Internet DRAFT - draft-celi-wiggers-tls-authkem
draft-celi-wiggers-tls-authkem
TLS Working Group T. Wiggers
Internet-Draft PQShield
Intended status: Informational S. Celi
Expires: 19 February 2024 Brave Software
P. Schwabe
Radboud University and MPI-SP
D. Stebila
University of Waterloo
N. Sullivan
18 August 2023
KEM-based Authentication for TLS 1.3
draft-celi-wiggers-tls-authkem-02
Abstract
This document gives a construction for a Key Encapsulation Mechanism
(KEM)-based authentication mechanism in TLS 1.3. This proposal
authenticates peers via a key exchange protocol, using their long-
term (KEM) public keys.
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-celi-wiggers-tls-authkem/.
Discussion of this document takes place on the tlswg Working Group
mailing list (mailto:tls@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/tls/. Subscribe at
https://www.ietf.org/mailman/listinfo/tls/.
Source for this draft and an issue tracker can be found at
https://github.com/kemtls/draft-celi-wiggers-tls-authkem.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 19 February 2024.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Using key exchange instead of signatures for
authentication . . . . . . . . . . . . . . . . . . . . . 4
1.2. Evaluation of handshake sizes . . . . . . . . . . . . . . 4
1.3. Related work . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1. OPTLS . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2. Compressing certificates and certificate chains . . . 6
1.4. Organization . . . . . . . . . . . . . . . . . . . . . . 7
2. Conventions and definitions . . . . . . . . . . . . . . . . . 7
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Key Encapsulation Mechanisms . . . . . . . . . . . . . . 8
3. Full 1.5-RTT AuthKEM Handshake Protocol . . . . . . . . . . . 8
3.1. Client authentication . . . . . . . . . . . . . . . . . . 10
3.2. Relevant handshake messages . . . . . . . . . . . . . . . 12
3.3. Overview of key differences with RFC8446 TLS 1.3 . . . . 12
3.4. Implicit and explicit authentication . . . . . . . . . . 12
3.5. Authenticating CertificateRequest . . . . . . . . . . . . 13
4. Implementation . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Negotiation of AuthKEM . . . . . . . . . . . . . . . . . 13
4.2. Protocol messages . . . . . . . . . . . . . . . . . . . . 14
4.3. Cryptographic computations . . . . . . . . . . . . . . . 15
4.3.1. Key schedule for full AuthKEM handshakes . . . . . . 15
4.3.2. Computations of KEM shared secrets . . . . . . . . . 17
4.3.3. Explicit Authentication Messages . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
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5.1. Implicit authentication . . . . . . . . . . . . . . . . . 18
5.2. Authentication of Certificate Request . . . . . . . . . . 19
5.3. Other security considerations . . . . . . . . . . . . . . 19
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Normative References . . . . . . . . . . . . . . . . . . 20
6.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Open points of discussion . . . . . . . . . . . . . 24
A.1. Authentication concerns for client authentication
requests. . . . . . . . . . . . . . . . . . . . . . . . . 24
A.2. Interaction with signing certificates . . . . . . . . . . 24
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
*Note:* This is a work-in-progress draft. We welcome discussion,
feedback and contributions through the IETF TLS working group mailing
list or directly on GitHub.
This document gives a construction for KEM-based authentication in
TLS 1.3 [RFC8446]. Authentication happens via asymmetric
cryptography by the usage of KEMs advertised as the long-term KEM
public keys in the Certificate.
TLS 1.3 is in essence a signed key exchange protocol (if using
certificate-based authentication). Authentication in TLS 1.3 is
achieved by signing the handshake transcript with digital signatures
algorithms. KEM-based authentication provides authentication by
deriving a shared secret that is encapsulated against the public key
contained in the Certificate. Only the holder of the private key
corresponding to the certificate's public key can derive the same
shared secret and thus decrypt its peer's messages.
This approach is appropriate for endpoints that have KEM public keys.
Though this is currently rare, certificates can be issued with (EC)DH
public keys as specified for instance in [RFC8410], or using a
delegation mechanism, such as delegated credentials
[I-D.ietf-tls-subcerts].
In this proposal, we build on [RFC9180]. This standard currently
only covers Diffie-Hellman based KEMs, but the first post-quantum
algorithms have already been put forward
[I-D.draft-westerbaan-cfrg-hpke-xyber768d00]. This proposal uses
Kyber [KYBER] [I-D.draft-cfrg-schwabe-kyber], the first selected
algorithm for key exchange in the NIST post-quantum standardization
project [NISTPQC].
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1.1. Using key exchange instead of signatures for authentication
The elliptic-curve and finite-field-based key exchange and signature
algorithms that are currently widely used are very similar in sizes
for public keys, ciphertexts and signatures. As an example, RSA
signatures are famously "just" RSA encryption backwards.
This changes in the post-quantum setting. Post-quantum key exchange
and signature algorithms have significant differences in
implementation, performance characteristics, and key and signature
sizes.
This also leads to increases in code size: For example, implementing
highly efficient polynomial multiplication for post-quantum KEM Kyber
and signature scheme Dilithium [DILITHIUM] requires significantly
different approaches, even though the algorithms are related [K22].
Using the protocol proposed in this draft allows to reduce the amount
of data exchanged for handshake authentication. It also allows to
re-use the implementation that is used for ephemeral key exchange for
authentication, as KEM operations replace signing. This decreases
the code size requirements, which is especially relevant to protected
implementations. Finally, KEM operations may be more efficient than
signing, which might especially affect embedded platforms.
1.2. Evaluation of handshake sizes
*Should probably be removed before publishing*
In the following table, we compare the sizes of TLS 1.3- and AuthKEM-
based handshakes. We give the transmission requirements for
handshake authentication (public key + signature), and certificate
chain (intermediate CA certificate public key and signature + root CA
signature). For clarity, we are not listing post-quantum/traditional
hybrid algorithms; we also omit mechanisms such as Certificate
Transparency [RFC6962] or OCSP stapling [RFC6960]. We use Kyber-768
instead of the smaller Kyber-512 parameterset, as the former is
currently used in experimental deployments. For signatures, we use
Dilithium, the "primary" algorithm selected by NIST for post-quantum
signatures, as well as Falcon [FALCON], the algorithm that offers
smaller public key and signature sizes, but which NIST indicates can
be used if the implementation requirements can be met.
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+===========+===================+=========+===============+======+
| Handshake | HS auth algorithm | HS Auth | Certificate | Sum |
| | | bytes | chain bytes | |
+===========+===================+=========+===============+======+
| TLS 1.3 | RSA-2048 | 528 | 784 (RSA- | 1312 |
| | | | 2048) | |
+-----------+-------------------+---------+---------------+------+
| TLS 1.3 | Dilithium-2 | 3732 | 6152 | 9884 |
| | | | (Dilithium-2) | |
+-----------+-------------------+---------+---------------+------+
| TLS 1.3 | Falcon-512 | 1563 | 2229 (Falcon- | 3792 |
| | | | 512) | |
+-----------+-------------------+---------+---------------+------+
| TLS 1.3 | Dilithium-2 | 3732 | 2229 (Falcon- | 5961 |
| | | | 512) | |
+-----------+-------------------+---------+---------------+------+
| AuthKEM | Kyber-768 | 2272 | 6152 | 8424 |
| | | | (Dilithium-2) | |
+-----------+-------------------+---------+---------------+------+
| AuthKEM | Kyber-768 | 2272 | 2229 (Falcon- | 4564 |
| | | | 512) | |
+-----------+-------------------+---------+---------------+------+
Table 1: Size comparison of public-key cryptography in TLS 1.3
and AuthKEM handshakes.
Note that although TLS 1.3 with Falcon-512 is the smallest
instantiation, Falcon is very challenging to implement: signature
generation requires (emulation of) 64-bit floating point operations
in constant time. It is also very difficult to protect against other
side-channel attacks, as there are no known methods of masking
Falcon. In light of these difficulties, use of Falcon-512 in online
handshake signatures may not be wise.
Using AuthKEM with Falcon-512 in the certificate chain remains an
attractive option, however: the certificate issuance process, because
it is mostly offline, could perhaps be set up in a way to protect the
Falcon implementation against attacks. Falcon signature verification
is fast and does not require floating-point arithmetic. Avoiding
online usage of Falcon in TLS 1.3 requires two implementations of the
signature verification routines, i.e., Dilithium and Falcon, on top
of the key exchange algorithm.
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In all examples, the size of the certificate chain still dominates
the TLS handshake, especially if Certificate Transparency SCT
statements are included, which is relevant in the context of the
WebPKI. However, we believe that if proposals to reduce transmission
sizes of the certificate chain in the WebPKI context are implemented,
the space savings of AuthKEM naturally become relatively larger and
more significant. We discuss this in Section 1.3.2.
1.3. Related work
1.3.1. OPTLS
This proposal draws inspiration from [I-D.ietf-tls-semistatic-dh],
which is in turn based on the OPTLS proposal for TLS 1.3 [KW16].
However, these proposals require a non-interactive key exchange: they
combine the client's public key with the server's long-term key.
This imposes an extra requirement: the ephemeral and static keys MUST
use the same algorithm, which this proposal does not require.
Additionally, there are no post-quantum proposals for a non-
interactive key exchange currently considered for standardization,
while several KEMs are on the way.
1.3.2. Compressing certificates and certificate chains
AuthKEM reduces the amount of data required for authentication in
TLS. In recognition of the large increase in handshake size that a
naive adoption of post-quantum signatures would affect, several
proposals have been put forward that aim to reduce the size of
certificates in the TLS handshake. [RFC8879] proposes a certificate
compression mechanism based on compression algorithms, but this is
not very helpful to reduce the size of high-entropy public keys and
signatures. Proposals that offer more significant reductions of
sizes of certificate chains, such as
[I-D.draft-jackson-tls-cert-abridge], [I-D.ietf-tls-ctls],
[I-D.draft-kampanakis-tls-scas-latest], and
[I-D.draft-davidben-tls-merkle-tree-certs] all mainly rely on some
form of out-of-band distribution of intermediate certificates or
other trust anchors in a way that requires a robust update mechanism.
This makes these proposals mainly suitable for the WebPKI setting;
although this is also the setting that has the largest number of
certificates due to the inclusion of SCT statements [RFC6962] and
OSCP staples [RFC6960].
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AuthKEM complements these approaches in the WebPKI setting. On its
own the gains that AuthKEM offers may be modest compared to the large
sizes of certificate chains. But when combined with compression or
certificate suppression mechanisms such as those proposed in the
referenced drafts, the reduction in handshake size when replacing
Dilithium-2 by Kyber-768 becomes significant again.
1.4. Organization
After a brief introduction to KEMs, we will introduce the AuthKEM
authentication mechanism. For clarity, we discuss unilateral and
mutual authentication separately. In the remainder of the draft, we
will discuss the necessary implementation mechanics, such as code
points, extensions, new protocol messages and the new key schedule.
The draft concludes with ah extensive discussion of relevant security
considerations.
A related mechanism for KEM-based PSK-style handshakes is discussed
in [I-D.draft-wiggers-tls-authkem-psk].
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.
2.1. Terminology
The following terms are used as they are in [RFC8446]
client: The endpoint initiating the TLS connection.
connection: A transport-layer connection between two endpoints.
endpoint: Either the client or server of the connection.
handshake: An initial negotiation between client and server that
establishes the parameters of their subsequent interactions within
TLS.
peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is not the primary subject of
discussion.
receiver: An endpoint that is receiving records.
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sender: An endpoint that is transmitting records.
server: The endpoint that responded to the initiation of the TLS
connection. i.e. the peer of the client.
2.2. Key Encapsulation Mechanisms
As this proposal relies heavily on KEMs, which are not originally
used by TLS, we will provide a brief overview of this primitive.
Other cryptographic operations will be discussed later.
A Key Encapsulation Mechanism (KEM) is a cryptographic primitive that
defines the methods Encapsulate and Decapsulate. In this draft, we
extend these operations with context separation strings, per HPKE
[RFC9180]:
Encapsulate(pkR, context_string):
Takes a public key, and produces a shared secret and
encapsulation.
Decapsulate(enc, skR, context_string):
Takes the encapsulation and the private key. Returns the shared
secret.
We implement these methods through the KEMs defined in [RFC9180] to
export shared secrets appropriate for using with the HKDF in TLS 1.3:
def Encapsulate(pk, context_string):
enc, ctx = HPKE.SetupBaseS(pk, "tls13 auth-kem " + context_string)
ss = ctx.Export("", HKDF.Length)
return (enc, ss)
def Decapsulate(enc, sk, context_string):
return HPKE.SetupBaseR(enc,
sk,
"tls13 auth-kem " + context_string)
.Export("", HKDF.Length)
Keys are generated and encoded for transmission following the
conventions in [RFC9180]. The values of context_string are defined
in Section 4.3.2.
3. Full 1.5-RTT AuthKEM Handshake Protocol
Figure 1 below shows the basic KEM-authentication (AuthKEM)
handshake, without client authentication:
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Client Server
Key ^ ClientHello
Exch | + key_share
v + signature_algorithms
-------->
ServerHello ^ Key
+ key_share v Exch
<EncryptedExtensions>
<Certificate> ^
<KEMEncapsulation> --------> |
{Finished} --------> | Auth
[Application Data] --------> |
<-------- {Finished} v
[Application Data] <-------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
<> Indicates messages protected using keys
derived from a [sender]_handshake_traffic_secret.
{} Indicates messages protected using keys
derived from a
[sender]_authenticated_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N.
Figure 1: Message Flow for KEM-Authentication (KEM-Auth)
Handshake without client authentication.
This basic handshake captures the core of AuthKEM. Instead of using
a signature to authenticate the handshake, the client encapsulates a
shared secret to the server's certificate public key. Only the
server that holds the private key corresponding to the certificate
public key can derive the same shared secret. This shared secret is
mixed into the handshake's key schedule. The client does not have to
wait for the server's Finished message before it can send data. The
client knows that its message can only be decrypted if the server was
able to derive the authentication shared secret encapsulated in the
KEMEncapsulation message.
Finished messages are sent as in TLS 1.3, and achieve full explicit
authentication.
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3.1. Client authentication
For client authentication, the server sends the CertificateRequest
message as in [RFC8446]. This message can not be authenticated in
the AuthKEM handshake: we will discuss the implications below.
As in [RFC8446], section 4.4.2, if and only if the client receives
CertificateRequest, it MUST send a Certificate message. If the
client has no suitable certificate, it MUST send a Certificate
message containing no certificates. If the server is satisfied with
the provided certificate, it MUST send back a KEMEncapsulation
message, containing the encapsulation to the client's certificate.
The resulting shared secret is mixed into the key schedule. This
ensures any messages sent using keys derived from it are covered by
the authentication.
The AuthKEM handshake with client authentication is given in
Figure 2.
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Client Server
Key ^ ClientHello
Exch | + key_share
v + signature_algorithms
-------->
ServerHello ^ Key
+ key_share v Exch
<EncryptedExtensions> ^ Server
<CertificateRequest> v Params
<Certificate> ^
^ <KEMEncapsulation> |
| {Certificate} --------> |
Auth | <-------- {KEMEncapsulation} | Auth
v {Finished} --------> |
[Application Data] --------> |
<------- {Finished} v
[Application Data] <-------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
<> Indicates messages protected using keys
derived from a [sender]_handshake_traffic_secret.
{} Indicates messages protected using keys
derived from a
[sender]_authenticated_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N.
Figure 2: Message Flow for KEM-Authentication (KEM-Auth)
Handshake with client authentication.
If the server is not satisfied with the client's certificates, it
MAY, at its discretion, decide to continue or terminate the
handshake. If it decides to continue, it MUST NOT send back a
KEMEncapsulation message and the client and server MUST compute the
encryption keys as in the server-only authenticated AuthKEM
handshake. The Certificate remains included in the transcript. The
client MUST NOT assume it has been authenticated.
Unfortunately, AuthKEM client authentication requires an extra round-
trip. Clients that know the server's long-term public KEM key MAY
choose to use the abbreviated AuthKEM handshake and opportunistically
send the client certificate as a 0-RTT-like message. This mechanism
is discussed in [I-D.draft-wiggers-authkem-psk-latest].
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3.2. Relevant handshake messages
After the Key Exchange and Server Parameters phase of TLS 1.3
handshake, the client and server exchange implicitly authenticated
messages. KEM-based authentication uses the same set of messages
every time that certificate-based authentication is needed.
Specifically:
* Certificate: The certificate of the endpoint and any per-
certificate extensions. This message MUST be omitted by the
client if the server did not send a CertificateRequest message
(thus indicating that the client should not authenticate with a
certificate). For AuthKEM, Certificate MUST include the long-term
KEM public key. Certificates MUST be handled in accordance with
[RFC8446], section 4.4.2.4.
* KEMEncapsulation: A key encapsulation against the certificate's
long-term public key, which yields an implicitly authenticated
shared secret.
3.3. Overview of key differences with RFC8446 TLS 1.3
* New types of signature_algorithms for KEMs.
* Public keys in certificates are KEM algorithms.
* New handshake message KEMEncapsulation.
* The key schedule mixes in the shared secrets from the
authentication.
* The Certificate is sent encrypted with a new handshake encryption
key.
* The client sends Finished before the server.
* The client sends data before the server has sent Finished.
3.4. Implicit and explicit authentication
The data that the client MAY transmit to the server before having
received the server's Finished is encrypted using ciphersuites chosen
based on the client's and server's advertised preferences in the
ClientHello and ServerHello messages. The ServerHello message can
however not be authenticated before the Finished message from the
server is verified. The full implications of this are discussed in
the Security Considerations section.
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Upon receiving the client's authentication messages, the server
responds with its Finished message, which achieves explicit
authentication. Upon receiving the server's Finished message, the
client achieves explicit authentication. Receiving this message
retroactively confirms the server's cryptographic parameter choices.
3.5. Authenticating CertificateRequest
The CertificateRequest message can not be authenticated during the
AuthKEM handshake; only after the Finished message from the server
has been processed, it can be proven as authentic. The security
implications of this are discussed later.
*This is discussed in GitHub issue #16 (https://github.com/kemtls/
draft-celi-wiggers-tls-authkem/issues/16). We would welcome feedback
there.*
Clients MAY choose to only accept post-handshake authentication.
*TODO: Should they indicate this? TLS Flag?*
4. Implementation
In this section we will discuss the implementation details such as
extensions and key schedule.
4.1. Negotiation of AuthKEM
Clients will indicate support for this mode by negotiating it as if
it were a signature scheme (part of the signature_algorithms
extension). We thus add these new signature scheme values (even
though, they are not signature schemes) for the KEMs defined in
[RFC9180] Section 7.1. Note that we will be only using their
internal KEM's API defined there.
enum {
dhkem_p256_sha256 => TBD,
dhkem_p384_sha384 => TBD,
dhkem_p521_sha512 => TBD,
dhkem_x25519_sha256 => TBD,
dhkem_x448_sha512 => TBD,
kem_x25519kyber768 => TBD, /*draft-westerbaan-cfrg-hpke-xyber768d00*/
}
*Please give feedback on which KEMs should be included*
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When present in the signature_algorithms extension, these values
indicate AuthKEM support with the specified key exchange mode. These
values MUST NOT appear in signature_algorithms_cert, as this
extension specifies the signing algorithms by which certificates are
signed.
4.2. Protocol messages
The handshake protocol is used to negotiate the security parameters
of a connection, as in TLS 1.3. It uses the same messages, expect
for the addition of a KEMEncapsulation message and does not use the
CertificateVerify one.
enum {
...
kem_encapsulation(tbd),
...
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* remaining bytes in message */
select (Handshake.msg_type) {
...
case kem_encapsulation: KEMEncapsulation;
...
};
} Handshake;
Protocol messages MUST be sent in the order defined in Section 4. A
peer which receives a handshake message in an unexpected order MUST
abort the handshake with a "unexpected_message" alert.
The KEMEncapsulation message is defined as follows:
struct {
opaque certificate_request_context<0..2^8-1>
opaque encapsulation<0..2^16-1>;
} KEMEncapsulation;
The encapsulation field is the result of a Encapsulate function. The
Encapsulate() function will also result in a shared secret (ssS or
ssC, depending on the peer) which is used to derive the AHS or MS
secrets (See Section 4.3.1).
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If the KEMEncapsulation message is sent by a server, the
authentication algorithm MUST be one offered in the client's
signature_algorithms extension. Otherwise, the server MUST terminate
the handshake with an "unsupported_certificate" alert.
If sent by a client, the authentication algorithm used in the
signature MUST be one of those present in the
supported_signature_algorithms field of the signature_algorithms
extension in the CertificateRequest message.
In addition, the authentication algorithm MUST be compatible with the
key(s) in the sender's end-entity certificate.
The receiver of a KEMEncapsulation message MUST perform the
Decapsulate() operation by using the sent encapsulation and the
private key of the public key advertised in the end-entity
certificate sent. The Decapsulate() function will also result on a
shared secret (ssS or ssC, depending on the Server or Client
executing it respectively) which is used to derive the AHS or MS
secrets.
certificate_request_context is included to allow the recipient to
identify the certificate against which the encapsulation was
generated. It MUST be set to the value in the Certificate message to
which the encapsulation was computed.
4.3. Cryptographic computations
The AuthKEM handshake establishes three input secrets which are
combined to create the actual working keying material, as detailed
below. The key derivation process incorporates both the input
secrets and the handshake transcript. Note that because the
handshake transcript includes the random values from the Hello
messages, any given handshake will have different traffic secrets,
even if the same input secrets are used.
4.3.1. Key schedule for full AuthKEM handshakes
AuthKEM uses the same HKDF-Extract and HKDF-Expand functions as
defined by TLS 1.3, in turn defined by [RFC5869].
Keys are derived from two input secrets using the HKDF-Extract and
Derive-Secret functions. The general pattern for adding a new secret
is to use HKDF-Extract with the Salt being the current secret state
and the Input Keying Material (IKM) being the new secret to be added.
The notable differences are:
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* The addition of the Authenticated Handshake Secret and a new set
of handshake traffic encryption keys.
* The inclusion of the SSs and SSc (if present) shared secrets as
IKM to Authenticated Handshake Secret and Main Secret,
respectively.
The full key schedule proceeds as follows:
0
|
v
PSK -> HKDF-Extract = Early Secret
|
+--> Derive-Secret(., "ext binder" | "res binder", "")
| = binder_key
|
+--> Derive-Secret(., "c e traffic", ClientHello)
| = client_early_traffic_secret
|
+--> Derive-Secret(., "e exp master", ClientHello)
| = early_exporter_master_secret
v
Derive-Secret(., "derived", "")
|
v
(EC)DHE -> HKDF-Extract = Handshake Secret
|
+--> Derive-Secret(., "c hs traffic",
| ClientHello...ServerHello)
| = client_handshake_traffic_secret
|
+--> Derive-Secret(., "s hs traffic",
| ClientHello...ServerHello)
| = server_handshake_traffic_secret
v
Derive-Secret(., "derived", "") = dHS
|
v
SSs -> HKDF-Extract = Authenticated Handshake Secret
|
+--> Derive-Secret(., "c ahs traffic",
| ClientHello...KEMEncapsulation)
| = client_handshake_authenticated_traffic_secret
|
+--> Derive-Secret(., "s ahs traffic",
| ClientHello...KEMEncapsulation)
| = server_handshake_authenticated_traffic_secret
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v
Derive-Secret(., "derived", "") = dAHS
|
v
SSc||0 * -> HKDF-Extract = Main Secret
|
+--> Derive-Secret(., "c ap traffic",
| ClientHello...server Finished)
| = client_application_traffic_secret_0
|
+--> Derive-Secret(., "s ap traffic",
| ClientHello...server Finished)
| = server_application_traffic_secret_0
|
+--> Derive-Secret(., "exp master",
| ClientHello...server Finished)
| = exporter_master_secret
|
+--> Derive-Secret(., "res master",
ClientHello...client Finished)
= resumption_master_secret
*: if client authentication was requested, the `SSc` value should
be used. Otherwise, the `0` value is used.
4.3.2. Computations of KEM shared secrets
The operations to compute SSs or SSc from the client are:
SSs, encapsulation <- Encapsulate(public_key_server,
"server authentication")
SSc <- Decapsulate(encapsulation, private_key_client,
"client authentication")
The operations to compute SSs or SSc from the server are:
SSs <- Decapsulate(encapsulation, private_key_server
"server authentication")
SSc, encapsulation <- Encapsulate(public_key_client,
"client authentication")
4.3.3. Explicit Authentication Messages
AuthKEM upgrades implicit to explicit authentication through the
Finished message. Note that in the full handshake, AuthKEM achieves
explicit authentication only when the server sends the final Finished
message (the client is only implicitly authenticated when they send
their Finished message).
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Full downgrade resilience and forward secrecy is achieved once the
AuthKEM handshake completes.
The key used to compute the Finished message MUST be computed from
the MainSecret using HKDF (instead of a key derived from HS as in
[RFC8446]). Specifically:
server/client_finished_key =
HKDF-Expand-Label(MainSecret,
server/client_label,
"", Hash.length)
server_label = "tls13 server finished"
client_label = "tls13 client finished"
The verify_data value is computed as follows. Note that instead of
what is specified in [RFC8446], we use the full transcript for both
server and client Finished messages:
server/client_verify_data =
HMAC(server/client_finished_key,
Transcript-Hash(Handshake Context,
Certificate*,
KEMEncapsulation*,
Finished**))
* Only included if present.
** The party who last sends the finished message in terms of flights
includes the other party's Finished message.
Any records following a Finished message MUST be encrypted under the
appropriate application traffic key as described in [RFC8446]. In
particular, this includes any alerts sent by the server in response
to client Certificate and KEMEncapsulation messages.
See [SSW20] for a full treatment of implicit and explicit
authentication.
5. Security Considerations
5.1. Implicit authentication
Because preserving a 1/1.5RTT handshake in KEM-Auth requires the
client to send its request in the same flight when the ServerHello
message is received, it can not yet have explicitly authenticated the
server. However, through the inclusion of the key encapsulated to
the server's long-term secret, only an authentic server should be
able to decrypt these messages.
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However, the client can not have received confirmation that the
server's choices for symmetric encryption, as specified in the
ServerHello message, were authentic. These are not authenticated
until the Finished message from the server arrived. This may allow
an adversary to downgrade the symmetric algorithms, but only to what
the client is willing to accept. If such an attack occurs, the
handshake will also never successfully complete and no data can be
sent back.
If the client trusts the symmetric algorithms advertised in its
ClientHello message, this should not be a concern. A client MUST NOT
accept any cryptographic parameters it does not include in its own
ClientHello message.
If client authentication is used, explicit authentication is reached
before any application data, on either client or server side, is
transmitted.
Application Data MUST NOT be sent prior to sending the Finished
message, except as specified in Section 2.3 of [RFC8446]. Note that
while the client MAY send Application Data prior to receiving the
server's last explicit Authentication message, any data sent at that
point is, being sent to an implicitly authenticated peer.
5.2. Authentication of Certificate Request
Due to the implicit authentication of the server's messages during
the full AuthKEM handshake, the CertificateRequest message can not be
authenticated before the client received Finished.
The key schedule guarantees that the server can not read the client's
certificate message (as discussed above). An active adversary that
maliciously inserts a CertificateRequest message will also result in
a mismatch in transcript hashes, which will cause the handshake to
fail.
However, there may be side effects. The adversary might learn that
the client has a certificate by observing the length of the messages
sent. There may also be side effects, especially in situations where
the client is prompted to e.g. approve use or unlock a certificate
stored encrypted or on a smart card.
5.3. Other security considerations
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* Because the Main Secret is derived from both the ephemeral key
exchange, as well as from the key exchanges completed for server
and (optionally) client authentication, the MS secret always
reflects the peers' views of the authentication status correctly.
This is an improvement over TLS 1.3 for client authentication.
* The academic works proposing AuthKEM (KEMTLS) contains an in-depth
technical discussion of and a proof of the security of the
handshake protocol without client authentication [SSW20].
* The work proposing the variant protocol [SSW21] with pre-
distributed public keys (the abbreviated AuthKEM handshake) has a
proof for both unilaterally and mutually authenticated handshakes.
* We have proofs of the security of KEMTLS and KEMTLS-PDK in
Tamarin. [CHSW22]
* Application Data sent prior to receiving the server's last
explicit authentication message (the Finished message) can be
subject to a client certificate suite downgrade attack. Full
downgrade resilience and forward secrecy is achieved once the
handshake completes.
* The client's certificate is kept secret from active observers by
the derivation of the client_authenticated_handshake_secret, which
ensures that only the intended server can read the client's
identity.
* If AuthKEM client authentication is used, the resulting shared
secret is included in the key schedule. This ensures that both
peers have a consistent view of the authentication status, unlike
[RFC8446].
6. References
6.1. 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>.
[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>.
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[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[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>.
6.2. Informative References
[CHSW22] Celi, S., Hoyland, J., Stebila, D., and T. Wiggers, "A
tale of two models: formal verification of KEMTLS in
Tamarin", ESORICS 2022 , DOI 10.1007/978-3-031-17143-7_4,
IACR ePrint https://ia.cr/2022/1111, August 2022,
<https://doi.org/10.1007/978-3-031-17143-7_4>.
[DILITHIUM]
Ducas, L., Kiltz, E., Lepoint, T., Lyubashevsky, V.,
Schwabe, P., Seiler, G., and D. Stehlé, "CRYSTALS-
Dilithium", 2021, <https://pq-crystals.org/dilithium/>.
[FALCON] Fouque, P., Hoffstein, J., Kirchner, P., Lyubashevsky, V.,
Pornin, T., Ricosset, T., Seiler, G., Whyte, W., and Z.
Zhang, "Falcon", 2021, <https://falcon-sign.info>.
[I-D.draft-cfrg-schwabe-kyber]
Schwabe, P. and B. Westerbaan, "Kyber Post-Quantum KEM",
Work in Progress, Internet-Draft, draft-cfrg-schwabe-
kyber-02, 31 March 2023,
<https://datatracker.ietf.org/doc/html/draft-cfrg-schwabe-
kyber-02>.
[I-D.draft-davidben-tls-merkle-tree-certs]
Benjamin, D., O'Brien, D., and B. Westerbaan, "Merkle Tree
Certificates for TLS", Work in Progress, Internet-Draft,
draft-davidben-tls-merkle-tree-certs-00, 10 March 2023,
<https://datatracker.ietf.org/doc/html/draft-davidben-tls-
merkle-tree-certs-00>.
[I-D.draft-jackson-tls-cert-abridge]
Jackson, D., "Abridged Compression for WebPKI
Certificates", Work in Progress, Internet-Draft, draft-
jackson-tls-cert-abridge-00, 6 July 2023,
<https://datatracker.ietf.org/doc/html/draft-jackson-tls-
cert-abridge-00>.
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[I-D.draft-kampanakis-tls-scas-latest]
Kampanakis, P., Bytheway, C., Westerbaan, B., and M.
Thomson, "Suppressing CA Certificates in TLS 1.3", Work in
Progress, Internet-Draft, draft-kampanakis-tls-scas-
latest-03, 5 January 2023,
<https://datatracker.ietf.org/doc/html/draft-kampanakis-
tls-scas-latest-03>.
[I-D.draft-westerbaan-cfrg-hpke-xyber768d00]
Westerbaan, B. and C. A. Wood, "X25519Kyber768Draft00
hybrid post-quantum KEM for HPKE", Work in Progress,
Internet-Draft, draft-westerbaan-cfrg-hpke-xyber768d00-02,
4 May 2023, <https://datatracker.ietf.org/doc/html/draft-
westerbaan-cfrg-hpke-xyber768d00-02>.
[I-D.draft-wiggers-authkem-psk-latest]
"*** BROKEN REFERENCE ***".
[I-D.draft-wiggers-tls-authkem-psk]
"*** BROKEN REFERENCE ***".
[I-D.ietf-tls-ctls]
Rescorla, E., Barnes, R., Tschofenig, H., and B. M.
Schwartz, "Compact TLS 1.3", Work in Progress, Internet-
Draft, draft-ietf-tls-ctls-08, 13 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
ctls-08>.
[I-D.ietf-tls-semistatic-dh]
Rescorla, E., Sullivan, N., and C. A. Wood, "Semi-Static
Diffie-Hellman Key Establishment for TLS 1.3", Work in
Progress, Internet-Draft, draft-ietf-tls-semistatic-dh-01,
7 March 2020, <https://datatracker.ietf.org/doc/html/
draft-ietf-tls-semistatic-dh-01>.
[I-D.ietf-tls-subcerts]
Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
"Delegated Credentials for TLS and DTLS", Work in
Progress, Internet-Draft, draft-ietf-tls-subcerts-15, 30
June 2022, <https://datatracker.ietf.org/doc/html/draft-
ietf-tls-subcerts-15>.
[K22] Kannwischer, M. J., "Polynomial Multiplication for Post-
Quantum Cryptography", Ph.D. thesis, 22 April 2022,
<https://kannwischer.eu/thesis/>.
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[KW16] Krawczyk, H. and H. Wee, "The OPTLS Protocol and TLS 1.3",
Proceedings of Euro S&P 2016 , 2016,
<https://ia.cr/2015/978>.
[KYBER] Avanzi, R., Bos, J., Ducas, L., Kiltz, E., Lepoint, T.,
Lyubashevsky, V., Schanck, J., Schwabe, P., Seiler, G.,
and D. Stehlé, "CRYSTALS-Kyber", 2021,
<https://pq-crystals.org/kyber/>.
[NISTPQC] NIST, "Post-Quantum Cryptography Standardization", 2020.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/rfc/rfc6960>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/rfc/rfc6962>.
[RFC8410] Josefsson, S. and J. Schaad, "Algorithm Identifiers for
Ed25519, Ed448, X25519, and X448 for Use in the Internet
X.509 Public Key Infrastructure", RFC 8410,
DOI 10.17487/RFC8410, August 2018,
<https://www.rfc-editor.org/rfc/rfc8410>.
[RFC8879] Ghedini, A. and V. Vasiliev, "TLS Certificate
Compression", RFC 8879, DOI 10.17487/RFC8879, December
2020, <https://www.rfc-editor.org/rfc/rfc8879>.
[SSW20] Stebila, D., Schwabe, P., and T. Wiggers, "Post-Quantum
TLS without Handshake Signatures", ACM CCS 2020 ,
DOI 10.1145/3372297.3423350, IACR
ePrint https://ia.cr/2020/534, November 2020,
<https://doi.org/10.1145/3372297.3423350>.
[SSW21] Stebila, D., Schwabe, P., and T. Wiggers, "More Efficient
KEMTLS with Pre-Shared Keys", ESORICS 2021 ,
DOI 10.1007/978-3-030-88418-5_1, IACR
ePrint https://ia.cr/2021/779, May 2021,
<https://doi.org/10.1007/978-3-030-88418-5_1>.
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Appendix A. Open points of discussion
The following are open points for discussion. The corresponding
Github issues will be linked.
A.1. Authentication concerns for client authentication requests.
Tracked by Issue #16 (https://github.com/kemtls/draft-celi-wiggers-
tls-authkem/issues/16).
The certificate request message from the server can not be
authenticated by the AuthKEM mechanism. This is already somewhat
discussed above and under security considerations. We might want to
allow clients to refuse client auth for scenarios where this is a
concern.
A.2. Interaction with signing certificates
Tracked by Issue #20 (https://github.com/kemtls/draft-celi-wiggers-
tls-authkem/issues/20).
In the current state of the draft, we have not yet discussed
combining traditional signature-based authentication with KEM-based
authentication. One might imagine that the Client has a sigining
certificate and the server has a KEM public key.
In the current draft, clients MUST use a KEM certificate algorithm if
the server negotiated AuthKEM.
Acknowledgements
Early versions of this work were supported by the European Research
Council through Starting Grant No. 805031 (EPOQUE).
Authors' Addresses
Thom Wiggers
PQShield
Nijmegen
Email: thom@thomwiggers.nl
Sofía Celi
Brave Software
Lisbon
Portugal
Email: cherenkov@riseup.net
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Peter Schwabe
Radboud University and MPI-SP
Email: peter@cryptojedi.org
Douglas Stebila
University of Waterloo
Waterloo, ON
Canada
Email: dstebila@uwaterloo.ca
Nick Sullivan
Email: nicholas.sullivan+ietf@gmail.com
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