC.
Implementations which support this feature MUST be able to handle the
representations of public keys for the algorithms specified in
section 7.3 of [PKIXPROF]. In this case, the digestedObjectType MUST
be publicKey and the otherObjectTypeID field MUST NOT be present.
In order to link an AC to a PKC via a digest, the digest MUST be
calculated over the DER encoding of the entire PKC, including the
signature value. In this case the digestedObjectType MUST be
publicKeyCert and the otherObjectTypeID field MUST NOT be present.
7.4 AA Controls
During AC validation a relying party has to answer the question: is
this AC issuer trusted to issue ACs containing this attribute? The
AAControls PKC extension MAY be used to help answer the question.
The AAControls extension is intended to be used in CA and AC issuer
PKCs.
id-pe-aaControls OBJECT IDENTIFIER ::= { id-pe 6 }
AAControls ::= SEQUENCE {
pathLenConstraint INTEGER (0..MAX) OPTIONAL,
permittedAttrs [0] AttrSpec OPTIONAL,
excludedAttrs [1] AttrSpec OPTIONAL,
permitUnSpecified BOOLEAN DEFAULT TRUE
}
AttrSpec::= SEQUENCE OF OBJECT IDENTIFIER
The AAControls extension is used as follows:
The pathLenConstraint, if present, is interpreted as in [PKIXPROF].
It restricts the allowed distance between the AA CA (a CA directly
trusted to include AAControls in its PKCs), and the AC issuer.
The permittedAttrs field specifies a set of attribute types that any
AC issuer below this AA CA is allowed to include in ACs. If this
field is not present, it means that no attribute types are explicitly
allowed.
Farrell & Housley Standards Track [Page 29]RFC 3281 An Internet Attribute Certificate April 2002
The excludedAttrs field specifies a set of attribute types that no AC
issuer is allowed to include in ACs. If this field is not present,
it means that no attribute types are explicitly disallowed.
The permitUnSpecified field specifies how to handle attribute types
which are not present in either the permittedAttrs or excludedAttrs
fields. TRUE (the default) means that any unspecified attribute type
is allowed in ACs; FALSE means that no unspecified attribute type is
allowed.
When AAControls are used, the following additional checks on an AA's
PKC chain MUST all succeed for the AC to be valid:
1. Some CA on the ACs certificate path MUST be directly trusted to
issue PKCs which precede the AC issuer in the certification path;
call this CA the "AA CA".
2. All PKCs on the path from the AA CA, down to and including the AC
issuer's PKC, MUST contain an AAControls extension; however, the
AA CA's PKC need not contain this extension.
3. Only those attributes in the AC which are allowed, according to
all of the AAControls extension values in all of the PKCs from the
AA CA to the AC issuer, may be used for authorization decisions;
all other attributes MUST be ignored. This check MUST be applied
to the set of attributes following attribute decryption, and the
id-aca-encAttrs type MUST also be checked.
name id-pe-aaControls
OID { id-pe 6 }
syntax AAControls
criticality MAY be TRUE
8. Security Considerations
The protection afforded for private keys is a critical factor in
maintaining security. Failure of AC issuers to protect their private
keys will permit an attacker to masquerade as them, potentially
generating false ACs or revocation status. Existence of bogus ACs
and revocation status will undermine confidence in the system. If
the compromise is detected, all ACs issued by the AC issuer MUST be
revoked. Rebuilding after such a compromise will be problematic, so
AC issuers are advised to implement a combination of strong technical
measures (e.g., tamper-resistant cryptographic modules) and
appropriate management procedures (e.g., separation of duties) to
avoid such an incident.
Farrell & Housley Standards Track [Page 30]RFC 3281 An Internet Attribute Certificate April 2002
Loss of an AC issuer's private signing key may also be problematic.
The AC issuer would not be able to produce revocation status or
perform AC renewal. AC issuers are advised to maintain secure backup
for signing keys. The security of the key backup procedures is a
critical factor in avoiding key compromise.
The availability and freshness of revocation status will affect the
degree of assurance that should be placed in a long-lived AC. While
long-lived ACs expire naturally, events may occur during its natural
lifetime which negate the binding between the AC holder and the
attributes. If revocation status is untimely or unavailable, the
assurance associated with the binding is clearly reduced.
The binding between an AC holder and attributes cannot be stronger
than the cryptographic module implementation and algorithms used to
generate the signature. Short key lengths or weak hash algorithms
will limit the utility of an AC. AC issuers are encouraged to note
advances in cryptology so they can employ strong cryptographic
techniques.
Inconsistent application of name comparison rules may result in
acceptance of invalid targeted or proxied ACs, or rejection of valid
ones. The X.500 series of specifications defines rules for comparing
distinguished names. These rules require comparison of strings
without regard to case, character set, multi-character white space
substrings, or leading and trailing white space. This specification
and [PKIXPROF] relaxes these requirements, requiring support for
binary comparison at a minimum.
AC issuers MUST encode the distinguished name in the AC
holder.entityName field identically to the distinguished name in the
holder's PKC. If different encodings are used, implementations of
this specification may fail to recognize that the AC and PKC belong
to the same entity.
If an attribute certificate is tied to the holder's PKC using the
baseCertificateID component of the Holder field and the PKI in use
includes a rogue CA with the same issuer name specified in the
baseCertificateID component, this rogue CA could issue a PKC to a
malicious party, using the same issuer name and serial number as the
proper holder's PKC. Then the malicious party could use this PKC in
conjunction with the AC. This scenario SHOULD be avoided by properly
managing and configuring the PKI so that there cannot be two CAs with
the same name. Another alternative is to tie ACs to PKCs using the
publicKeyCert type in the ObjectDigestInfo field. Failing this, AC
verifiers have to establish (using other means) that the potential
collisions cannot actually occur, for example, the CPSs of the CAs
involved may make it clear that no such name collisions can occur.
Farrell & Housley Standards Track [Page 31]
RFC 3281 An Internet Attribute Certificate April 2002
Implementers MUST ensure that following validation of an AC, only
attributes that the issuer is trusted to issue are used in
authorization decisions. Other attributes, which MAY be present MUST
be ignored. Given that the AA controls PKC extension is optional to
implement, AC verifiers MUST be provided with this information by
other means. Configuration information is a likely alternative
means. This becomes very important if an AC verifier trusts more
than one AC issuer.
There is often a requirement to map between the authentication
supplied by a particular security protocol (e.g. TLS, S/MIME) and the
AC holder's identity. If the authentication uses PKCs, then this
mapping is straightforward. However, it is envisaged that ACs will
also be used in environments where the holder may be authenticated
using other means. Implementers SHOULD be very careful in mapping
the authenticated identity to the AC holder.
9. IANA Considerations
Attributes and attribute certificate extensions are identified by
object identifiers (OIDs). Many of the OIDs used in this document
are copied from X.509 [X.509-2000]. Other OIDs were assigned from an
arc delegated by the IANA. No further action by the IANA is
necessary for this document or any anticipated updates.
10. References
[CMC] Myers, M., Liu, X., Schaad, J. and J. Weinstein,
"Certificate Management Messages over CMS", RFC 2797,
April 2000.
[CMP] Adams, C. and S. Farrell, "Internet X.509 Public Key
Infrastructure - Certificate Management Protocols", RFC
2510, March 1999.
[CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[KRB] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[LDAP] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.
Farrell & Housley Standards Track [Page 32]RFC 3281 An Internet Attribute Certificate April 2002
[OCSP] 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.
[PKIXALGS] Bassham, L., Polk, W. and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
Lists CRL Profile", RFC 3279, April 2002.
[PKIXPROF] Housley, R., Polk, T, Ford, W. and Solo, D., "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[URL] Berners-Lee, T., Masinter L. and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[X.208-1988] CCITT Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988.
[X.501-88] CCITT Recommendation X.501: The Directory - Models.
1988.
[X.501-1993] ITU-T Recommendation X.501 : Information Technology -
Open Systems Interconnection - The Directory: Models,
1993.
[X.509-1988] CCITT Recommendation X.509: The Directory -
Authentication Framework. 1988.
[X.509-1997] ITU-T Recommendation X.509: The Directory -
Authentication Framework. 1997.
[X.509-2000] ITU-T Recommendation X.509: The Directory - Public-Key
and Attribute Certificate Frameworks. 2000
Farrell & Housley Standards Track [Page 33]RFC 3281 An Internet Attribute Certificate April 2002
Appendix A: Object Identifiers
This (normative) appendix lists the new object identifiers which are
defined in this specification. Some of these are required only for
support of optional features and are not required for conformance to
this profile. This specification mandates support for OIDs which
have arc elements with values that are less than 2^32, (i.e. they
MUST be between 0 and 4,294,967,295 inclusive) and SHOULD be less
than 2^31 (i.e. less than or equal to 2,147,483,647). This allows
each arc element to be represented within a single 32 bit word.
Implementations MUST also support OIDs where the length of the dotted
decimal (see [LDAP], section 4.1.2) string representation can be up
to 100 bytes (inclusive). Implementations MUST be able to handle
OIDs with up to 20 elements (inclusive). AA's SHOULD NOT issue ACs
which contain OIDs that breach these requirements.
The following OIDs are imported from [PKIXPROF]:
id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
id-mod OBJECT IDENTIFIER ::= { id-pkix 0 }
id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }
id-ad OBJECT IDENTIFIER ::= { id-pkix 48 }
id-at OBJECT IDENTIFIER ::= { joint-iso-ccitt(2) ds(5) 4 }
id-ce OBJECT IDENTIFIER ::= { joint-iso-ccitt(2) ds(5) 29 }
The following new ASN.1 module OID is defined:
id-mod-attribute-cert OBJECT IDENTIFIER ::= { id-mod 12 }
The following AC extension OIDs are defined:
id-pe-ac-auditIdentity OBJECT IDENTIFIER ::= { id-pe 4 }
id-pe-ac-proxying OBJECT IDENTIFIER ::= { id-pe 10 }
id-ce-targetInformation OBJECT IDENTIFIER ::= { id-ce 55 }
The following PKC extension OIDs are defined:
id-pe-aaControls OBJECT IDENTIFIER ::= { id-pe 6 }
Farrell & Housley Standards Track [Page 34]RFC 3281 An Internet Attribute Certificate April 2002
The following attribute OIDs are defined:
id-aca OBJECT IDENTIFIER ::= { id-pkix 10 }
id-aca-authenticationInfo OBJECT IDENTIFIER ::= { id-aca 1 }
id-aca-accessIdentity OBJECT IDENTIFIER ::= { id-aca 2 }
id-aca-chargingIdentity OBJECT IDENTIFIER ::= { id-aca 3 }
id-aca-group OBJECT IDENTIFIER ::= { id-aca 4 }
id-aca-encAttrs OBJECT IDENTIFIER ::= { id-aca 6 }
id-at-role OBJECT IDENTIFIER ::= { id-at 72 }
id-at-clearance OBJECT IDENTIFIER ::=
{ joint-iso-ccitt(2) ds(5) module(1)
selected-attribute-types(5) clearance (55) }
Appendix B: ASN.1 Module
PKIXAttributeCertificate {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-attribute-cert(12)}
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
-- IMPORTed module OIDs MAY change if [PKIXPROF] changes
-- PKIX Certificate Extensions
Attribute, AlgorithmIdentifier, CertificateSerialNumber,
Extensions, UniqueIdentifier,
id-pkix, id-pe, id-kp, id-ad, id-at
FROM PKIX1Explicit88 {iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5)
pkix(7) id-mod(0) id-pkix1-explicit-88(1)}
GeneralName, GeneralNames, id-ce
FROM PKIX1Implicit88 {iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5)
pkix(7) id-mod(0) id-pkix1-implicit-88(2)} ;
id-pe-ac-auditIdentity OBJECT IDENTIFIER ::= { id-pe 4 }
id-pe-aaControls OBJECT IDENTIFIER ::= { id-pe 6 }
id-pe-ac-proxying OBJECT IDENTIFIER ::= { id-pe 10 }
id-ce-targetInformation OBJECT IDENTIFIER ::= { id-ce 55 }
id-aca OBJECT IDENTIFIER ::= { id-pkix 10 }
Farrell & Housley Standards Track [Page 35]RFC 3281 An Internet Attribute Certificate April 2002
id-aca-authenticationInfo OBJECT IDENTIFIER ::= { id-aca 1 }
id-aca-accessIdentity OBJECT IDENTIFIER ::= { id-aca 2 }
id-aca-chargingIdentity OBJECT IDENTIFIER ::= { id-aca 3 }
id-aca-group OBJECT IDENTIFIER ::= { id-aca 4 }
-- { id-aca 5 } is reserved
id-aca-encAttrs OBJECT IDENTIFIER ::= { id-aca 6 }
id-at-role OBJECT IDENTIFIER ::= { id-at 72}
id-at-clearance OBJECT IDENTIFIER ::=
{ joint-iso-ccitt(2) ds(5) module(1)
selected-attribute-types(5) clearance (55) }
-- Uncomment this if using a 1988 level ASN.1 compiler
-- UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
AttributeCertificate ::= SEQUENCE {
acinfo AttributeCertificateInfo,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING
}
AttributeCertificateInfo ::= SEQUENCE {
version AttCertVersion -- version is v2,
holder Holder,
issuer AttCertIssuer,
signature AlgorithmIdentifier,
serialNumber CertificateSerialNumber,
attrCertValidityPeriod AttCertValidityPeriod,
attributes SEQUENCE OF Attribute,
issuerUniqueID UniqueIdentifier OPTIONAL,
extensions Extensions OPTIONAL
}
AttCertVersion ::= INTEGER { v2(1) }
Holder ::= SEQUENCE {
baseCertificateID [0] IssuerSerial OPTIONAL,
-- the issuer and serial number of
-- the holder's Public Key Certificate
entityName [1] GeneralNames OPTIONAL,
-- the name of the claimant or role
objectDigestInfo [2] ObjectDigestInfo OPTIONAL
-- used to directly authenticate the
-- holder, for example, an executable
}
Farrell & Housley Standards Track [Page 36]RFC 3281 An Internet Attribute Certificate April 2002
ObjectDigestInfo ::= SEQUENCE {
digestedObjectType ENUMERATED {
publicKey (0),
publicKeyCert (1),
otherObjectTypes (2) },
-- otherObjectTypes MUST NOT
-- MUST NOT be used in this profile
otherObjectTypeID OBJECT IDENTIFIER OPTIONAL,
digestAlgorithm AlgorithmIdentifier,
objectDigest BIT STRING
}
AttCertIssuer ::= CHOICE {
v1Form GeneralNames, -- MUST NOT be used in this
-- profile
v2Form [0] V2Form -- v2 only
}
V2Form ::= SEQUENCE {
issuerName GeneralNames OPTIONAL,
baseCertificateID [0] IssuerSerial OPTIONAL,
objectDigestInfo [1] ObjectDigestInfo OPTIONAL
-- issuerName MUST be present in this profile
-- baseCertificateID and objectDigestInfo MUST
-- NOT be present in this profile
}
IssuerSerial ::= SEQUENCE {
issuer GeneralNames,
serial CertificateSerialNumber,
issuerUID UniqueIdentifier OPTIONAL
}
AttCertValidityPeriod ::= SEQUENCE {
notBeforeTime GeneralizedTime,
notAfterTime GeneralizedTime
}
Targets ::= SEQUENCE OF Target
Target ::= CHOICE {
targetName [0] GeneralName,
targetGroup [1] GeneralName,
targetCert [2] TargetCert
}
Farrell & Housley Standards Track [Page 37]RFC 3281 An Internet Attribute Certificate April 2002
TargetCert ::= SEQUENCE {
targetCertificate IssuerSerial,
targetName GeneralName OPTIONAL,
certDigestInfo ObjectDigestInfo OPTIONAL
}
IetfAttrSyntax ::= SEQUENCE {
policyAuthority[0] GeneralNames OPTIONAL,
values SEQUENCE OF CHOICE {
octets OCTET STRING,
oid OBJECT IDENTIFIER,
string UTF8String
}
}
SvceAuthInfo ::= SEQUENCE {
service GeneralName,
ident GeneralName,
authInfo OCTET STRING OPTIONAL
}
RoleSyntax ::= SEQUENCE {
roleAuthority [0] GeneralNames OPTIONAL,
roleName [1] GeneralName
}
Clearance ::= SEQUENCE {
policyId [0] OBJECT IDENTIFIER,
classList [1] ClassList DEFAULT {unclassified},
securityCategories
[2] SET OF SecurityCategory OPTIONAL
}
ClassList ::= BIT STRING {
unmarked (0),
unclassified (1),
restricted (2),
confidential (3),
secret (4),
topSecret (5)
}
SecurityCategory ::= SEQUENCE {
type [0] IMPLICIT OBJECT IDENTIFIER,
value [1] ANY DEFINED BY type
}
Farrell & Housley Standards Track [Page 38]RFC 3281 An Internet Attribute Certificate April 2002
AAControls ::= SEQUENCE {
pathLenConstraint INTEGER (0..MAX) OPTIONAL,
permittedAttrs [0] AttrSpec OPTIONAL,
excludedAttrs [1] AttrSpec OPTIONAL,
permitUnSpecified BOOLEAN DEFAULT TRUE
}
AttrSpec::= SEQUENCE OF OBJECT IDENTIFIER
ACClearAttrs ::= SEQUENCE {
acIssuer GeneralName,
acSerial INTEGER,
attrs SEQUENCE OF Attribute
}
ProxyInfo ::= SEQUENCE OF Targets
END
Author's Addresses
Stephen Farrell
Baltimore Technologies
39/41 Parkgate Street
Dublin 8
IRELAND
EMail: stephen.farrell@baltimore.ie
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: rhousley@rsasecurity.com
Acknowledgements
Russ Housley thanks the management at SPYRUS, who supported the
development of this specification while he was employed at SPYRUS.
Russ Housley also thanks the management at RSA Laboratories, who
supported the completion of the specification after a job change.
Farrell & Housley Standards Track [Page 39]RFC 3281 An Internet Attribute Certificate April 2002
Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Farrell & Housley Standards Track [Page 40]
