| Internet-Draft | AuthKEM-PSK | October 2024 |
| Wiggers, et al. | Expires 20 April 2025 | [Page] |
This document gives a construction in which (long-term) KEM public keys are used in the place of TLS PSK keys, avoiding concerns that may affect systems that use symmetric-key-based PSK, such as requiring key diversification and protection of symmetric-keys' confidentiality.¶
This mechanism is inspired by AuthKEM (and could use AuthKEM certificate public keys for resumption), but can be independently implemented.¶
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-wiggers-tls-authkem-psk/.¶
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.¶
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Copyright (c) 2024 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/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
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, PSK-style abbreviated TLS 1.3 [RFC8446] handshakes. It is similar in spirit to [I-D.celi-wiggers-tls-authkem], but can be independently implemented.¶
The abbreviated handshake is appropriate for endpoints that have KEM public keys, and where the client has the server's public key before initiation of the connection. 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]. The public keys need not necessarily be certificates, however. The client might be provided with the public key as a matter of configuration.¶
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.westerbaan-cfrg-hpke-xyber768d00]. This proposal uses ML-KEM [FIPS203] [I-D.cfrg-schwabe-kyber], the first selected algorithm for key exchange in the NIST post-quantum standardization project [NISTPQC].¶
This section should be removed prior to publication of a final version of this document.¶
After covering preliminaries, we introduce the abbreviated AuthKEM-PSK handshake, and its opportunistic client authentication mechanism. 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 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.¶
The following terms are used as they are in [RFC8446]¶
The endpoint initiating the TLS connection.¶
A transport-layer connection between two endpoints.¶
Either the client or server of the connection.¶
An initial negotiation between client and server that establishes the parameters of their subsequent interactions within TLS.¶
An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint that is not the primary subject of discussion.¶
An endpoint that is receiving records.¶
An endpoint that is transmitting records.¶
The endpoint that responded to the initiation of the TLS connection. i.e. the peer of the client.¶
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.¶
This definition matches the one from [I-D.celi-wiggers-tls-authkem].¶
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:¶
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 key schedule in TLS 1.3:¶
def Encapsulate(pk, context_string):
enc, ctx = HPKE.SetupBaseS(pk, "tls13 auth-kem")
ss = ctx.Export(context_string, HKDF.Length)
return (enc, ss)
def Decapsulate(enc, sk, context_string):
return HPKE.SetupBaseR(enc, sk, "tls13 auth-kem")
.Export(context_string, HKDF.Length)
¶
Keys are generated and encoded for transmission following the conventions in
[RFC9180]. The values of context_string are defined in
Section 4.4.2.¶
Open question: Should we keep using HPKE, or just use "plain" KEMs, as in the original KEMTLS works? Please see the discussion at Issue #32.¶
When the client already has the server's long-term public key, we can do a more
efficient handshake. The client will send the encapsulation to the server's
long-term public key in a ClientHello extension. An overview of the
abbreviated AuthKEM handshake is given in Figure 3.¶
A client that already knows the server, might also already know that it will be required to present a client certificate. This is expected to be especially useful in server-to-server scenarios. The abbreviated handshake allows to encrypt the certificate and send it similarly to early data.¶
Client Server
Key ^ ClientHello
Exch | + key_share
& | + stored_auth_key
Auth | + signature_algorithms
| + early_auth*
| + early_data*
| (Certificate*)
| (Application Data*) --------> ServerHello ^
| + key_share |
| + stored_auth_key | Key
| + early_auth* | Exch,
| + early_data* | Auth &
| {EncryptedExtensions} | Server
| {KEMEncapsulation*} | Params
| <-------- {Finished} v
| <-------- [Application Data*]
| (EndOfEarlyData)
v {Finished} -------->
[Application Data] <-------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages/extensions that are not always sent.
<> Indicates messages protected using keys
derived from a
client_early_handshake_traffic_secret.
() Indicates messages protected using keys derived
from a client_early_traffic_secret.
{} Indicates messages protected using keys
derived from a
[sender]_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N.
Figure 3: Abbreviated AuthKEM handshake, with optional
opportunistic client authentication.
¶
In an Appendix A.1, we sketch a variant based on the PSK extension.¶
A client that knows a server's long-term KEM public key MAY choose to attempt
the abbreviated AuthKEM handshake. If it does so, it MUST include the
stored_auth_key extension in the ClientHello message. This message MUST
contain the encapsulation against the long-term KEM public key. Details of the
extension are described below. The shared secret resulting from the
encapsulation is mixed in to the EarlySecret computation.¶
The client MAY additionally choose to send a certificate to the server. It MUST
know what ciphersuites the server accepts before it does so. If it chooses to do
so, it MUST send the early_auth extension to the server. The Certificate
is encrypted with the client_early_handshake_traffic_secret.¶
The server MAY accept the abbreviated AuthKEM handshake. If it does, it MUST
reply with a stored_auth_key extension. If it does not accept the
abbreviated AuthKEM handshake, for instance because it does not have access to
the correct secret key anymore, it MUST NOT reply with a stored_auth_key
extension. The server, if it accepts the abbreviated AuthKEM handshake, MAY
additionally accept the Certificate message. If it does, it MUST reply with
a early_auth extension.¶
If the client, who sent a stored_auth_key extension, receives a
ServerHello without stored_auth_key extension, it MUST recompute
EarlySecret without the encapsulated shared secret.¶
If the client sent a Certificate message, it MUST drop that message from its
transcript. The client MUST then continue with a full AuthKEM handshake.¶
The client MAY send 0-RTT data, as in [RFC8446] 0-RTT mode. The
Certificate MUST be sent before the 0-RTT data.¶
As the EarlySecret is derived only from a key encapsulated to a long-term
secret, it does not have forward secrecy. Clients MUST take this into
consideration before transmitting 0-RTT data or opting in to early client auth.
Certificates and 0-RTT data may also be replayed.¶
This will be discussed in full under Security Considerations.¶
In this section we will discuss the implementation details such as extensions and key schedule.¶
Clients and servers indicate support for AuthKEM authentication 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*/
}
¶
This matches the definition in [I-D.celi-wiggers-tls-authkem].¶
Please give feedback on which KEMs should be included¶
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.¶
A number of AuthKEM messages contain tag-length-value encoded extensions
structures. We are adding those extensions to the ExtensionType list from TLS
1.3.¶
enum {
...
stored_auth_key (TBD), /* RFC TBD */
early_auth (TBD), /* RFC TBD */
(65535)
} ExtensionType;
¶
The table below indicates the messages where a given extension may appear:¶
+---------------------------------------+-------------+ | Extension | KEM-Auth | +---------------------------------------+-------------+ | stored_auth_key [RFCTBD] | CH, SH | | | | | early_auth [RFCTBD] | CH, SH | | | | +---------------------------------------+-------------+¶
To transmit the early authentication encapsulation in the abbreviated AuthKEM
handshake, this document defines a new extension type
(stored_auth_key (TBD)). It is used in ClientHello and ServerHello
messages.¶
The extension_data field of this extension, when included in the
ClientHello, MUST contain the StoredInformation structure.¶
struct {
select (type) {
case client:
opaque key_fingerprint<1..255>;
opaque ciphertext<1..2^16-1>
case server:
AcceptedAuthKey '1';
} body;
} StoredInformation
¶
This extension MUST contain the following information when included in
ClientHello messages:¶
The client indicates the public key encapsulated to by its fingerprint¶
The client submits the ciphertext¶
The server MUST send the extension back as an acknowledgement, if and only if it wishes to negotiate the abbreviated AuthKEM handshake.¶
The fingerprint calculation proceeds this way:¶
Compute the SHA-256 hash of the input data. Note that the computed hash only covers the input data structure (and not any type and length information of the record layer).¶
Use the output of the SHA-256 hash.¶
If this extension is not present, the client and the server MUST NOT negotiate the abbreviated AuthKEM handshake.¶
The presence of the fingerprint might reveal information about the identity of the server that the client has. This is discussed further under Security Considerations (Section 5).¶
To indicate the client will attempt client authentication in the abbreviated
AuthKEM handshake, and for the server to indicate acceptance of attempting this
authentication mechanism, we define the ```early_auth (TDB)extension. It is
used inClientHelloandServerHello`` messages.¶
struct {} EarlyAuth
¶
This is an empty extension.¶
It MUST NOT be sent if the stored_auth_key extension is not present.¶
The handshake protocol is used to negotiate the security parameters
of a connection, as in TLS 1.3. It uses the same messages, except
for the addition of a KEMEncapsulation message and does not use
the CertificateVerify one.¶
Note that these definitions mirror [I-D.celi-wiggers-tls-authkem].¶
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 3. A peer which receives a handshake message in an unexpected order MUST abort the handshake with an "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.¶
If the KEMEncapsulation message is sent by a server, the authentication
algorithm MUST be one offered in the client's signature_algorithms extension
unless no valid certificate chain can be produced without unsupported
algorithms.¶
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.¶
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.¶
The AuthKEM-PSK handshake follows the [RFC8446] key schedule closely. We
change the computation of the EarlySecret as follows, and add a computation
for client_early_handshake_traffic_secret:¶
0
|
v
SSs -> HKDF-Extract = Early Secret
|
...
+--> Derive-Secret(., "c e traffic", ClientHello)
| = client_early_traffic_secret
|
+--> Derive-Secret(., "c e hs traffic", ClientHello)
| = client_early_handshake_traffic_secret
...
|
v
Derive-Secret(., "derived", "") = dES
...
¶
We change the computation of Main Secret as follows:¶
Derive-Secret(., "derived", "") = dHS
|
v
SSc||0 * -> HKDF-Extract = Main Secret
|
...
¶
SSc is included if client authentication is used; otherwise, the value 0 is
used.¶
As in [I-D.celi-wiggers-tls-authkem], 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")
¶
AuthKEM upgrades implicit to explicit authentication through the Finished
message. With AuthKEM-PSK, the server achieves explicit authentication when
sending their Finished message and the client when they send their
Finished message.¶
The key used to compute the Finished message MUST be computed from the
MainSecret using HKDF. 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:¶
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.
¶
These computations match [I-D.celi-wiggers-tls-authkem].¶
See Section 3.1 for special considerations for the abbreviated AuthKEM handshake.¶
Any records following a Finished message MUST be encrypted under the appropriate
application traffic key as described in TLS 1.3. In particular, this includes
any alerts sent by the server in response to client Certificate and
KEMEncapsulation messages.¶
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], [Wig24]).¶
The work proposing the variant protocol ([SSW21], [Wig24]) with pre-distributed public keys (the abbreviated AuthKEM handshake) has a proof for both unilaterally and mutually authenticated handshakes.¶
We have machine-verified proofs of the security of KEMTLS and KEMTLS-PDK in Tamarin. [CHSW22]¶
When the client opportunistically sends its certificate, it is not encrypted under a forward-secure key. This has similar considerations and trade-offs as 0-RTT data. If it is a replayed message, there are no expected consequences for security as the malicious replayer will not be able to decapsulate the shared secret.¶
A client that opportunistically sends its certificate, SHOULD send it encrypted with a ciphertext that it knows the server will accept. Otherwise, it will fail.¶
If AuthKEM-PSK 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].¶
The PDK extension identifies the public key to which the client has encapsulated via a hash. This reveals some information about which server identity the client has. [I-D.ietf-tls-esni] may help alleviate this.¶
An alternative approach could be the use of trial decryption. If the KEM used has anonymity, the ciphertext that the client sends is not linkable to the server public key. ML-KEM offers post-quantum anonymity [MX22].¶
The following are open points for discussion. The corresponding GitHub issues will be linked.¶
pre_shared_key extension
This is discussed in Issue #25.¶
[RFC8446] defines a PSK handshake that can be used with symmetric keys from
e.g. session tickets. In this section, we sketch an alternative approach to
AuthKEM-PSK based on the pre_shared_key extension.¶
A client needs to be set up with the following information:¶
struct {
uint32 authkem_psk_config_version;
uint32 config_lifetime;
opaque KEMPublicKey;
} AuthKEMPSKConfig;
¶
The client computes a KEM ciphertext and shared secret as follows:¶
SSs, encapsulation <- Encapsulate(public_key_server,
"server authentication")
¶
SSs is used in place of PSK in the TLS 1.3 key schedule, and binder_key is
derived as follows:¶
0
|
v
SSc -> HKDF-Extract = Early Secret
|
+-----> Derive-Secret(., "ext binder" | "res binder", "")
| = binder_key
...
¶
In the pre_shared_key extension's identities, the client sends the following
data:¶
struct {
uint32 authkem_psk_config_version;
opaque KEMCiphertext;
} AuthKEMPSKIdentity
¶
The server computes the shared secret SSs from
AuthKEMPSKIdentity.KEMCiphertext as follows:¶
SSs <- Decapsulate(encapsulation,
private_key_server
"server authentication")
¶
The PSK binder value is computed as specified in [RFC8446], section 4.2.11.2. The server MUST verify the binder before continuing and abort the handshake if verification fails.¶
To be determined: how to handle immediate client authentication.¶
It is currently open if there need to be made modifications to better support integration with DTLS. Discussion is at Issue #23.¶
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 signing 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.¶
This work has been supported by the European Research Council through Starting Grant No. 805031 (EPOQUE).¶
Part of this work was supported by the NLNet NGI Assure theme fund project "Standardizing KEMTLS"¶