]>

Austria hannes.tschofenig@gmx.net

Transmute

United States orie@transmute.industries

Japan dajiaji@gmail.com

Security Theory LLC

United States lgl@securitytheory.com

Security COSE Internet-Draft This specification defines hybrid public-key encryption (HPKE) for use with CBOR Object Signing and Encryption (COSE). HPKE offers a variant of public-key encryption of arbitrary-sized plaintexts for a recipient public key. HPKE works for any combination of an asymmetric key encapsulation mechanism (KEM), key derivation function (KDF), and authenticated encryption with additional data (AEAD) function. Authentication for HPKE in COSE is provided by COSE-native security mechanisms or by one of the authenticated variants of HPKE. This document defines the use of the HPKE with COSE.

Introduction Hybrid public-key encryption (HPKE) is a scheme that provides public key encryption of arbitrary-sized plaintexts given a recipient's public key. HPKE utilizes a non-interactive ephemeral-static Diffie-Hellman exchange to establish a shared secret. The motivation for standardizing a public key encryption scheme is explained in the introduction of . The HPKE specification provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key, one that authenticates possession of a key encapsulation mechanism (KEM) private key, and one that authenticates possession of both a pre-shared key and a KEM private key. This specification utilizes HPKE as a foundational building block and carries the output to COSE (, ).

Conventions and 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 when, and only when, they appear in all capitals, as shown here. This specification uses the following abbreviations and terms: Content-encryption key (CEK), a term defined in CMS . Hybrid Public Key Encryption (HPKE) is defined in . pkR is the public key of the recipient, as defined in . skR is the private key of the recipient, as defined in . Key Encapsulation Mechanism (KEM), see . Key Derivation Function (KDF), see . Authenticated Encryption with Associated Data (AEAD), see . Additional Authenticated Data (AAD), see .

HPKE for COSE

Overview This specification supports two uses of HPKE in COSE, namely HPKE in a single recipient setup. This use case utilizes a one layer COSE structure. provides the details. HPKE in a multiple recipient setup. This use case requires a two layer COSE structure. provides the details. While it is possible to support the single recipient use case with a two layer structure, the single layer setup is more efficient. In both cases a new COSE header parameter, called 'encapsulated_key', is used to convey the content of the enc structure defined in the HPKE specification. "Enc" represents the serialized public key. For use with HPKE the 'encapsulated_key' header parameter MUST be present in the unprotected header parameter and MUST contain the encapsulated key, which is output of the HPKE KEM, and it is a bstr.

Single Recipient / One Layer Structure With the one layer structure the information carried inside the COSE_recipient structure is embedded inside the COSE_Encrypt0. HPKE is used to directly encrypt the plaintext and the resulting ciphertext is either included in the COSE_Encrypt0 or is detached. If a payload is transported separately then it is called "detached content". A nil CBOR object is placed in the location of the ciphertext. See Section 5 of for a description of detached payloads. The sender MUST set the alg parameter in the protected header, which indicates the use of HPKE. The sender MUST place the 'encapsulated_key' parameter into the unprotected header. Although the use of the 'kid' parameter in COSE_Encrypt0 is discouraged by RFC 9052, this profile allows the use of the 'kid' parameter (or other parameters) to identify the static recipient public key used by the sender. If the COSE_Encrypt0 contains the 'kid' then the recipient may use it to select the appropriate private key. HPKE defines an API and this API uses an "aad" parameter as input. When COSE_Encrypt0 is used then there is no AEAD function executed by COSE natively and HPKE offers this functionality. The "aad" parameter provided to the HPKE API is constructed as follows (and the design has been re-used from ):

The protected field in the Enc_structure contains the protected attributes from the COSE_Encrypt0 structure at layer 0, encoded in a bstr type. The external_aad field in the Enc_structure contains the Externally Supplied Data described in Section 4.3 and Section 5.3 in RFC 9052. If this field is not supplied, it defaults to a zero-length byte string. The HPKE APIs also use an "info" parameter as input and the details are provided in . shows the COSE_Encrypt0 CDDL structure.

The COSE_Encrypt0 MAY be tagged or untagged. An example is shown in .

Multiple Recipients / Two Layer Structure With the two layer structure the HPKE information is conveyed in the COSE_recipient structure, i.e. one COSE_recipient structure per recipient. In this approach the following layers are involved: Layer 0 (corresponding to the COSE_Encrypt structure) contains the content (plaintext) encrypted with the CEK. This ciphertext MAY be detached. If not detached, then it is included in the COSE_Encrypt structure. Layer 1 (corresponding to a recipient structure) contains parameters needed for HPKE to generate a shared secret used to encrypt the CEK. This layer conveys the encrypted CEK in the encCEK structure. The protected header MUST contain the HPKE alg parameter and the unprotected header MUST contain the 'encapsulated_key' parameter. The unprotected header MAY contain the kid parameter to identify the static recipient public key the sender has been using with HPKE. This two-layer structure is used to encrypt content that can also be shared with multiple parties at the expense of a single additional encryption operation. As stated above, the specification uses a CEK to encrypt the content at layer 0. The COSE_recipient structure, shown in , is repeated for each recipient.

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The COSE_Encrypt MAY be tagged or untagged. An example is shown in .

Info Parameter The HPKE specification defines the "info" parameter as a context information structure that is used to ensure that the derived keying material is bound to the context of the transaction. This section provides a suggestion for constructing the info structure. HPKE leaves the info parameter for these two functions as optional. Application profiles of this specification MAY populate the fields of the COSE_KDF_Context structure or MAY use a different structure as input to the "info" parameter. If no content for the "info" parameter is not supplied, it defaults to a zero-length byte string. This specification re-uses the context information structure defined in as a foundation for the info structure. This payload becomes the content of the info parameter for the HPKE functions, when utilized. For better readability of this specification the COSE_KDF_Context structure is repeated in .

Ciphersuite Registration This specification registers a number of ciphersuites for use with HPKE. A ciphersuite is thereby a combination of several algorithm configurations: HPKE Mode KEM algorithm KDF algorithm AEAD algorithm The "KEM", "KDF", and "AEAD" values are conceptually taken from the HPKE IANA registry . Hence, COSE-HPKE cannot use a algorithm combination that is not already available with HPKE. For better readability of the algorithm combination ciphersuites labels are build according to the following scheme:

---- ]]>

The "Mode" indicator may be populated with the following values from Table 1 of : "Base" refers to "mode_base" described in Section 5.1.1 of , which only enables encryption to the holder of a given KEM private key. "PSK" refers to "mode_psk", described in Section 5.1.2 of , which authenticates using a pre-shared key. "Auth" refers to "mode_auth", described in Section 5.1.3 of , which authenticates using an asymmetric key. "Auth_Psk" refers to "mode_auth_psk", described in Section 5.1.4 of , which authenticates using both a PSK and an asymmetric key. For a list of ciphersuite registrations, please see . The following table summarizes the relationship between the ciphersuites registered in this document and the values registered in the HPKE IANA registry .

Note that the last four entries in the table refer to the compact encoding of the public keys defined in . As the list indicates, the ciphersuite labels have been abbreviated at least to some extend to maintain the tradeoff between readability and length.

Examples This section provides a set of examples that shows all COSE message types (COSE_Encrypt0, COSE_Encrypt and COSE_MAC) to which the COSE-HPKE can be applied. Each example includes the following information that can be used to check the interoperability of COSE-HPKE implementations: plaintext: Original data of the encrypted payload. external_aad: Externally supplied AAD. skR: A recipient private key. skE: An ephemeral sender private key paired with the encapsulated_key.

Single Recipient / One Layer Example This example assumes that a sender wants to communicate an encrypted payload to a single recipient in the most efficient way. An example of the COSE_Encrypt0 structure using the HPKE scheme is shown in . Line breaks and comments have been inserted for better readability. This example uses the following: alg: HPKE-Base-P256-SHA256-AES128GCM plaintext: "This is the content." external_aad: "COSE-HPKE app" skR: h'57c92077664146e876760c9520d054aa93c3afb04e306705db6090308507b4d3' skE: h'42dd125eefc409c3b57366e721a40043fb5a58e346d51c133128a77237160218'

Multiple Recipients / Two Layer In this example we assume that a sender wants to transmit a payload to two recipients using the two-layer structure. Note that it is possible to send two single-layer payloads, although it will be less efficient.

COSE_Encrypt An example of the COSE_Encrypt structure using the HPKE scheme is shown in . Line breaks and comments have been inserted for better readability. This example uses the following: Encryption alg: AES-128-GCM plaintext: "This is the content." detatched ciphertext: h'cc168c4e148c52a83010a75250935a47ccb8682deebcef8fce5d60c161e849f53a2dc664' kid:"01" alg: HPKE-Base-P256-SHA256-AES128GCM external_aad: "COSE-HPKE app" skR: h'57c92077664146e876760c9520d054aa93c3afb04e306705db6090308507b4d3' skE: h'97ad883f949f4cdcb1301b9446950efd4eb519e16c4a3d78304eec832692f9f6' kid:"02" alg: HPKE-Base-X25519-SHA256-CHACHA20POLY1305 external_aad: "COSE-HPKE app" skR: h'bec275a17e4d362d0819dc0695d89a73be6bf94b66ab726ae0b1afe3c43f41ce' skE: h'b8ed3f4df56c230e36fa6620a47f24d08856d242ea547c5521ff7bd69af8fd6f'

To offer authentication of the sender the payload in is signed with a COSE_Sign1 wrapper, which is outlined in . The payload in is meant to contain the content of .

COSE_MAC An example of the COSE_MAC structure using the HPKE scheme is shown in . This example uses the following: MAC alg: HMAC 256/256 payload: "This is the content." kid:"01" alg: HPKE-Base-P256-SHA256-AES128GCM external_aad: "COSE-HPKE app" skR: h'57c92077664146e876760c9520d054aa93c3afb04e306705db6090308507b4d3' skE: h'e5dd9472b5807636c95be0ba2575020ba91cbb3561b52be141da89678c664307' kid:"02" alg: HPKE-Base-X25519-SHA256-CHACHA20POLY1305 external_aad: "COSE-HPKE app" skR: h'bec275a17e4d362d0819dc0695d89a73be6bf94b66ab726ae0b1afe3c43f41ce' skE: h'78a49d7af71b5244498e943f361aa0250184afc48b8098a68ae97ccd2cd7e56f'

Security Considerations This specification is based on HPKE and the security considerations of are therefore applicable also to this specification. HPKE assumes the sender is in possession of the public key of the recipient and HPKE COSE makes the same assumptions. Hence, some form of public key distribution mechanism is assumed to exist but outside the scope of this document. HPKE relies on a source of randomness to be available on the device. Additionally, with the two layer structure the CEK is randomly generated and it MUST be ensured that the guidelines in for random number generations are followed. HPKE in Base mode does not offer authentication as part of the HPKE KEM. In this case COSE constructs like COSE_Sign, COSE_Sign1, COSE_MAC, or COSE_MAC0 can be used to add authentication. HPKE also offers modes that offer authentication. If COSE_Encrypt or COSE_Encrypt0 is used with a detached ciphertext then the subsequently applied integrity protection via COSE_Sign, COSE_Sign1, COSE_MAC, or COSE_MAC0 does not cover this detached ciphertext. Implementers MUST ensure that the detached ciphertext also experiences integrity protection. This is, for example, the case when an AEAD cipher is used to produce the detached ciphertext but may not be guaranteed by non-AEAD ciphers.

IANA Considerations This document requests IANA to add new values to the 'COSE Algorithms' and to the 'COSE Header Parameters' registries.

COSE Algorithms Registry Name: HPKE-Base-P256-SHA256-AES128GCM Value: TBD1 (Assumed: 35) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(P-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the AES-128-GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-P256-SHA256-ChaCha20Poly1305 Value: TBD2 (Assumed: 36) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(P-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-P384-SHA384-AES256GCM Value: TBD3 (Assumed: 37) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(P-384, HKDF-SHA384) KEM, the HKDF-SHA384 KDF, and the AES-256-GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-P384-SHA384-ChaCha20Poly1305 Value: TBD4 (Assumed: 38) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(P-384, HKDF-SHA384) KEM, the HKDF-SHA384 KDF, and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-P521-SHA512-AES256GCM Value: TBD5 (Assumed: 39) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(P-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the AES-256-GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-P521-SHA512-ChaCha20Poly1305 Value: TBD6 (Assumed: 40) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(P-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-X25519-SHA256-AES128GCM Value: TBD7 (Assumed: 41) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and the AES-128-GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-X25519-SHA256-ChaCha20Poly1305 Value: TBD8 (Assumed: 42) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-X448-SHA512-AES256GCM Value: TBD9 (Assumed: 43) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the AES-256-GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-X448-SHA512-ChaCha20Poly1305 Value: TBD10 (Assumed: 44) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-X25519Kyber768-SHA256-AES256GCM Value: TBD11 (Assumed: 250) Description: Cipher suite for COSE-HPKE in Base Mode that uses the X25519Kyber768Draft00 KEM, the HKDF-SHA256 KDF, and the AES-256-GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: No Name: HPKE-Base-X25519Kyber768-SHA256-ChaCha20Poly1305 Value: TBD12 (Assumed: 251) Description: Cipher suite for COSE-HPKE in Base Mode that uses the X25519Kyber768Draft00 KEM, the HKDF-SHA256 KDF, and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: No Name: HPKE-Base-CP256-SHA256-ChaCha20Poly1305 Value: TBD13 (Assumed: 45) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(CP-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-CP521-SHA512-ChaCha20Poly1305 Value: TBD14 (Assumed: 46) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(CP-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the ChaCha20Poly1305 AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-CP256-SHA256-AES128GCM Value: TBD15 (Assumed: 47) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(CP-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the AES128GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes Name: HPKE-Base-CP521-SHA512-AES256GCM Value: TBD16 (Assumed: 47) Description: Cipher suite for COSE-HPKE in Base Mode that uses the DHKEM(CP-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the AES256GCM AEAD. Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

COSE Header Parameters Name: encapsulated_key Label: TBDX (Assumed: -4) Value type: bstr Value Registry: N/A Description: HPKE encapsulated key Reference: [[This specification]]

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. This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. Randomness is a crucial ingredient for Transport Layer Security (TLS) and related security protocols. Weak or predictable "cryptographically secure" pseudorandom number generators (CSPRNGs) can be abused or exploited for malicious purposes. An initial entropy source that seeds a CSPRNG might be weak or broken as well, which can also lead to critical and systemic security problems. This document describes a way for security protocol implementations to augment their CSPRNGs using long-term private keys. This improves randomness from broken or otherwise subverted CSPRNGs. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. This document describes the Cryptographic Message Syntax. This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary messages. [STANDARDS-TRACK] Hewlett-Packard Enterprise This document describes enhancements to the Hybrid Public Key Encryption standard published by CFRG. These include use of "compact representation" of relevant public keys, support for key-wrapping, and two ways to address the use of HPKE on lossy networks: a determinstic, nonce-less AEAD scheme, and use of a rolling sequence number with existing AEAD schemes. IANA

Contributors We would like thank the following individuals for their contributions to the design of embedding the HPKE output into the COSE structure following a long and lively mailing list discussion: Richard Barnes Ilari Liusvaara Finally, we would like to thank Russ Housley and Brendan Moran for their contributions to the draft as co-authors of initial versions.

Acknowledgements We would like to thank John Mattsson, Mike Prorock, Michael Richardson, and Goeran Selander for their review feedback.