Internet DRAFT - draft-ra-cose-hybrid-encrypt
draft-ra-cose-hybrid-encrypt
COSE T. Reddy
Internet-Draft A. Banerjee
Intended status: Standards Track Nokia
Expires: 22 April 2024 20 October 2023
Hybrid key exchange in JOSE and COSE
draft-ra-cose-hybrid-encrypt-02
Abstract
Hybrid key exchange refers to using multiple key exchange algorithms
simultaneously and combining the result with the goal of providing
security even if all but one of the component algorithms is broken.
It is motivated by transition to post-quantum cryptography. This
document provides a construction for hybrid key exchange in JOSE and
COSE. It defines the use of traditional and PQC algorithms, a hybrid
post-quantum KEM, for JOSE and COSE.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ra-cose-hybrid/.
Discussion of this document takes place on the cose Working Group
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Status of This Memo
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This Internet-Draft will expire on 22 April 2024.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
2.1. Key Encapsulation Mechanisms . . . . . . . . . . . . . . 4
3. Construction . . . . . . . . . . . . . . . . . . . . . . . . 4
4. KEM Combiner . . . . . . . . . . . . . . . . . . . . . . . . 5
5. KEM PQC Algorithms . . . . . . . . . . . . . . . . . . . . . 7
5.1. Kyber . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Hybrid Key Representation with JOSE . . . . . . . . . . . . . 8
6.1. "kem" . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Example Hybrid Key Agreement Computation . . . . . . . . 11
7. Hybrid Key Representation with COSE . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9.1. JOSE . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1.1. JSON PQC KEM Registry . . . . . . . . . . . . . . . . 21
9.2. COSE . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.2.1. COSE PQC KEM Registry . . . . . . . . . . . . . . . . 26
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 27
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Normative References . . . . . . . . . . . . . . . . . . . . . 28
Informative References . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
The migration to PQC is unique in the history of modern digital
cryptography in that neither the traditional algorithms nor the post-
quantum algorithms are fully trusted to protect data for the required
data lifetimes. The traditional algorithms, such as RSA and elliptic
curve, will fall to quantum cryptalanysis, while the post-quantum
algorithms face uncertainty about the underlying mathematics,
compliance issues, unknown vulnerabilities, hardware and software
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implementations that have not had sufficient maturing time to rule
out classical cryptanalytic attacks and implementation bugs.
During the transition from traditional to post-quantum algorithms,
there is a desire or a requirement for protocols that use both
algorithm types. [I-D.ietf-pquip-pqt-hybrid-terminology] defines
terminology for the Post-Quantum and Traditional Hybrid Schemes.
This document gives a construction for hybrid key exchange in
Javascript Object Signing and Encryption (JOSE) and CBOR Object
Signing and Encryption (COSE). The overall design approach is a
simple, "hash and concatenation" based approach to use a “hybrid”
shared secret.
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.
This document makes use of the terms defined in
[I-D.ietf-pquip-pqt-hybrid-terminology]. For the purposes of this
document, it is helpful to be able to divide cryptographic algorithms
into two classes:
"Traditional Algorithm": An asymmetric cryptographic algorithm based
on integer factorisation, finite field discrete logarithms or
elliptic curve discrete logarithms. In the context of JOSE, examples
of traditional key exchange algorithms include Elliptic Curve Diffie-
Hellman Ephemeral Static [RFC6090] [RFC8037]. In the context of
COSE, examples of traditional key exchange algorithms include
Ephemeral-Static (ES) DH and Static-Static (SS) DH [RFC9052].
"Post-Quantum Algorithm": An asymmetric cryptographic algorithm that
is believed to be secure against attacks using quantum computers as
well as classical computers. Examples of PQC key exchange algorithms
include Kyber.
"Hybrid" key exchange, in this context, means the use of two key
exchange algorithms based on different cryptographic assumptions,
e.g., one traditional algorithm and one Post-Quantum algorithm, with
the purpose of the final shared secret key being secure as long as at
least one of the component key exchange algorithms remains unbroken.
It is referred to as PQ/T Hybrid Scheme in
[I-D.ietf-pquip-pqt-hybrid-terminology].
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PQ/T Hybrid Key Encapsulation Mechanism: A Key Encapsulation
mechanism (KEM) made up of two or more component KEM algorithms where
at least one is a post-quantum algorithm and at least one is a
traditional algorithm.
2.1. Key Encapsulation Mechanisms
For the purposes of this document, we consider a Key Encapsulation
Mechanism (KEM) to be any asymmetric cryptographic scheme comprised
of algorithms satisfying the following interfaces [PQCAPI].
* def kemKeyGen() -> (pk, sk)
* def kemEncaps(pk) -> (ct, ss)
* def kemDecaps(ct, sk) -> ss
where pk is public key, sk is secret key, ct is the ciphertext
representing an encapsulated key, and ss is shared secret.
KEMs are typically used in cases where two parties, hereby refereed
to as the "encapsulater" and the "decapsulater", wish to establish a
shared secret via public key cryptography, where the decapsulater has
an asymmetric key pair and has previously shared the public key with
the encapsulater.
3. Construction
Building a PQ/T hybrid KEM requires a secure function which combines
the output of both component KEMs to form a single output. Several
IETF protocols are adding PQ/T hybrid KEM mechanisms as part of their
overall post-quantum migration strategies, examples include TLS 1.3
[I-D.ietf-tls-hybrid-design], IKEv2 [RFC9370].
The migration to PQ/T Hybrid KEM calls for performing multiple key
encapsulations in parallel and then combining their outputs to derive
a single shared secret. It is compatible with NIST SP 800-56Cr2
[SP800-56C] when viewed as a key derivation function. The hybrid
scheme defined in this document is the combination of Traditional and
Post-Quantum Algorithms. The Key agreement Traditional and Post-
Quantum Algorithms are used in parallel to generate shared secrets.
The two shared secrets are hashed and concatenated together and used
as the shared secret in JOSE and COSE.
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 ECDH algorithm
will be the content encryption key (CEK). When Key Agreement with
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Key Wrapping is employed, the shared secret established through the
ECDH algorithm will wrap the CEK. Simiarly, COSE in Sections 8.5.4
and 8.5.5 [RFC9052] define the Direct Key Agreement and Key Agreement
with Key Wrap classes. If multiple recipients are needed, then the
version with key wrap is used.
It is essential to note that in the PQ/T hybrid KEM mode, one needs
to apply Fujisaki-Okamoto [FO] transform or its variant [HHK] on the
PQC KEM part to ensure that the overall scheme is IND-CCA2 secure as
mentioned in [I-D.ietf-tls-hybrid-design]. The FO transform is
performed using the KDF such that the PQC KEM shared secret achieved
is IND-CCA2 secure. In this case, one can re-use the PQC KEM public
keys but depending on some upper bound that must adhered to.
4. KEM Combiner
The specification uses the KEM combiner defined in
[I-D.ounsworth-cfrg-kem-combiners] that takes in two or more shared
secrets and returns a combined shared secret. In case of PQ/T Hybrid
KEM, the shared secrets are the output of the traditional key
exchange (Key Agreement with Elliptic Curve Diffie-Hellman Ephemeral
Static defined in Section 4.6 of [RFC9370] for JOSE and Key Agreement
with Ephemeral-Static (ES) Diffie-Hellman (DH) defined in
Section 6.3.1 of [RFC9053] for COSE) and PQC KEM. The KEM combiner
function is defined in Section 3 of
[I-D.ounsworth-cfrg-kem-combiners]. The KDF and Hash functions will
be KMAC and SHA3 and the counter will be initialized with a value of
0x00000001 (Section 4 of [I-D.ounsworth-cfrg-kem-combiners]). The
KMAC functions used with the PQ/T hybrid algorithms are specified in
the table below:
+==============+=========+=========+
| PQ/T hybrid algorithm | KDF |
+========================+=========+
| x25519-ES_kyber512 | KMAC256 |
+------------------------+---------+
| secp384r1-ES_kyber768 | KMAC256 |
+------------------------+---------+
| x25519-ES_kyber768 | KMAC256 |
+-----------------------+----------+
| secp256r1-ES_kyber512 | KMAC256 |
+------------------------+---------+
Table 1
KMAC is defined in NIST SP 800-56Cr2 [SP800-56C]. The KMAC(K, X, L,
S) parameters are instantiated as follows:
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* K: context-specific string. In case of JOSE, the context-specific
string will be set to
concat("JOSE_PostQuantum_Traditional_Hybrid", "_", Name of the PQ/
T hybrid algorithm). In case of COSE, the context-specific string
will be set to concat("COSE_PostQuantum_Traditional_Hybrid", "_",
Name of the PQ/T hybrid algorithm). For example,
concat("JOSE_PostQuantum_Traditional_Hybrid", "_",
"x25519-ES_kyber512") =
"JOSE_PostQuantum_Traditional_Hybrid_x25519-ES_kyber512". Note:
The suggested maximum byte length of K (salt) can be 132 bytes,
and the salt can be a multiple of 132 bytes, excluding an
additional 4 bytes for length encoding, as discussed in Table 3 of
[SP800-56C]. However, in this document, K is of variable length.
The size of "K" will change based on the PQ/T hybrid algorithm.
For instance, "x25519-ES_kyber512" and "secp256r1-ES_kyber512"
will result in two different sizes of K after the concat
operation. A shorter key K will be padded by appending an all-
zero bit string to obtain a 132-byte output.
* X: concat(0x00000001, k_1, ... , k_n, fixedInfo). The fixedInfo
parameter is a fixed-format string containing context-specific
information.
* L: length of the output key in bits.
* S: utf-8 string "KDF".
In the case of a traditional key exchange algorithm (e.g., x25519,
secp384r1) since there is no associated ciphertext present when
calculating the constant-length input key (k1) defined in Section 3.2
of [I-D.ounsworth-cfrg-kem-combiners], the key derivation process
defined in Section 4.6.2 of [RFC7518] for JOSE would be used to
construct k. However, in case of COSE, the HKDF (HMAC based Key
Derivation Function) defined in Section 5 of [RFC9053] would be used.
The HKDF algorithm leverages HMAC SHA-256 as the underlying PRF
(Pseudo-Random function) for secp256r1 and x25519, and HMAC SHA-384
for secp384r1. The context structure defined in Section 5.2 of
[RFC9053], salt and secret from DH key agreement are used as inputs
to the HKDF. In case of JOSE, the fixedInfo parameter will carry the
JOSE context specific data defined in Section 4.6.2 of [RFC7518]. In
case of COSE, the fixedInfo parameter will carry the COSE context
structure defined in Section 5.2 of [RFC9053]. Note that the result
of an ECDH key agreement process does not provide a uniformly random
secret and it needs to be run through a KDF in order to produce a
usable key (see Section 6.3.1 of [RFC9053]).
The KEM combiner instantiation of the first entry of Table 1 would
be:
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ss = KMAC256("COSE_PostQuantum_Traditional_Hybrid_X25519-ES_kyber512", "0x00000001 ||
HKDF-256(DH-Shared-Secret, salt, context) ||
ct_1 || rlen(ct_1) || ss_1 || rlen(ss_1) || context" , 256, "KDF")
Where ss_1 is shared secret and its corresponding ciphertext ct_1
generated from kemEncaps(pk). If ss_1 or ct_1 are not guaranteed to
have constant length, rlen encoded length is appended when
concatenating as discussed in Section 3.2 of
[I-D.ounsworth-cfrg-kem-combiners].
In Direct Key Agreement mode, the output of the KEM combiner MUST be
a key of the same length as that used by encryption algorithm. In
Key Agreement with Key Wrapping mode, the output of the KEM combiner
MUST be a key of the length needed for the specified key wrap
algorithm.
5. KEM PQC Algorithms
The National Institute of Standards and Technology (NIST) started a
process to solicit, evaluate, and standardize one or more quantum-
resistant public-key cryptographic algorithms, as seen here
(https://csrc.nist.gov/projects/post-quantum-cryptography). Said
process has reached its first announcement
(https://csrc.nist.gov/publications/detail/nistir/8413/final) in July
5, 2022, which stated which candidates to be standardized for KEM:
* Key Encapsulation Mechanisms (KEMs): CRYSTALS-Kyber (https://pq-
crystals.org/kyber/): Kyber is a module learning with errors
(MLWE)-based key encapsulation mechanism.
NIST announced as well that they will be opening a fourth round
(https://csrc.nist.gov/csrc/media/Projects/post-quantum-
cryptography/documents/round-4/guidelines-for-submitting-tweaks-
fourth-round.pdf) to standardize an alternative KEM, and a call
(https://csrc.nist.gov/csrc/media/Projects/pqc-dig-sig/documents/
call-for-proposals-dig-sig-sept-2022.pdf) for new candidates for a
post-quantum signature algorithm.
5.1. Kyber
Kyber offers several parameter sets with varying levels of security
and performance trade-offs. This document specifies the use of the
Kyber algorithm at two security levels: Kyber512 and Kyber768. Kyber
key generation, encapsulation and decaspulation functions are defined
in [I-D.cfrg-schwabe-kyber].
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6. Hybrid Key Representation with JOSE
The parameter "kty" MUST be present and set to "OKP" defined in
Section 2 of [RFC7518] for expressing the cryptographic keys for PQ/T
Hybrid KEM, the following rules apply:
* The parameter "alg" MUST be specified, and its value MUST be one
of the values specified in the table below:
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+===============================+===================================+
| alg | Description |
+===============================+===================================+
| x25519-ES_kyber512 | Curve25519 elliptic curve + |
| | Kyber512 parameter |
| | Direct Key Agreement |
+===============================+===================================+
| secp384r1-ES_kyber768 | P-384 + Kyber768 parameter |
| | Direct Key Agreement |
+===============================+===================================+
| x25519-ES_kyber768 | Curve25519 elliptic curve + |
| | Kyber768 parameter |
| | Direct Key Agreement |
+===============================+===================================+
| secp256r1-ES_kyber512 | P-256 + Kyber512 parameter |
| | Direct Key Agreement |
+===============================+===================================+
| x25519-ES_kyber512+A128KW | Curve25519 elliptic curve + |
| | Kyber512 parameter + CEK wrapped |
| | with "A128KW" |
+========+==============---=====+===================================+
| secp384r1-ES_kyber768+A128KW | P-384 + Kyber768 parameter |
| | + CEK wrapped with "A128KW" |
+========+===================+======================================+
| x25519-ES_kyber768+A128KW | Curve25519 elliptic curve + |
| | Kyber768 parameter + CEK wrapped |
| | with "A128KW" |
+========+======================+===================================+
| secp256r1-ES_kyber512+A128KW | P-256 + Kyber512 parameter |
| | + CEK wrapped with "A128KW" |
+===============================+===================================+
| x25519-ES_kyber512+A256KW | Curve25519 elliptic curve + |
| | Kyber512 parameter + CEK wrapped |
| | with "A256KW" |
+===============================+===================================+
| secp384r1-ES_kyber768+A256KW | P-384 + Kyber768 parameter |
| | + CEK wrapped with "A256KW" |
+===============================+===================================+
| x25519-ES_kyber768+A256KW | Curve25519 elliptic curve + |
| | Kyber768 parameter + CEK wrapped |
| | with "A256KW" |
+===============================+===================================+
| secp256r1-ES_kyber512+A256KW | P-256 + Kyber512 parameter |
| | + CEK wrapped with "A256KW" |
+===============================+===================================+
Table 2
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* The parameter "kem" MUST be present and set to the PQC KEM
algorithm.
* The parameter "kem-pk" MUST be present and contains the PQC KEM
public key encoded using the base64url [RFC4648] encoding.
* The parameter "kem-sk" MUST be present for private key and
contains the PQC KEM private key encoded using the base64url
encoding. This parameter MUST NOT be present for public key.
* The parameter "kem-ct" MUST be present for KEM ciphertext encoded
using the base64url [RFC4648] encoding.
* The parameter "crv" MUST be present and contains the Elliptic
Curve Algorithm used (from the "JSON Web Key Elliptic Curve"
registry).
* The parameter "x" MUST be present and contains the x coordinate
for the Elliptic Curve point encoded using the base64url [RFC4648]
encoding.
* The parameter "y" MUST be present and contains the y coordinate
for the Elliptic Curve point encoded using the base64url [RFC4648]
encoding. This parameter is not present for "X25519".
* The parameter "d" MUST be present for private keys and contains
the Elliptic Curve Algorithm private key encoded using the
base64url encoding. This parameter MUST NOT be present for public
keys.
In Table 2, "A128KW" and "A256KW" are AES Key Wrap with 128-bit key
and 256-bit key respectively. Encryption of the plaintext is
accomplished with AES symmetric key cryptography. In Table 2, 'ES'
indicates that the traditional key agreement process is performed
using an ephemeral key on the sender's side, and for each key
agreement operation, the sender will generate a new ephemeral key.
The specification allows a small number of "known good" PQ/T hybrid
algorithms listed in Table 2 instead of allowing arbitrary
combinations of traditional and PQC algorithms. It follows the
recent trend in protocols to only allow a small number of "known
good" configurations that make sense, instead of allowing arbitrary
combinations of individual configuration choices that may interact in
dangerous ways.
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6.1. "kem"
The "kem" (KEM) parameter identifies PQC KEM algorithm used with the
"kem-pk" key. KEM values used by this specification are:
"kem" PQC KEM Applied
Kyber512 Kyber512
Kyber768 Kyber768
These values are registered in the IANA "JSON PQC KEM" registry
defined in Section 9.1.1. Additional "kem" values can be registered
by other specifications.
6.2. Example Hybrid Key Agreement Computation
This example uses secp256r1-ES_kyber512, i.e., ECDH-ES Key Agreement
with the P-256 curve and PQC KEM kyber512. The KEM Combiner is used
to derive the CEK in the manner described in Section 4.
In this example, the secp256r1_kyber512 Key Agreement mode ("alg"
value "secp256r1_kyber512") is used to produce an agreed-upon key for
AES GCM with a 128-bit key ("enc" value "A128GCM").
In this example, a producer Alice is encrypting content to a consumer
Bob. The producer (Alice) generates an ephemeral key for the key
agreement computation. Alice's ephemeral key is used for the key
agreement computation in this example (including the private part)
is:
{"kty":"OKP",
"crv":"P-256",
"x":"alice_eph_public_key_x",
"y":"alice_eph_public_key_y",
"d":"alice_eph_private_key"
}
The consumer's (Bob's) key used for the key agreement computation in
this example (including the private part) is:
{"kty":"OKP",
"kem": "kyber512",
"kem-pk":"bob_kyber_public_key",
"kem-sk":"bob_kyber_private_key"
"crv":"P-256",
"x":"bob_public_key_x",
"y":"bob_public_key_y",
"d":"bob_private_key"
}
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Header Parameter values used in this example are as follows.
{"alg":"secp256r1-ES_kyber512",
"enc":"A128GCM",
"apu":"QWxpY2U", // base64url encoding of the UTF-8 string "Alice"
"apv":"Qm9i", // base64url encoding of the UTF-8 string "Bob"
"epk":
{"kty":"OKP",
"crv":"P-256",
"x":"alice_eph_public_key_x",
"y":"alice_eph_public_key_y",
}
}
7. Hybrid Key Representation with COSE
The approach taken here matches the work done to support secp256k1 in
JOSE and COSE in [RFC8812]. The following table maps terms between
JOSE and COSE for Key Type Parameters.
+======================+========================================+==+
| Name | Value | Description |
+======================+===========================================+
| crv | -1 | EC used |
+----------------------+-------------------------------------------+
| d | -4 | Private key |
+----------------------+-------------------------------------------+
| x | -2 | x coordinate for the public key |
+----------------- ----+-------------------------------------------+
| y | -3 | y coordinate for the public key |
+---------------- -----+-------------------------------------------+
| kem | TBD2 | PQC KEM Algorithm |
+----------------------+-------------------------------------------+
| kem-pk | TBD3 | PQC KEM Public Key |
+----------------------+-------------------------------------------+
| kem-sk | TBD4 | PQC KEM Private Key |
+----------------------+-------------------------------------------+
| kem-ct | TBD5 | PQC KEM ciphertext |
+----------------------+-------------------------------------------+
Table 3
The following table maps terms between JOSE and COSE for PQ/T Hybrid
KEM.
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+==============+===================+====================+============================+
| Name | Value | Description | Recommended |
+===================+===========+========+=============================+=============+
| x25519-ES_kyber512 | TBD10 | Curve25519 elliptic curve + | No |
| | | Kyber512 parameter | |
+-------------------------------+--------+-----------------------------+-------------+
| secp384r1-ES_kyber768 | TBD11 | P-384 + Kyber768 parameter | No |
| | | | |
+-------------------------------+--------+-----------------------------+-------------+
| x25519-ES_kyber768 | TBD12 | Curve25519 elliptic curve | No |
| | | + Kyber768 parameter | |
+-------------------------------+--------+-----------------------------+-------------+
| secp256r1-ES_kyber512 | TBD13 | P-256 + Kyber512 parameter | No |
| | | | |
+-------------------------------+--------+-----------------------------+-------------+
| x25519-ES_kyber512+A128KW | TBD14 | Curve25519 elliptic curve + | No |
| | | Kyber512 parameter + | |
| | | CEK wrapped with "A128KW" | |
+-------------------------------+--------+-----------------------------+-------------+
| secp384r1-ES_kyber768+A128KW | TBD15 | P-384 + Kyber768 parameter | No |
| | | + CEK wrapped with "A128KW" | |
+-------------------------------+--------+-----------------------------+-------------+
| x25519-ES_kyber768+A128KW | TBD16 | Curve25519 elliptic curve | No |
| | | + Kyber768 parameter | |
| | | + CEK wrapped with "A128KW" | |
+-------------------------------+--------+-----------------------------+-------------+
| secp256r1-ES_kyber512+A128KW | TBD17 | P-256 + Kyber512 parameter | No |
| | | + CEK wrapped with "A128KW" | |
+-------------------------------+--------+-----------------------------+-------------+
| x25519-ES_kyber512+A256KW | TBD18 | Curve25519 elliptic curve + | No |
| | | Kyber512 parameter + | |
| | | CEK wrapped with "A256KW" | |
+-------------------------------+--------+-----------------------------+-------------+
| secp384r1-ES_kyber768+A256KW | TBD19 | P-384 + Kyber768 parameter | No |
| | | + CEK wrapped with "A256KW" | |
+-------------------------------+--------+-----------------------------+-------------+
| x25519-ES_kyber768+A256KW | TBD20 | Curve25519 elliptic curve | No |
| | | + Kyber768 parameter | |
| | | + CEK wrapped with "A256KW" | |
+-------------------------------+--------+-----------------------------+-------------+
| secp256r1_-ES_kyber512+A256KW | TBD21 | P-256 + Kyber512 parameter | No |
| | | + CEK wrapped with "A256KW" | |
+-------------------------------+--------+-----------------------------+-------------+
Table 4
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The following table maps terms between JOSE and COSE for PQC KEM
algorithms.
+==============+=======+====================+===============================+
| Name | Value | Description | Recommended |
+==============+=======+====================+=============--------==========+
| Kyber512 | TBD7 | Kyber512 | No |
| | | | |
+---------------------+--------+-----------------------------+--------------+
| Kyber768 | TBD9 | Kyber768 | No |
| | | | |
+---------------------+--------+-----------------------------+--------------+
Table 5
This example uses the following parameters:
* Algorithm for payload encryption: AES-GCM-128
* IV: 0x26, 0x68, 0x23, 0x06, 0xd4, 0xfb, 0x28, 0xca, 0x01, 0xb4,
0x3b, 0x80
* Algorithm for content key distribution: secp256r1_kyber512
* KID: "kid-4"
The COSE_Encrypt structure
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96(
[
/ protected h'a10101' / << {
/ alg / 1:1 / AES-GCM 128 /
} >>,
/ unprotected / {
/ iv / 5:h'26682306D4FB28CA01B43B80'
},
/ null because of detached ciphertext /
null,
/ recipients / [
[
/ protected h'Assuming -50 is assigned' / << {
/ alg / 1:-50 / secp256r1-ES_kyber512 /
} >>,
/ unprotected / {
/ ephemeral / -1:{
/ kty / 1:1 /OKP/,
/ crv / -1:1 /secp256r1 or P-256/,
/ x / -2:h'415A8ED270C4B1F10B0A2D42B28EE6028CE25D74552CB4291A4069A2E989B0F6',
/ y / -3:h'CCC9AAF60514B9420C80619A4FF068BC1D77625BA8C90200882F7D5B73659E76'
},
/ kid / 4:'kid-10'
}
]
]
]
)
Figure 1: COSE_Encrypt Example for secp256r1-ES_kyber512
8. Security Considerations
Security considerations from [RFC7748] and
[I-D.ounsworth-cfrg-kem-combiners] apply here. The shared secrets
computed in the hybrid key exchange should be computed in a way that
achieves the "hybrid" property: the resulting secret is secure as
long as at least one of the component key exchange algorithms is
unbroken.
PQC KEMs used in the manner described in this document MUST
explicitly be designed to be secure in the event that the public key
is reused, such as achieving IND-CCA2 security. Kyber has such
security properties.
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9. IANA Considerations
9.1. JOSE
The following has to be added to the "JSON Web Key Parameters"
registry:
* Parameter Name: "kem"
* Parameter Description: PQC KEM Algorithm
* Parameter Information Class: Public
* Used with "kty" Value(s): "OKP"
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Parameter Name: "kem-pk"
* Parameter Description: PQC KEM Public Key
* Parameter Information Class: Public
* Used with "kty" Value(s): "OKP"
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Parameter Name: "kem-sk"
* Parameter Description: PQC KEM Private Key
* Parameter Information Class: Private
* Used with "kty" Value(s): "OKP"
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Parameter Name: "kem-ct"
* Parameter Description: PQC KEM ciphertext
* Parameter Information Class: Public
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* Used with "kty" Value(s): "OKP"
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
The following has to be added to the "JSON Web Signature and
Encryption Algorithms" registry:
* Algorithm Name: "x25519-ES_kyber768"
* Algorithm Description: Curve25519 elliptic curve + Kyber768
parameter
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "secp384r1-ES_kyber768"
* Algorithm Description: P-384 + Kyber768 parameter
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "x25519-ES_kyber512"
* Algorithm Description: Curve25519 elliptic curve + Kyber512
parameter
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
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* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "secp256r1-ES_kyber512"
* Algorithm Description: P-256 + Kyber512 parameter
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "x25519-ES_kyber768+A128KW"
* Algorithm Description: Curve25519 elliptic curve + Kyber768
parameter and CEK wrapped with "A128KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "secp384r1-ES_kyber768+A128KW"
* Algorithm Description: P-384 + Kyber768 parameter and CEK wrapped
with "A128KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
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* Algorithm Name: "x25519-ES_kyber512+A128KW"
* Algorithm Description: Curve25519 elliptic curve + Kyber512
parameter and CEK wrapped with "A128KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "secp256r1-ES_kyber512+A128KW"
* Algorithm Description: P-256 + Kyber512 parameter and CEK wrapped
with "A128KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "x25519-ES_kyber768+A256KW"
* Algorithm Description: Curve25519 elliptic curve + Kyber768
parameter and CEK wrapped with "A256KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "secp384r1-ES_kyber768+A256KW"
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* Algorithm Description: P-384 + Kyber768 parameter and CEK wrapped
with "A256KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): SSection 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "x25519-ES_kyber512+A256KW"
* Algorithm Description: Curve25519 elliptic curve + Kyber512
parameter and CEK wrapped with "A256KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
* Algorithm Name: "secp256r1-ES_kyber512+A256KW"
* Algorithm Description: P-256 + Kyber512 parameter and CEK wrapped
with "A256KW"
* Algorithm Usage Location(s): "alg"
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6 of this document (TBD)
* Algorithm Analysis Documents(s): (TBD20)
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9.1.1. JSON PQC KEM Registry
This section establishes the IANA "JSON PQC KEM" registry for JWK
"kem" member values. The registry records the PQC KEM name,
implementation requirements, and a reference to the specification
that defines it. This specification registers the PQC KEM algorithms
defined in Section 6.1.
The implementation requirements of a PQC KEM may be changed over time
as the cryptographic landscape evolves, for instance, to change the
status of a PQC KEM to Deprecated or to change the status of a PQC
KEM from Optional to Recommended+ or Required. Changes of
implementation requirements are only permitted on a Specification
Required basis after review by the Designated Experts, with the new
specification defining the revised implementation requirements level.
9.1.1.1. Registration Template
PQC KEM name: The name requested (e.g., "Kyber512"). Because a core
goal of this specification is for the resulting representations to be
compact, it is RECOMMENDED that the name be short -- not to exceed 12
characters without a compelling reason to do so. This name is case
sensitive. Names may not match other registered names in a case-
insensitive manner unless the Designated Experts state that there is
a compelling reason to allow an exception.
PQC KEM Description: Brief description of the PQC KEM (e.g.,
"Kyber512").
JOSE Implementation Requirements: The PQC KEM implementation
requirements for JWE, which must be one the words Required,
Recommended, Optional, Deprecated, or Prohibited. Optionally, the
word can be followed by a "+" or "-". The use of "+" indicates that
the requirement strength is likely to be increased in a future
version of the specification. The use of "-" indicates that the
requirement strength is likely to be decreased in a future version of
the specification.
Change Controller: For Standards Track RFCs, list "IESG". For
others, give the name of the responsible party. Other details (e.g.,
postal address, email address, home page URI) may also be included.
Specification Document(s): Reference to the document or documents
that specify the parameter, preferably including URIs that can be
used to retrieve copies of the documents. An indication of the
relevant sections may also be included but is not required.
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9.1.1.2. Initial Registry Contents
* PQC KEM name: "Kyber512"
* PQC KEM Description: Kyber512
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6.1
* PQC KEM name: "Kyber768"
* PQC KEM Description: Kyber768
* JOSE Implementation Requirements: Optional
* Change Controller: IESG
* Specification Document(s): Section 6.1
9.2. COSE
The following has to be added to the "COSE Key Type Parameters"
registry:
* Key Type: OKP
* Name: kem
* Label : TBD2
* CBOR Type: int / tstr
* Description: PQC KEM Algorithm
* Reference: This document (TBD)
* Key Type: OKP
* Name: kem-pk
* Label : TBD3
* CBOR Type: bstr
* Description: PQC KEM Public Key
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* Reference: This document (TBD)
* Key Type: OKP
* Name: kem-sk
* Label : TBD4
* CBOR Type: bstr
* Description: PQC KEM Private Key
* Reference: This document (TBD)
* Key Type: OKP
* Name: kem-ct
* Label : TBD5
* CBOR Type: bstr
* Description: PQC KEM ciphertext
* Reference: This document (TBD)
The following has to be added to the "COSE Algorithms" registry:
* Name: x25519-ES_kyber768
* Value: TBD10
* Description: Curve25519 elliptic curve + Kyber768 parameter
* Reference: This document (TBD)
* Recommended: No
* Name: secp384r1-ES_kyber768
* Value: TBD11
* Description: P-384 + Kyber768 parameter
* Reference: This document (TBD)
* Recommended: No
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* Name: x25519-ES_kyber512
* Value: TBD12
* Description: Curve25519 elliptic curve + Kyber512 parameter
* Reference: This document (TBD)
* Recommended: No
* Name: secp256r1-ES_kyber512
* Value: TBD13
* Description: P-256 + Kyber512 parameter
* Reference: This document (TBD)
* Recommended: No
* Name: x25519-ES_kyber768+A128KW
* Value: TBD14
* Description: Curve25519 elliptic curve + Kyber768 parameter and
CEK wrapped with "A128KW"
* Reference: This document (TBD)
* Recommended: No
* Name: secp384r1-ES_kyber768+A128KW
* Value: TBD15
* Description: P-384 + Kyber768 parameter and CEK wrapped with
"A128KW"
* Reference: This document (TBD)
* Recommended: No
* Name: x25519-ES_kyber512+A128KW
* Value: TBD16
* Description: Curve25519 elliptic curve + Kyber512 parameter and
CEK wrapped with "A128KW"
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* Reference: This document (TBD)
* Recommended: No
* Name: secp256r1-ES_kyber512+A128KW
* Value: TBD17
* Description: P-256 + Kyber512 parameter and CEK wrapped with
"A128KW"
* Reference: This document (TBD)
* Recommended: No
* Name: x25519-ES_kyber768+A256KW
* Value: TBD18
* Description: Curve25519 elliptic curve + Kyber768 parameter and
CEK wrapped with "A256KW"
* Reference: This document (TBD)
* Recommended: No
* Name: secp384r1-ES_kyber768+A256KW
* Value: TBD19
* Description: P-384 + Kyber768 parameter and CEK wrapped with
"A256KW"
* Reference: This document (TBD)
* Recommended: No
* Name: x25519-ES_kyber512+A256KW
* Value: TBD20
* Description: Curve25519 elliptic curve + Kyber512 parameter and
CEK wrapped with "A256KW"
* Reference: This document (TBD)
* Recommended: No
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* Name: secp256r1-ES_kyber512+A256KW
* Value: TBD21
* Description: P-256 + Kyber512 parameter and CEK wrapped with
"A256KW"
* Reference: This document (TBD)
* Recommended: No
9.2.1. COSE PQC KEM Registry
This section establishes the IANA "COSE PQC KEM" registry for "kem"
member values. The registry records the PQC KEM name, implementation
requirements, and a reference to the specification that defines it.
This specification registers the PQC KEM algorithms defined in
Section 7.
The implementation requirements of a PQC KEM may be changed over time
as the cryptographic landscape evolves, for instance, to change the
status of a PQC KEM to Deprecated or to change the status of a PQC
KEM from Optional to Recommended+ or Required. Changes of
implementation requirements are only permitted on a Specification
Required basis after review by the Designated Experts, with the new
specification defining the revised implementation requirements level.
9.2.1.1. Registration Template
Name: The name requested (e.g., "Kyber512"). Because a core goal of
this specification is for the resulting representations to be
compact, it is RECOMMENDED that the name be short -- not to exceed 12
characters without a compelling reason to do so. This name is case
sensitive. Names may not match other registered names in a case-
insensitive manner unless the Designated Experts state that there is
a compelling reason to allow an exception.
Value: This is the value used for the label. The label can be either
an integer or a string. Registration in the table is based on the
value of the label requested. Integer values between 1 and 255 and
strings of length 1 are designated as "Standards Action". Integer
values from 256 to 65535 and strings of length 2 are designated as
"Specification Required". Integer values of greater than 65535 and
strings of length greater than 2 are designated as "Expert Review".
Integer values in the range -1 to -65536 are "delegated to the COSE
Header Algorithm Parameters registry". Integer values less than
-65536 are marked as private use.
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Description: Brief description of the PQC KEM (e.g., "Kyber512").
Change Controller: For Standards Track RFCs, list "IESG". For
others, give the name of the responsible party. Other details (e.g.,
postal address, email address, home page URI) may also be included.
Reference: Reference to the document or documents that specify the
parameter, preferably including URIs that can be used to retrieve
copies of the documents. An indication of the relevant sections may
also be included but is not required.
Recommended: Does the IETF have a consensus recommendation to use
the algorithm? The legal values are 'Yes', 'No', and
'Deprecated'.
9.2.1.2. Initial Registry Contents
* Name: "Kyber512"
* Value: TBD7
* Description: Kyber512
* Change Controller: IESG
* Reference: This document (TBD)
* Recommended: No
* Name: "Kyber768"
* Value: TBD9
* Description: Kyber768
* Change Controller: IESG
* Reference: This document (TBD)
* Recommended: No
Acknowledgments
Thanks to Mike Ounsworth and Ilari Liusvaara for the discussion and
comments.
References
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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>.
[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>.
Informative References
[FO] "Secure Integration of Asymmetric and Symmetric Encryption
Schemes", <https://link.springer.com/article/10.1007/
s00145-011-9114-1>.
[HHK] "A Modular Analysis of the Fujisaki-Okamoto
Transformation", <https://link.springer.com/
chapter/10.1007/978-3-319-70500-2_12>.
[I-D.cfrg-schwabe-kyber]
Schwabe, P. and B. Westerbaan, "Kyber Post-Quantum KEM",
Work in Progress, Internet-Draft, draft-cfrg-schwabe-
kyber-03, 25 September 2023,
<https://datatracker.ietf.org/doc/html/draft-cfrg-schwabe-
kyber-03>.
[I-D.ietf-pquip-pqt-hybrid-terminology]
D, F., "Terminology for Post-Quantum Traditional Hybrid
Schemes", Work in Progress, Internet-Draft, draft-ietf-
pquip-pqt-hybrid-terminology-00, 4 May 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
pqt-hybrid-terminology-00>.
[I-D.ietf-tls-hybrid-design]
Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
exchange in TLS 1.3", Work in Progress, Internet-Draft,
draft-ietf-tls-hybrid-design-09, 7 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
hybrid-design-09>.
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[I-D.ounsworth-cfrg-kem-combiners]
Ounsworth, M., Wussler, A., and S. Kousidis, "Combiner
function for hybrid key encapsulation mechanisms (Hybrid
KEMs)", Work in Progress, Internet-Draft, draft-ounsworth-
cfrg-kem-combiners-04, 8 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ounsworth-
cfrg-kem-combiners-04>.
[PQCAPI] "PQC - API notes",
<https://csrc.nist.gov/CSRC/media/Projects/Post-Quantum-
Cryptography/documents/example-files/api-notes.pdf>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/rfc/rfc6090>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/rfc/rfc7518>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/rfc/rfc7748>.
[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>.
[RFC8812] Jones, M., "CBOR Object Signing and Encryption (COSE) and
JSON Object Signing and Encryption (JOSE) Registrations
for Web Authentication (WebAuthn) Algorithms", RFC 8812,
DOI 10.17487/RFC8812, August 2020,
<https://www.rfc-editor.org/rfc/rfc8812>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.
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[RFC9370] Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
Key Exchanges in the Internet Key Exchange Protocol
Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
2023, <https://www.rfc-editor.org/rfc/rfc9370>.
[SP800-185]
"SHA-3 Derived Functions: cSHAKE, KMAC, TupleHash, and
ParallelHash", <https://doi.org/10.6028/NIST.SP.800-185>.
[SP800-56C]
"Recommendation for Key-Derivation Methods in Key-
Establishment Schemes",
<https://doi.org/10.6028/NIST.SP.800-56Cr2>.
Authors' Addresses
Tirumaleswar Reddy
Nokia
Bangalore
Karnataka
India
Email: kondtir@gmail.com
Aritra Banerjee
Nokia
Munich
Germany
Email: aritra.banerjee@nokia.com
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