Use of Hybrid Public-Key Encryption (HPKE) with Javascript Object Signing and Encryption (JOSE)

Internet-Draft	Use of HPKE in JOSE	January 2024
Reddy, et al.	Expires 25 July 2024	[Page]

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1. Introduction

Hybrid public-key encryption (HPKE) [RFC9180] 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 [RFC9180].¶

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 JOSE ([RFC7516], [RFC7518]).¶

2. Conventions and Definitions

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Conventions and Terminology

This specification uses the following abbreviations and terms:¶

Content-encryption key (CEK), a term defined in CMS [RFC2630].¶
Hybrid Public Key Encryption (HPKE) is defined in [RFC9180].¶
pkR is the public key of the recipient, as defined in [RFC9180].¶
skR is the private key of the recipient, as defined in [RFC9180].¶
Key Encapsulation Mechanism (KEM), see [RFC9180].¶
Key Derivation Function (KDF), see [RFC9180].¶
Authenticated Encryption with Associated Data (AEAD), see [RFC9180].¶
Additional Authenticated Data (AAD), see [RFC9180].¶
Key Type (kty), see [RFC7517].¶

4. HPKE for JOSE

4.1. Overview

The JSON Web Algorithms (JWA) [RFC7518] in Section 4.6 defines two ways using the key agreement result. When Direct Key Agreement is employed, the shared secret established through the HPKE will be the content encryption key (CEK). When Key Agreement with Key Wrapping is employed, the shared secret established through the HPKE will wrap the CEK. If multiple recipients are needed, then Key Agreement with Key Wrapping mode is used.¶

In both cases a new JOSE 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.¶

When the alg value is set to any of algorithms registered by this specification then the 'encapsulated_key' header parameter MUST be present in the unprotected header parameter.¶

The 'encapsulated_key' parameter contains the encapsulated key, which is output of the HPKE KEM, and is represented as a base64url encoded string. The parameter "kty" MUST be present and set to "OKP" defined in Section 2 of [RFC8037].¶

4.1.1. HPKE Usage in Direct and Key Agreement with Key Wrapping

In Direct Key Agreement mode, HPKE is employed to directly encrypt the plaintext, and the resulting ciphertext is included in the JWE ciphertext. In Key Agreement with Key Wrapping mode, HPKE is used to encrypt the Content Encryption Key (CEK), and the resulting ciphertext is included in the JWE ciphertext.¶

4.1.1.1. HPKE Usage in Direct Key Agreement mode

In Direct Key Agreement mode, the sender MUST specify the 'encapsulated_key' and 'alg' parameters in the protected header to indicate the use of HPKE. In this mode, the 'enc' (Encryption Algorithm) parameter MUST NOT be present because the ciphersuite (KEM, KDF, AEAD) is fully-specified in the 'alg' parameter itself. If the 'enc' parameter is present, it MUST be ignored by implementations. This is a deviation from the rule in Section 4.1.2 of [RFC7516]. Optionally, the protected header MAY contain the 'kid' parameter used to identify the static recipient public key used by the sender.¶

4.1.1.2. HPKE Usage in Key Agreement with Key Wrapping mode

In the JWE JSON Serialization, the sender MUST place the 'encapsulated_key' and 'alg' parameters in the per-recipient unprotected header to indicate the use of HPKE. Optionally, the per-recipient unprotected header MAY contain the 'kid' parameter used to identify the static recipient public key used by the sender. In the JWE Compact Serialization, the sender MUST place the 'encapsulated_key' and 'alg' parameters in the protected header to indicate the use of HPKE.¶

5. 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 [HPKE-IANA]. Hence, JOSE-HPKE cannot use an 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:¶

HPKE-<Mode>-<KEM>-<KDF>-<AEAD>

The "Mode" indicator may be populated with the following values from Table 1 of [RFC9180]:¶

"Base" refers to "mode_base" described in Section 5.1.1 of [RFC9180], 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 [RFC9180], which authenticates using a pre-shared key.¶
"Auth" refers to "mode_auth", described in Section 5.1.3 of [RFC9180], which authenticates using an asymmetric key.¶
"Auth_Psk" refers to "mode_auth_psk", described in Section 5.1.4 of [RFC9180], which authenticates using both a PSK and an asymmetric key.¶

For a list of ciphersuite registrations, please see Section 9.¶

6. HPKE Encryption and Decryption

6.1. 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). If "zip" parameter is present, compression is applied to the plaintext pt using the specified compression algorithm before invoking SealBase.¶

Two cases of plaintext need to be distinguished:¶

In Direct Key Agreement mode, the plaintext "pt" passed into SealBase is the content to be encrypted. Hence, there is no intermediate layer utilizing a CEK.¶
In Key Agreement with Key Wrapping mode, the plaintext "pt" passed into SealBase is the CEK. The CEK is a random byte sequence of length appropriate for the encryption algorithm. For example, AES-128-GCM requires a 16 byte key and the CEK would therefore be 16 bytes long.¶

In the JWE Compact Serialization, the "aad" parameter in SealBase function will take the Additional Authenticated Data encryption parameter defined in Section 5.1 of [RFC7516] as input. In the JWE JSON Serialization, SealBase function will be invoked with empty associated data "aad".¶

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. The "info" parameter in SealBase function will take the JOSE context specific data defined in Section 4.6.2 of [RFC7518] as input.¶

The SealBase function internally creates the sending HPKE context by invoking SetupBaseS() (Section 5.1.1 of [RFC9180]) with "pkR" and "info". This yields the context "sctxt" and an encapsulation key "enc". The SealBase function then invokes the Seal() method on "sctxt" (Section 5.2 of [RFC9180]) with "aad", yielding ciphertext "ct". Note that Section 6 of [RFC9180] discusses Single-Shot APIs for encryption and decryption; SetupBaseS internally invokes Seal() method to return both "ct" and "enc".¶

In summary, if SealBase() is successful, it will output a ciphertext "ct" and an encapsulated key "enc".¶

In both modes, 'encapsulated_key' will contain the value of "enc". In Direct Key Agreement mode, the JWE Ciphertext will contain the value of 'ct'. In Key Agreement with Key Wrapping mode, the JWE Encrypted Key will contain the value of 'ct'. In Direct Key Agreement mode, the JWE Encrypted Key will use the value of an empty octet sequence. In both modes, the JWE Initialization Vector value will be an empty octet sequence. In both modes, the JWE Authentication Tag MUST be absent.¶

In both JWE Compact Serialization and the JWE JSON Serialization, "ct" and "enc" will be base64url encoded (see Section 7.1 and 7.2 of [RFC7518]), since JSON lacks a way to directly represent arbitrary octet sequences.¶

6.2. HPKE Decryption with OpenBase

The recipient will use the OpenBase(enc, skR, info, aad, ct) function with the base64url decoded "encapsulated_key" and the "ciphertext" parameters received from the sender. The "aad" and the "info" parameters are constructed from Additional Authenticated Data encryption parameter and JOSE context, respectively.¶

The OpenBase internally creates the receiving HPKE context by invoking SetupBaseR() (Section 5.1.1 of [RFC9180]) with "skR", "enc", and "info". This yields the context "rctxt". The OpenBase function then decrypts "ct" by invoking the Open() method on "rctxt" (Section 5.2 of [RFC9180]) with "aad", yielding "pt" or an error on failure.¶

The OpenBase function will, if successful, decrypts "ct". When decrypted, the result will be either the CEK (when Key Agreement with Key Wrapping mode is used), or the content (if Direct Key Agreement mode is used). The CEK is the symmetric key used to decrypt the ciphertext.¶

6.3. Example Hybrid Key Agreement Computation

This example uses HPKE-Base-P256-SHA256-AES128GCM which corresponds to the following HPKE algorithm combination:¶

KEM: DHKEM(P-256, HKDF-SHA256)¶
KDF: HKDF-SHA256¶
AEAD: AES-128-GCM¶
Mode: Base¶
payload: "This is the content"¶
aad: ""¶

{
   "alg": "HPKE-Base-P256-SHA256-AES128GCM",
   "kid": "7"
   "encapsulated_key": "BIxvdeRjp3MILzyw06cBNIpXjGeAq6ZYZGaCqa9ykd/
    Cd+yTw9WHB4GChsEJeCVFczjcPcr/Nn4pUTQunbMNwOc=",
}

              JWE Protected Header JSON

7. Security Considerations

This specification is based on HPKE and the security considerations of [RFC9180] are therefore applicable also to this specification.¶

HPKE assumes the sender is in possession of the public key of the recipient and HPKE JOSE makes the same assumptions. Hence, some form of public key distribution mechanism is assumed to exist but outside the scope of this document.¶

HPKE in Base mode does not offer authentication as part of the HPKE KEM. In this case JOSE constructs like JWS and JSON Web Tokens (JWTs) can be used to add authentication. HPKE also offers modes that offer authentication.¶

HPKE relies on a source of randomness to be available on the device. In Key Agreement with Key Wrapping mode, CEK has to be randomly generated and it MUST be ensured that the guidelines in [RFC8937] for random number generations are followed.¶

9. IANA Considerations

This document requests IANA to add new values to the 'JOSE Algorithms' and to the 'JOSE Header Parameters' registries in the 'Standards Action With Expert Review category'.¶

9.1. JOSE Algorithms Registry (Direct Key Agreement)

Algorithm Name: HPKE-Base-P256-SHA256-AES128GCM¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(P-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and the AES-128-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-P384-SHA384-AES256GCM¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(P-384, HKDF-SHA384) KEM, the HKDF-SHA384 KDF, and the AES-256-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-P521-SHA512-AES256GCM¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(P-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the AES-256-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X25519-SHA256-AES128GCM¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and the AES-128-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X25519-SHA256-ChaCha20Poly1305¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and the ChaCha20Poly1305 AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X448-SHA512-AES256GCM¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the AES-256-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X448-SHA512-ChaCha20Poly1305¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and the ChaCha20Poly1305 AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶

9.2. JOSE Algorithms Registry (Key Agreement with Key Wrapping)

Algorithm Name: HPKE-Base-P256-SHA256-AES128GCMKW¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(P-256, HKDF-SHA256) KEM, the HKDF-SHA256 KDF and Key wrapping with the AES-128-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-P384-SHA384-AES256GCMKW¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(P-384, HKDF-SHA384) KEM, the HKDF-SHA384 KDF, and Key wrapping with the AES-256-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-P521-SHA512-AES256GCMKW¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(P-521, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and Key wrapping with the AES-256-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X25519-SHA256-AES128GCMKW¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and Key wrapping with the AES-128-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X25519-SHA256-ChaCha20Poly1305KW¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X25519, HKDF-SHA256) KEM, the HKDF-SHA256 KDF, and Key wrapping with the ChaCha20Poly1305 AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X448-SHA512-AES256GCMKW¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and Key wrapping with the AES-256-GCM AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶
Algorithm Name: HPKE-Base-X448-SHA512-ChaCha20Poly1305KW¶
Algorithm Description: Cipher suite for JOSE-HPKE in Base Mode that uses the DHKEM(X448, HKDF-SHA512) KEM, the HKDF-SHA512 KDF, and Key wrapping with the ChaCha20Poly1305 AEAD.¶
Algorithm Usage Location(s): "alg"¶
JOSE Implementation Requirements: Optional¶
Change Controller: IESG¶
Specification Document(s): [[TBD: This RFC]]¶
Algorithm Analysis Documents(s): TODO¶

9.3. JOSE Header Parameters

Parameter Name: "encapsulated_key"¶
Parameter Description: HPKE encapsulated key¶
Parameter Information Class: Public¶
Used with "kty" Value(s): "OKP"¶
Change Controller: IESG¶
Specification Document(s): [[This specification]]¶

10.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC7516]: Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015, <https://www.rfc-editor.org/rfc/rfc7516>.
[RFC7517]: Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7517, May 2015, <https://www.rfc-editor.org/rfc/rfc7517>.
[RFC7518]: Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015, <https://www.rfc-editor.org/rfc/rfc7518>.
[RFC8037]: Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH) and Signatures in JSON Object Signing and Encryption (JOSE)", RFC 8037, DOI 10.17487/RFC8037, January 2017, <https://www.rfc-editor.org/rfc/rfc8037>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9180]: Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180, February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

10.2. Informative References

[HPKE-IANA]: IANA, "Hybrid Public Key Encryption (HPKE) IANA Registry", October 2023, <https://www.iana.org/assignments/hpke/hpke.xhtml>.
[I-D.ietf-cose-hpke]: Tschofenig, H., Steele, O., Daisuke, A., and L. Lundblade, "Use of Hybrid Public-Key Encryption (HPKE) with CBOR Object Signing and Encryption (COSE)", Work in Progress, Internet-Draft, draft-ietf-cose-hpke-07, 22 October 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-cose-hpke-07>.
[RFC2630]: Housley, R., "Cryptographic Message Syntax", RFC 2630, DOI 10.17487/RFC2630, June 1999, <https://www.rfc-editor.org/rfc/rfc2630>.
[RFC8937]: Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N., and C. Wood, "Randomness Improvements for Security Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020, <https://www.rfc-editor.org/rfc/rfc8937>.

Acknowledgments

This specification leverages text from [I-D.ietf-cose-hpke]. We would like to thank Matt Chanda, Ilari Liusvaara and Aaron Parecki for their feedback.¶

Authors' Addresses

Tirumaleswar Reddy

Nokia

Bangalore

Karnataka

India

Email: kondtir@gmail.com

Hannes Tschofenig

Austria

Email: hannes.tschofenig@gmx.net

Aritra Banerjee

Nokia

Munich

Germany

Email: aritra.banerjee@nokia.com

Orie Steele

Transmute

United States

Email: orie@transmute.industries

Michael B. Jones

independent