Hash Of Root Key Certificate Extension
draft-ietf-lamps-hash-of-root-key-cert-extn-04

Network Working Group                                         R. Housley
Internet-Draft                                            Vigil Security
Intended status: Informational                          January 15, 2019
Expires: July 19, 2019

                 Hash Of Root Key Certificate Extension
             draft-ietf-lamps-hash-of-root-key-cert-extn-04

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

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

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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .   8
   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 Hashed 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] are generated using ASN.1 [X680]; certificates
   are always encoded with the Distinguished Encoding Rules (DER)
   [X690].

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
   the 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

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   continues to use the current Root CA certificate.  If both checks
   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        AlgorithmIdentifier,  -- 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 DER-encoded
   SubjectPublicKeyInfo as defined in [RFC5280].

4.  IANA Considerations

   This document makes no requests of the IANA.

5.  Operational Considerations

   Guidance on the transition from one trust anchor to another is
   available in Section 4.4 of [RFC4210].  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

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   transition.  The notAfter field in the 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 oldWithNew and newWithOld certificates, the Root CA
   MUST stop using the old private key to sign certificates.

   In enterprise and application-specific environments where a directory
   service or 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.  In
   environments without such a directory service or repository,
   recipients SHOULD keep both the old and replacement Root CA self-
   signed certificate 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 multiple self-
   signed certificates in the trust anchor store have 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.

   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.

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   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
   published hash value.  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 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 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.  The first
   transition takes the Root CA to the key pair that suffered the early
   release, and it 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 have 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.

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7.  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.

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

8.  References

8.1.  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>.

   [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>.

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   [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, 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, 2015.

8.2.  Informative References

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

Appendix A.  ASN.1 Module

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

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

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- EXPORTS All

   IMPORTS

   AlgorithmIdentifier{}, DIGEST-ALGORITHM
     FROM AlgorithmInformation-2009  -- [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-algorithmInformation-02(58) }

   EXTENSION
     FROM PKIX-CommonTypes-2009
       { 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 [RFC5912]
   --

   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        HashAlgorithmId,   -- Hash algorithm used
      hashValue      OCTET STRING }     -- Hash of DER-encoded
                                        --   SubjectPublicKeyInfo

   HashAlgorithmId  ::=  AlgorithmIdentifier {DIGEST-ALGORITHM,{ ... }}

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

   END

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Author's Address

   Russ Housley
   Vigil Security
   516 Dranesville Road
   Herndon, VA  20170
   US

   Email: housley@vigilsec.com

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