Limited Additional Mechanisms for PKIX and SMIME G. Wang, Ed. Internet-Draft Y. Yang Intended status: Standards Track Huawei Int. Pte Ltd Expires: 6 May 2027 J. Zhang Huawei Tech. Ltd 2 November 2026 Root CA Certificate Rekeying in the Scenario of Post Quantum Migration draft-wang-lamps-root-ca-cert-rekeying-05 Abstract In the public key infrastructures (PKIs), root certification authority (CA) certificate rekeying is crucial to guarantee business continuity. Two approaches are given in [RFC4210] for entities belonging to different generations to verify each other's certificate chain. However, these approaches rely on the assumption that the old entities can be updated. In this draft, we propose a One-way Link Certificate based solution such that old entities are transparent to root CA certificate rekeying. Namely, during the overlapping lifetime of two root CA certificates, without any update in old entities, old and new entities can verify each other's certificate chain smoothly. Furthermore, the proposed solution works in both traditional PKIs, and post-quantum (PQ) PKIs, where the certificates can be pure PQ or hybrid. 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 6 May 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Wang, et al. Expires 6 May 2027 [Page 1] Internet-Draft Root CA Cert Rekeying November 2026 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Notes of Change . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Changes in v05 . . . . . . . . . . . . . . . . . . . . . 2 1.2. Changes in v04 . . . . . . . . . . . . . . . . . . . . . 2 1.3. Changes in v03 . . . . . . . . . . . . . . . . . . . . . 3 1.4. Changes in v02 . . . . . . . . . . . . . . . . . . . . . 3 1.5. Changes in v01 . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Requirements Language . . . . . . . . . . . . . . . . . . . . 6 4. Design Goals of a New Solution for Root CA Rekeying . . . . . 6 5. The Proposed Solution Based on One-way Link Certificate . . . 7 6. Testing Results . . . . . . . . . . . . . . . . . . . . . . . 9 7. An Extension for Switching Root CA Certificate More Smoothly . . . . . . . . . . . . . . . . . . . . . . . . 12 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 8.1. Impact of Old Root CA Key Compromise . . . . . . . . . . 14 8.2. Legacy Cryptographic Constraints on the Link Certificate . . . . . . . . . . . . . . . . . . . . . . . 14 8.3. Path Length Validation Anomalies . . . . . . . . . . . . 15 8.4. Lifetime Alignment and Revocation . . . . . . . . . . . . 15 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 10. Normative References . . . . . . . . . . . . . . . . . . . . 15 11. Informative References . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 1. Notes of Change 1.1. Changes in v05 * Added security considerations in Section 8. * Editorial changes through the whole draft. 1.2. Changes in v04 * Editorial changes through the whole draft. Wang, et al. Expires 6 May 2027 [Page 2] Internet-Draft Root CA Cert Rekeying November 2026 1.3. Changes in v03 * Minor changes made from v02 to make the draft live. Plan to have a major update in the next version. 1.4. Changes in v02 * Added an extension that allows root CA certificate can be switched more smoothly in Section 7, by assuming that there is a maximum time difference (MTD) T between any two entities when one sending a certificate chain and the other receiving. 1.5. Changes in v01 * Added the testing results using OpenSSL library in Section 6. 2. Introduction In the public key infrastructures (PKIs), root certification authority (CA) certificate management is crucial to guarantee business continuity, as the CA certificate is the trust anchor for certificate chains, which establish the trust paths in PKIs from end entities to the trusted authority, i.e., the root CA. However, just like the certificates for end entities, the root CA certificate has a limited lifetime as well, which normally varies from a few years to several decades to satisfy various applications. Furthermore, while new end user certificates can be issued when old ones are nearly expiring, a new root CA certificate should be issued when the old root CA certificate has been used for about half of its lifetime. Here are the main reasons behind this practice. A root CA certificate is generally used to issue one or more subordinate CA (sub-CA) certificates (for different departments in a given organization, for example), and then each of such sub-CA certificates is used to issue lower level sub-CA certificates or end user certificates. The lifetime of end user certificates are expected to be around a given period, say 5 years (for some specific products, for example). According to [RFC4210], to issue end user certificates with validity of 5 years, the remaining lifetime of the sub-CA certificate must be 5 years or more, such that the lifetime of each end user certificate MUST be covered by that of the sub-CA certificate. However, to avoid frequently apply and manage multiple or even many sub-CA certificates, the lifetime of each sub-CA certificate could be set as 10 years, which means for every 5 years sub-CA certificates should be updated so that end certificates for new entities or products can still be continuously issued to guarantee business continuity. Similarly, for continously issuing Wang, et al. Expires 6 May 2027 [Page 3] Internet-Draft Root CA Cert Rekeying November 2026 sub-CA certificates with validity of 10 years without frequently changing the root CA certificate, it can be expected that each root CA certificate may have a lifetime of 20 years, which also implies that each root CA certificate should be updated for every 10 years, not nearly the expiry date of the root CA certificate. Each such new root CA certificate can be called a new generation of root CA certificate, though all of them still belong to the same legal entity, a particular CA. However, this also implies that different sub-CA certificates and end user certificates issued under different generations of root CA certificate co-exist during the overlapping period of those root CA certificates. So, it may not be easy to verify each other's certificate chain between entities that hold different generations of certificates issued by the same CA from a legal perspective but under different generations of root CA certificates. Say, for the above example, the sub-CA certificates and end user certificates issued under the first generation of root CA certificate may still be valid in the year of 12 when a given CA has been established, but the newly issued sub-CA certificates and end user certificates are actually under the second generation of root CA certificate. For simplicity, we call the owner of a sub-CA certificate or such an end user as an entity or a device. Moreover, a device or entity that holds certificate chain issued by an old generation of root CA certificate is called as an old device or entity, while a device or entity hold certificate chain issued by a new generation of root CA certificate is called as a new device or entity. These terms are used relatively. Actually, to address the above issue, there are two solutions given in [RFC4210]: * Old and new entities upload both old and new root CA certificates, * Or upload two-way link certificates which introduce old CA to new CA and vice versa. However, these two solutions do not work if old entities cannot upload the new root certificates or the necessary link certificate, or old devices are even out of maintenance. In fact, such worse cases are possible, due to either limited foresight in product design such that old devices may do not support adding multiple root CA or link certificates (automatically), or still running old devices are not maintained well and automatically update is not supported. Wang, et al. Expires 6 May 2027 [Page 4] Internet-Draft Root CA Cert Rekeying November 2026 Conceptually, there are two approaches to update root CA certificate, i.e., renewing and rekeying. The former means to extend the validity of an existing root CA certificate but the root CA key pair and the associated cryptographic algorithms are still the same. In this case, the validity of the same (old) key pair of the root CA will be extended for a longer period. So, attackers shall have longer time to cryptanalyze the same key pairs. Also, as time goes, the security strength of the associated cryptographical algorithm for the root CA certificate may become weaker and weaker. So, sooner or later, it still needs to issue a brand new CA certificate. For the latter, namely rekeying approach, a brand new root CA certificate will be issued to replace the old key pair. In this case, no just a new pair of keys, even different key length or new algorithm can be used for generating the new root CA certificate by considering the progress of cryptanalysis and potential security threats in the near future, like quantum computing. However, in this situation, a big challenge is to manage two or even multiple root CA certificates during the overlapping periods, which could be 20 years or more, as mentioned in the above. In particular, some old devices may not be able to install the new root CA or link certificates, such that the two solutions given in [RFC4210] do not work for such systems. Therefore, old devices may not be able to verify the new certificate chain from a new device, though a new device can install all necessary certificates and verify the old certificate chains from old devices. Motivated by the above observation, this draft proposes a One-way Link Certificate based solution such that root CA certificate rekeying is transparent to old entities: * During the overlapping period of two or multiple root CA certificates, without any update in old entities, old and new entities can verify each other's certificate chain smoothly. * The proposed solution works in the scenario of traditional PKIs, pure post-quantum PKIs, and also hybrid PKIs [RFC9794], as the rationale of the solution does not rely on the type of underlying cryptographic algorithms. * Essentially, the solution can be viewed as an extension of the approach for root CA certificate issuing and managnement specified in [RFC4210] and [RFC5280]. Wang, et al. Expires 6 May 2027 [Page 5] Internet-Draft Root CA Cert Rekeying November 2026 3. Requirements Language 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. 4. Design Goals of a New Solution for Root CA Rekeying Basically speaking, to design the root CA rekeying solution based on One-way Link Certificate, the following principles are followed to maximize the potential deployment of the solution in practice: * Requirements on old devices are minimized such that the solution is applicable to as many as possible scenarios. * New devices are supposed to do more as they are normally more powerful and designed with better prediction. * When possible, the usage of the new root CA keys will be maximized as normally they are associated with cryptographically stronger algorithms. * The existing standards [RFC4210] and [RFC5280] will be followed as much as possible, so that the deployment of the new solution for root CA rekeying can be implemented as easy as possible. In Section 6.1 of [RFC5280], the following is specified: "A prospective certification path (a sequence of n certificates) satisfies the following conditions: * (a) for all x in {1, ..., n-1}, the subject of certificate x is the issuer of certificate x+1; * (b) certificate 1 is issued by the trust anchor; * (c) certificate n is the certificate to be validated (i.e., the target certificate); and * (d) for all x in {1, ..., n}, the certificate was valid at the time in question. " Link certificates were introduced in Section 4.4.1 for specifying CA operator actions to rekey root CA certificate [RFC4210] as follows. " To change the key of the CA, the CA operator does the following: Wang, et al. Expires 6 May 2027 [Page 6] Internet-Draft Root CA Cert Rekeying November 2026 * 1. Generate a new key pair; * 2. Create a certificate containing the old CA public key signed with the new private key (the "old with new" certificate); * 3. Create a certificate containing the new CA public key signed with the old private key (the "new with old" certificate); * 4. Create a certificate containing the new CA public key signed with the new private key (the "new with new" certificate); * 5. Publish these new certificates via the repository and/or other means (perhaps using a CAKeyUpdAnn message); * 6. Export the new CA public key so that end entities may acquire it using the "out-of-band" mechanism (if required). The old CA private key is then no longer required. " However, as we mentioned in Section 1, the above solution specified in [RFC4210] assumes that (new and old) end entities may acquire the new CA public key (using the "out-of-band" mechanism, if needed), as the above item 6 depicts. 5. The Proposed Solution Based on One-way Link Certificate Here is the basic idea of the proposed root CA key rekeying based on One-way Link Certificate: * Use the One-way Link Certificate, called newWithOld, which is the link certificate of the new public key signed by the old private key of the root CA. * The newWithOld certifies the new root CA key by the old one. * So, during the overlapping period, old devices can verify a link certificate chain from a new device by using the old root CA certificate as the trust anchor. * Other cases are simple … Wang, et al. Expires 6 May 2027 [Page 7] Internet-Draft Root CA Cert Rekeying November 2026 Old Device A New Device C ******************* *************************** * oldCACet * * oldCACert newCACert * * ^ * * ^ ^ * * oldSubCACert * * newWithOld ^ * * ^ * * ^ ^ * * oldCertA * * newSubCAcert * ******************* * ^ * ^ * newCertC * | B sends its *************************** | link certificate ^ B sends either of its two | chain to A | certificate chains to C | | ***************************** * oldCACert newCACert * * ^ ^ * * newWithOld ^ * * ^ ^ * * newSubCACert * * ^ * * newCertB * ***************************** New Device B Figure 1. Illustration to the Solution Based on One-way Link Certificate. Figure 1 shows how a new device B can send out its link certificate chain, (oldCACert, newWithOld, newSubCACert, newCertB), to an old device A such that A can verify B's certificate chain by using the old root CA certificate, which is A's trust anchor. To respond to B, A just sends its old certificate chain, (oldCACert, oldSubCACert, oldCertA), to new device B, which can verify A's certificate chain by using the old root CA certificate, which is one of B's two trust anchors, namely, the old or new root CA certificate. To communicate with another new device C, new device B can send out either its link certificate chain, (oldCACert, newWithOld, newSubCACert, newCertB), or the new certificate chain, (newCAcert, newSubCACert, newCertB), to new device C, which can verify either of them by using one of its two trust anchors, namely, the old or new root CA certificate. In this case, as both B and C are new devices, C can do as B does such that B can verify either of C's two certificate chains similarly. The case of one old device communicates with another old device is simple, as they just behave as normal by sending their own old certificate chain to the other. Wang, et al. Expires 6 May 2027 [Page 8] Internet-Draft Root CA Cert Rekeying November 2026 More detailed description will be provided later. 6. Testing Results The proposed solution has been tested using OpenSSL library. Here are the testing environments: * 3 generations of PKIs with different root CA certificates.: G1 (2016-2025), G2 (2018-2027), and G3 (2020-2029). * G1 and G2 implement RSA, with RSA 4096 for root CA certificate, RSA 3092 for subject CA certificate, and RSA 2048 for end entity certificate. * G3 implements ECDSA, with ECDSA 384 for root CA certificate, and ECDSA 256 for both subject CA certificate and end entity certificate. * Cross verifying tests are experimented among implmentations of running OpenSSL 1.0.1c, OpenSSL 1.0.2u、OpenSSL 1.1.1g and JDK 8u251. More specicically, G1 implements RSA with all these four softeare packages, while G2 and G3 implement RSA and ECDA respectively in the last three software packages, but not OpenSSL 1.0.1c. This is becuase OpenSSL 1.0.1c is a relativley old version of openSSL, so new system may not employ it. Table 1 given below lists the life times of three generations of PKIs with different root CA certificates, namely, G1, G2 and G3. That is, the life time of the first generation PKI G1 is valid from 2016 to 2025. More specifically, the root CA certificate of G1 is valid from the first moment of the year of 2016 to the last moment of the year of 2025. Therefore, in 2015, G1 is invalid, namely, not issued yet. Similiarly, G1 is expired from the first moment of year 2026. The root CA certificate of the 2nd generation PKI G2 is valid from 2018 to 2027, while the root CA certificate of the 3rd generation PKI G3 is valid from 2020 to 2029. +====+=======+=======+=======+========+========+=======+=======+=======+ | | 2015 | 2016 | 2018 | 2020 | 2025 | 2026 | 2028 | 2030 | +----+-------+-------+-------+--------+--------+-------+-------+-------+ | G1 |Invalid| Valid | Valid | Valid | Valid |Expired|Expired|Expired| +----+-------+-------+-------+--------+--------+-------+-------+-------+ | G2 |Invalid|Invalid| Valid | Valid | Valid | Valid |Expired|Expired| +----+---------------+-------+--------+--------+-------+-------+-------+ | G3 |Invalid|Invalid|Invalid| Valid | Valid | Valid | Valid |Expired| +----+-------+-------+-------+--------+--------+-------+-------+-------+ Table 1: Life Times of 3 Generations of Root CA Certificate Wang, et al. Expires 6 May 2027 [Page 9] Internet-Draft Root CA Cert Rekeying November 2026 The testing results for eight selected years from 2015 to 20230 are given in the following four tables, namely, from Table 2 to Table 5. The ruslts show positive answers for all the cases considered, as explanied below in more detail. These tables show all representive cases needed to be considered. That is, for all three generateions of PKIs G1, G2 and G3, we have selected 8 years from 2015 to 2030 to include all possible combinations of some of PKIs G1, G2, and G3 are not issued, valid and/or expired. The complete list of these seven combinations is given as following: (G1 invalid, G2 invalid, G3 invaid), (G1 valid, G2 invalid, G3 invaid), (G1 valid, G2 valid, G3 invaid), (G1 valid, G2 valid, G3 vaid), (G1 expired, G2 valid, G3 vaid), (G1 expired, G2 expired, G3 vaid), and (G1 expired, G2 expired, G3 expired). Note that other combinations in theory do not exist in practice, like (G1 valid, G2 expired, G3 vaid), as in the above test G1, G2, and G3 are issued and expired gradually in this order. This means that it is not possible for G1 valid and G2 expired. From Table 1, we actually can see that only for case (G1 valid, G2 valid, G3 vaid), twe years, namely 2020 and 2025, are selected. This is because 2020 is the year when G3 is issued, while 2025 is the year when G1 is in service for the last year. If fact, these eight years selected include all such critical years for these three generations of PKIs, namely the years when each G1, G2 or G3 is issued, and the years when each G1, G2 or G3 is in service for the last year. +=========+=============================+=============================+ | | 2015 | 2016 | +---------+---------+---------+---------+---------+---------+---------+ | |G1 Client|G2 Client|G3 Client|G1 Client|G2 Client|G3 Client| +---------+---------+---------+---------+---------+---------+---------+ | |Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.| +---------+---------+---------+---------+---------+---------+---------+ |G1 Server| - | - | - | - | - | - | OK | OK | - | - | - | - | +---------+---------+---------+---------+---------+---------+---------+ |G2 Server| - | - | - | - | - | - | - | - | - | - | - | - | +---------+-----------------------------+-----------------------------+ |G3 Server| - | - | - | - | - | - | - | - | - | - | - | - | +=========+=========+=========+=========+=========+=========+=========+ Table 2: Testing Results for 2015 and 2016 Each table from Table 2 Table 5 gives the testing rsults what a client and a server output for verifying the certificate chain of the other, for each case where during each of the eight years selected, both the server and the client can be any of G1, G2 or G3 server and client. Namely, these servers and clients hold the certificate chains issued by the respective root CA, together with the one-way Wang, et al. Expires 6 May 2027 [Page 10] Internet-Draft Root CA Cert Rekeying November 2026 link certifates NewWithOld which link the current root CA public key to the previous one or two old root CA public keys. The verification results from the client and the server are listed in the colums of "Cli." and "Ser.", respectivley. "-" means that such a case actually doest not exist, while "OK" means that such a verification of the other peer's certificate chain is possitive, and "No" means that such a verification of the other peer's certificate chain is negative as one or more certificates in such a certificate chain are expired already. As the first example, in Table 2, the "-" shown in the six cells of the fifth row under "2016" mean that in the year of 2016, when a G2 server interacts with a G1, G2 or G3 client, they cannot verify and accept each other's certificate chains, as such a G2 or G3 server, G2 or G3 client should not exist. As the second example, in Table 4, the "NO" shown in the six cells of the fourth row under "2026" mean that in the year of 2026, when a G1 server interacts with a G1, G2 or G3 client, they cannot verify and accept each other's certificate chains, as the root CA certificate of PKI G1 is expired already. As the third example, in Table 3, the "OK" shown in the six cells from the fourth row to the sixth row under "2020" mean that in the year of 2020, when any G1, G2 or G3 server interacts with any G1, G2 or G3 client, both the server and the client can verify and accept each other's certificate chains, which includes the One-way Link Certificates specified in this draft. +=========+=============================+=============================+ | | 2018 | 2020 | +---------+---------+---------+---------+---------+---------+---------+ | |G1 Client|G2 Client|G3 Client|G1 Client|G2 Client|G3 Client| +---------+---------+---------+---------+---------+---------+---------+ | |Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.| +---------+---------+---------+---------+---------+---------+---------+ |G1 Server| OK | OK | OK | OK | - | - | OK | OK | OK | OK | OK | OK | +---------+---------+---------+---------+---------+---------+---------+ |G2 Server| OK | OK | OK | OK | - | - | OK | OK | OK | OK | OK | OK | +---------+---------+---------+---------+---------+---------+---------+ |G3 Server| - | - | - | - | - | - | OK | OK | OK | OK | OK | OK | +=========+=========+=========+=========+=========+=========+=========+ Table 3: Testing Results for 2018 and 2020 Wang, et al. Expires 6 May 2027 [Page 11] Internet-Draft Root CA Cert Rekeying November 2026 +=========+=============================+=============================+ | | 2025 | 2026 | +---------+---------+---------+---------+---------+---------+---------+ | |G1 Client|G2 Client|G3 Client|G1 Client|G2 Client|G3 Client| +---------+---------+---------+---------+---------+---------+---------+ | |Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.| +---------+---------+---------+---------+---------+---------+---------+ |G1 Server| OK | OK | OK | OK | OK | OK | NO | NO | NO | NO | NO | NO | +---------+---------+---------+---------+---------+---------+---------+ |G2 Server| OK | OK | OK | OK | OK | OK | NO | NO | OK | OK | OK | OK | +---------+---------+---------+---------+---------+---------+---------+ |G3 Server| OK | OK | OK | OK | OK | OK | NO | NO | OK | OK | OK | OK | +=========+=========+=========+=========+=========+=========+=========+ Table 4: Testing Results for 2025 and 2026 +=========+=============================+=============================+ | | 2028 | 2030 | +---------+---------+---------+---------+---------+---------+---------+ | |G1 Client|G2 Client|G3 Client|G1 Client|G2 Client|G3 Client| +---------+---------+---------+---------+---------+---------+---------+ | |Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.|Cli.|Ser.| +---------+---------+---------+---------+---------+---------+---------+ |G1 Server| NO | NO | NO | NO | NO | NO | NO | NO | NO | NO | NO | NO +---------+---------+---------+---------+---------+---------+---------+ |G2 Server| NO | NO | NO | NO | NO | NO | NO | NO | NO | NO | NO | NO | +---------+---------+---------+---------+---------+---------+---------+ |G3 Server| NO | NO | NO | NO | OK | OK | NO | NO | NO | NO | NO | NO | +=========+=========+=========+=========+=========+=========+=========+ Table 5: Testing Results for 2028 and 2030 7. An Extension for Switching Root CA Certificate More Smoothly There will be a time difference between when a device A sends its certificate chain and when another device B receives the certificate chain. This time difference is due to a number of reasons, including communnicaiton delay, operation time (especially for PQ cryptographic operations) needed at the receiving device B, the clock drift between devices A and B. In practice, the solution introduced in Section 5 requires that a new device SHOULD switch its link certificate chain to its new certificate chain around the expiry time of the old root CA certificate. Therfore, two abnormal cases may happen, as explained below. * Case 1: A new decive A switches its certificate chain a little late. In this case, for a new receiving device B, it may be not able to verify this link certificate chain from device A as the Wang, et al. Expires 6 May 2027 [Page 12] Internet-Draft Root CA Cert Rekeying November 2026 old root certificate is expired when device B recieved the certificate chain, though such a connection is expected to be successful. In fact, as a new device, if B could try to use the new root CA cerificate verifying the link certificate chain, the result SHALL be positive. However, such a disturbance has been caused to new device B. * Case 2: A new decive A switches its certificate chain a little early. In this case, for an old device B, it may receive a new ccertificate chain from A beore the expiry time of the old root CA certificate. Then, naturally, as an old device, B will be not able to verify this certificate chain when it received the certificate chain from device A, though such a connection is supposed to be successful as old device B does not retire yet. To address the above issue due to switching the link certificate chain early or late, this section describes an exension to the solution proposed in Section 5. The basic idea is to introduce a maximum time difference (MTD) T between any two entities in a given domain when one entity sending a certificate chain and the other receiving it. The extension works as follows. * For any certificate issued by the old root CA certificate, its validity time MUST be at least 2T less than T3, the expected expiry time of the old root CA certificate and the newWithOld link certificate. * Each new device is supposed to complete its certificate chain switching on T3-T. * When a new device A sends out its link certificate chain before T3-T, it will be recieved by a device B by time T3 in B's local time. At this moment, does not matter if B is a new or old old device, the old root CA certificate is still valid according to B's local time. Therefore, B can verify A's link certificate chain and the extension works. * When a new device A sends out its new certificate chain after T3-T, it will be recieved by a device B after time T3-2T in B's local time. At this moment, if B is an old device, it retired or should be retired already. If B is a new device, the new root CA certificate is valid alredy, so B can verify A's new certificate chain. Therefore, the exension works to avoid both abnormal cases described in the above. Wang, et al. Expires 6 May 2027 [Page 13] Internet-Draft Root CA Cert Rekeying November 2026 8. Security Considerations The security of the One-way Link Certificate solution defined in this document relies on the structural integrity of X.509 certification path validation [RFC5280] and the cryptographic strength of the keys used to sign the cross-generational bridge components. Implementers and CA operators MUST consider the following security aspects when deploying this mechanism. 8.1. Impact of Old Root CA Key Compromise The primary security trade-off of the one-way link certificate (newWithOld) is that the validation path for new entities remains bound to the security of the old Root CA private key. If an attacker compromises the private key of the old Root CA during the overlapping lifetime period, they can forge a malicious newWithOld link certificate. Such a compromise would allow an attacker to sign an arbitrary public key and masquerade as a valid subordinate CA of the new infrastructure to any legacy entity that has not been updated. Therefore, CA operators MUST maintain strict, high-assurance physical and cryptographic protection over the old Root CA private key until its corresponding old root certificate has officially expired, even if that key is no longer actively used to issue new operational subordinate CA certificates. 8.2. Legacy Cryptographic Constraints on the Link Certificate As noted in Section 2, this solution is intended to support the transition from traditional cryptographic algorithms to post-quantum (PQ) or hybrid algorithms. However, legacy entities are inherently incapable of parsing or verifying signatures generated by cryptographic algorithms introduced after their deployment. Consequently, the signature on the newWithOld link certificate MUST be generated using an algorithm and key length that the oldest targeted legacy entities can fully parse and verify (e.g., traditional RSA or ECDSA). CA operators must accept that during the transition period, the bridge validating new entities to old entities cannot be cryptographically stronger than the legacy algorithm's security bounds. Wang, et al. Expires 6 May 2027 [Page 14] Internet-Draft Root CA Cert Rekeying November 2026 8.3. Path Length Validation Anomalies The introduction of a newWithOld certificate increases the path length of the certification chain for new devices trying to authenticate to old devices. Legacy devices enforcing strict pathLenConstraint limits within their basic constraints processing [RFC5280] may fail to validate the extended path if the maximum path depth is exceeded. CA operators MUST carefully calculate and configure the path length constraints of both old and new generations of subordinate CAs to accommodate the extra cross-certificate intermediate step. 8.4. Lifetime Alignment and Revocation The validity period of the newWithOld link certificate MUST NOT extend past the expiration date of the old Root CA trust anchor certificate. If the old Root CA private key is compromised, or if the migration strategy changes, revocation of the newWithOld certificate via standard CRLs or OCSP profiles may not be effective if the legacy client lacks an operational online revocation checking mechanism. In such cases, the compromise of the legacy anchor invalidates the entire bridging architecture, necessitating out-of- band removal of the old trust anchor from remaining legacy systems. 9. Acknowledgments The authors would like to thank the helpful comments from the following experts: Hendrik Brockhaus, Abel C. H. Chen, Russ Housley, David von Oheimb, and Michael Richardson. 10. 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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . Wang, et al. Expires 6 May 2027 [Page 15] Internet-Draft Root CA Cert Rekeying November 2026 [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, . 11. Informative References [RFC9794] Driscoll, F., Parsons, M., and B. Hale, "Terminology for Post-Quantum Traditional Hybrid Schemes", RFC 9794, DOI 10.17487/RFC9794, June 2025, . Authors' Addresses Guilin Wang (editor) Huawei Int. Pte Ltd 9 North Buona Vista Drive, #13-01 The Metropolis Tower 1 SINGAPORE 138588 Singapore Email: wang.guilin@huawei.com Yanjiang Yang Huawei Int. Pte Ltd Email: yang.yanjiang@huawei.com Jie Zhang Huawei Tech. Ltd Email: zhangjie184@huawei.com Wang, et al. Expires 6 May 2027 [Page 16]