Network Working Group R. Housley Internet-Draft Vigil Security Intended status: Informational January 31, 2019 Expires: August 4, 2019 Hash Of Root Key Certificate Extension draft-ietf-lamps-hash-of-root-key-cert-extn-05 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 4, 2019. 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 Housley Expires August 4, 2019 [Page 1] Internet-Draft Hash Of Root Key Extension January 2019 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 . . . . . . . . . . . . . . . . . . . 6 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. Housley Expires August 4, 2019 [Page 2] Internet-Draft Hash Of Root Key Extension January 2019 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 Housley Expires August 4, 2019 [Page 3] Internet-Draft Hash Of Root Key Extension January 2019 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 Housley Expires August 4, 2019 [Page 4] Internet-Draft Hash Of Root Key Extension January 2019 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. 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, 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. Housley Expires August 4, 2019 [Page 5] Internet-Draft Hash Of Root Key Extension January 2019 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 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 Housley Expires August 4, 2019 [Page 6] Internet-Draft Hash Of Root Key Extension January 2019 pair. Of course, the second transition also causes the Root CA to generate another key pair that is reserved 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 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, . [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, . [RFC4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes in Internet Protocols", RFC 4270, DOI 10.17487/RFC4270, November 2005, . [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, . Housley Expires August 4, 2019 [Page 7] Internet-Draft Hash Of Root Key Extension January 2019 [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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. Housley Expires August 4, 2019 [Page 8] Internet-Draft Hash Of Root Key Extension January 2019 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 Housley Expires August 4, 2019 [Page 9] Internet-Draft Hash Of Root Key Extension January 2019 Author's Address Russ Housley Vigil Security 516 Dranesville Road Herndon, VA 20170 US Email: housley@vigilsec.com Housley Expires August 4, 2019 [Page 10]