]> IMT Atlantique

dmytro.ochkas@imt-atlantique.fr

IMT Atlantique

helene.le-bouder@imt-atlantique.fr

IMT Atlantique

alexander.pelov@imt-atlantique.fr

Infineon

Stefan.Hristozov@infineon.com

General COSE Working Group Internet-Draft This document describes CBOR Object Signing and Encryption (COSE) serialization with Ascon, a NIST standard for lightweight cryptography. In 2019, as a part of CAESAR competition, Ascon-128 and Ascon-128a were selected as the first choice for the lightweight authenticated encryption. After, in 2023, National Institute of Standards and Technology (NIST) selected Ascon family of cryptographic algorithms to be the standard for lightweight cryptography. In August 2025, NIST Special Publication 800-232 was released, defining Ascon-based lightweight cryptography standards for constrained devices. This recognition makes it particularly interesting to enable using Ascon with COSE structures.

Introduction Constrained networks such as Internet of Things (IoT) networks most of the time are characterized by the limited computational power and energy. In this context, the choice of suitable cryptographic suites that provide reliable security without consuming large amount of resources is essential. As a winner of the lightweight cryptography standardization process conducted by NIST, Ascon algorithms are the perfect candidates to fit into the described use case. The Ascon family offers a low computational cost that translates into lower latency, reduced energy consumption, and higher throughput compared to traditional crypto algorithms. Moreover, all Ascon algorithms share a single permutation function allowing for a drastic reduction of memory footprint. Ascon-Based Lightweight Cryptography Standards for Constrained Devices introduces a suite of algorithms consisting of Ascon-AEAD128, an authenticated encryption with associated data (AEAD) algorithm; Ascon-Hash256, a hash function; and two eXtendable Output Functions (XOFs). This document focuses on the usage of Ascon-AEAD128 for content encryption, Ascon-Hash256 for hashing, Key Derivation Function (KDF) and Message Authentication Code (MAC) operations, and Ascon-MAC for KDF and MAC operations. This document does not define any new cryptography, only serializations of existing cryptographic systems described in .

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.

Ascon algorithms for COSE

Ascon-AEAD128 This document introduces Ascon-AEAD128 for COSE content encryption. Ascon AEAD algorithm is parametrized by the key size k, the nonce size n, the tag size t, the rate r, and the number of outer permutation rounds a and the number of inner permutation rounds b. specifies the Ascon-AEAD128 parameters with the following numerical values. Key size, k Nonce size, n Tag size, t Rate, r Outer permutation rounds, a Inner permutation rounds, b 128 bits 128 bits 32-128 bits 128 bits 12 8 Note that the standard permits tag sizes t between 32 and 128 bits. Tags of less than 64 bits SHALL only be selected after a careful risk analysis. Since COSE can be used in a variety of scenarios, this document requests to register three configurations of Ascon-AEAD128 in with tag sizes of 32, 64, and 128 bits (see ). Using the 32-bit variation is discouraged for typical scenarios and SHOULD be introduced only in extremely constrained use cases with justified security. Name alg Description Ascon-AEAD128 TBD (requested assignment 35) Ascon-AEAD128 with 128-bit tag as the CBOR Object Encryption Algorithm Ascon-AEAD128/64 TBD (requested assignment 36) Ascon-AEAD128 with 64-bit tag as the CBOR Object Encryption Algorithm Ascon-AEAD128/32 TBD (requested assignment 37) Ascon-AEAD128 with 32-bit tag as the CBOR Object Encryption Algorithm In COSE, keys may be obtained from either a key structure or a recipient structure . When using a COSE key for this algorithm, the following checks are made: The "kty" field MUST be present, and it MUST be "Symmetric". If the "alg" field is present, it MUST match the variation of Ascon-AEAD128 algorithm being used. If the "key_ops" field is present, it MUST include "encrypt" when encrypting. If the "key_ops" field is present, it MUST include "decrypt" when decrypting. COSE encryption and decryption with Ascon-AEAD128 MUST be done in accordance with Section 5.3 of .

Ascon-Hash256 This document introduces Ascon-Hash256 for direct operations, Key Derivation Function (KDF) and Message Authentication Code (MAC) COSE operations. Ascon hash algorithm is parametrized by the rate r, the number of permutation rounds a, and the length of output l. specifies the Ascon-Hash256 parameters with the following numerical values. Rate, r Permutation rounds, a Output length, l 64 bits 12 256 bits

Ascon-Hash256 for direct usage In COSE, hash algorithms are used directly, for example, for object identification with a fingerprint, or indirect signatures (see ). specifies two Ascon-Hash256 algorithms for COSE to satisfy these use cases. Name Value Description   Recommended Ascon-Hash256 TBD (requested assignment -20) Ascon 256-bit hash Yes   Ascon-Hash256/64 TBD (requested assignment -21) Ascon 256-bit hash truncated to 64 bits Filter Only   The truncated version the Ascon hash allows for a smaller transmission overhead but with a higher risk of collision. Using this algorithm requires risk analysis or must be limited to noncryptographic use cases (see Section 2 of ).

Ascon-Hash256 for MAC operations In COSE, integration Ascon-Hash256 for MAC operations SHALL be implemented via HMAC algorithm . This algorithm is described in . Name Value Hash Tag Length Description HMAC Ascon-Hash256 TBD (requested assigment 8) Ascon-Hash256 256 bits HMAC w/ Ascon-Hash256 HMAC Ascon-Hash256/64 TBD (requested assigment 9) Ascon-Hash256 64 bits HMAC w/ Ascon-Hash256 truncated to 64 bits The creation and verification of COSE MAC using the algorithms from MUST follow Section 6.3 of .

Ascon-Hash256 for KDF operations In COSE, integration Ascon-Hash256 for KDF operations SHALL be done via HMAC-based Extract-and-Expand KDF (HKDF) defined in . HKDF leverages HMAC mechanism described in . HMAC is an algorithm allowing usage of arbirtary cryptographic hash functions for message authentication. This KDF is described in . Name PRF Description HKDF Ascon-Hash256 HMAC with Ascon-Hash256 HKDF using HMAC with Ascon-Hash256 In addition to application-specific usage, COSE KDFs are used for Direct Key with KDF and Direct Key Agreement key distribution methods. To introduce this functionality with Ascon, this document requests the registration of the following recipient algorithms: Direct Key with HKDF Ascon-Hash256, Key Agreement with Ephemeral-Static (ES) Ellyptic-Curve Diffie-Hellman (ECDH), and Key Agreement with Static-Static (SS) ECDH. These algorithms are defined in . Name Value KDF Description direct+HKDF-Ascon-Hash256 TBD (requested assigment -22) HKDF Ascon-Hash256 Shared secret w/ HKDF Ascon-Hash256 ECDH-ES+HKDF-Ascon-Hash256 TBD (requested assigment -23) HKDF Ascon-Hash256 ECDH ES w/ HKDF Ascon-Hash256 ECDH-SS+HKDF-Ascon-Hash256 TBD (requested assigment -24) HKDF Ascon-Hash256 ECDH SS w/ HKDF Ascon-Hash256 To accomodate the new algorithms specified in , this document extends the relevant header algorithm parameters as shown in . Name Label Type New Algorithm Description salt -20 bstr direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256 Random salt PartyU identity -21 bstr direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256 PartyU identity information PartyU nonce -22 bstr/int direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256 PartyU provided nonce PartyU other -23 bstr direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256 PartyU other provided information PartyV identity -24 bstr direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256 PartyV identity information PartyV nonce -25 bstr/int direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256 PartyV provided nonce PartyV other -26 bstr direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256 PartyV other provided information ephemeral key -1 COSE_Key ECDH-ES+HKDF-Ascon-Hash256 Ephemeral public key for the sender static key -2 COSE_Key ECDH-SS+HKDF-Ascon-Hash256 Static public key for the sender static key id -3 bstr ECDH-SS+HKDF-Ascon-Hash256 Static public key identifier for the sender The direct+HKDF-Ascon-Hash256 algorithm MUST be used as described in Section 8.5.1 of and Section 6.1.2 of . The ECDH-ES+HKDF-Ascon-Hash256 and ECDH-SS+HKDF-Ascon-Hash256 algorithms MUST be used as described in Section 8.5.4 of and Section 6.3.1 of .

Ascon-MAC Note: NIST is yet to standardize Ascon algorithms for MAC and KDF. So this section can be modified based on the upcoming Ascon NIST publication This document introduces Ascon-MAC for KDF and MAC COSE operations.

Ascon-MAC for MAC operations In COSE, Ascon-MAC for MAC operations SHALL be used directly. This algorithm is described in . Name Value MAC Tag Length Description Ascon-MAC TBD Ascon-MAC 256 bits Ascon MAC Ascon-MAC/64 TBD Ascon-MAC 64 bits Ascon MAC truncated to 64 bits The creation and verification of COSE MAC using the algorithms from MUST follow Section 6.3 of .

Ascon-MAC for KDF operations In COSE, Ascon-MAC SHALL be used directly as PRF for KDF operations, similarly to KMAC . Name PRF Description Ascon-MAC KDF Ascon-MAC KDF using Ascon-MAC as PRF In addition to application-specific usage, COSE KDFs are used for Direct Key with KDF and Direct Key Agreement key distribution methods. To introduce this functionality with Ascon, this document requests the registration of the following recipient algorithms: Direct Key with HKDF Ascon-Hash256, Key Agreement with Ephemeral-Static (ES) Ellyptic-Curve Diffie-Hellman (ECDH), and Key Agreement with Static-Static (SS) ECDH. These algorithms are defined in . Name Value KDF Description direct+KDF-Ascon-MAC TBD Ascon-MAC KDF Shared secret w/ Ascon-MAC KDF ECDH-ES+KDF-Ascon-MAC TBD Ascon-MAC KDF ECDH ES w/ Ascon-MAC KDF ECDH-SS+KDF-Ascon-MAC TBD Ascon-MAC KDF ECDH SS w/ Ascon-MAC KDF To accomodate the new algorithms specified in , this document extends the relevant header algorithm parameters as shown in . Name Label Type New Algorithm Description salt -20 bstr direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC Random salt PartyU identity -21 bstr direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC PartyU identity information PartyU nonce -22 bstr/int direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC PartyU provided nonce PartyU other -23 bstr direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC PartyU other provided information PartyV identity -24 bstr direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC PartyV identity information PartyV nonce -25 bstr/int direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC PartyV provided nonce PartyV other -26 bstr direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC PartyV other provided information ephemeral key -1 COSE_Key ECDH-ES+KDF-Ascon-MAC Ephemeral public key for the sender static key -2 COSE_Key ECDH-SS+KDF-Ascon-MAC Static public key for the sender static key id -3 bstr ECDH-SS+KDF-Ascon-MAC Static public key identifier for the sender The direct+HKDF-Ascon-Hash256 algorithm MUST be used as described in Section 8.5.1 of and Section 6.1.2 of . The ECDH-ES+HKDF-Ascon-Hash256 and ECDH-SS+HKDF-Ascon-Hash256 algorithms MUST be used as described in Section 8.5.4 of and Section 6.3.1 of . {:/comment}

Security Considerations The security considerations of , and apply to this specification as well. According to the most recent security analysis publications, Ascon did not show any security vulnerabilities so far and the best attacks target the initialization of Ascon reduced to 7 (out of 12) rounds, concluding that Ascon has a security margin of 5 rounds (42 % of the 12 rounds). More details are available at List of Published Analysis section of .

IANA Considerations

Additions to COSE Algorithms Registry IANA is requested to add the following entries to the COSE Algorithms Registry. The following completed registration templates are provided as described in . The "Recommended" fields for Ascon-AEAD128/32 and HMAC Ascon-Hash256/64 are set to "Filter Only" to discourage unreflected usage.

Ascon-AEAD128 for COSE Name: Ascon-AEAD128 Value: TBD (requested assignment 35) Description: Ascon-AEAD128 with 128-bit tag Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

Ascon-AEAD128/64 for COSE Name: Ascon-AEAD128/64 Value: TBD (requested assignment 36) Description: Ascon-AEAD128 with 64-bit tag Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

Ascon-AEAD128/32 for COSE Name: Ascon-AEAD128/32 Value: TBD (requested assignment 37) Description: Ascon-AEAD128 with 32-bit tag Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Filter Only

Ascon-Hash256 for COSE Name: Ascon-Hash256 Value: TBD (requested assignment -20) Description: Ascon 256-bit hash Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

Ascon-Hash256/64 for COSE Name: Ascon-Hash256/64 Value: TBD (requested assignment -21) Description: Ascon 256-bit hash truncated to 64 bits Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Filter Only

HMAC Ascon-Hash256 for COSE Name: HMAC Ascon-Hash256 Value: TBD (requested assignment 8) Description: HMAC w/ Ascon-Hash256 Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

HMAC Ascon-Hash256/64 for COSE Name: HMAC Ascon-Hash256/64 Value: TBD (requested assignment 9) Description: HMAC w/ Ascon-Hash256 truncated to 64 bits Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Filter Only

direct+HKDF-Ascon-Hash256 for COSE Name: direct+HKDF-Ascon-Hash256 Value: TBD (requested assignment -22) Description: Shared secret w/ HKDF Ascon-Hash256 Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

ECDH-ES+HKDF-Ascon-Hash256 for COSE Name: ECDH-ES+HKDF-Ascon-Hash256 Value: TBD (requested assignment -23) Description: ECDH ES w/ HKDF Ascon-Hash256 Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

ECDH-SS+HKDF-Ascon-Hash256 for COSE Name: ECDH-SS+HKDF-Ascon-Hash256 Value: TBD (requested assignment -24) Description: ECDH SS w/ HKDF Ascon-Hash256 Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

Ascon-MAC for COSE Name: Ascon-MAC Value: TBD Description: Ascon MAC Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

Ascon-MAC/64 for COSE Name: Ascon-MAC/64 Value: TBD Description: Ascon MAC truncated to 64 bits Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Filter Only

direct+KDF-Ascon-MAC for COSE Name: direct+KDF-Ascon-MAC Value: TBD Description: Shared secret w/ Ascon-MAC KDF Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

ECDH-ES+KDF-Ascon-MAC for COSE Name: ECDH-ES+KDF-Ascon-MAC Value: TBD Description: ECDH ES w/ Ascon-MAC KDF Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

ECDH-SS+KDF-Ascon-MAC for COSE Name: ECDH-SS+KDF-Ascon-MAC Value: TBD Description: ECDH SS w/ Ascon-MAC KDF Capabilities: [kty] Reference: NIST SP 800-232 Recommended: Yes

Additions to COSE Header Algorithm Parameters Registry

salt Algorithm: ..., direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256, direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC

PartyU identity Algorithm: ..., direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256, direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC

PartyU nonce Algorithm: ..., direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256, direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC

PartyU other Algorithm: ..., direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256, direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC

PartyV identity Algorithm: ..., direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256, direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC

PartyV nonce Algorithm: ..., direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256, direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC

PartyV other Algorithm: ..., direct+HKDF-Ascon-Hash256,ECDH-ES+HKDF-Ascon-Hash256,ECDH-SS+HKDF-Ascon-Hash256, direct+KDF-Ascon-MAC,ECDH-ES+KDF-Ascon-MAC,ECDH-SS+KDF-Ascon-MAC

ephemeral key Algorithm: ..., ECDH-ES+HKDF-Ascon-Hash256, ECDH-ES+KDF-Ascon-MAC

static key Algorithm: ..., ECDH-SS+HKDF-Ascon-Hash256, ECDH-SS+KDF-Ascon-MAC

static key id Algorithm: ..., ECDH-SS+HKDF-Ascon-Hash256, ECDH-SS+KDF-Ascon-MAC

This document describes HMAC, a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, e.g., MD5, SHA-1, in combination with a secret shared key. The cryptographic strength of HMAC depends on the properties of the underlying hash function. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind 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. This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications. The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions. This document is not an Internet Standards Track specification; it is published for informational purposes. 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. 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. The CBOR Object Signing and Encryption (COSE) syntax (see RFC 9052) does not define any direct methods for using hash algorithms. There are, however, circumstances where hash algorithms are used, such as indirect signatures, where the hash of one or more contents are signed, and identification of an X.509 certificate or other object by the use of a fingerprint. This document defines hash algorithms that are identified by COSE algorithm identifiers. IANA

Examples This appendix provides some examples of various Ascon-AEAD128 encryptions with COSE.

COSE Content Encryption

Simple Ascon-AEAD128 encryption

Direct Ascon-AEAD128 encryption with recipient

Direct Ascon-AEAD128 encryption with HKDF-SHA-256