Network Working Group R. Housley
Internet-Draft Vigil Security
Intended status: Informational January 03, 2019
Expires: July 7, 2019
Hash Of Root Key Certificate Extension
draft-ietf-lamps-hash-of-root-key-cert-extn-03
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, replacing the current one.
Status of This Memo
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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 . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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 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 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 intends; 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 potential Root CA
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certificate is not a valid replacement, and the recipient continues
to use the current Root CA certificate.
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 [RFC2510]. 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. Further, this
technique avoids the need for all relying parties to make the
transition at the same time.
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
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that module fails, the Root CA remains committed to the now
unavailable key pair. The remedy is 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 loosing 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. The remedy is to
deploy a new public key and algorithm in the same manner as the
initial Root CA self-signed certificate, thus loosing 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 the
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 becomes 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
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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 the Root key pair for future use.
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 Jim Schaad, Stefan Santesson, and Paul Hoffman. 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>.
[RFC2510] Adams, C. and S. Farrell, "Internet X.509 Public Key
Infrastructure Certificate Management Protocols",
RFC 2510, DOI 10.17487/RFC2510, March 1999,
<https://www.rfc-editor.org/info/rfc2510>.
[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>.
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[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, 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
Housley Expires July 7, 2019 [Page 9]
