LAMPS L. Perret Internet-Draft J. Prat Intended status: Standards Track CryptoNext Security Expires: 26 May 2022 M. Ounsworth Entrust Limited 22 November 2021 Use of Post-Quantum KEM in the Cryptographic Message Syntax (CMS) draft-perret-prat-lamps-cms-pq-kem-00 Abstract This document describes the conventions for using a Key Encapsulation Mechanism algorithm (KEM) within the Cryptographic Message Syntax (CMS). The CMS specifies the enveloped-data content type, which consists of an encrypted content and encrypted content-encryption keys for one or more recipients. The mechanism proposed here can rely on either post-quantum KEMs, hybrid KEMs or classical KEMs. 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 26 May 2022. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Perret, et al. Expires 26 May 2022 [Page 1] Internet-Draft PQ KEM in CMS November 2021 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Design Rationales . . . . . . . . . . . . . . . . . . . . . . 3 4. KEM Key Transport Mechanism (KEM-TRANS) . . . . . . . . . . . 4 4.1. Underlying Components . . . . . . . . . . . . . . . . . . 4 4.1.1. KEM . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1.2. KDF . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1.3. WRAP . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Recipient's Key Generation and Distribution . . . . . . . 6 4.3. Sender's Operations . . . . . . . . . . . . . . . . . . . 6 4.4. Recipient's Operations . . . . . . . . . . . . . . . . . 7 5. Use in CMS . . . . . . . . . . . . . . . . . . . . . . . . . 7 5.1. RecipientInfo Conventions . . . . . . . . . . . . . . . . 7 5.2. Certificate Conventions . . . . . . . . . . . . . . . . . 8 5.2.1. Key Usage Extension . . . . . . . . . . . . . . . . . 8 5.2.2. Subject Public Key Info . . . . . . . . . . . . . . . 9 5.3. SMIME Capabilities Attribute Conventions . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 9. Annex A : ASN.1 Syntax . . . . . . . . . . . . . . . . . . . 10 9.1. Annex A1 : KEM-TRANS Key Transport Mechanism . . . . . . 10 9.2. Annex A2 : Underlying Components . . . . . . . . . . . . 11 9.2.1. Key Encapsulation Mechanisms . . . . . . . . . . . . 11 9.2.2. Key Derivation Functions . . . . . . . . . . . . . . 11 9.2.3. Key Wrapping Schemes . . . . . . . . . . . . . . . . 12 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 10.1. Normative References . . . . . . . . . . . . . . . . . . 12 10.2. Informative References . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Perret, et al. Expires 26 May 2022 [Page 2] Internet-Draft PQ KEM in CMS November 2021 1. Introduction In recent years, there has been a substantial amount of research on quantum computers -- machines that exploit quantum mechanical phenomena to solve mathematical problems that are difficult or intractable for conventional computers. If large-scale quantum computers are ever built, they will be able to break many of the public-key cryptosystems currently in use. This would seriously compromise the confidentiality and integrity of digital communications on the Internet and elsewhere. Under such a threat model, the current key encapsulation mechanisms would be vulnerable. Post-quantum key encapsulation mechanisms (PQ-KEM) are being developed in order to provide secure key establishment against an adversary with access to a quantum computer. As the National Institute of Standards and Technology (NIST) is still in the process of selecting the new post-quantum cryptographic algorithms that are secure against both quantum and classical computers, the purpose of this document is to propose a generic "algorithm-agnostic" solution to protect in confidentiality the CMS envelopped-data content against the quantum threat : the KEM-TRANS mechanism. This mechanism could thus be used with any key encapsulation mechanism, including post-quantum KEMs or hybrid KEMs. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. The following terms are used in this document: BER: Basic Encoding Rules (BER) as defined in [X.690]. DER: Distinguished Encoding Rules as defined in [X.690]. 3. Design Rationales In the Envelopped-Data Content section of [RFC5652], two levels of encryptions are defined for CMS: * the Content-encryption process which protects the data thanks to a symmetric algorithm used with a content encryption key (CEK); Perret, et al. Expires 26 May 2022 [Page 3] Internet-Draft PQ KEM in CMS November 2021 * the Key-encryption process which protects this CEK thanks to a key transport mechanism. One of the typical use case of the CMS Envelopped-Data Content is to randomly generate a CEK, encrypt the data with a symmetric algorithm using this CEK and individually send the CEK to several recipients. In this scenario the CEK must be protected individually to the different recipients. To achieve this scenario with KEM primitives, it is necessary to define a new key transport mechanism that will fulfil the following requirements: * the Key Transport Mechanism SHALL be secure againt quantum computers. * the Key Transport Mechanism SHALL take the Content-Encryption Key (CEK) as input. According to NIST, a KEM encapsulation algorithm generates a random secret and a ciphertext from which the recipient can extract the shared secret, meaning that a KEM can not be used straightforward as a key transport mechanism in the CMS "multi-recipients" context. The KEM-TRANS mechanism defined in this document aims to turn a KEM into a key transport scheme allowing the sender to distribute a randomly generated key to several recipients. The KEM-TRANS Key transport mechanism described in the following section fulfils the requirements listed above and is an adaptation of the RSA-KEM algorithm previously specified in [RFC5652]. 4. KEM Key Transport Mechanism (KEM-TRANS) The KEM Key Transport Mechanism (KEM-TRANS) is a one-pass (store-and- forward) mechanism for transporting keying data to a recipient. With this type of mechanism, a sender encrypts the keying data using the recipient's public key to obtain encrypted keying data. The recipient can then decrypt the encrypted keying data using his private key to recover the plaintext keying data. 4.1. Underlying Components The KEM-TRANS mechanism has the following underlying components: * KEM, a Key Encapsulation Mechanism; * KDF, a Key Derivation Function, which derives keying data of a specified length from a shared secret value; Perret, et al. Expires 26 May 2022 [Page 4] Internet-Draft PQ KEM in CMS November 2021 * WRAP, a symmetric key-wrapping scheme, which encrypts keying Data using a key-encrypting key (KEK). 4.1.1. KEM A KEM is cryptographic algorithm consisting of three functions : * a key generation function *KEM.keygen* taking as input a security level and returning a key pair (private key and the associated public key) for this security level. * an encapsulation function *KEM.encaps* taking a public key as input and returning a random session key and a ciphertext that is an encapsulation of the session key. * a decaspulation function *KEM.decaps* taking as input a private key and a ciphertext and returning a session key. 4.1.2. KDF A key derivation function (KDF) is a cryptographic function that derives one or more secret keys from a secret value using a pseudorandom function. KDFs can be used to stretch keys into longer keys or to obtain keys of a required format. 4.1.3. WRAP A wrapping algorithm is a symmetric algorithm protecting data in confidentiality and integrity. It is especially designed to transport key material. the WRAP algorithm consists of two functions : * a wrapping function *Wrap* taking a wrapping key and a plaintext key as input and returning a wrapped key. * a decaspulation function *Unwrap* taking as input a wrapping key and a wraped key and returning the plaintext key. In the following, _kekLen_ denotes the length in bytes of the wrapping key for the underlying symmetric key-wrapping scheme. In this scheme, the length of the keying data to be transported MUST be among the lengths supported by the underlying symmetric key- wrapping scheme. Usage and formatting of the keying data is outside the scope of this algorithm. With some key derivation functions, it is possible to include other information besides the shared secret value in the Perret, et al. Expires 26 May 2022 [Page 5] Internet-Draft PQ KEM in CMS November 2021 input to the function. Also, with some symmetric key-wrapping schemes, it is possible to associate a label with the keying data. Such uses are outside the scope of this document, as they are not directly supported by CMS. 4.2. Recipient's Key Generation and Distribution The KEM-TRANS mechanism described in the next sections assumes that the recipient has previously generated a key pair (_recipPrivKey_ and _recipPubKey_) and has distributed this public key to the sender. The protocols and mechanisms by which the key pair is securely generated and the public key is securely distributed are out of the scope of this document. 4.3. Sender's Operations Let _recipPubKey_ be the recipient's public key, and let _K_ be the keying data to be transported. The sender performs the following operations: 1. Generate a shared secret _SS_ and the associated ciphertext _CT_ thanks to the KEM encaspulation function and the recipient's public key _recipPubKey_: (SS, CT) = KEM.encaps(recipPubKey) 2. Derive a key-encrypting key _KEK_ of length _kekLen_ bytes from the shared secret _SS_ using the underlying key derivation function: KEK = KDF(SS, kekLen) 3. Wrap the keying data _K_ with the key-encrypting key _KEK_ using the underlying key-wrapping scheme to obtain wrapped keying data _WK_ of length _wrappedKekLen_: WK = Wrap(KEK, K) 4. Concatenate the wrapped keying data _WK_ of length _wrappedKekLen_ and the ciphertext _CT_ to obtain the encrypted keying data _EK_: EK = (WK || CT) 5. Output the encrypted keying data _EK_. Perret, et al. Expires 26 May 2022 [Page 6] Internet-Draft PQ KEM in CMS November 2021 4.4. Recipient's Operations Let _recipPrivKey_ be the recipient's private key and let _EK_ be the encrypted keying data. The recipient performs the following operations: 1. Separate the encrypted keying data _EK_ into wrapped keying data _WK_ of length _wrappedKekLen_ and a ciphertext _CT_ : (WK || CT) = EK 2. Decapsulate the ciphertext _CT_ thanks to the KEM decaspulation function and the recipient's private key to retrieve the shared secret _SS_: SS = KEM.decaps(recipPrivKey, CT) If the decapsulation operation outputs an error, output "decryption error", and stop. 3. Derive a key-encrypting key _KEK_ of length _kekLen_ bytes from the shared secret _SS_ using the underlying key derivation function: KEK = KDF(SS, kekLen) 4. Unwrap the wrapped keying data _WK_ with the key-encrypting key _KEK_ using the underlying key-wrapping scheme to recover the keying data _K_: K = Unwrap(KEK, WK) If the unwrapping operation outputs an error, output "decryption error", and stop. 5. Output the keying data _K_. 5. Use in CMS The KEM Key Transport Mechanism MAY be employed for one or more recipients in the CMS enveloped-data content type (Section 6 of [RFC5652]), where the keying data processed by the mechanism is the CMS content-encryption key (_CEK_). 5.1. RecipientInfo Conventions Perret, et al. Expires 26 May 2022 [Page 7] Internet-Draft PQ KEM in CMS November 2021 EDITOR'S NOTE' - TO BE DISCUSSED 3 possibilities to communicate info: -Use KeyTransRecipientInfo (as it is done in RSA-KEM and in here) -Define in this RFC a KemRecipientInfo as instance of OtherRecipientInfo -Define in this RFC a new top-level KemRecipientInfo When the KEM Key Transport Mechanism is employed for a recipient, the RecipientInfo alternative for that recipient MUST be KeyTransRecipientInfo. The mechanism satisfies the KeyTransportRecipientInfo interface defined in RFC 5652 by taking in a Content Encryption Key (CEK) and a recipient public key, and encrypting the CEK for that recipient. The algorithm-specific fields of the KeyTransRecipientInfo value MUST have the following values: * keyEncryptionAlgorithm.algorithm MUST be id-kem-trans (see Appendix A); * keyEncryptionAlgorithm.parameters MUST be a value of type GenericHybridParameters (see Appendix A), identifying: - the key encapsulation mechanism (id-kem); - the key derivation function (id-kdf); - the symmetric wrapping mechanism (id-wrap). * encryptedKey MUST be the encrypted keying data output by the KEM- TRANS Mechanism, where the keying data is the content-encryption key (CEK). 5.2. Certificate Conventions The conventions specified in this section augment [RFC5280]. 5.2.1. Key Usage Extension The intended application for the key MAY be indicated in the key usage certificate extension (see [RFC5280], Section 4.2.1.3). If the keyUsage extension is present in a certificate that conveys a public key with the id-kem object identifier as discussed above, then the key usage extension MUST contain only the value _keyEncipherment_ _digitalSignature_, _nonRepudiation_, _dataEncipherment_, _keyAgreement_, _keyCertSign_, _cRLSign_, _encipherOnly_ and _decipherOnly_ SHOULD NOT be present. Perret, et al. Expires 26 May 2022 [Page 8] Internet-Draft PQ KEM in CMS November 2021 A key intended to be employed only with the KEM-TRANS Mechanism SHOULD NOT also be employed for data encryption. Good cryptographic practice employs a given key pair in only one scheme. This practice avoids the risk that vulnerability in one scheme may compromise the security of the other, and may be essential to maintain provable security. 5.2.2. Subject Public Key Info EDITOR'S NOTE' - TODO Update this section according to the future PQC-PKIX RFCs If the recipient wishes only to employ the KEM-TRANS Mechanism with a given public key, the recipient MUST identify the public key in the certificate using the _id-kem_ object identifier. EDITOR'S NOTE' - TODO Clarify that id-kem refers to the KEM algo while KEM-TRANS refers to the KEM Transport mechanism 5.3. SMIME Capabilities Attribute Conventions [RFC8551], Section 2.5.2 defines the SMIMECapabilities signed attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be used to specify a partial list of algorithms that the software announcing the SMIMECapabilities can support. When constructing a signedData object, compliant software MAY include the SMIMECapabilities signed attribute announcing that it supports the KEM Key Transport Mechanism. The SMIMECapability SEQUENCE representing the KEM Key Transport Mechanism MUST include the id-kem-trans object identifier in the capabilityID field and MUST include a GenericHybridParameters value in the parameters field identifying the components with which the mechanism is to be employed. The DER encoding of a SMIMECapability SEQUENCE is the same as the DER encoding of an AlgorithmIdentifier. Example DER encodings for typical sets of components are given in Appendix A. 6. Security Considerations EDITOR'S NOTE' - TODO section to be completed Perret, et al. Expires 26 May 2022 [Page 9] Internet-Draft PQ KEM in CMS November 2021 7. IANA Considerations Within the CMS, algorithms are identified by object identifiers (OIDs). With one exception, all of the OIDs used in this document were assigned in other IETF documents, in ISO/IEC standards documents, by the National Institute of Standards and Technology (NIST). The two exceptions are the ASN.1 module's identifier and id- kem-transport that are both assigned in this document. 8. Acknowledgements This document incorporates contributions and comments from a large group of experts. The Editors would especially like to acknowledge the expertise and tireless dedication of the following people, who attended many long meetings and generated millions of bytes of electronic mail and VOIP traffic over the past year in pursuit of this document: EDITOR'S NOTE' - TODO section to be completed We are grateful to all, including any contributors who may have been inadvertently omitted from this list. This document borrows text from similar documents, including those referenced below. Thanks go to the authors of those documents. "Copying always makes things easier and less error prone" - [RFC8411]. 9. Annex A : ASN.1 Syntax The ASN.1 syntax for identifying the KEM Key Transport Mechanism is an extension of the syntax for the "generic hybrid cipher" in ANS X9.44 [X9.44]. The syntax for the scheme is given in Appendix A.1. The syntax for selected underlying components including those mentioned above is given in Appendix A.2. 9.1. Annex A1 : KEM-TRANS Key Transport Mechanism The object identifier for the KEM Key Transport Mechanism is id-kem- trans, which is defined in this document as: id-kem-trans OID ::= { TBD } Perret, et al. Expires 26 May 2022 [Page 10] Internet-Draft PQ KEM in CMS November 2021 When id-kem-trans is used in an AlgorithmIdentifier, the parameters MUST employ the GenericHybridParameters syntax. The syntax for GenericHybridParameters is as follows: GenericHybridParameters ::= { kem KeyEncapsulationMechanism, kdf KeyDerivationFunction, wrap KeyWrappingMechanism } The fields of type GenericHybridParameters have the following meanings: * kem identifies the underlying key encapsulation mechanism (KEM). This can be any NIST KEM. * kdf identifies the underlying key derivation function (KDF). This can be any KDF from [SP-800-56C-r2]. kdf can be equal to _null_ if the key encaspulation mechanism outputs a shared secret _SS_ of size _kekLen_. * wrap identifies the underlying key wrapping mechanism (WRAP). This can be any wrapping mechanism from [RFC5649]. 9.2. Annex A2 : Underlying Components 9.2.1. Key Encapsulation Mechanisms The KEM-TRANS Mechanism can support any NIST KEM, including post- quantum KEMs. The object identifier for KEM is (TBD) EDITOR'S NOTE' - TODO : PQ-KEMs OID have to be defined 9.2.2. Key Derivation Functions The KEM-TRANS Mechanism can support any KDF from [SP-800-56C-r2]. The KDF can be bypassed if the key encaspulation mechanism outputs a shared secret _SS_ of size _kekLen_. kdf is then equal to _null_. EDITOR'S NOTE' - TO BE DISCUSSED : Is there a RFC defining KDF with SHA3? Should we limit the compatible KDFs to [SP-800-56C-r2]? Perret, et al. Expires 26 May 2022 [Page 11] Internet-Draft PQ KEM in CMS November 2021 9.2.3. Key Wrapping Schemes The KEM-TRANS Mechanism can support any wrapping mechanism from [RFC5649]. EDITOR'S NOTE' - TO BE DISCUSSED : Should we limit the compatible wrapping modes to [RFC5649]? 10. References 10.1. 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, . [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, . [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/ Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification", RFC 8551, DOI 10.17487/RFC8551, April 2019, . [SP-800-56C-r2] NIST, "Recommendation for Key-Derivation Methods in Key- Establishment Schemes", 2020. [X.690] ASC, "Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", 2007. Perret, et al. Expires 26 May 2022 [Page 12] Internet-Draft PQ KEM in CMS November 2021 [X9.44] ASC, "American National Standard X9.44: Public Key Cryptography for the Financial Services Industry -- Key Establishment Using Integer Factorization Cryptography", 2007. 10.2. Informative References [RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, DOI 10.17487/RFC2986, November 2000, . [RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard (AES) Key Wrap with Padding Algorithm", RFC 5649, DOI 10.17487/RFC5649, September 2009, . [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, . [RFC5990] Randall, J., Kaliski, B., Brainard, J., and S. Turner, "Use of the RSA-KEM Key Transport Algorithm in the Cryptographic Message Syntax (CMS)", RFC 5990, DOI 10.17487/RFC5990, September 2010, . [RFC8411] Schaad, J. and R. Andrews, "IANA Registration for the Cryptographic Algorithm Object Identifier Range", RFC 8411, DOI 10.17487/RFC8411, August 2018, . [SP-800-108] NIST, "Recommendation for Key Derivation Using Pseudorandom Functions", 2009. Authors' Addresses Ludovic Perret CryptoNext Security Email: ludovic.perret@cryptonext-security.com Julien Prat CryptoNext Security Perret, et al. Expires 26 May 2022 [Page 13] Internet-Draft PQ KEM in CMS November 2021 Email: julien.prat@cryptonext-security.com Mike Ounsworth Entrust Limited Email: mike.ounsworth@entrust.com Perret, et al. Expires 26 May 2022 [Page 14]