Internet Engineering Task Force (IETF)
Request for Comments: 8649
Category: Informational
ISSN: 2070-1721

R. Housley
Vigil Security
August 2019

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

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.

   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.

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 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 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.

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 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>.
   
   [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.

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
   
   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