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Versions: 00 01 02 03 04 05 06 RFC 6024
Network Working Group R. Reddy
Internet-Draft National Security Agency
Intended status: Informational C. Wallace
Expires: December 22, 2008 Cygnacom Solutions
June 20, 2008
Trust Anchor Management Requirements
draft-ietf-pkix-ta-mgmt-reqs-00
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Abstract
A trust anchor represents an authoritative entity via a public key
and associated data. The public key is used to verify digital
signatures and the associated data is used to constrain the types of
information for which the trust anchor is authoritative. A relying
party uses trust anchors to determine if a digitally signed object is
valid by verifying a digital signature using the trust anchor's
public key, and by enforcing the constraints expressed in the
associated data for the trust anchor. This document describes some
of the problems associated with the lack of a standard trust anchor
management mechanism and defines requirements for data formats and
protocols designed to address these problems. This document
discusses only public keys as trust anchors; symmetric key trust
anchors are not considered.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Transport independence . . . . . . . . . . . . . . . . . . 8
3.1.1. Functional Requirements . . . . . . . . . . . . . . . 8
3.1.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Basic management operations . . . . . . . . . . . . . . . 8
3.2.1. Functional Requirements . . . . . . . . . . . . . . . 8
3.2.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Management targets . . . . . . . . . . . . . . . . . . . . 9
3.3.1. Functional Requirements . . . . . . . . . . . . . . . 9
3.3.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Delegation of TA Management Authority . . . . . . . . . . 10
3.4.1. Functional Requirements . . . . . . . . . . . . . . . 10
3.4.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 10
3.5. RFC 5280 Support . . . . . . . . . . . . . . . . . . . . . 10
3.5.1. Functional Requirements . . . . . . . . . . . . . . . 10
3.5.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 10
3.6. Support Purposes Other Than Certification Path
Validation . . . . . . . . . . . . . . . . . . . . . . . . 10
3.6.1. Functional Requirements . . . . . . . . . . . . . . . 11
3.6.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 11
3.7. Trust Anchor Format . . . . . . . . . . . . . . . . . . . 11
3.7.1. Functional Requirements . . . . . . . . . . . . . . . 11
3.7.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 11
3.8. Authentication of Trust Anchor Store Contents . . . . . . 12
3.8.1. Functional Requirements . . . . . . . . . . . . . . . 12
3.8.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 12
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3.9. Source Authentication . . . . . . . . . . . . . . . . . . 12
3.9.1. Functional Requirements . . . . . . . . . . . . . . . 12
3.9.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 12
3.10. Reduce Reliance on Out-of-Band Trust Mechanisms . . . . . 12
3.10.1. Functional Requirements . . . . . . . . . . . . . . . 12
3.10.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 12
3.11. Replay Detection . . . . . . . . . . . . . . . . . . . . . 13
3.11.1. Functional Requirements . . . . . . . . . . . . . . . 13
3.11.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 13
3.12. Compromise or Disaster Recovery . . . . . . . . . . . . . 13
3.12.1. Functional Requirements . . . . . . . . . . . . . . . 13
3.12.2. Rationale . . . . . . . . . . . . . . . . . . . . . . 13
4. Security Considerations . . . . . . . . . . . . . . . . . . . 14
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Normative References . . . . . . . . . . . . . . . . . . . 16
6.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
Digital signatures are used in many applications. For digital
signatures to provide integrity and authentication, the public key
used to verify the digital signature must be "trusted", i.e.,
accepted by a relying party (RP) as appropriate for use in the given
context. A public key used to verify a signature must be configured
as a trust anchor or contained in a certificate that can be
transitively verified by a certification path terminating at a trust
anchor. A Trust Anchor is a public key and associated data used by a
relying party to validate a signature on a signed object where the
object is either:
o a public key certificate that begins a certification path
terminated by a signature certificate or encryption certificate
o an object (other than a public key certificate) that cannot be
validated via use of a certification path
Trust anchors have only local significance, i.e., each RP is
configured with a set of trust anchors, either by the RP or by an
entity that manages TAs in the context in which the RP operates. The
associated data defines the scope of a trust anchor by imposing
constraints on the signatures the trust anchor may be used to verify.
For example, if a trust anchor is used to verify signatures on X.509
certificates, these constraints may include a combination of name
spaces, certificate policies, or application/usage types.
One use of digital signatures is the verification of signatures on
firmware packages loaded into hardware modules, such as cryptographic
modules, cable boxes, routers, etc. Since such devices are often
managed remotely, the devices must be able to authenticate the source
of management interactions and can use trust anchors to perform this
authentication. However, trust anchors require management as well.
All applications that rely upon digital signatures rely upon some
means of managing one or more sets of trust anchors. These sets of
trust anchors are referred to in this document as trust anchor
stores. Often, the means of managing trust anchor stores are
application-specific and rely upon out-of-band means to establish and
maintain trustworthiness. An application may use multiple trust
anchor stores and a given trust anchor store may be used by multiple
applications. Trust anchor stores are managed by trust anchor
managers.
This section provides an introduction and defines basic terminology.
Section 2 describes problems with current trust anchor management
methods. Sections 3 and 4 describe requirements and security
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considerations for a trust anchor management solution.
1.1. Terminology
The following terms are defined in order to provide a vocabulary for
describing requirements for trust anchor management.
Trust Anchor: A trust anchor represents an authoritative entity via
a public key and associated data. The public key is used to
verify digital signatures and the associated data is used to
constrain the types of information for which the trust anchor is
authoritative. A relying party uses trust anchors to determine if
a digitally signed object is valid by verifying a digital
signature using the trust anchor's public key, and by enforcing
the constraints expressed in the associated data for the trust
anchor.
Trust Anchor Manager: Trust anchor manager is a role responsible
for managing the contents of a trust anchor store. Throughout
this document, trust anchor managers are assumed to be represented
as trust anchors.
Trust Anchor Store: A trust anchor store is a set of one or more
trust anchors. A trust anchor store may be managed by one or more
trust anchor managers. A device may have more than one trust
anchor store.
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2. Problem Statement
Trust anchors are used to support many application scenarios. Most
Internet browsers and email clients use trust anchors when
authenticating TLS sessions, verifying signed email and generating
encrypted email by validating a certification path to a server's
certificate, an e-mail originator's certificate or an e-mail
recipient's certificate. Many software distributions are digitally
signed to enable authentication of the software source to be
performed prior to installation. Trust anchors that support these
applications are typically installed as part of the operating system
(OS) or application, installed using an enterprise configuration
management system or installed directly by an OS or application user.
Trust anchors are typically stored in application-specific or
operating system-specific trust anchor stores. Often, a single
machine may have a number of different trust anchor stores that may
not be synchronized. Reviewing the contents of a particular trust
anchor store typically involves use of a proprietary tool that
interacts with a particular type of trust store.
The presence of a trust anchor in a particular store often conveys
implicit authorization to validate signatures for any contexts from
which the store is accessed. For example, the public key of a
timestamp authority (TSA) may be installed in a trust anchor store to
validate signatures on timestamps. However, if the store containing
this TA is used by multiple applications that serve different
purposes, the same key may be used (inappropriately) to validate
other types of objects such as certificates or OCSP responses. There
is currently no standard means of limiting the applicability (scope)
of a trust anchor except by placing different TAs in different stores
and limiting the set of applications that access a given TA store.
Trust relationships between PKIs are negotiated by policy
authorities. Negotiations frequently require significant time to
ensure all participating parties' requirements are satisfied. These
requirements are expressed, to some extent, in public key
certificates via policy constraints, name constraints and etc. In
order for these requirements to be enforced, trust anchor stores must
be managed in accord with policy authority intentions and avoid
circumventing constraints defined in a cross-certificate by
recognizing the subject of the cross certificate as a trust anchor.
Trust anchors are often represented as self-signed certificates,
which provide no useful means of establishing the validity of the
information contained in the certificate. Confidence in the
integrity of a trust anchor is typically established through out-of-
band means, often by checking the "fingerprint" (one-way hash) of the
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self-signed certificate with an authoritative source. Routine trust
anchor re-key operations typically require similar out-of-band
checks. Ideally, only the initial set of trust anchors installed in
a particular trust anchor store should require out-of-band
verification, particularly when the costs of performing out-of-band
checks commensurate with the security requirements of applications
using the trust anchor store are high.
Despite the prevalent use of trust anchors, there is neither a
standard means for discovering which trust anchors installed in a
particular trust anchor store nor a standard means of managing those
trust anchors. The remainder of this document describes requirements
for a solution to this problem along with some security
considerations.
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3. Requirements
This section describes the requirements for a trust anchor management
protocol. Requirements are provided for trust anchor contents as
well as for trust anchor store management operations.
3.1. Transport independence
3.1.1. Functional Requirements
A general-purpose solution for the management of trust anchors must
be transport independent in order to apply to a range of device
communications environments. It should be applicable in both
session-oriented and store-and-forward contexts. Message integrity
must be assured in all cases.
3.1.2. Rationale
Not all devices that use trust anchors are available for online
management operations; some devices may require manual interaction
for trust anchor management. Message integrity is required to
authenticate the originator of a TA management transaction and
confirm the authorization of the originator for that transaction.
3.2. Basic management operations
3.2.1. Functional Requirements
At a minimum, a protocol used for trust anchor management must enable
a trust anchor manager to perform the following operations:
o Determine which trust anchors are installed in a particular trust
anchor store
o Add one or more trust anchors to a trust anchor store
o Remove one or more trust anchors from a trust anchor store
o Replace an entire trust anchor store
A trust anchor management protocol must provide support for these
basic operations, however, not all implementations must support each
option. For example, some implementations may only support
replacement of trust anchor stores.
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3.2.2. Rationale
These requirements describe the core operations required to manage
the contents of a trust anchor store. An edit operation was omitted
for sake of simplicity, with consecutive remove and add operations
used for this purpose. Add and remove operations are compound to
avoid unnecessary round trips and are provided to avoid always
replacing an entire trust anchor store. Trust anchor store
replacement may be useful as a simple, higher bandwidth alternative
to add and remove operations. Many devices and some applications
utilize multiple trust anchor stores. Trust anchor store discovery
is required to determine which trust anchor stores are present in a
particular context.
3.3. Management targets
3.3.1. Functional Requirements
A protocol for TA management must allow a TA management transaction
to be directed to:
All TA stores for which the manager is responsible
An enumerated list of one or more groups of trust anchor stores
An individual trust anchor store
3.3.2. Rationale
Trust anchor configurations may be uniform across an enterprise, or
they may be unique to a single application or small set of
applications.
Connections between PKIs can be accomplished using different means.
Unilateral or bilateral cross-certification can be performed, or a
community may simply elect to explicitly accept a trust anchor from
another community. Typically, these decisions occur at the
enterprise level. In some scenarios, it can be useful to establish
these connections for a small community within an enterprise.
Enterprise-wide mechanisms such as cross-certificates are ill-suited
for this purpose since certificate revocation or expiration affects
the entire enterprise. A trust anchor management protocol can
address this issue by supporting limited installation of trust
anchors and by supporting expression of constraints on trust anchor
usage. Limited installation requires the ability to identify the
members of the community that are authorized to rely upon a
particular trust anchor, as well as the ability to query and report
on the contents of trust anchor stores. The trust anchor constraints
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can represent the limitations that would have been expressed in a
cross-certificate and limited installation ensures the recognition of
the trust anchor does not necessarily encompass an entire enterprise.
3.4. Delegation of TA Management Authority
3.4.1. Functional Requirements
A trust anchor management protocol must enable secure transfer of
control of a trust anchor store from one trust anchor manager to
another. It must also enable delegation for specific operations
without requiring delegation of the overall trust anchor management
capability itself.
3.4.2. Rationale
Trust anchor re-key is one type of transfer that must be supported.
In this case, the new key will be assigned the same privileges as the
old key. Creation of trust anchors for specific purposes, such as
firmware signing, is another example of delegation. For example, a
trust anchor manager may delegate only the authority to sign firmware
and disallow further delegation of the privilege, or the trust anchor
manager may allow its delegate to delegate firmware signing to other
entities.
3.5. RFC 5280 Support
3.5.1. Functional Requirements
A trust anchor management protocol must enable management of trust
anchors that can be used to validate certification paths in
accordance with [RFC5280] and [RFC5055]. A trust anchor format must
enable the representation of constraints that influence certification
path validation or otherwise establish the scope of usage of the
trust anchor public key. Examples of such constraints are name
constraints, certificate policies and key usage.
3.5.2. Rationale
Certification path validation is one of the most common applications
of trust anchors. The rules for using trust anchors for path
validation are established in [RFC5280]. [RFC5055] describes the use
of trust anchors for delegated path validation.
3.6. Support Purposes Other Than Certification Path Validation
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3.6.1. Functional Requirements
A trust anchor management protocol must enable management of trust
anchors that can be used for purposes other than certification path
validation, including trust anchors that cannot be used for
certification path validation. It should be possible to authorize a
trust anchor to delegate authority (to other TAs or certificate
holders) and to prevent a trust anchor from delegating authority.
3.6.2. Rationale
Trust anchors are used to validate a variety of objects other than
public key certificates and CRLs. For example, a trust anchor may be
used to verify firmware packages [RFC4108], OCSP responses [RFC2560],
SCVP responses [RFC5055] or timestamps [RFC3161]. TAs authorized for
these operations may not be authorized to sign public key
certificates or CRLs.
3.7. Trust Anchor Format
3.7.1. Functional Requirements
Minimally, a trust anchor management protocol must support management
of trust anchors represented as self-signed certificates or as a
distinguished name and public key information. The definition of a
trust anchor must include a public key, a public key algorithm and,
if necessary, public key parameters. When the public key is used to
validate certification paths, a distinguished name also must be
included per [RFC3852]. A trust anchor format should enable
specification of public key identifier to enable other applications
of the trust anchor, for example, verification of data signed using
the Cryptographic Message Syntax (CMS) SignedData structure
[RFC3852]. A trust anchor format should also enable the use of
constraints that can be applied to specify the type/usage of a trust
anchor.
3.7.2. Rationale
There is no standardized format for trust anchors. Self-signed X.509
certificates are typically used but [RFC5280] does not mandate a
particular trust anchor representation. It requires only that a
trust anchor's public key information and distinguished name be
available during certification path validation. CMS is widely used
to protect a variety of types of content using digital signatures,
including contents that may verified directly using a trust anchor,
such as firmware packages [RFC4108].
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3.8. Authentication of Trust Anchor Store Contents
3.8.1. Functional Requirements
A trust anchor manager must be able to authenticate which trust
anchor store produced a report listing the contents of the trust
anchor store and be able to confirm the contents of the report have
not been subsequently altered. Replay of old reports (from the same
trust anchor store) must be detectable by a TA manager.
3.8.2. Rationale
Authentication of trust anchor store-generated reports is required to
support remote management operations.
3.9. Source Authentication
3.9.1. Functional Requirements
An entity receiving trust anchor management data must be able to
authenticate the party providing the information and must be able to
confirm the party is authorized to provide that trust anchor
information.
3.9.2. Rationale
A trust anchor manager may be authorized to participate in trust
anchor management protocol exchanges, but be limited to managing
trust anchors within a particular scope. Alternatively, a trust
anchor manager may be authorized to participate in trust anchor
management protocol exchanges without any constraints on the types of
trust anchors that may be managed.
3.10. Reduce Reliance on Out-of-Band Trust Mechanisms
3.10.1. Functional Requirements
A trust anchor management protocol should enable TA integrity to be
checked automatically without relying on out-of-band trust
mechanisms.
3.10.2. Rationale
Traditionally, a trust anchor is distributed out-of-band with its
integrity checked manually prior to installation. Installation
typically is performed by anyone with sufficient administrative
privilege on the system receiving the trust anchor. Reliance on out-
of-band trust mechanisms is one problem with current trust anchor
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management approaches and reduction of the need to use out-of-band
trust mechanisms is a primary motivation for developing a trust
anchor management protocol. Ideally, out-of-band trust mechanisms
will be required only during trust anchor store initialization.
3.11. Replay Detection
3.11.1. Functional Requirements
A trust anchor management protocol must enable participants engaged
in a trust anchor management protocol exchange to detect replay
attacks. Replay detection mechanisms should not introduce a
requirement for a reliable source of time as some devices that
utilize trust anchors have no access to a reliable source of time.
3.11.2. Rationale
Replay of old trust anchor management messages could result in the
addition of compromised trust anchors to a trust anchor store,
potentially exposing applications to malicious signed objects or
certification paths.
3.12. Compromise or Disaster Recovery
3.12.1. Functional Requirements
A trust anchor management protocol must enable recovery from the
compromise or loss of a trust anchor private key, including the
private key authorized to serve as a source of trust anchor
information.
3.12.2. Rationale
Compromise or loss of a private key corresponding to a trust anchor
can have significant negative consequences. Currently, in some
cases, re-initialization of all effected trust anchor stores is
required to recover from a lost or compromised trust anchor key. A
trust anchor management protocol should support recovery options that
do not require trust anchor store re-initialization.
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4. Security Considerations
The public key used to authenticate a TA management transaction may
have been placed in the client as the result of an earlier TA
management transaction or during an initial bootstrap configuration
operation. In most scenarios, at least one public key authorized for
trust anchor management must be placed in each trust anchor store to
be managed during the initial configuration of the trust anchor
store. This public key may be transported and checked using out-of-
band means. In all scenarios, regardless of the authentication
mechanism, at least one trust anchor manager must be established for
each trust anchor store during the initial configuration of the trust
anchor store.
Many of the security considerations from [RFC5280] are also
applicable to trust anchor management.
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5. IANA Considerations
None. Please remove this section prior to publication as an RFC.
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6. References
6.1. Normative References
[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.
6.2. Informative References
[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.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
[RFC4108] Housley, R., "Using Cryptographic Message Syntax (CMS) to
Protect Firmware Packages", RFC 4108, August 2005.
[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, December 2007.
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Authors' Addresses
Raksha Reddy
National Security Agency
Suite 6599
9800 Savage Road
Fort Meade, MD 20755
Email: r.reddy@radium.ncsc.mil
Carl Wallace
Cygnacom Solutions
Suite 5200
7925 Jones Branch Drive
McLean, VA 22102
Email: cwallace@cygnacom.com
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