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hannes.tschofenig@gmx.net

Arm Limited

Brendan.Moran@arm.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. This document defines the use of the HPKE base mode with COSE. Other modes are supported by HPKE but not by this specification.

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 defines several features for use with public key encryption and a subset of those features is applied to COSE (, ). Since COSE provides constructs for authentication, those are not re-used from the HPKE specification. This specification uses the "base" mode, as it is called in HPKE specification language.

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 cases uses 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. HPKE in Base mode requires little information to be provided by the sender, namely algorithm information (KEM, KDF, and AEAD identifiers), an encapsulated key generated by the sender, and an identifier of the static recipient key. In the subsections below we explain how this information is carried inside the COSE_Encrypt0 and the COSE_Encrypt for the one layer and the two layer structure, respectively. In both cases a new structure is used to convey information about the HPKE sender, namely the HPKE sender information structure (sender_info). When the alg value is set to 'HPKE-v1-BASE', the sender_info structure MUST be present in the unprotected header parameter. The CDDL grammar describing the sender_info structure is:

The fields have the following meaning:

kem_id: This parameter is used to identify the KEM. The registry for KEM ids has been established with RFC 9180. kdf_id: This parameter contains the KDF identifier. The registry containing the KDF ids has been established with RFC 9180. aead_id: This parameter contains the AEAD identifier. The registry containing the AEAD ids has been established with RFC 9180. enc: This parameter contains the encapsulated key, which is output of the HPKE KEM.

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. The resulting ciphertext may be included in the COSE_Encrypt0 or may be 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 kid parameter and the sender_info structure into the unprotected header. The kid identifies the static recipient public key used by the sender. The recipient uses the kid to determine the appropriate private key. 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 sender_info structure as well as 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. For example, the content encrypted at layer 0 may be a firmware image. The same encrypted firmware image may need to be sent to many recipients; however, each recipient uses their own private key to obtain the CEK. 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 .

HPKE Encryption and Decryption

HPKE Encryption with SealBase The SealBase(pkR, info, aad, pt) function is used to encrypt a plaintext pt to a recipient's public key (pkR). Two cases of plaintext need to be distinguished: For use in COSE_Encrypt, the plaintext "pt" passed into
SealBase is the CEK. The CEK is a random byte sequence of length appropriate for the encryption algorithm selected in layer 0. For example, AES-128-GCM requires a 16 byte key and the CEK would therefore be 16 bytes long. In case of COSE_Encrypt0, the plaintext "pt" passed into SealBase is the content to be encrypted. Hence, there is no intermediate layer utilizing a CEK. The "aad" and the "info" parameters are described in and , respectively. If SealBase() is successful, it will output a ciphertext "ct" and an encapsulated key "enc".

HPKE Decryption with OpenBase The recipient will use the OpenBase(enc, skR, info, aad, ct) function with the "enc" and the "ct" parameters received from the sender. The "aad" and the "info" parameters are assumed to be constructed from the context and described in and , respectively. The OpenBase function will, if successful, decrypt "ct". When decrypted, the result will be either the CEK (when COSE_Encrypt is used), or the content (if COSE_Encrypt0 is used). The CEK is the symmetric key used to decrypt the ciphertext at layer 0.

AAD Parameter HPKE requires an "aad" parameter to be provided to the SealBase and OpenBase functions. Note that there are three types of additional authenticated data used by this specification: AAD provided to HPKE for COSE_Encrypt0. AAD provided to HPKE for COSE_Encrypt at the recipient layer. AAD provided to the AEAD cipher used for content encryption at layer 0 by COSE_Encrypt. We describe the three variants in the subsections below.

AAD provided to HPKE for COSE_Encrypt0 When COSE_Encrypt0 is used then there is no separate AEAD function at the content encryption layer provided by COSE natively and HPKE offers this functionality. The "aad" parameter of provided to the SealBase and OpenBase functions is constructed as follows (again intentionally aligned with COSE by re-using the Enc_structure):

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 is populated with the API caller provided AAD information. If this field is not supplied, it defaults to a zero-length byte string.

AAD provided to HPKE for COSE_Encrypt at the Recipient Layer The AAD used at the recipient layer re-uses Enc_structure from and populates it with the following content:

The protected field in the Enc_structure contains the protected attributes from the COSE_recipient structure at layer 1, encoded in a bstr type. The external_aad field in the Enc_structure is populated with the API caller provided AAD information. In the COSE_Encrypt case this AAD information is also input to the AAD at layer 0, if an AEAD cipher is used at layer 0. If this field is not supplied, it defaults to a zero-length byte string.

AAD provided to the AEAD cipher used for Content Encryption at Layer 0 by COSE_Encrypt The construction of AAD is defined in Section 5.3 of (see Enc_structure structure).

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, when used with SealBase() and OpenBase(). 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 .

Examples

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. It uses the following algorithm combination: - KEM: DHKEM(P-256, HKDF-SHA256) - KDF: HKDF-SHA256 - AEAD: AES-128-GCM

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. An example of the COSE_Encrypt structure using the HPKE scheme is shown in . Line breaks and comments have been inserted for better readability. It uses the following algorithm combination: At layer 0 AES-128-GCM is used for encryption of the detached plaintext "This is the content.". At the recipient structure at layer 1, DHKEM(P-256, HKDF-SHA256) (as the KEM), with AES-128-GCM (as the AEAD) and HKDF-SHA256 (as the KDF) is used. The algorithm selection is based on the registry of the values offered by the alg parameters (see ).

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

Security Considerations This specification is based on HPKE and the security considerations of HPKE 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. 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 the it MUST be ensured that the guidelines for random number generations are followed. The COSE_Encrypt structure MUST be authenticated using COSE constructs like COSE_Sign, COSE_Sign1, COSE_MAC, or COSE_MAC0. When 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 Algorithm Parameters' registries in the 'Standards Action With Expert Review category.

COSE Algorithms Registry Name: HPKE-v1-BASE Value: TBD1 (Assumed: -1) Description: HPKE in version 1 in base mode for use with COSE Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

COSE Header Algorithm Parameters Name: sender_info Label: TBD2 (Assumed: -4) Value type: sender_info Value Registry: N/A Description: HPKE Sender Information structure for the Base mode.

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]

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. Daisuke Ajitomi Richard Barnes Ilari Liusvaara Finally, we would like to thank Russ Housley for his contributions to the draft as a co-author of initial versions.

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