ACME End User Client and Code Signing Certificates

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Author Kathleen Moriarty 
Last updated 2019-05-30
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IETF                                                         K. Moriarty
Internet-Draft                                                  Dell EMC
Intended status: Standards Track                            May 30, 2019
Expires: December 1, 2019

           ACME End User Client and Code Signing Certificates


   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 1, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Identity Proofing for Client Certificates . . . . . . . . . .   2
   3.  Device Certificates . . . . . . . . . . . . . . . . . . . . .   4
   4.  End User Client Certificates  . . . . . . . . . . . . . . . .   5
   5.  CodeSigning Certificates  . . . . . . . . . . . . . . . . . .   6
   6.  Pre-authorization . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Challenge Types . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  One Time Password (OTP) . . . . . . . . . . . . . . . . .  10
     7.2.  Certificate . . . . . . . . . . . . . . . . . . . . . . .  10
     7.3.  FIDO or Public/Private Key Pairs  . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
     11.3.  URL References . . . . . . . . . . . . . . . . . . . . .  12
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  14
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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.

   The core ACME protocol defined challenge types specific to web server
   certificates with the possibility to create extensions, or additional
   challenge types for other use cases and certificate types.  Client
   certificates, such as end user and Code SIgning may also benefit from
   automated management to ease the deployment and maintenance of these
   certificates type, thus the definition of this extension defining
   challenge types specific to that usage.

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

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   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).
   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:

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   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
   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.  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
   general workflow is similar to that described in RFC8555 with the

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   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.

   [There is some work happening in possibly 2 different drafts on
   device certificates, so no further definition is provided here at
   this time.]

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

4.  End User Client Certificates

   A client certificate used to authenticate an end user may be used for
   mutual authentication in TLS, EAP-TLS, or messaging.  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

   [We need to determine if it is important in ACME to define an
   automated method that tests the identity or the user or to just have
   consistent credentials for the authentication challenges.  The
   credentials may be distributed through an out-of-band method that
   involves identity proofing.]

   [Several authentication options with identity proofing are
   intentionally provided for review and discussion by the ACME working

   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.  This would be similar to the signup
   process used in some financial applications.

   Existing credentials - for instance, FIDO.  FIDO uses a public key
   pair and does not perform identity proofing.  FIDO authentication
   provides a different key pair to each service using FIDO for

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   authentication, which are generated at the client and registered by
   the server.  This may require using the FIDO credentials from a
   specific service for authentication to gain ACME issued crededentials
   (not advised based on how FIDO credentials are supposed to be used).
   Are there instances where the same provider would issue both sets of
   credentials?  You wouldn't want to expose your FIDO credentials to a
   different party, that's why each service has their own.  Would you
   set up a mechanism to get FIDO credentials to then obtain a
   certificate?  (What use cases would this be necessary?  When do you
   need a certificate where you already have a specific public/private
   key pair?)  This can be defined as an auth type, but should it be?

   One-time password (OTP) authentication is a secure option.  In cases
   where a higher assurance level is needed, OTP may be a good choice
   and many options exist today for OTP that could use an app on a phone
   for instance tied to an existing (or newly established) password.
   The OTP may be tied to an out-of-band process and may be associated
   with a username/password and other accounts.

   One consideration is to understand if the use case could just use
   FIDO and not create anything new (ACME client certificates).  FIDO
   provides a mechanism to have unique public key pair 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
   decoupled where it could be automated by a service offering or is
   there a need for a standardized mechanism to support it for user

   Three methods for ACME client authentication, not identity proofing,
   are proposed in the Challenge Type Section.

5.  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.]

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   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 and identity proofing.

   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 out-of-band 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 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.

   [For standard certificates, is it worth defining SMS and email for
   the challenge?  Obviously, EV needs more, so a few choices are
   suggested in this revision.]

   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.

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   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).  Additional
   challenge types are added to provide higher levels of security for
   this issuance verification step.  The use of OTP, FIDO credentials
   (public/private key pairs), or validation from a certificate issued
   at account setup time are defined in Section 8.  Pre-Authoriziation.

   Questions for reviewers:

   [Is there interest to set a specific or default 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, some other challenge type like public/private key
   pairs or OTP 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
   to limit or reduce the possibility of a third party gaining access to
   a code signing certificate inappropriately.  Development of
   additional challenge types is included in this document to support
   this for pre-authorization, which would better match the security
   considerations for this certificate type.  Additional types may be
   added if agreed upon by the working group.

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   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.

6.  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.  Additional pre-authorization
   types are defined that provide a higher level of assurance to
   authorize a request.

7.  Challenge Types

   The challenge types are defined in the following subsections are for
   use to authenticate individuals or holders of specific pre-issued
   credentials (users acting in roles for an organization).  The
   challenge types can be used to obtain end user certificate types or
   as a pre-authorization challenges with certificate types such as the
   Code Signing Certificate.  Please note that the pre-authorization
   challenge is also coupled with the account certificate in ACME for
   verification.  The process for obtaining EV Code Signing Certificates
   typically requires authorization from one or more individuals in a
   role for the organization.  The use of pre-issued secure credentials,
   at an assurance level appropriate for the certificate type being
   issued, provides a way to automate the issuance and renewal process.

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7.1.  One Time Password (OTP)

   There are numerous one time password technologies with slight
   variations between implementations.  The response to the challenge is
   entered in the provided URL, offering flexibility to those using this
   challenge type to accomodate the specific requirements of their
   solution.  Looking at 2 OTP solutions, the challenge response is
   provided via a tool or app without any user interaction of
   information required from the server to generate the challenge.  The
   2 solutions that operate in this manner include SecureID and Duo
   Security.  If a challenge is required to generate the reponse to be
   provided in the URL, the token can supply the challenge.

      type (required, string): The string "otp-01".

      token (required, string): A random value that uniquely identifies
      the challenge.  OTP types and input vary between technologies.
      The token value will match the type expected for the pre-issued
      OTP credential.  The user will be able to supply a response in the
      provided URL from this challenge.  It MUST NOT contain any
      characters outside the base64url alphabet and MUST NOT include
      base64 padding characters ("=").

        "type": "otp-01",
        "url": "",
        "status": "pending",
        "token": "challenge"

7.2.  Certificate

   Certificates may be pre-issued and stored according to assurance
   level requirements for the purpose of identiying a user's identity.
   If a higher assurance level is needed for a user serving in a
   specific role or for that individual, it is posisble for identity
   proofing to occur in person using identifiers acceptable for the
   specified process and then stored appropriately for the required
   assurance level.  PKCS#11 software or hardware tokens are both
   possible options.  This model assumes that there may be multiple
   authorized users with different certificates that can be used for the
   authorization or pre-authentication challenge.  As such, the user
   first provides the digital signature, so the account management can
   determine if one of the acceptable certificates was used to digitally
   sign the token.

      type (required, string): The string "cert-01".

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      token (required, string): A random value that uniquely identifies
      the challenge.  The token for a certificate authentication
      challenge includes a value for the recipeint to digitally sign
      using their private key and post to the provided URL.  The ACME
      server then uses the digitally signed content to verify that the
      challenge was signed using authorized credentials (certificate
      issued and authorized for this challenge type).  It MUST NOT
      contain any characters outside the base64url alphabet and MUST NOT
      include base64 padding characters ("=").

        "type": "cert-01",
        "url": "",
        "status": "pending",
        "token": "Some challenge to digitally sign"

7.3.  FIDO or Public/Private Key Pairs

   FIDO uses public/private key pairs that are issued specific to a
   service.  If FIDO or public/private key pairs (PPKP) are selected as
   the challenge type, the account and credential issuance will have to
   occur prior to use of this challenge type.  The FIDO or public/
   private key pair credentials would be specific to the certificate
   management account and would be created by the client, then
   registered with the service as occurs with normal FIDO regisration of
   credentials.  As with normal FIDO and puiblic/private key pairs, the
   token or challenge is digitally signed to prove possession of the
   private key.

      type (required, string): The string "ppkp-01".

      token (required, string): A random value that uniquely identifies
      the challenge.  This challenge will operate much in the same way
      as the certificate challenge as the operations are largely the
      same.  The user will be able to supply a response in the provided
      URL from this challenge.  It MUST NOT contain any characters
      outside the base64url alphabet and MUST NOT include base64 padding
      characters ("=").

        "type": "ppkp-01",
        "url": "",
        "status": "pending",
        "token": "Some challenge to sign"

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8.  Security Considerations

   This will likely be full of considerations and is TBD for this
   revision until challenge types are settled.

9.  IANA Considerations

   This memo includes no request to IANA, yet.

10.  Contributors

11.  References

11.1.  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,

11.2.  Informative References

              Shoemaker, R., "ACME IP Identifier Validation Extension",
              draft-ietf-acme-ip-06 (work in progress), May 2019.

11.3.  URL References

              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,

              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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