





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.



Housley                       Informational                     [Page 1]

RFC 8649               Hash Of Root Key Extension            August 2019


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.







Housley                       Informational                     [Page 2]

RFC 8649               Hash Of Root Key Extension            August 2019


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



Housley                       Informational                     [Page 3]

RFC 8649               Hash Of Root Key Extension            August 2019


   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



Housley                       Informational                     [Page 4]

RFC 8649               Hash Of Root Key Extension            August 2019


   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.




Housley                       Informational                     [Page 5]

RFC 8649               Hash Of Root Key Extension            August 2019


   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



Housley                       Informational                     [Page 6]

RFC 8649               Hash Of Root Key Extension            August 2019


   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>.





Housley                       Informational                     [Page 7]

RFC 8649               Hash Of Root Key Extension            August 2019


   [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.


















Housley                       Informational                     [Page 8]

RFC 8649               Hash Of Root Key Extension            August 2019


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



Housley                       Informational                     [Page 9]

RFC 8649               Hash Of Root Key Extension            August 2019


   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



















Housley                       Informational                    [Page 10]

