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IETF RFC 8649
Last modified on Monday, August 26th, 2019
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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/RFC 8649.
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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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 [RFC 5280]. 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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
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1.2. ASN.1
Certificates [RFC 5280] 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 [RFC 5280]. 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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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
[RFC 5912].
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 [RFC 5280].
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 [RFC 4210]. 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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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 [RFC 5280] 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 [RFC 5280] 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 [RFC 5272] 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 [RFC 5280] 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 [RFC 4270]. 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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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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210,
DOI 10.17487/RFC 4210, September 2005,
<https://www.rfc-editor.org/info/RFC 4210>.
[RFC 4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270,
DOI 10.17487/RFC 4270, November 2005,
<https://www.rfc-editor.org/info/RFC 4270>.
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RFC 8649 Hash Of Root Key Extension August 2019
[RFC 5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, DOI 10.17487/RFC 5272, June 2008,
<https://www.rfc-editor.org/info/RFC 5272>.
[RFC 5280] 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/RFC 5280, May 2008,
<https://www.rfc-editor.org/info/RFC 5280>.
[RFC 5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
DOI 10.17487/RFC 5912, June 2010,
<https://www.rfc-editor.org/info/RFC 5912>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[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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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
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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
RFC TOTAL SIZE: 22410 bytes
PUBLICATION DATE: Monday, August 26th, 2019
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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