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 .

This specification supports two uses of HPKE in COSE, namely HPKE in a single sender - single recipient setup. This use cases uses a one layer structure for efficiency. provides the details. HPKE in a single sender - multiple recipient setup. This use case requires a two layer structure. provides the details. HPKE in “base” mode requires little information to be exchanged between a sender and a recipient, namely algorithm information, the ephemeral public key, 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_Encrypt1 for the one layer and the two layer structure, respectively.

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. A sender MUST set the alg parameter in the protected header, which indicates the use of HPKE. The values for the alg parameter MUST be taken from , or values registered in the future with the COSE_ALG_HPKE_* prefix. The sender MUST place the kid and ephemeral public key into the unprotected header. shows the COSE_Encrypt0 CDDL structure.

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

The SealBase(pkR, info, aad, pt) function is used to encrypt a plaintext pt to a recipient’s public key (pkR). For use in COSE_Encrypt0, the plaintext “pt” passed into the SealBase is the raw plaintext. In the absence of an application profile standard specifying otherwise a COSE-HPKE-compliant application MUST use an empty “info” parameter. The Enc_structure, defined in Section 5.3 of , is used as input to the “aad” parameter. The CDDL fragment is defined as:

The “external_aad” is empty, unless an application profile standard specifies otherwise. 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 recipient will use the OpenBase(enc, skR, info, aad, ct) function with the enc and ct parameters received from the sender. In the absence of an application profile standard specifying otherwise a COSE-HPKE-compliant application MUST use an empty “info” parameter. The Enc_structure, defined in Section 5.3 of , is used as input to the “aad” parameter. The CDDL fragment is shown in the previous section. The OpenBase function will, if successful, decrypt “ct”. When decrypted, the result is the raw plaintext.

This example shows a COSE_Encrypt0 structure. HPKE was used to encrypt plaintext with AES-128-GCM. The ephemeral NIST P-256 key key generated by the HPKE SealBase().

>, { / ephemeral public key structure / -1: << { / kty set to EC2 / 1: 2, / crv set to P-256 / -1: 1, / x-coordinate / -2: h'985E2FDE3E67E1F7146AB305AA98FE89 B1CFE545965B6CFB066C0BB19DE7E489', / y-coordinate / -3: h'4AC5E777A7C96CB5D70B8A40E2951562 F20C21DB021AAD12E54A8DBE7EF9DF10' } >>, 4: 'kid-2' }, / encrypted plaintext / h'4123E7C3CD992723F0FA1CD3A903A588 42B1161E02D8E7FD842C4DA3B984B9CF' ] ) ]]>

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 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 algorithm information and the unprotected header MUST contain the ephemeral public key and the key id (kid) of the static recipient public key. 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 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.

values, } ]]>

The COSE_Encrypt MAY be tagged or untagged. HPKE algorithms take an info parameter that can be used to influence the generation of keys (e.g., to fold in identity information) and an aad parameter that provides additional authenticated data to the AEAD algorithm in use. An example is shown in .

The SealBase(pkR, info, aad, pt) function is used to encrypt a plaintext pt to a recipient’s public key (pkR). For use in COSE_Encrypt, 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. In the absence of an application profile standard specifying otherwise, a COSE-HPKE-compliant implementation MUST leave the info and the aad parameters empty when used with the two layer structure. 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 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. In the absence of an application profile standard specifying otherwise, a COSE-HPKE-compliant implementation MUST leave the info and the aad parameters empty when used with the two layer structure. The OpenBase function will, if successful, decrypt “ct”. When decrypted, the result will be the CEK. The CEK is the symmetric key used to decrypt the ciphertext in layer 0 of the COSE_Encrypt structure.

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. Encryption of the CEK in layer 1 utilizing HPKE 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 (see ).

>, / unprotected header / { / nonce value / 5: h'938b528516193cc7123ff037809f4c2a' }, / detached ciphertext / null, / recipient structure / [ / algorithm id TBD4 for COSE_ALG_HPKE_AES_128_GCM / << {1: TBD4} >>, / unprotected header / { / ephemeral public key structure / -1: << { / kty set to EC2 / 1: 2, / crv set to P-256 / -1: 1, / x-coordinate / -2: h'985E2FDE3E67E1F7146AB305AA98FE89 B1CFE545965B6CFB066C0BB19DE7E489', / y-coordinate / -3: h'4AC5E777A7C96CB5D70B8A40E2951562 F20C21DB021AAD12E54A8DBE7EF9DF10' } >>, 4: 'kid-2' }, / encrypted CEK / h'9aba6fa44e9b2cef9d646614dcda670dbdb31a3b9d37c7a 65b099a8152533062', ], ]) ]]>

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 .

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.

This document requests IANA to add new values to the COSE Algorithms registry and to the COSE Elliptic Curves registry, defined in (in the Standards Action With Expert Review category).

Name: COSE_ALG_HPKE_AES_128_GCM Value: TBD1 Description: HPKE with AES-128-GCM Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: COSE_ALG_HPKE_AES_256_GCM Value: TBD2 Description: HPKE with AES-256-GCM Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: COSE_ALG_HPKE_CHACHA20_POLY1305 Value: TBD3 Description: HPKE with CHACHA20-POLY1305 Capabilities: [kty] Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: COSE_CRV_HPKE_P256_SHA256 Value: TBD4 Key Type: Description: NIST P256 and SHA256 for use with HPKE Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: COSE_CRV_HPKE_P384_SHA384 Value: TBD5 Key Type: Description: NIST P384 and SHA384 for use with HPKE Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: COSE_CRV_HPKE_P521_SHA512 Value: TBD6 Key Type: Description: NIST P521 and SHA512 for use with HPKE Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: COSE_CRV_HPKE_X25519_SHA256 Value: TBD7 Key Type: Description: X25519 and SHA256 for use with HPKE Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

Name: COSE_CRV_HPKE_X448_SHA512 Value: TBD8 Key Type: Description: X448 and SHA512 for use with HPKE Change Controller: IESG Reference: [[TBD: This RFC]] Recommended: Yes

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 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.