    Internet Engineering Task Force (IETF)                        R. Housley
    Request for Comments: 8649                                Vigil Security
    Category: Informational                                      August 2019
    ISSN: 2070-1721

                     Hash Of Root Key Certificate Extension

    Abstract

       This document specifies the Hash Of Root Key certificate extension.
       This certificate extension is carried in the self-signed certificate
       for a trust anchor, which is often called a Root Certification
       Authority (CA) certificate.  This certificate extension unambiguously
       identifies the next public key that will be used at some point in the
       future as the next Root CA certificate, eventually replacing the
       current one.

    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 candidates for any level of Internet
       Standard; see Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8649.

    Copyright Notice

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

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

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    Table of Contents

       1. Introduction ....................................................2
          1.1. Terminology ................................................2
          1.2. ASN.1 ......................................................3
       2. Overview ........................................................3
       3. Hash Of Root Key Certificate Extension ..........................4
       4. IANA Considerations .............................................4
       5. Operational Considerations ......................................4
       6. Security Considerations .........................................6
       7. References ......................................................7
          7.1. Normative References .......................................7
          7.2. Informative References .....................................8
       Appendix A.  ASN.1 Module ..........................................9
       Acknowledgements ..................................................10
       Author's Address ..................................................10

    1.  Introduction

       This document specifies the Hash Of Root Key X.509 version 3
       certificate extension.  The extension is an optional addition to the
       Internet X.509 Public Key Infrastructure Certificate and Certificate
       Revocation List (CRL) Profile [RFC5280].  The certificate extension
       facilitates the orderly transition from one Root Certification
       Authority (CA) public key to the next.  It does so by publishing the
       hash value of the next-generation public key in the current self-
       signed certificate.  This hash value is a commitment to a particular
       public key in the next-generation self-signed certificate.  This
       commitment allows a relying party to unambiguously recognize the
       next-generation self-signed certificate when it becomes available,
       install the new self-signed certificate in the trust anchor store,
       and eventually remove the previous one from the trust anchor store.

       A Root CA certificate MAY include the Hash Of Root Key certificate
       extension to provide the hash value of the next public key that will
       be used by the Root CA.

    1.1.  Terminology

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

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    1.2.  ASN.1

       Certificates [RFC5280] use ASN.1 [X680]; Distinguished Encoding Rules
       (DER) [X690] are REQUIRED for certificate signing and validation.

    2.  Overview

       Before the initial deployment of the Root CA, the following are
       generated:

          R1 = The initial Root key pair
          R2 = The second-generation Root key pair
          H2 = Thumbprint (hash) of the public key of R2
          C1 = Self-signed certificate for R1, which also contains H2

       C1 is a self-signed certificate, and it contains H2 within the
       HashOfRootKey extension.  C1 is distributed as part of the initial
       system deployment.  The HashOfRootKey certificate extension is
       described in Section 3.

       When the time comes to replace the initial Root CA certificate, R1,
       the following are generated:

          R3 = The third-generation Root key pair
          H3 = Thumbprint (hash) the public key of R3
          C2 = Self-signed certificate for R2, which contains H3

       This is an iterative process.  That is, R4 and H4 are generated when
       it is time for C3 to replace C2, and so on.

       The successor to the Root CA self-signed certificate can be delivered
       by any means.  Whenever a new Root CA self-signed certificate is
       received, the recipient is able to verify that the potential Root CA
       certificate links back to a previously authenticated Root CA
       certificate with the HashOfRootKey certificate extension.  That is,
       the recipient verifies the signature on the self-signed certificate
       and verifies that the hash of the DER-encoded SubjectPublicKeyInfo
       from the potential Root CA certificate matches the value from the
       HashOfRootKey certificate extension of the current Root CA
       certificate.  Checking the self-signed certificate signature ensures
       that the certificate contains the subject name, public key algorithm
       identifier, and public key algorithm parameters intended by the key
       owner; these are important inputs to certification path validation as
       defined in Section 6 of [RFC5280].  Checking the hash of the
       SubjectPublicKeyInfo ensures that the certificate contains the
       intended public key.  If either check fails, then the potential Root
       CA certificate is not a valid replacement, and the recipient
       continues to use the current Root CA certificate.  If both checks

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       succeed, then the recipient adds the potential Root CA certificate to
       the trust anchor store.  As discussed in Section 5, the recipient can
       remove the current Root CA certificate immediately in some
       situations.  In other situations, the recipient waits an appropriate
       amount of time to ensure that existing certification paths continue
       to validate.

    3.  Hash Of Root Key Certificate Extension

       The HashOfRootKey certificate extension MUST NOT be critical.

       The following ASN.1 [X680] [X690] syntax defines the HashOfRootKey
       certificate extension:

       ext-HashOfRootKey EXTENSION ::= {    -- Only in Root CA certificates
          SYNTAX         HashedRootKey
          IDENTIFIED BY  id-ce-hashOfRootKey
          CRITICALITY    {FALSE} }

       HashedRootKey ::= SEQUENCE {
          hashAlg        HashAlgorithm,        -- Hash algorithm used
          hashValue      OCTET STRING }        -- Hash of DER-encoded
                                               --   SubjectPublicKeyInfo

       id-ce-hashOfRootKey  ::=  OBJECT IDENTIFIER { 1 3 6 1 4 1 51483 2 1 }

       The definitions of EXTENSION and HashAlgorithm can be found in
       [RFC5912].

       The hashAlg indicates the one-way hash algorithm that was used to
       compute the hash value.

       The hashValue contains the hash value computed from the next-
       generation public key.  The public key is the DER-encoded
       SubjectPublicKeyInfo as defined in [RFC5280].

    4.  IANA Considerations

       This document has no IANA actions.

    5.  Operational Considerations

       Guidance on the transition from one root key to another is available
       in Section 4.4 of [RFC4210].  Of course, a root key is also known as
       a trust anchor.  In particular, the oldWithNew and newWithOld advice
       ensures that relying parties are able to validate certificates issued
       under the current Root CA certificate and the next-generation Root CA
       certificate throughout the transition.  The notAfter field in the

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       oldWithNew certificate MUST cover the validity period of all
       unexpired certificates issued under the old Root CA private key.
       Further, this advice SHOULD be followed by Root CAs to avoid the need
       for all relying parties to make the transition at the same time.

       After issuing the newWithOld certificate, the Root CA MUST stop using
       the old private key to sign certificates.

       Some enterprise and application-specific environments offer a
       directory service or certificate repository to make certificate and
       CRLs available to relying parties.  Section 3 in [RFC5280] describes
       a certificate repository.  When a certificate repository is
       available, the oldWithNew and newWithOld certificates SHOULD be
       published before the successor to the current Root CA self-signed
       certificate is released.  Recipients that are able to obtain the
       oldWithNew certificate SHOULD immediately remove the old Root CA
       self-signed certificate from the trust anchor store.

       In environments without such a directory service or repository, like
       the Web PKI, recipients need a way to obtain the oldWithNew and
       newWithOld certificates.  The Root CA SHOULD include the subject
       information access extension [RFC5280] with the accessMethod set to
       id-ad-caRepository and the assessLocation set to the HTTP URL that
       can be used to fetch a DER-encoded "certs-only" (simple PKI response)
       message as specified in [RFC5272] in all of their self-signed
       certificates.  The Root CA SHOULD publish the "certs-only" message
       with the oldWithNew certificate and the newWithOld certificate before
       the subsequent Root CA self-signed certificate is released.  The
       "certs-only" message format allows certificates to be added and
       removed from the bag of certificates over time, so the same HTTP URL
       can be used throughout the lifetime of the Root CA.

       In environments without such a directory service or repository,
       recipients SHOULD keep both the old and replacement Root CA self-
       signed certificates in the trust anchor store for some amount of time
       to ensure that all end-entity certificates can be validated until
       they expire.  The recipient MAY keep the old Root CA self-signed
       certificate until all of the certificates in the local cache that are
       subordinate to it have expired.

       Certification path construction is more complex when the trust anchor
       store contains multiple self-signed certificates with the same
       distinguished name.  For this reason, the replacement Root CA self-
       signed certificate SHOULD contain a different distinguished name than
       the one it is replacing.  One approach is to include a number as part
       of the name that is incremented with each generation, such as
       "Example CA", "Example CA G2", "Example CA G3", and so on.

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       Changing names from one generation to another can lead to confusion
       when reviewing the history of a trust anchor store.  To assist with
       such review, a recipient MAY create an audit entry to capture the old
       and replacement self-signed certificates.

       The Root CA must securely back up the yet-to-be-deployed key pair.
       If the Root CA stores the key pair in a hardware security module and
       that module fails, the Root CA remains committed to the key pair that
       is no longer available.  This leaves the Root CA with no alternative
       but to deploy a new self-signed certificate that contains a newly
       generated key pair in the same manner as the initial self-signed
       certificate, thus losing the benefits of the Hash Of Root Key
       certificate extension altogether.

    6.  Security Considerations

       The security considerations from [RFC5280] apply, especially the
       discussion of self-issued certificates.

       The Hash Of Root Key certificate extension facilitates the orderly
       transition from one Root CA public key to the next by publishing the
       hash value of the next-generation public key in the current
       certificate.  This allows a relying party to unambiguously recognize
       the next-generation public key when it becomes available; however,
       the full public key is not disclosed until the Root CA releases the
       next-generation certificate.  In this way, attackers cannot begin to
       analyze the public key before the next-generation Root CA self-signed
       certificate is released.

       The Root CA needs to ensure that the public key in the next-
       generation certificate is as strong or stronger than the key that it
       is replacing.  Of course, a significant advance in cryptoanalytic
       capability can break the yet-to-be-deployed key pair.  Such advances
       are rare and difficult to predict.  If such an advance occurs, the
       Root CA remains committed to the now broken key.  This leaves the
       Root CA with no alternative but to deploy a new self-signed
       certificate that contains a newly generated key pair, most likely
       using a different signature algorithm, in the same manner as the
       initial self-signed certificate, thus losing the benefits of the Hash
       Of Root Key certificate extension altogether.

       The Root CA needs to employ a hash function that is resistant to
       preimage attacks [RFC4270].  A first-preimage attack against the hash
       function would allow an attacker to find another input that results
       in the hash value of the next-generation public key that was
       published in the current certificate.  For the attack to be
       successful, the input would have to be a valid SubjectPublicKeyInfo
       that contains a public key that corresponds to a private key known to

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       the attacker.  A second-preimage attack becomes possible once the
       Root CA releases the next-generation public key, which makes the
       input to the hash function available to the attacker and everyone
       else.  Again, the attacker needs to find a valid SubjectPublicKeyInfo
       that contains the public key that corresponds to a private key known
       to the attacker.  If the employed hash function is broken after the
       Root CA publishes the self-signed certificate with the HashOfRootKey
       certificate extension, an attacker would be able to trick the
       recipient into installing the incorrect next-generation certificate
       in the trust anchor store.

       If an early release of the next-generation public key occurs and the
       Root CA is concerned that attackers were given too much lead time to
       analyze that public key, then the Root CA can transition to a freshly
       generated key pair by rapidly performing two transitions.  After the
       first transition, the Root CA is using the key pair that suffered the
       early release, and that transition causes the Root CA to generate the
       subsequent Root key pair.  The second transition occurs when the Root
       CA is confident that the population of relying parties has completed
       the first transition, and it takes the Root CA to the freshly
       generated key pair.  Of course, the second transition also causes the
       Root CA to generate another key pair that is reserved for future use.
       Queries for the CRLs associated with certificates that are
       subordinate to the self-signed certificate can give some indication
       of the number of relying parties that are still actively using the
       self-signed certificates.

    7.  References

    7.1.  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,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
                  "Internet X.509 Public Key Infrastructure Certificate
                  Management Protocol (CMP)", RFC 4210,
                  DOI 10.17487/RFC4210, September 2005,
                  <https://www.rfc-editor.org/info/rfc4210>.

       [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
                  Hashes in Internet Protocols", RFC 4270,
                  DOI 10.17487/RFC4270, November 2005,
                  <https://www.rfc-editor.org/info/rfc4270>.

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       [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
                  (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
                  <https://www.rfc-editor.org/info/rfc5272>.

       [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, DOI 10.17487/RFC5280, May 2008,
                  <https://www.rfc-editor.org/info/rfc5280>.

       [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
                  Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
                  DOI 10.17487/RFC5912, June 2010,
                  <https://www.rfc-editor.org/info/rfc5912>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

       [X680]     ITU-T, "Information technology -- Abstract Syntax Notation
                  One (ASN.1): Specification of basic notation",
                  ITU-T Recommendation X.680, August 2015.

       [X690]     ITU-T, "Information Technology -- ASN.1 encoding rules:
                  Specification of Basic Encoding Rules (BER), Canonical
                  Encoding Rules (CER) and Distinguished Encoding Rules
                  (DER)", ITU-T Recommendation X.690, August 2015.

    7.2.  Informative References

       [SET]      MasterCard and VISA, "SET Secure Electronic Transaction
                  Specification -- Book 2: Programmer's Guide, Version 1.0",
                  May 1997.

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    Appendix A.  ASN.1 Module

       The following ASN.1 module provides the complete definition of the
       HashOfRootKey certificate extension.

       <CODE BEGINS>

       HashedRootKeyCertExtn { 1 3 6 1 4 1 51483 0 1 }

       DEFINITIONS IMPLICIT TAGS ::=
       BEGIN

       -- EXPORTS All

       IMPORTS

       HashAlgorithm
         FROM PKIX1-PSS-OAEP-Algorithms-2009  -- RFC 5912
              { iso(1) identified-organization(3) dod(6) internet(1)
                security(5) mechanisms(5) pkix(7) id-mod(0)
                id-mod-pkix1-rsa-pkalgs-02(54) }

       EXTENSION
         FROM PKIX-CommonTypes-2009  -- RFC 5912
           { iso(1) identified-organization(3) dod(6) internet(1)
             security(5) mechanisms(5) pkix(7) id-mod(0)
             id-mod-pkixCommon-02(57) } ;

       --
       -- Expand the certificate extensions list in RFC 5912
       --

       CertExtensions EXTENSION ::= {
          ext-HashOfRootKey, ... }

       --
       -- HashOfRootKey Certificate Extension
       --

       ext-HashOfRootKey EXTENSION ::= {    -- Only in Root CA certificates
          SYNTAX         HashedRootKey
          IDENTIFIED BY  id-ce-hashOfRootKey
          CRITICALITY    {FALSE} }

       HashedRootKey  ::=  SEQUENCE {
          hashAlg        HashAlgorithm,     -- Hash algorithm used
          hashValue      OCTET STRING }     -- Hash of DER-encoded
                                            --   SubjectPublicKeyInfo

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       id-ce-hashOfRootKey OBJECT IDENTIFIER  ::=  { 1 3 6 1 4 1 51483 2 1 }

       END

       <CODE ENDS>

    Acknowledgements

       The Secure Electronic Transaction (SET) [SET] specification published
       by MasterCard and VISA in 1997 includes a very similar certificate
       extension.  The SET certificate extension has essentially the same
       semantics, but the syntax fairly different.

       CTIA - The Wireless Association - is developing a public key
       infrastructure that will make use of the certificate extension
       described in this document; the object identifiers used in the ASN.1
       module were assigned by CTIA.

       Many thanks to Stefan Santesson, Jim Schaad, Daniel Kahn Gillmor,
       Joel Halpern, Paul Hoffman, Rich Salz, and Ben Kaduk.  Their reviews
       and comments greatly improved the document, especially the
       "Operational Considerations" and "Security Considerations" sections.

    Author's Address

       Russ Housley
       Vigil Security
       516 Dranesville Road
       Herndon, VA  20170
       United States of America

       Email: housley@vigilsec.com

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