Trust Anchor Management Requirements

Internet Engineering Task Force (IETF)                          R. Reddy
Request for Comments: 6024                      National Security Agency
Category: Informational                                       C. Wallace
ISSN: 2070-1721                                       Cygnacom Solutions
                                                            October 2010


                  Trust Anchor Management Requirements

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
   push-based protocols designed to address these problems.

Status of This Memo

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

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6024.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Requirements Notation  . . . . . . . . . . . . . . . . . .  4
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Transport Independence . . . . . . . . . . . . . . . . . .  6
     3.2.  Basic Management Operations  . . . . . . . . . . . . . . .  7
     3.3.  Management Targets . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Delegation of TA Manager Authority . . . . . . . . . . . .  8
     3.5.  RFC 5280 Support . . . . . . . . . . . . . . . . . . . . .  9
     3.6.  Support Purposes other than Certification Path
           Validation . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.7.  Trust Anchor Format  . . . . . . . . . . . . . . . . . . . 10
     3.8.  Source Authentication  . . . . . . . . . . . . . . . . . . 10
     3.9.  Reduce Reliance on Out-of-Band Trust Mechanisms  . . . . . 11
     3.10. Replay Detection . . . . . . . . . . . . . . . . . . . . . 11
     3.11. Compromise or Disaster Recovery  . . . . . . . . . . . . . 12
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 13

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 (TA) 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 or certificate
      revocation list (CRL), 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 that 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.
   Other applications requiring trust anchor management include web
   browsers (which use trust anchors when authenticating web servers)
   and email clients (which use trust anchors when validating signed
   email and when authenticating recipients of encrypted email).

   All applications that rely upon digital signatures rely upon some
   means of managing one or more sets of trust anchors.  Each set of
   trust anchors is referred to in this document as a trust anchor
   store.  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.  Each trust anchor store is managed by at least one TA
   manager; a TA manager may manage multiple TA stores.

   The requirements stated in this document were prepared prior to the
   publication of [RFC5914] and [RFC5934].  The document was not
   published at that time to allow for changes in requirements during
   the development of the associated technical specifications.  The
   requirements described below are those that were considered during
   the development of [RFC5914] and [RFC5934].

   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
   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:   A trust anchor manager is an entity
      responsible for managing the contents of a trust anchor store.
      Throughout this document, each trust anchor manager is assumed to
      be represented as or delegated by a distinct trust anchor.

   Trust Anchor Store:   A trust anchor store is a set of one or more
      trust anchors stored in a device.  A trust anchor store may be
      managed by one or more trust anchor managers.  A device may have
      more than one trust anchor store, each of which may be used by one
      or more applications.

1.2.  Requirements Notation

   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 RFC 2119 [RFC2119].

2.  Problem Statement

   Trust anchors are used to support many application scenarios.  Most
   Internet browsers and email clients use trust anchors when
   authenticating Transport Layer Security (TLS) sessions, verifying
   signed email, and generating encrypted email by validating a
   certification path to a server's certificate, an email originator's
   certificate, or an email recipient's certificate, respectively.  Many
   software distributions are digitally signed to enable authentication
   of the software source 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
   OS-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 [RFC3161].  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 Online
   Certificate Status Protocol (OCSP) responses.  Prior to publication
   of [RFC5914], there was no standard general-purpose mechanism for
   limiting the applicability (scope) of a trust anchor.  A common
   practice to address this problem is to place different TAs in
   different stores and limit the set of applications that access a
   given TA store.

   Trust relationships between Public Key Infrastructures (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,
   etc.  In order for these requirements to be enforced, trust anchor
   stores must be managed in accord with policy authority intentions.
   Otherwise, the constraints defined in a cross-certificate could be
   circumvented by recognizing the subject of the cross certificate as a
   trust anchor, which would enable path processing implementations to
   avoid the cross-certificate.

   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
   self-signed certificate with an authoritative source.  Routine trust
   anchor rekey operations typically require similar out-of-band checks,
   though in-band rekey of a trust anchor is supported by the
   Certificate Management Protocol (CMP) [RFC4210].  Ideally, only the
   initial set of trust anchors are 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 the set of 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.

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 MUST work in both session-oriented
   and store-and-forward communications environments as well as in both
   push and pull distribution models.  To accommodate both communication
   models in a uniform fashion, connectionless integrity and data origin
   authentication for TA transactions MUST be provided at the
   application layer.  Confidentiality MAY be provided for such
   transactions.

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.  Data origin authentication and
   integrity are required to ensure that the transaction has not been
   modified en route.  Only connectionless integrity is required, for
   compatibility with store-and-forward contexts.

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 support only
   replacement of trust anchor stores.

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 the sake of simplicity, with consecutive remove and add
   operations used for this purpose.  A single add or remove operation
   can act upon more than one trust anchor to avoid unnecessary round
   trips and are provided to avoid the need to always replace an entire
   trust anchor store.  Trust anchor store replacement may be useful as
   a simple, higher-bandwidth alternative to add and remove operations.

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 named groups of trust anchor
      stores

      An individual trust anchor store

3.3.2.  Rationale

   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 (i.e., installation
   of TAs in subsets of the enterprise user community), and by
   supporting expression of constraints on trust anchor use by relying
   parties.  Limited installation requires the ability to identify the
   members of the community that are intended to rely upon a particular
   trust anchor, as well as the ability to query and report on the
   contents of trust anchor stores.  Trust anchor constraints can be
   used to represent the limitations that might otherwise be expressed
   in a cross-certificate, and limited installation ensures the
   recognition of the trust anchor does not necessarily encompass an
   entire enterprise.

   Trust anchor configurations may be uniform across an enterprise, or
   they may be unique to a single application or small set of
   applications.  Many devices and some applications utilize multiple
   trust anchor stores.  By providing means of addressing a specific
   store or collections of stores, a trust anchor management protocol
   can enable efficient management of all stores under a trust anchor
   manager's control.

3.4.  Delegation of TA Manager 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 also SHOULD enable delegation for specific operations
   without requiring delegation of the overall trust anchor management
   capability itself.

3.4.2.  Rationale

   Trust anchor manager rekey 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 to an
   entity, but disallow further delegation of that privilege, or the
   trust anchor manager may allow its delegate to further delegate
   firmware signing authority 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 will be used to validate certification paths and CRLs 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.  Trust anchors used
   to validate certification paths are responsible for providing,
   possibly through a delegate, the revocation status information of
   certificates it issues; this is often accomplished by signing a CRL.

3.6.  Support Purposes other than Certification Path Validation

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 signed objects, not
   just public key certificates and CRLs.  For example, a trust anchor
   may be used to verify firmware packages [RFC4108], OCSP responses
   [RFC2560], Server-Based Certificate Validation Protocol (SCVP)
   responses [RFC5055], or timestamps [RFC3161].  TAs that are
   authorized for use with some or all of these other types of
   operations may not be authorized to verify public key certificates or
   CRLs.  Thus, it is important to be able to impose constraints on the
   ways in which a given TA is employed.

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 and trust
   anchors represented as a distinguished name, public key information,
   and, optionally, associated data.  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 or CRLs, a distinguished name also MUST be
   included per [RFC5280].  A trust anchor format SHOULD enable
   specification of a 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
   [RFC5652].  A trust anchor format also SHOULD enable the
   representation of constraints that can be applied to restrict the use
   of a trust anchor.

3.7.2.  Rationale

   Prior to the publication of [RFC5914], there was 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 be verified directly using a trust anchor, such as
   firmware packages [RFC4108].  Constraints may include a validity
   period, constraints on certification path validation, etc.

3.8.  Source Authentication

3.8.1.  Functional Requirements

   An entity receiving trust anchor management data MUST be able to
   authenticate the identity of the party providing the information and
   MUST be able to confirm the party is authorized to provide that trust
   anchor information.

   A trust anchor manager MUST be able to authenticate which trust
   anchor store corresponds to 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.

3.8.2.  Rationale

   Data origin authentication and integrity are required to support
   remote management operations, even when TA management transactions
   are effected via store-and-forward communications.

3.9.  Reduce Reliance on Out-of-Band Trust Mechanisms

3.9.1.  Functional Requirements

   When performing add operations, a trust anchor management protocol
   SHOULD enable TA integrity to be checked automatically by a relying
   party without relying on out-of-band trust mechanisms.

3.9.2.  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
   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.

3.10.  Replay Detection

3.10.1.  Functional Requirements

   A trust anchor management protocol MUST enable participants engaged
   in a trust anchor management protocol exchange to detect replay
   attacks.  A replay detection mechanism that does not introduce a
   requirement for a reliable source of time MUST be available.
   Mechanisms that do require a reliable source of time MAY be
   available.

3.10.2.  Rationale

   Detection of replays of trust anchor management transactions is
   required to support remote management operations.  Replay of old
   trust anchor management transactions could result in the
   reintroduction of compromised trust anchors to a trust anchor store,
   potentially exposing applications to malicious signed objects or
   certification paths.

   Some devices that utilize trust anchors have no access to a reliable
   source of time, so a replay detection mechanism that requires a
   reliable time source is insufficient.

3.11.  Compromise or Disaster Recovery

3.11.1.  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 trust anchor manager, without
   requiring re-initialization of the trust store.

3.11.2.  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 affected trust anchor stores is
   required to recover from a lost or compromised trust anchor key.  Due
   to the costs associated with re-initialization, a trust anchor
   management protocol should support recovery options that do not
   require trust anchor store re-initialization.

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.

   Compromise of a trust anchor's private key can result in many
   security problems including issuance of bogus certificates or
   installation of rogue trust anchors.

   Usage of trust anchor-based constraints requires great care when
   defining trust anchors.  Errors on the part of a trust anchor manager
   could result in denial of service or have serious security
   consequences.  For example, if a name constraint for a trust anchor
   that serves as the root of a PKI includes a typo, denial of service
   results for certificate holders and relying parties.  If a trust
   anchor manager inadvertently delegates all of its privileges and the
   delegate subsequently removes the trust anchor manager from trust
   anchor stores now under its control, recovery may require
   re-initialization of all effected trust anchor stores.

   RFC 5280 requires that certificate path validation be initialized
   with a TA subject name and public key, but does not require
   processing of other information, such as name constraints extensions.
   Inclusion of constraints in trust anchors is optional.  When
   constraints are explicitly included by a trust anchor manager using a
   trust anchor management protocol, there exists an expectation that
   the certificate path validation algorithm will make use of the
   constraints.  Application owners must confirm the path processing
   implementations support the processing of TA-based constraints, where
   required.

   Many of the security considerations from [RFC5280] are also
   applicable to trust anchor management.

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5055]  Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
              Polk, "Server-Based Certificate Validation Protocol
              (SCVP)", RFC 5055, December 2007.

   [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.

5.2.  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.

   [RFC4108]  Housley, R., "Using Cryptographic Message Syntax (CMS) to
              Protect Firmware Packages", RFC 4108, August 2005.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210, September 2005.

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

   [RFC5914]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Format", RFC 5914, June 2010.

   [RFC5934]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Management Protocol (TAMP)", RFC 5934, August 2010.

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 5400
   7925 Jones Branch Drive
   McLean, VA  22102

   EMail: cwallace@cygnacom.com