]> Cisco Systems

sfluhrer@cisco.com

genua GmbH

ietf@gazdag.de

ISARA Corporation

daniel.vangeest@isara.com

BSI

stavros.kousidis@bsi.bund.de

sec LAMPS - Limited Additional Mechanisms for PKIX and SMIME Internet-Draft This document specifies algorithm identifiers and ASN.1 encoding formats for the Hash-Based Signature (HBS) schemes Hierarchical Signature System (HSS), eXtended Merkle Signature Scheme (XMSS), and XMSS^MT, a multi-tree variant of XMSS, as well as SPHINCS+, the latter being the only stateless scheme. 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 . Discussion of this document takes place on the LAMPS Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Hash-Based Signature (HBS) Schemes combine Merkle trees with One/Few Time Signatures (OTS/FTS) in order to provide digital signature schemes that remain secure even when quantum computers become available. There 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 HSS, XMSS and XMSS^MT 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. Due to thise statefulness of the private key and the limited number of signatures that can be created, these signature algorithms might not be appropriate for use in interactive protocols. 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. Since these algorithms are already known to be secure against quantum attacks, and because roots of trust are generally long-lived and can take longer to be deployed than end-entity certificates, these signature algorithms are more appropriate to be used in root and subordinate CA certificates. They are also appropriate in non-interactive contexts such as code signing. In particular, there are multi-party IoT ecosystems where publicly trusted code signing certificates are useful. The private key of SPHINCS+ is a finite but very large collection of FTS keys and hence stateless. This typically comes at the cost of larger signatures compared to the stateful HBS variants. Thus SPHINCS+ is suitable for more use-cases if the signature sizes fit the requirements.

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. 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, SPHINCS+).

Subject 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 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 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 level L=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 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 for more information on the contents and format of an XMSS^MT public key.

SPHINCS+ Public Keys The object and public key algorithm identifiers for SPHINCS+ are defined in . The definitions are repeated here for reference. The SPHINCS+ public key identifier is as follows: The SPHINCS+ public key is defined as follows: See for more information on the contents and format of a SPHINCS+ 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 an end-entity certificate that indicates id-alg-xmss-hashsig or id-alg-xmssmt-hashsig in SubjectPublicKeyInfo, then the keyUsage extension MUST contain one or both of the following values: If the keyUsage extension is present in a certification authority certificate that indicates id-alg-xmss-hashsig or id-alg-xmssmt-hashsig, then the keyUsage extension MUST contain one or more of the following values: defines the key usage for id-alg-hss-lms-hashsig, which is the same as for the keys above.

Signature Algorithms This section identifies OIDs for signing using HSS, XMSS, XMSS^MT, and SPHINCS+. 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 below. 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. 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. For SPHINCS+ the signature values are described in . The octet string representing the signature is encoded directly in the BIT STRING without adding any additional ASN.1 wrapping. For the Certificate and CertificateList structures, the signature value is wrapped in the "signatureValue" BIT 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 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 for more information on the contents and format of an XMSS signature.

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 for more information on the contents and format of an XMSS^MT signature.

SPHINCS+ Signature Algorithm The SPHINCS+ public key OID is also used to specify that a SPHINCS+ signature was generated on the full message, i.e. the message was not hashed before being processed by the SPHINCS+ signature algorithm. The SPHINCS+ signature is defined as follows: See for more information on the contents and format of a SPHINCS+ signature.

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 } id-alg-sphincs-plus-128 OBJECT IDENTIFIER ::= { TBD } id-alg-sphincs-plus-192 OBJECT IDENTIFIER ::= { TBD } id-alg-sphincs-plus-256 OBJECT IDENTIFIER ::= { TBD } -- -- 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 } SMIME-CAPS { IDENTIFIED BY id-alg-xmssmt-hashsig } } -- sa-sphincs-plus-128 is defined in [I-D.ietf-lamps-cms-sphincs-plus] sa-sphincs-plus-128 SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-sphincs-plus-128 PARAMS ARE absent PUBLIC-KEYS { pk-sphincs-plus-128 } SMIME-CAPS { IDENTIFIED BY id-alg-sphincs-plus-128 } } -- sa-sphincs-plus-192 is defined in [I-D.ietf-lamps-cms-sphincs-plus] sa-sphincs-plus-192 SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-sphincs-plus-192 PARAMS ARE absent PUBLIC-KEYS { pk-sphincs-plus-192 } SMIME-CAPS { IDENTIFIED BY id-alg-sphincs-plus-192 } } -- sa-sphincs-plus-256 is defined in [I-D.ietf-lamps-cms-sphincs-plus] sa-sphincs-plus-256 SIGNATURE-ALGORITHM ::= { IDENTIFIER id-alg-sphincs-plus-256 PARAMS ARE absent PUBLIC-KEYS { pk-sphincs-plus-256 } SMIME-CAPS { IDENTIFIED BY id-alg-sphincs-plus-256 } } -- 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-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } XMSSMT-HashSig-PublicKey ::= OCTET STRING -- pk-sphincs-plus-128 is defined in [I-D.ietf-lamps-cms-sphincs-plus] pk-sphincs-plus-128 PUBLIC-KEY ::= { IDENTIFIER id-alg-sphincs-plus-128 KEY SPHINCS-Plus-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } -- pk-sphincs-plus-192 is defined in [I-D.ietf-lamps-cms-sphincs-plus] pk-sphincs-plus-192 PUBLIC-KEY ::= { IDENTIFIER id-alg-sphincs-plus-192 KEY SPHINCS-Plus-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } -- pk-sphincs-plus-256 is defined in [I-D.ietf-lamps-cms-sphincs-plus] pk-sphincs-plus-256 PUBLIC-KEY ::= { IDENTIFIER id-alg-sphincs-plus-256 KEY SPHINCS-Plus-PublicKey PARAMS ARE absent CERT-KEY-USAGE { digitalSignature, nonRepudiation, keyCertSign, cRLSign } } SPHINCS-Plus-PublicKey ::= OCTET STRING END ]]>

Security Considerations

Algorithm Security Considerations The cryptographic security of the signatures generated by the algorithms mentioned in this document depends only on the hash algorithms used within the signature algorithms and the pre-hash algorithm used to create an X.509 certificate's message digest. Grover's algorithm [Grover96] is a quantum search algorithm which gives a quadratic improvement in search time to brute-force pre-image attacks. The results of [BBBV97] show that this improvement is optimal, however [Fluhrer17] notes that Grover's algorithm doesn't parallelize well. Thus, given a bounded amount of time to perform the attack and using a conservative estimate of the performance of a real quantum computer, the pre-image quantum security of SHA-256 is closer to 190 bits. All parameter sets for the signature algorithms in this document currently use SHA-256 internally and thus have at least 128 bits of quantum pre-image resistance, or 190 bits using the security assumptions in [Fluhrer17]. [Zhandry15] shows that hash collisions can be found using an algorithm with a lower bound on the number of oracle queries on the order of 2^(n/3) on the number of bits, however [DJB09] demonstrates that the quantum memory requirements would be much greater. Therefore a parameter set using SHA-256 would have at least 128 bits of quantum collision-resistance as well as the pre-image resistance mentioned in the previous paragraph. Given the quantum collision and pre-image resistance of SHA-256 estimated above, the current parameter sets used by id-alg-hss-lms-hashsig, id-alg-xmss-hashsig and id-alg-xmssmt-hashsig provide 128 bits or more of quantum security. This is believed to be secure enough to protect X.509 certificates for well beyond any reasonable certificate lifetime.

Implementation Security Considerations Implementations MUST protect the private keys. Compromise of the private keys may result in the ability to forge signatures. Along with the private key, the implementation MUST keep track of which leaf nodes in the tree have been used. Loss of integrity of this tracking data can cause a one-time key to be used more than once. As a result, when a private key and the tracking data are stored on non- volatile media or stored in a virtual machine environment, care must be taken to preserve confidentiality and integrity. The generation of private keys relies on random numbers. The use of inadequate pseudo-random number generators (PRNGs) to generate these values can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys, searching the resulting small set of possibilities, rather than brute force searching the whole key space. The generation of quality random numbers is difficult. offers important guidance in this area. The generation of hash-based signatures also depends on random numbers. While the consequences of an inadequate pseudo-random number generator (PRNG) to generate these values is much less severe than the generation of private keys, the guidance in remains important.

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. Vigil Security, LLC Cisco Systems Amazon Web Services Cloudflare SPHINCS+ is a stateless hash-based signature scheme. This document specifies the conventions for using the SPHINCS+ stateless hash-based signature algorithm with the Cryptographic Message Syntax (CMS). In addition, the algorithm identifier and public key syntax are provided. 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] Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the External Data Representation Standard (XDR) protocol as it is currently deployed and accepted. This document obsoletes RFC 1832. [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. 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.

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" - .