IETF                                                         K. Moriarty
Internet-Draft                                                  Dell EMC
Intended status: Standards Track                          March 29, 2019
Expires: September 19, 2019

                         ACME Client Extension


   Automated Certificate Management Environment (ACME) core protocol
   addresses the use case of web server certificates for TLS.  This
   document extends the ACME protocol to support end user client, device
   client, and code signing certificates.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 19, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Identity Proofing for Client Certificates . . . . . . . . . .   2
   3.  Key Storage . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Why Not EST . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Device Certificates . . . . . . . . . . . . . . . . . . . . .   5
   6.  End USer Client Certificates  . . . . . . . . . . . . . . . .   6
   7.  CodeSigning Certificates  . . . . . . . . . . . . . . . . . .   7
   8.  Pre-authorization . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  10
     12.3.  URL References . . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   ACME [RFC8555] is a mechanism for automating certificate management
   on the Internet.  It enables administrative entities to prove
   effective control over resources like domain names, and automates the
   process of generating and issuing certificates.

   ACME was designed for web server certificates with the possibility to
   create extensions for other use cases and certificate types.  End
   user and device certificates may also benefit from automated
   management to ease the deployment and maintenance of these
   certificates type, thus the definition of the extension for that
   purpose in this document.

2.  Identity Proofing for Client Certificates

   As with the TLS certificates defined in the core ACME document,
   identity proofing for ACME issued end user client, device client, and
   code signing certificates was not covered in RFC8555.

   Identity proofing for these certificate types present some challenges
   for process automation.  NIST SP 800-63 r3 [NIST800-63r3] serves as
   guidance for identity proofing further detailed in NIST SP 800-63A
   [NIST800-63A] that may occur prior to the ability to automate
   certificate management via ACME or may obviate the need for it
   weighing end user privacy as a higher concern and allowing for
   credential issuance to be decoupled from identity proofing (IAL1).

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   Using this guidance, a CA might select from the identity proofing
   levels to assert claims on the issued certificates as follows from
   NIST SP 800-63 r3 [NIST800-63r3]:

   "IAL1: There is no requirement to link the applicant to a specific
   real-life identity.  Any attributes provided in conjunction with the
   authentication process are self-asserted or should be treated as such
   (including attributes a Credential Service Provider, or CSP, asserts
   to an RP).

   IAL2: Evidence supports the real-world existence of the claimed
   identity and verifies that the applicant is appropriately associated
   with this real-world identity.  IAL2 introduces the need for either
   remote or physically-present identity proofing.  Attributes can be
   asserted by CSPs to RPs in support of pseudonymous identity with
   verified attributes.

   IAL3: Physical presence is required for identity proofing.
   Identifying attributes must be verified by an authorized and trained
   representative of the CSP.  As with IAL2, attributes can be asserted
   by CSPs to RPs in support of pseudonymous identity with verified

   The certificate issuing CA may make this choice by certificate type
   issued.  Once identity proofing has been performed, in cases where
   this is part of the process, and certificates have been issued, NIST
   SP 800-63 r3 [NIST800-63r3] has the following recommendations for
   authentication or in the context of ACME, management of issuance for
   subsequent client, device, or code-signing certificates:

   "For services in which return visits are applicable, a successful
   authentication provides reasonable risk-based assurances that the
   subscriber accessing the service today is the same as that which
   accessed the service previously.  The robustness of this confidence
   is described by an AAL categorization.  NIST SP 800-63 B
   [NIST800-63B] addresses how an individual can securely authenticate
   to a CSP to access a digital service or set of digital services.  SP
   800-63B contains both normative and informative material.

   The three AALs define the subsets of options agencies can select
   based on their risk profile and the potential harm caused by an
   attacker taking control of an authenticator and accessing agencies?
   systems.  The AALs are as follows:

   AAL1: AAL1 provides some assurance that the claimant controls an
   authenticator bound to the subscriber?s account.  AAL1 requires
   either single-factor or multi-factor authentication using a wide
   range of available authentication technologies.  Successful

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   authentication requires that the claimant prove possession and
   control of the authenticator through a secure authentication

   AAL2: AAL2 provides high confidence that the claimant controls
   authenticator(s) bound to the subscriber?s account.  Proof of
   possession and control of two distinct authentication factors is
   required through secure authentication protocol(s).  Approved
   cryptographic techniques are required at AAL2 and above.

   AAL3: AAL3 provides very high confidence that the claimant controls
   authenticator(s) bound to the subscriber?s account.  Authentication
   at AAL3 is based on proof of possession of a key through a
   cryptographic protocol.  AAL3 authentication SHALL use a hardware-
   based authenticator and an authenticator that provides verifier
   impersonation resistance; the same device MAY fulfill both these
   requirements.  In order to authenticate at AAL3, claimants SHALL
   prove possession and control of two distinct authentication factors
   through secure authentication protocol(s).  Approved cryptographic
   techniques are required."

   If federations and assertions are used for authorizing certificate
   issuance, NIST SP 800-63 C [NIST800-63C] may be referenced for
   guidance on levels of assurance.

   Existing PKI certification authorities (CAs) tend to use a set of ad
   hoc protocols for certificate issuance and identity verification.
   For each certificate usage type, a basic process will be described to
   obtain an initial certificate and for the certificate renewal
   process.  If higher assurance levels are desired, the guidance from
   NIST SP 800-63 r3 [NIST800-63r3] may be useful and out-of-band
   identity proofing options are possible options for pre-authorization
   challenges or notifications.

3.  Key Storage

   [The following text may be left out in the next revision as it is
   decoupled already: A design goal for the automated workflow for these
   certificate types via ACME is to allow for use of the Key Management
   Interoperability Protocol (KMIP) for key management and storage or
   PKCS-11 for key storage.  In the case of KMIP, the KMIP enterprise
   key manager could use ACME to communicate with the CA server, leaving
   the device communications between devices and the KMIP server.
   However, the use of ACME can be standalone integrating with the
   available client key storage method (for example, PKCS-#11) provided
   for accessibility and to prevent cost barriers for automating key
   management for some implementations.  The ACME client on the device
   or system storing the code signing certificate would authenticate to

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   the CA running an ACME server to obtain initial certificates or renew
   certificates.  With the proliferation of open source implementations
   of ACME for TLS server certificates, this seems like a reasonable

4.  Why Not EST

   [These discussions have happened already for the core ACME protocol,
   expect this to be removed for the next version:

   Enrollment over Secure Transport (EST) [RFC7030] and OpenStack's
   Keystone are options for automating client certificates.  [OpenSSL
   can be combined with libest to automate the management of client

   The authentication options used in EST to obtain a client certificate
   are described in [RFC7030] Section 2.2 and are stated as follows:

      TLS with a previously issued client certificate (e.g., an existing
      certificate issued by the EST CA);

      TLS with a previously installed certificate (e.g., manufacturer-
      installed certificate or a certificate issued by some other

      Certificate-less TLS (e.g., with a shared credential distributed

      HTTP-based with a username/password distributed out-of-band.

   Although a fine a protocol, none of these options enable the protocol
   to establish authentication of the entity (device, user, owner of
   code signing certificate) without a pre-established and external
   process to the protocol.  In some cases, higher levels of assertion
   are necessary and EST may be more suited for those purposes or
   additional out-of-band processing could be used in conjunction with
   ACME if adopted widely for the automation of client certificate

5.  Device Certificates

   A device certificate is a client certificate issued to a device
   identified through device credentials such as an IP address,
   hostname, or MAC address.  This process is separate from an end user
   client certificate that may be stored on a device, but identifies a
   person using the device described in the next subsection.  While
   there are automated processes in place today for device certificate
   renewal, most are specific to the CA and not open standards.  The

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   general workflow is similar to that described in RFC8555 with the
   differences being in the CSR, requesting a client certificate.  [IP
   addresses may be necessary for some devices and it may be best to
   extend [I-D.ietf-acme-ip] to cover varying CSR types that include
   client certificates for devices explicitly.]

   A typical process to obtain a device certificate may be similar to
   the following workflow described in the introduction of RFC8555 with
   the exception of certificate type and usage.

   [Is an additional type definition helpful to distinguish that this is
   for a client certificate?]

6.  End USer Client Certificates

   [Should this be done in ACME?  I'm leaning towards no.]

   A client certificate used to authenticate an end user may be used for
   mutual authentication in TLS or another example would be with EAP-
   TLS.  The client certificate in this case may be stored in a browser,
   PKCS-#11 container, KMIP, or another key container.  To obtain an end
   user client certificate, there are several possibilities to automate
   authentication of an identity credential presumably tied to an end

   [Several authentication options are intentionally provided for review
   and discussion by the ACME working group.]

   A trusted federated service that ties the user to an email address
   with a reputation of the user attached to the email may be possible.
   One such example might be the use of a JWT signed OAuth token.

   Risk based authentication used for identity proofing with red herring
   questions is a third option that could utilize public information on
   individuals to authenticate.

   Just use FIDO and don't create anything new.  FIDO provides a
   mechanism to have unique certificate based access for client
   authentication to web sites and they are working on non-web.
   Identity proofing is intentionally decoupled from authentication in
   this model as that is in line with NIST 800-63r3 recommendations for
   privacy protections of the user.  The credential in this case is
   authenticated and would be consistent for it's use, but the identity
   proofing for that credential is not performed.  Obviously, identity
   proofing is more important for some services, like financial
   applications where tying the user to the identity for access to
   financial information is important.  However, is automated identity
   proofing important for any user certificate or should it remain

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   decoupled where it could be automated by a service offering or is
   there a need for a standardized mechanism to support it for user

7.  CodeSigning Certificates

   The process to retrieve a code signing certificate is similar to that
   of a web server certificate, with differences primarily in the CSR
   request and the resulting certificate properties.  [The storage and
   access of a code signing certificate must be protected and is
   typically done through hardware, a hardware security module (HSM)
   which likely has a PKCS#11 interface.  A code signing certificate may
   either be a standard one or an extended validation (EV) certificate.]

   [For automation purposes, the process described in this document will
   follow the standard process and any out-of-band preprocessing can
   increase the level of the issued certificate if the CA offers such
   options and has additional identity proofing mechanisms (in band or

   Strict vetting processes are necessary for many code signing
   certificates to provide a high assurance on the signer.  In some
   cases, issuance of a standard CodeSigning certificate will be
   appropriate and no additional "challenges" [RFC8555 Section 8] will
   be necessary.  In this case, the standard option could be automated
   very similar to Web server certificates with the only changes being
   in the CSR properties.  However, this may not apply to all scenarios,
   such as those requiring EV certificates with the possibility for
   required out-of-band initial authentication.

   Organization validation is required for standard code signing
   certificates from most issuers.  The CSR is used to identify the
   organization from the included domain name in the request.  The
   resulting certificate, however, instead contains the organization's
   name and for EV certificates, other identifying information for the
   organization.  For EV certificates, this typically requires that the
   domain is registered with the Certificate Authority provider, listed
   in CAA [RFC6844], and administrators for the account are named with
   provided portal access for certificate issuance and management

   While ACME allows for the client to directly establish an account
   with a CA, an initial process for this step may assist with the
   additional requirements for EV certificates and assurance levels
   typically required for code signing certificates.  For standard
   certificates, with a recommendation for additional vetting through
   extended challenge options to enable ACME to establish the account
   directly.  In cases where code signing certificates are used heavily

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   for an organization, having the portal access accessible replaced
   with ACME authenticated client access with extra challenges for
   authentication may be an option to automate the functionality.

   To improve the vetting process, ACME's optional use of CAA [RFC6844]
   with the Directory "meta" data "caaIdentities" ([RFC8555]
   Section 9.7.6) assists with the validation that a CA may have issue
   certificates for any particular domain and is RECOMMENDED for use
   with code signing certificates for this additional level of
   validation checking on issued certificates.

   CAA helps as anyone verifying a certificate used for code signing can
   verify that the CA used has been authorized to issue certificates for
   that organization.  CSR requests for code signing certificates
   typically contain a Common Name (CN) using a domain name that is
   replaced with the organization name to have the expected details
   displayed in the resulting certificate.  Since this work flow already
   occurs, there is a path to automation and validation via an existing
   ACME type, "dns".

   As noted in RFC8555, "the external account binding feature (see
   Section 7.3.4) can allow an ACME account to use authorizations that
   have been granted to an external, non-ACME account.  This allows ACME
   to address issuance scenarios that cannot yet be fully automated,
   such as the issuance of "Extended Validation" certificates."

   The ACME challenge object, [RFC8555] Section 7.1.5 is RECOMMENDED for
   use for Pre-authorization ([RFC8555] Section 7.4.1).

   Questions for reviewers:

   [Is there interest to set a specific challenge object for CodeSigning
   Certificates?  Or should this be left to individual CAs to decide and
   differentiate?  The current challenge types defined in RFC8555
   include HTTPS (provisioning HTTP resources) and DNS (provisioning a
   TXT resource record).  Use of DNS may be possible, but the HTTP
   resource doesn't necessarily make sense.  Since the process to
   retrieve an EV CodeSigning certificate usually requires proof of the
   organization and validation from one of 2 named administrators, SMS
   or email may be needed as defined challenge types.  AN organization
   may want to tie this contact to a role rather than a person and that
   consideration should be made in the design as well as implementation
   by organizations.]

   ACME provides an option for notification of the operator via email or
   SMS upon issuance/renewal of a certificate after the domain has been
   validated as owned by the requestor.  This option is RECOMMENDED due
   to the security considerations of code signing certificates as a way

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   to limit or reduce the possibility of a third party gaining access to
   a code signing certificate inappropriately.  [Development of
   additional challenge types is likely to support this for pre-
   authorization, which would better match the security considerations
   for this certificate type.]

   Since DNS is used to identify the organization in the request, the
   identifier "type" ([RFC8555]Section 7.4) is set to dns, not requiring
   any additions to the ACME protocol for this type of certificate.  The
   distinction lies in the CSR, where the values are set to request a
   CodeSigning certificate for a client certificate.  [Question: Is it
   helpful to define an identifier for the administrator or for the
   developer to distinguish the certificate type in ACME and not just
   the CSR?]

   KeyUsage (DigitalSignature) and ExtendedKeyUsage (CodeSigning) in the
   CSR MUST be set to the correct values for the CA to see the request
   is for a Code Signing certificate.  The Enhanced Key Usage SHOULD be
   set to show this is a client certificate., using OID
   "".  The CN MUST be set to the expected registered
   domain with the CA account.

   An advantage of ACME is the ability to automate rollover to allow for
   easy management of short expiry times on certificates.  The lifetime
   of CodeSigning certificates is typically a year or two, but
   automation could allow for shorter expiry times becoming feasible.

   Automation of storage to an HSM, which typically requires
   authentication is intentionally left out-of-scope.

8.  Pre-authorization

   Additional challenge types are defined here for the verification of
   administrors at an organization requesting CodeSigning certificates.
   SMS and email are both defined and may be used singularly or in
   combination as the ACME protocol allows for multiple pre-
   authorization challenges to be issued.


9.  Security Considerations

   This will likely be full of considerations and is TBD for revision

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10.  IANA Considerations

   This memo includes no request to IANA, yet.

11.  Contributors

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,

12.2.  Informative References

              Shoemaker, R., "ACME IP Identifier Validation Extension",
              draft-ietf-acme-ip-05 (work in progress), February 2019.

12.3.  URL References

              US National Institute of Standards and Technology,

              US National Institute of Standards and Technology,

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              US National Institute of Standards and Technology,

              US National Institute of Standards and Technology,

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Appendix A.  Change Log

   Note to RFC Editor: if this document does not obsolete an existing
   RFC, please remove this appendix before publication as an RFC.

Appendix B.  Open Issues

   Note to RFC Editor: please remove this appendix before publication as
   an RFC.

Author's Address

   Kathleen M. Moriarty
   Dell EMC
   176 South Street


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