Network Working Group K. Bashiri Internet-Draft BSI Intended status: Informational S. Fluhrer Expires: 25 August 2024 Cisco Systems S. Gazdag genua GmbH D. Van Geest CryptoNext Security S. Kousidis BSI 22 February 2024 Internet X.509 Public Key Infrastructure: Algorithm Identifiers for HSS and XMSS draft-gazdag-x509-shbs-00 Abstract This document specifies algorithm identifiers and ASN.1 encoding formats for the Stateful Hash-Based Signature Schemes (S-HBS) Hierarchical Signature System (HSS), eXtended Merkle Signature Scheme (XMSS), and XMSS^MT, a multi-tree variant of XMSS. This specification applies to the Internet X.509 Public Key infrastructure (PKI) when those digital signatures are used in Internet X.509 certificates and certificate revocation lists. About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-gazdag-x509-shbs/. Source for this draft and an issue tracker can be found at https://github.com/x509-hbs/draft-x509-shbs. 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/. Bashiri, et al. Expires 25 August 2024 [Page 1] Internet-Draft HSS and XMSS for X.509 February 2024 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 25 August 2024. Copyright Notice Copyright (c) 2024 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 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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Use Cases of S-HBS in X.509 . . . . . . . . . . . . . . . . . 4 5. Public Key Algorithms . . . . . . . . . . . . . . . . . . . . 4 5.1. HSS Public Keys . . . . . . . . . . . . . . . . . . . . . 5 5.2. XMSS Public Keys . . . . . . . . . . . . . . . . . . . . 5 5.3. XMSS^MT Public Keys . . . . . . . . . . . . . . . . . . . 6 6. Key Usage Bits . . . . . . . . . . . . . . . . . . . . . . . 7 7. Signature Algorithms . . . . . . . . . . . . . . . . . . . . 7 7.1. HSS Signature Algorithm . . . . . . . . . . . . . . . . . 8 7.2. XMSS Signature Algorithm . . . . . . . . . . . . . . . . 8 7.3. XMSS^MT Signature Algorithm . . . . . . . . . . . . . . . 9 8. Key Generation . . . . . . . . . . . . . . . . . . . . . . . 9 9. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . . . 9 10. Security Considerations . . . . . . . . . . . . . . . . . . . 11 11. Backup and Restore Management . . . . . . . . . . . . . . . . 12 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 13.1. Normative References . . . . . . . . . . . . . . . . . . 12 13.2. Informative References . . . . . . . . . . . . . . . . . 13 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 Bashiri, et al. Expires 25 August 2024 [Page 2] Internet-Draft HSS and XMSS for X.509 February 2024 1. Introduction Stateful Hash-Based Signature Schemes (S-HBS) such as HSS, XMSS and XMSS^MT combine Merkle trees with One Time Signatures (OTS) in order to provide digital signature schemes that remain secure even when quantum computers become available. Their theoretic security is well understood and depends only on the security of the underlying hash function. As such they can serve as an important building block for quantum computer resistant information and communication technology. The private key of S-HBS is a finite collection of OTS keys, hence only a limited number of messages can be signed and the private key's state must be updated and persisted after signing to prevent reuse of OTS keys. While the right selection of algorithm parameters would allow a private key to sign a virtually unbounded number of messages (e.g. 2^60), this is at the cost of a larger signature size and longer signing time. Due to the statefulness of the private key and the limited number of signatures that can be created, S-HBS might not be appropriate for use in interactive protocols. However, in some use cases the deployment of S-HBS may be appropriate. Such use cases are described and discussed later in Section 4. 2. Conventions and Definitions 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. 3. Notation The parameter 'n' is the security parameter, given in bytes. In practice this is typically aligned to the standard output length of the hash function in use, i.e. either 24, 32 or 64 bytes. The height of a single tree is typically given by the parameter 'h'. The number of levels of trees is either called 'L' (HSS) or 'd' (XMSS, XMSS^MT). [EDNOTE: Should we delete this section? The parameters are not used in this document.] Bashiri, et al. Expires 25 August 2024 [Page 3] Internet-Draft HSS and XMSS for X.509 February 2024 4. Use Cases of S-HBS in X.509 As many cryptographic algorithms that are considered to be quantum- resistant, S-HBS have several pros and cons regarding their practical usage. On the positive side they are considered to be secure against a classical as well as a quantum adversary, and a secure instantiation of S-HBS may always be built as long as a cryptographically secure hash function exists. Moreover, S-HBS offer small public key sizes, and, in comparison to other post-quantum signature schemes, the S-HBS can offer relatively small signature sizes (for certain parameter sets). While key generation and signature generation may take longer than classical alternatives, fast and minimal verification routines can be built. The major negative aspect is the statefulness. Private keys always have to be handled in a secure manner, S-HBS necessitate a special treatment of the private key in order to avoid security incidents like signature forgery [MCGREW], [SP800208]. Therefore, for S-HBS, a secure environment MUST be used for key generation and key management. Note that, in general, root CAs offer such a secure environment and the number of issued signatures (including signed certificates and CRLs) is often moderate due to the fact that many root CAs delegate OCSP services or the signing of end-entity certificates to other entities (such as subordinate CAs) that use stateless signature schemes. Therefore, many root CAs should be able to handle the required state management, and S-HBS offer a viable solution. As the above reasoning for root CAs usually does not apply for subordinate CAs, it is NOT RECOMMENDED for subordinate CAs to use S-HBS for issuing end-entity certificates. Moreover, S-HBS MUST NOT be used for end-entity certificates. However, S-HBS MAY be used for code signing certificates, since they are suitable and recommended in such non-interactive contexts. For example, see the recommendations for software and firmware signing in [CNSA2.0]. Some manufactures use common and well-established key formats like X.509 for their code signing and update mechanisms. Also there are multi-party IoT ecosystems where publicly trusted code signing certificates are useful. 5. Public Key Algorithms Certificates conforming to [RFC5280] can convey a public key for any public key algorithm. The certificate indicates the algorithm through an algorithm identifier. An algorithm identifier consists of an OID and optional parameters. Bashiri, et al. Expires 25 August 2024 [Page 4] Internet-Draft HSS and XMSS for X.509 February 2024 In this document, we define new OIDs for identifying the different stateful hash-based signature algorithms. An additional OID is defined in [RFC8708] and repeated here for convenience. For all of the OIDs, the parameters MUST be absent. 5.1. HSS Public Keys The object identifier and public key algorithm identifier for HSS is defined in [RFC8708]. The definitions are repeated here for reference. The object identifier for an HSS public key is id-alg-hss-lms- hashsig: id-alg-hss-lms-hashsig OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) alg(3) 17 } Note that the id-alg-hss-lms-hashsig algorithm identifier is also referred to as id-alg-mts-hashsig. This synonym is based on the terminology used in an early draft of the document that became [RFC8554]. The HSS public key identifier is as follows: pk-HSS-LMS-HashSig PUBLIC-KEY ::= { IDENTIFIER id-alg-hss-lms-hashsig KEY HSS-LMS-HashSig-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } The HSS public key is defined as follows: HSS-LMS-HashSig-PublicKey ::= OCTET STRING See [SP800208] and [RFC8554] for more information on the contents and format of an HSS public key. Note that the single-tree signature scheme LMS is instantiated as HSS with number of levels being equal to 1. 5.2. XMSS Public Keys The object identifier for an XMSS public key is id-alg-xmss-hashsig: Bashiri, et al. Expires 25 August 2024 [Page 5] Internet-Draft HSS and XMSS for X.509 February 2024 id-alg-xmss-hashsig OBJECT IDENTIFIER ::= { itu-t(0) identified-organization(4) etsi(0) reserved(127) etsi-identified-organization(0) isara(15) algorithms(1) asymmetric(1) xmss(13) 0 } The XMSS public key identifier is as follows: pk-XMSS-HashSig PUBLIC-KEY ::= { IDENTIFIER id-alg-xmss-hashsig KEY XMSS-HashSig-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } The XMSS public key is defined as follows: XMSS-HashSig-PublicKey ::= OCTET STRING See [SP800208] and [RFC8391] for more information on the contents and format of an XMSS public key. 5.3. XMSS^MT Public Keys The object identifier for an XMSS^MT public key is id-alg-xmssmt- hashsig: id-alg-xmssmt-hashsig OBJECT IDENTIFIER ::= { itu-t(0) identified-organization(4) etsi(0) reserved(127) etsi-identified-organization(0) isara(15) algorithms(1) asymmetric(1) xmssmt(14) 0 } The XMSS^MT public key identifier is as follows: pk-XMSSMT-HashSig PUBLIC-KEY ::= { IDENTIFIER id-alg-xmssmt-hashsig KEY XMSSMT-HashSig-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } The XMSS^MT public key is defined as follows: XMSSMT-HashSig-PublicKey ::= OCTET STRING See [SP800208] and [RFC8391] for more information on the contents and format of an XMSS^MT public key. Bashiri, et al. Expires 25 August 2024 [Page 6] Internet-Draft HSS and XMSS for X.509 February 2024 6. Key Usage Bits The intended application for the key is indicated in the keyUsage certificate extension [RFC5280]. If the keyUsage extension is present in a certificate that indicates id-alg-hss-lms-hashsig, id- alg-xmss-hashsig, or id-alg-xmssmt-hashsig, then the following requirements given in this section MUST be fulfilled. If the keyUsage extension is present in a code signing certificate, then it MUST contain at least one of the following values: nonRepudiation; or digitalSignature. However, it MUST NOT contain other values. If the keyUsage extension is present in a certification authority certificate, then it MUST contain at least one of the following values: nonRepudiation; or digitalSignature; or keyCertSign; or cRLSign. However, it MUST NOT contain other values. Note that for certificates that indicate id-alg-hss-lms-hashsig the above definitions are more restrictive than the requirement defined in Section 4 of [RFC8708]. 7. Signature Algorithms This section identifies OIDs for signing using HSS, XMSS, and XMSS^MT. When these algorithm identifiers appear in the algorithm field as an AlgorithmIdentifier, the encoding MUST omit the parameters field. That is, the AlgorithmIdentifier SHALL be a SEQUENCE of one component, one of the OIDs defined in the following subsections. The data to be signed is prepared for signing. For the algorithms used in this document, the data is signed directly by the signature algorithm, the data is not hashed before processing. Then, a private key operation is performed to generate the signature value. [EDNOTE: Should we delete the preceding paragraph?] Bashiri, et al. Expires 25 August 2024 [Page 7] Internet-Draft HSS and XMSS for X.509 February 2024 For HSS, the signature value is described in section 6.4 of [RFC8554]. For XMSS and XMSS^MT the signature values are described in sections B.2 and C.2 of [RFC8391], respectively. The octet string representing the signature is encoded directly in the OCTET STRING without adding any additional ASN.1 wrapping. For the Certificate and CertificateList structures, the signature value is wrapped in the "signatureValue" OCTET STRING field. 7.1. HSS Signature Algorithm The HSS public key OID is also used to specify that an HSS signature was generated on the full message, i.e. the message was not hashed before being processed by the HSS signature algorithm. id-alg-hss-lms-hashsig OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) alg(3) 17 } The HSS signature is defined as follows: HSS-LMS-HashSig-Signature ::= OCTET STRING See [SP800208] and [RFC8554] for more information on the contents and format of an HSS signature. 7.2. XMSS Signature Algorithm The XMSS public key OID is also used to specify that an XMSS signature was generated on the full message, i.e. the message was not hashed before being processed by the XMSS signature algorithm. id-alg-xmss-hashsig OBJECT IDENTIFIER ::= { itu-t(0) identified-organization(4) etsi(0) reserved(127) etsi-identified-organization(0) isara(15) algorithms(1) asymmetric(1) xmss(13) 0 } The XMSS signature is defined as follows: XMSS-HashSig-Signature ::= OCTET STRING See [SP800208] and [RFC8391] for more information on the contents and format of an XMSS signature. The signature generation MUST be performed according to 7.2 of [SP800208]. Bashiri, et al. Expires 25 August 2024 [Page 8] Internet-Draft HSS and XMSS for X.509 February 2024 7.3. XMSS^MT Signature Algorithm The XMSS^MT public key OID is also used to specify that an XMSS^MT signature was generated on the full message, i.e. the message was not hashed before being processed by the XMSS^MT signature algorithm. id-alg-xmssmt-hashsig OBJECT IDENTIFIER ::= { itu-t(0) identified-organization(4) etsi(0) reserved(127) etsi-identified-organization(0) isara(15) algorithms(1) asymmetric(1) xmssmt(14) 0 } The XMSS^MT signature is defined as follows: XMSSMT-HashSig-Signature ::= OCTET STRING See [SP800208] and [RFC8391] for more information on the contents and format of an XMSS^MT signature. The signature generation MUST be performed according to 7.2 of [SP800208]. 8. Key Generation The key generation for XMSS and XMSS^MT MUST be performed according to 7.2 of [SP800208] 9. ASN.1 Module For reference purposes, the ASN.1 syntax is presented as an ASN.1 module here. This ASN.1 Module builds upon the conventions established in [RFC5911]. -- -- ASN.1 Module -- Hashsigs-pkix-0 -- TBD - IANA assigned module OID DEFINITIONS IMPLICIT TAGS ::= BEGIN EXPORTS ALL; IMPORTS PUBLIC-KEY, SIGNATURE-ALGORITHM FROM AlgorithmInformation-2009 {iso(1) identified-organization(3) dod(6) internet(1) security(5) Bashiri, et al. Expires 25 August 2024 [Page 9] Internet-Draft HSS and XMSS for X.509 February 2024 mechanisms(5) pkix(7) id-mod(0) id-mod-algorithmInformation-02(58)}; -- -- Object Identifiers -- -- id-alg-hss-lms-hashsig is defined in [RFC8708] id-alg-hss-lms-hashsig OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) alg(3) 17 } id-alg-xmss-hashsig OBJECT IDENTIFIER ::= { itu-t(0) identified-organization(4) etsi(0) reserved(127) etsi-identified-organization(0) isara(15) algorithms(1) asymmetric(1) xmss(13) 0 } id-alg-xmssmt-hashsig OBJECT IDENTIFIER ::= { itu-t(0) identified-organization(4) etsi(0) reserved(127) etsi-identified-organization(0) isara(15) algorithms(1) asymmetric(1) xmssmt(14) 0 } -- -- Signature Algorithms and Public Keys -- -- sa-HSS-LMS-HashSig is defined in [RFC8708] sa-HSS-LMS-HashSig SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-hss-lms-hashsig PARAMS ARE absent PUBLIC-KEYS { pk-HSS-LMS-HashSig } SMIME-CAPS { IDENTIFIED BY id-alg-hss-lms-hashsig } } sa-XMSS-HashSig SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-xmss-hashsig PARAMS ARE absent PUBLIC-KEYS { pk-XMSS-HashSig } SMIME-CAPS { IDENTIFIED BY id-alg-xmss-hashsig } } sa-XMSSMT-HashSig SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-xmssmt-hashsig PARAMS ARE absent PUBLIC-KEYS { pk-XMSSMT-HashSig } SMIME-CAPS { IDENTIFIED BY id-alg-xmssmt-hashsig } } -- pk-HSS-LMS-HashSig is defined in [RFC8708] Bashiri, et al. Expires 25 August 2024 [Page 10] Internet-Draft HSS and XMSS for X.509 February 2024 pk-HSS-LMS-HashSig PUBLIC-KEY ::= { IDENTIFIER id-alg-hss-lms-hashsig KEY HSS-LMS-HashSig-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } HSS-LMS-HashSig-PublicKey ::= OCTET STRING pk-XMSS-HashSig PUBLIC-KEY ::= { IDENTIFIER id-alg-xmss-hashsig KEY XMSS-HashSig-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } XMSS-HashSig-PublicKey ::= OCTET STRING pk-XMSSMT-HashSig PUBLIC-KEY ::= { IDENTIFIER id-alg-xmssmt-hashsig KEY XMSSMT-HashSig-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } XMSSMT-HashSig-PublicKey ::= OCTET STRING END 10. Security Considerations The security requirements of [SP800208] MUST be taken into account. For S-HBS it is crucial to stress the importance of a correct state management. If an attacker were able to obtain signatures for two different messages created using the same OTS key, then it would become computationally feasible for that attacker to create forgeries [BH16]. As noted in [MCGREW] and [ETSI-TR-103-692], extreme care needs to be taken in order to avoid the risk that an OTS key will be reused accidentally. This is a new requirement that most developers will not be familiar with and requires careful handling. Various strategies for a correct state management can be applied: Bashiri, et al. Expires 25 August 2024 [Page 11] Internet-Draft HSS and XMSS for X.509 February 2024 * Implement a track record of all signatures generated by a key pair associated to a S-HBS instance. This track record may be stored outside the device which is used to generate the signature. Check the track record to prevent OTS key reuse before a new signature is released. Drop the new signature and hit your PANIC button if you spot OTS key reuse. * Use a S-HBS instance only for a moderate number of signatures such that it is always practical to keep a consistent track record and be able to unambiguously trace back all generated signatures. * Apply the state reservation strategy described in Section 5 of [MCGREW], where upcoming states are reserved in advance by the signer. In this way the number of state synchronisations between nonvolatile and volatile memory is reduced. 11. Backup and Restore Management Certificate Authorities have high demands in order to ensure the availability of signature generation throughout the validity period of signing key pairs. Usual backup and restore strategies when using a stateless signature scheme (e.g. SLH-DSA) are to duplicate private keying material and to operate redundant signing devices or to store and safeguard a copy of the private keying material such that it can be used to set up a new signing device in case of technical difficulties. For S-HBS such straightforward backup and restore strategies will lead to OTS reuse with high probability as a correct state management is not guaranteed. Strategies for maintaining availability and keeping a correct state are described in Section 7 of [SP800208]. 12. IANA Considerations IANA is requested to assign a module OID from the "SMI for PKIX Module Identifier" registry for the ASN.1 module in Section 6. 13. References 13.1. 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, . Bashiri, et al. Expires 25 August 2024 [Page 12] Internet-Draft HSS and XMSS for X.509 February 2024 [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, . [RFC5911] Hoffman, P. and J. Schaad, "New ASN.1 Modules for Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911, DOI 10.17487/RFC5911, 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, . [RFC8391] Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A. Mohaisen, "XMSS: eXtended Merkle Signature Scheme", RFC 8391, DOI 10.17487/RFC8391, May 2018, . [RFC8554] McGrew, D., Curcio, M., and S. Fluhrer, "Leighton-Micali Hash-Based Signatures", RFC 8554, DOI 10.17487/RFC8554, April 2019, . [RFC8708] Housley, R., "Use of the HSS/LMS Hash-Based Signature Algorithm in the Cryptographic Message Syntax (CMS)", RFC 8708, DOI 10.17487/RFC8708, February 2020, . [SP800208] National Institute of Standards and Technology (NIST), "Recommendation for Stateful Hash-Based Signature Schemes", 29 October 2020, . 13.2. Informative References [BH16] Bruinderink, L. and S. Hülsing, "Oops, I did it again – Security of One-Time Signatures under Two-Message Attacks.", 2016, . [CNSA2.0] National Security Agency (NSA), "Commercial National Security Algorithm Suite 2.0 (CNSA 2.0) Cybersecurity Advisory (CSA)", 7 September 2022, . Bashiri, et al. Expires 25 August 2024 [Page 13] Internet-Draft HSS and XMSS for X.509 February 2024 [ETSI-TR-103-692] European Telecommunications Standards Institute (ETSI), "State management for stateful authentication mechanisms", November 2021, . [MCGREW] McGrew, D., Kampanakis, P., Fluhrer, S., Gazdag, S., Butin, D., and J. Buchmann, "State Management for Hash- Based Signatures", 2 November 2016, . [RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April 2002, . [RFC8410] Josefsson, S. and J. Schaad, "Algorithm Identifiers for Ed25519, Ed448, X25519, and X448 for Use in the Internet X.509 Public Key Infrastructure", RFC 8410, DOI 10.17487/RFC8410, August 2018, . [RFC8411] Schaad, J. and R. Andrews, "IANA Registration for the Cryptographic Algorithm Object Identifier Range", RFC 8411, DOI 10.17487/RFC8411, August 2018, . Acknowledgments Thanks for Russ Housley and Panos Kampanakis for helpful suggestions. This document uses a lot of text from similar documents [SP800208], ([RFC3279] and [RFC8410]) as well as [RFC8708]. Thanks go to the authors of those documents. "Copying always makes things easier and less error prone" - [RFC8411]. Authors' Addresses Kaveh Bashiri BSI Email: kaveh.bashiri.ietf@gmail.com Scott Fluhrer Cisco Systems Email: sfluhrer@cisco.com Bashiri, et al. Expires 25 August 2024 [Page 14] Internet-Draft HSS and XMSS for X.509 February 2024 Stefan Gazdag genua GmbH Email: ietf@gazdag.de Daniel Van Geest CryptoNext Security Email: daniel.vangeest.ietf@gmail.com Stavros Kousidis BSI Email: kousidis.ietf@gmail.com Bashiri, et al. Expires 25 August 2024 [Page 15]