Arm Limited

hannes.tschofenig@arm.com

Vigil Security, LLC

housley@vigilsec.com

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) encryption function. Authentication for HPKE in COSE is provided by COSE-native security mechanisms.

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.

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 .

The CDDL for the COSE_Encrypt structure, as used with this specification, is shown in . HPKE, when used with COSE, follows a two layer structure: Layer 0 (corresponding to the COSE_Encrypt structure) contains 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 includes the encrypted CEK. 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 is a firmware image. The same ciphertext firmware image is processed by all of the recipients; however, each recipient uses their own private key to obtain the CEK.

The COSE_recipient structure shown in is repeated for each recipient, and it includes the encrypted CEK as well as the sender-generated ephemeral public key in the unprotected header structure.

The SealBase(pkR, info, aad, pt) function is used to encrypt a plaintext pt to a recipient’s public key (pkR). IMPORTANT: For use in this specification, the plaintext “pt” passed into the 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. The “info” parameter can be used to influence the generation of keys and the “aad” parameter provides additional authenticated data to the AEAD algorithm in use. This specification does not mandate the use of the info and the aad parameters. If SealBase() is successful, it will output a ciphertext “ct” and an encapsulated key “enc”. The content of enc is the ephemeral public key. The content of the info parameter is based on the ‘COSE_KDF_Context’ structure, which is detailed in .

The recipient will use the OpenBase(enc, skR, info, aad, ct) function with the enc and ct parameters received from the sender. The “aad” and the “info” parameters are obtained via the context of the usage. The OpenBase function will, if successful, decrypt “ct”. When decrypted, the result will be the CEK. The CK is the symmetric key used to decrypt the ciphertext in layer 0 of the COSE_Encrypt structure.

This section provides a suggestion for constructing the info structure, when used with SealBase() and OpenBase(). Note that the use of the aad and the info structures for these two functions is optional. Profiles of this specification may require their use and may define different info structure. 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 .

The fields in are populated as follows: PartyUInfo.identity corresponds to the kid found in the COSE_Sign_Tagged or COSE_Sign1_Tagged structure (when a digital signature is used). When utilizing a MAC, then the kid is found in the COSE_Mac_Tagged or COSE_Mac0_Tagged structure. PartyVInfo.identity corresponds to the kid used for the respective recipient from the inner-most recipients array. The value in the AlgorithmID field corresponds to the alg parameter in the unprotected header structure of the recipient structure. keyDataLength is set to the number of bits of the desired output value. protected refers to the protected structure of the inner-most array.

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: AES-GCM-128 for encryption of detached ciphertext in layer 0. AES-GCM-128 for encryption of the CEK in layer 1 as well as ECDH with NIST P-256 and HKDF-SHA256 as a Key Encapsulation Mechanism (KEM). The algorithm selection is based on the registry of the values offered by the alg parameters.

Note that the COSE_Sign1 wrapper outside the COSE_Encrypt structure is not shown in the example above.

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. Some form of public key distribution mechanism is assumed to exist. Since the CEK is randomly generated it must be ensured that the guidelines for random number generations are followed, see . The COSE_Encrypt structure must be authenticated using COSE constructs like COSE_Sign, or COSE_Sign1.

This document requests IANA to add new values to the COSE Algorithms registry defined in (in the Standards Action With Expert Review category):

Name: HPKE_P256_HKDF256_AES128_GCM Value: TBD1 Description: HPKE/P-256+HKDF-256 and AES-128-GCM Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: HPKE_P521_HKDF512_AES256_GCM Value: TBD2 Description: HPKE/P-512+HKDF-512 and AES-256-GCM Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes TBD: More values to be added.

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. Cisco Inria Inria Cloudflare 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 for the ability to have basic security services defined 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. 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]

We would like to thank Goeran Selander, John Mattsson and Ilari Liusvaara for their review feedback.