]> BSI

kaveh.bashiri.ietf@gmail.com

Cisco Systems

sfluhrer@cisco.com

genua GmbH

ietf@gazdag.de

CryptoNext Security

daniel.vangeest.ietf@gmail.com

BSI

kousidis.ietf@gmail.com

Internet-Draft 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 Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

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 .

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 when, and only when, they appear in all capitals, as shown here.

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.]

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 , . 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 . 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.

Public Key Algorithms Certificates conforming to 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. In this document, we define new OIDs for identifying the different stateful hash-based signature algorithms. An additional OID is defined in and repeated here for convenience. For all of the OIDs, the parameters MUST be absent.

HSS Public Keys The object identifier and public key algorithm identifier for HSS is defined in . The definitions are repeated here for reference. The object identifier for an HSS public key is id-alg-hss-lms-hashsig: 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 . The HSS public key identifier is as follows: The HSS public key is defined as follows: See and 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.

XMSS Public Keys The object identifier for an XMSS public key is id-alg-xmss-hashsig: The XMSS public key identifier is as follows: The XMSS public key is defined as follows: See and for more information on the contents and format of an XMSS public key.

XMSS^MT Public Keys The object identifier for an XMSS^MT public key is id-alg-xmssmt-hashsig: The XMSS^MT public key identifier is as follows: The XMSS^MT public key is defined as follows: See and for more information on the contents and format of an XMSS^MT public key.

Key Usage Bits The intended application for the key is indicated in the keyUsage certificate extension . 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: 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: 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 .

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?] For HSS, the signature value is described in section 6.4 of . For XMSS and XMSS^MT the signature values are described in sections B.2 and C.2 of , 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.

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. The HSS signature is defined as follows: See and for more information on the contents and format of an HSS signature.

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. The XMSS signature is defined as follows: See and for more information on the contents and format of an XMSS signature. The signature generation MUST be performed according to 7.2 of .

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. The XMSS^MT signature is defined as follows: See and for more information on the contents and format of an XMSS^MT signature. The signature generation MUST be performed according to 7.2 of .

Key Generation The key generation for XMSS and XMSS^MT MUST be performed according to 7.2 of

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 . 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) 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] 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 ]]>

Security Considerations The security requirements of 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 . As noted in and , 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:

• 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 , 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.

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 .

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.

References Normative References The Cryptographic Message Syntax (CMS) format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This note describes the eXtended Merkle Signature Scheme (XMSS), a hash-based digital signature system that is based on existing descriptions in scientific literature. This note specifies Winternitz One-Time Signature Plus (WOTS+), a one-time signature scheme; XMSS, a single-tree scheme; and XMSS^MT, a multi-tree variant of XMSS. Both XMSS and XMSS^MT use WOTS+ as a main building block. XMSS provides cryptographic digital signatures without relying on the conjectured hardness of mathematical problems. Instead, it is proven that it only relies on the properties of cryptographic hash functions. XMSS provides strong security guarantees and is even secure when the collision resistance of the underlying hash function is broken. It is suitable for compact implementations, is relatively simple to implement, and naturally resists side-channel attacks. Unlike most other signature systems, hash-based signatures can so far withstand known attacks using quantum computers. This note describes a digital-signature system based on cryptographic hash functions, following the seminal work in this area of Lamport, Diffie, Winternitz, and Merkle, as adapted by Leighton and Micali in 1995. It specifies a one-time signature scheme and a general signature scheme. These systems provide asymmetric authentication without using large integer mathematics and can achieve a high security level. They are suitable for compact implementations, are relatively simple to implement, and are naturally resistant to side-channel attacks. Unlike many other signature systems, hash-based signatures would still be secure even if it proves feasible for an attacker to build a quantum computer. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. This has been reviewed by many researchers, both in the research group and outside of it. The Acknowledgements section lists many of them. This document specifies the conventions for using the Hierarchical Signature System (HSS) / Leighton-Micali Signature (LMS) hash-based signature algorithm with the Cryptographic Message Syntax (CMS). In addition, the algorithm identifier and public key syntax are provided. The HSS/LMS algorithm is one form of hash-based digital signature; it is described in RFC 8554. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document specifies algorithm identifiers and ASN.1 encoding formats for digital signatures and subject public keys used in the Internet X.509 Public Key Infrastructure (PKI). Digital signatures are used to sign certificates and certificate revocation list (CRLs). Certificates include the public key of the named subject. [STANDARDS-TRACK] This document specifies algorithm identifiers and ASN.1 encoding formats for elliptic curve constructs using the curve25519 and curve448 curves. The signature algorithms covered are Ed25519 and Ed448. The key agreement algorithms covered are X25519 and X448. The encoding for public key, private key, and Edwards-curve Digital Signature Algorithm (EdDSA) structures is provided. When the Curdle Security Working Group was chartered, a range of object identifiers was donated by DigiCert, Inc. for the purpose of registering the Edwards Elliptic Curve key agreement and signature algorithms. This donated set of OIDs allowed for shorter values than would be possible using the existing S/MIME or PKIX arcs. This document describes the donated range and the identifiers that were assigned from that range, transfers control of that range to IANA, and establishes IANA allocation policies for any future assignments within that range.

Acknowledgments Thanks for Russ Housley and Panos Kampanakis for helpful suggestions. This document uses a lot of text from similar documents , ( and ) as well as . Thanks go to the authors of those documents. "Copying always makes things easier and less error prone" - .