| LURK | D. Migault |
| Internet-Draft | Ericsson |
| Intended status: Standards Track | April 24, 2020 |
| Expires: October 26, 2020 |
LURK Extension version 1 for (D)TLS 1.3 Authentication
draft-mglt-lurk-tls13-02
This document describes the LURK Extension 'tls13' which enables interactions between a LURK Client and a LURK Server in a context of authentication with (D)TLS 1.3.
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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 26, 2020.
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
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This document defines a LURK extension for TLS 1.3 [RFC8446].
This document assumes the reader is familiar with TLS 1.3 the LURK architecture [I-D.mglt-lurk-lurk].
The motivations for the LURK Extension TLS 1.3 are similar to those for the LURK use cases [I-D.mglt-lurk-tls-use-cases].
Interactions with the Cryptographic Service (CS) can be performed by the TLS Client as well as by the TLS Server.
LURK defines an interface to a CS that stores the security credentials which include the PSK involved in a PSK or PSK-ECDHE authentication or the key used for signing in an ECDHE authentication. In the case of session resumption the PSK is derived from the resumption_master_secret during the key schedule [RFC8446] section 7.1, this secret MAY require similar protection as well. On the other hand session resumption MAY also be delegated as in the LURK extension of TLS 1.2 [I-D.mglt-lurk-tls12].
The current document extends the scope of the LURK extension for TLS 1.2 in that it defines the CS on the TLS server as well as on the TLS client and the CS can operate in non delegating scenarios.
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 uses the terms defined [RFC8446] and [I-D.mglt-lurk-tls12].
LURK / TLS 1.3 is a LURK Extension that introduces a new designation "tls13". This document assumes that Extension is defined with designation set to "tls13" and version set to 1. The LURK Extension extends the LURKHeader structure defined in [I-D.mglt-lurk-lurk] as follows:
enum {
tls13 (2), (255)
} Designation;
enum {
capabilities(0),
ping(1),
s_init_early_secret(2),
s_init_cert_verify(3),
s_hand_and_app_secret(4),
s_new_ticket(5),
c_binder_key(6),
c_init_early_secret(7),
c_init_hand_secret(8),
c_hand_secret(9),
c_app_secret(10),
c_cert_veri913fy(11),
c_register_ticket(12),
c_post_hand(13), (255)
}TLS13Type;
enum {
// generic values reserved or aligned with the
// LURK Protocol
request (0), success (1), undefined_error (2),
invalid_payload_format (3),
invalid_psk
invalid_freshness_funct
invalid_request
invalid_key_id_type
invalid_key_id
invalid_signature_scheme
invalid_certificate_type
invalid_certificate
invalid_certificate_verify
invalid_key_request
invalid_handshake
invalid_extension
invalid_ephemeral
invalid_cookie_h
}TLS13Status
struct {
Designation designation = "tls13";
int8 version = 1;
} Extension;
struct {
Extension extension;
select( Extension ){
case ("tls13", 1):
TLS13Type;
} type;
select( Extension ){
case ("tls13", 1):
TLS13Status;
} status;
uint64 id;
unint32 length;
} LURKHeader;
The CS is not expected to perform any policies such as choosing the appropriated authentication method. Such choices are performed by the TLS client or TLS server that instruct the LURK client accordingly. Such enforced policies between the TLS client and the TLS server are those performed on a standard TLS exchange.
On the other hand, some CS MAY be optimized by implementing a subset of the specified possibilities described in this document. Typically some implementations MAY not implement the session resumption or the post handshake authentication to avoid keeping states of a given session once the handshake has been performed. These capabilities of the CS MAY also in return impact the policies of the TLS client or TLS server.
These limitations are mentioned throughout the document, and even represented in the state diagrams, the recommendation is that the CS SHOULD NOT impact the policies of the TLS client or TLS server. Instead they SHOULD be able to optimize the CS to their policies via some configuration parameters presented in section Section 10.1. Such parameters are implementation dependent and only provided here as informative.
This document defines the role to specify whether the CS runs on a TLS client or a TLS service. The CS MUST be associated a single role.
LURK exchanges falls into three categories: 1) request of keys or secrets, 2) request of signing operations, and 3) requests for ticket (NewSessionTicket) management purposes. In some cases, these operations are combined into a single LURK exchange. Table Figure 1 below summarizes the operations associated for each exchange.
+--------+-----------------------+------------------------+ | Role | LURK exchange | secret | sign | ticket | +--------+-----------------------+------------------------+ | server | s_init_early_secret | yes | - | - | | server | s_init_cert_verify | yes | yes | - | | server | s_hand_and_app_secret | yes | - | - | | server | s_new_ticket | yes | - | yes | | client | c_binder_key | yes | - | - | | client | c_init_early_secret | yes | - | - | | client | c_init_hand_secret | yes | - | - | | client | c_hand_secret | yes | - | - | | client | c_app_secret | yes | - | - | | client | c_cert_verify | yes | yes | - | | client | c_register_ticket | yes | - | yes | | client | c_post_hand | - | yes | - | +--------+-----------------------+------------------------+
Figure 1: Operation associated to LURK exchange
This section describes structures that are widely re-used across the multiple LURK exchanges.
key_request is a 16 bit structure described in Table Figure 2 that indicates the requested key or secrets by the LURK client. The same structure is used across all LURK exchanges, but each LURK exchange only permit a subset of values described in Table Figure 3.
A LURK client MUST NOT set key_request to key or secrets that are not permitted. The CS MUST check the key_request has only permitted values and has all mandatory keys or secrets set. If these two criteria are not met the CS MUST NOT perform the LURK exchange and SHOULD return a invalid_key_request error. If the CS is not able to compute an optional key or secret, the CS MUST proceed the LURK exchange and ignore the optional key or secret.
+------+-------------------------------------------+ | Bit | key or secret (designation) | +------+-------------------------------------------+ | 0 | binder_key (b) | | 1 | client_early_traffic_secret (e_c) | | 2 | early_exporter_master_secret (e_x) | | 3 | client_handshake_traffic_secret (h_c) | | 4 | server_handshake_traffic_secret (h_s) | | 5 | client_application_traffic_secret_0 (a_c) | | 6 | server_application_traffic_secret_0 (a_s) | | 7 | exporter_master_secret (x) | | 8 | resumption_master_secret (r) | | 9-15 | reserved and set to zero | +------+-------------------------------------------+
Figure 2: key_request structure
+-----------------------+--------------------------+ | LURK exchange | Permitted key/secrets | +-----------------------+--------------------------+ | s_init_early_secret | b,e_c*, e_x* | | s_init_cert_verify | h_c, h_s, a_c*, a_s*, x* | | s_hand_and_app_secret | h_c, h_s, a_c*, a_s*, x* | | s_new_ticket | r* | | c_binder_key | b | | c_init_early_secret | e_c*, e_x* | | c_init_hand_secret | h_c, h_s | | c_hand_secret | h_c, h_s | | c_app_secret | a_c*, a_s*, x* | | c_cert_verify | a_c*, a_s*, x* | | c_register_ticket | r* | | c_post_hand | | +-----------------------+--------------------------+ (*) indicates an optional value, other values are mandatory
Figure 3: key_request permitted values per LURK exchange
The Secret structure carries a secret designated by its type and value.
enum {
binder_key (0),
client_early_traffic_secret(1),
early_exporter_master_secret(2),
client_handshake_traffic_secret(3),
server_handshake_traffic_secret(4),
client_application_traffic_secret_0(5),
server_application_traffic_secret_0(6),
exporter_master_secret(7),
esumption_master_secret(8),
(255)
} SecretType;
struct {
SecretType secret_type;
opaque secret_data<0..2^8-1>;
} Secret;
secret_type: The type of the secret or key
secret_data: The value of the secret.
Secrets derivation takes Handshake Context as input. It is the responsibility of the CS to maintain this variable in an internal variable. On the other hand, it is the responsibility of the LURK client to provide the necessary element so the Cryptographic Service got the necessary Handshake Context. The Handshake Context evolves during the key derivation schedule, the LURK client implements an incremental approach where only the missing part of the Handshake Context are provided. The main intention is to prevent the LURK client from providing multiple time the same information as well as to perform extensive compatibility checks between the duplicated information provided.
The handshake_context variable is based on the Handshake structure defined in [RFC8446] section 4. The table below lists the values of the handshake_context associated to each LURK exchange.
+-----------------------+--------------------------------+ | LURK exchange | handshake_context | +-----------------------+--------------------------------+ | s_init_early_secret | ClientHello | | s_init_cert_verify | ClientHello ... later of | | | server EncryptedExtensions / | | | CertificateRequest | | s_hand_and_app_secret | ServerHello ... later of | | | server EncryptedExtensions / | | | CertificateRequest | | s_new_ticket | earlier of client Certificate /| | | client CertificateVerify / | | | Finished ... Finished | | c_binder_key | | | c_init_early_secret | ClientHello | | c_init_hand_secret | ClientHello ... ServerHello | | c_hand_secret | ServerHello | | c_app_secret | server EncryptedExtensions ... | | | server Finished | | c_cert_verify | server EncryptedExtensions ... | | | later of server Finished/ | | | EndOfEarlyData | | c_register_ticket | earlier of client Certificate | | | client CertificateVerify ... | | | client Finished | | c_post_hand | CertificateRequest | +-----------------------+--------------------------------+
Figure 4: handshake values per LURK exchange
Secrets are derived from the key schedule of [RFC8446] section 7.
The derivation of secrets requires an optional PSK that is provided in the psk_id extension described in section Section 4.4.2 as well as a ECDHE value which is provided by the ecdhe extension described in section Section 4.4.1.
The extensions considered in this document are defined as below:
enum { psk_id(1), ephemeral(2), freshness(3), session_id(4) ... (255)
} LURK13ExtensionType;
struct {
LURK13ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} LURK13Extension
struct {
uint16 key_request;
Handshake handshake_context<0..2^32> //RFC8446 section 4.
LURK13Extension extention_list<0...2^16>
} SecretsRequest;
struct {
Secret secret_list<0..2^16-1>;
Extension extention_list<0...2^16>
} SecretsResponse;
key_request: designates the requested secrets (see section Section 4.1).
handshake_context: designates the necessary messages so the CS is aware of the appropriated Handshake Context to generate the secrets (see section Section 4.3).
extension_list: the list of extensions.
secret_list: the list of requested secrets (see section Section 4.2).
The Ephemeral structure carries the necessary information to generate the ECDHE input used to derive the secrets. This document describes two ways the shared secret can be generated: shared_secret_provided: When Diffie Hellman or ECDH keys and shared secret are generated by the TLS server and the shared secret is provided to the CS
secret_generated: When the DH / ECDH keys and shared secret are generated by the CS.
When ECDHE shared secret are not generated by the CS, the LURK client provides the shared secret value to the CS via the ephemeral extension. The shared secret is transmitted via the EphemeralSharedSecret, constructed similarly to the key_exchange parameter of the KeyShareEntry described in [RFC8446] section 4.2.8. The CS MUST NOT return any data.
struct {
NamedGroup group;
opaque shared_secret[coordinate_length];
} EphemeralSharedSecret;
Where coordinate_length depends on the chosen group. For secp256r1, secp384r1, secp521r1, x25519, x448, the coordinate_length is respectively 32 bytes, 48 bytes, 66 bytes, 32 bytes and 56 bytes. Upon receiving the shared_secret, the CS MUST check group is proposed in the KeyShareClientHello and agreed in the KeyShareServerHello.
When the ECDHE public/private keys are generated by the CS, the LURK client requests the CS the associated public value. Note that in such cases the CS would receive an incomplete Handshake Context from the LURK client with the public part of the ECDHE missing. Typically the ServerHello message would present a KeyShareServerHello that consists of a KeyShareEntry with an empty key_exchange field, but the field group is present.
The CS MUST check the group field in the KeyShareServerHello, and get the public value of the TLS client from the KeyShareCLientHello. The CS performs the same checks as described in [RFC8446] section 4.2.8. The CS generates the private and public Diffie Hellman or ECDH keys, computes the shared key and return the KeyShareEntry server_share structure defined in [RFC8446] section section 4.2.8.
Other methods may be defined in the future.
This extension MUST NOT be sent outside the LURK exchanges mentioned below. When received outside these exchanges, the CS SHOULD return an invalid_extension error. When the ephemeral is not supported, an invalid_ephemeral error SHOULD be returned. The ephemeral extension MUST NOT appear more than once in a LURK session. When the extensions appears in more than one LURK exchange an invalid_ephemeral error SHOULD be returned
+-----------------------+----------+ | LURK exchange | Presence | +-----------------------+----------+ | s_init_early_secret | - | | s_init_cert_verify | M | | s_hand_and_app_secret | * | | s_new_ticket | - | | c_binder_key | * | | c_init_early_secret | - | | c_init_hand_secret | * | | c_hand_secret | * | | c_app_secret | - | | c_cert_verify | - | | c_register_ticket | - | | c_post_hand | - | +-----------------------+----------+ M indicates the extension is mandatory - indicates the extension MUST NOT be provided * indicates the extension MAY be provided
Figure 5: Ephemeral Extension presence per LURK exchange
The extension data is defined as follows:
enum { secret_provided(0), secret_generated(1) (255)} EphemeralMethod;
EphemeralDataRequest {
EphemeralMethod method;
select(method) {
case secret_provided:
EphemeralSharedSecret shared_secret<0..2^16>;
}
}
EphemeralDataResponse {
select(method) {
case secret_generated:
KeyShareEntry server_share
}
}
The psk_id indicates the identity of the PSK used in the key schedule.
The LURK client MUST provide this extension only when PSK or PSK-authentication is envisioned and when the PSK has not been provided earlier. These exchanges are s_init_early_secret on the TLS server. On the TLS client side, these exchanges are c_binder_key, c_init_early_secret and c_init_hand_secret. The LURK client MUST NOT provide this extension outside these exchanges. When receiving the PSK extension outside these messages, the CS MUST NOT proceed to the exchange and SHOULD return a invalid_format error.
+-----------------------+----------+ | LURK exchange | Presence | +-----------------------+----------+ | s_init_early_secret | M | | s_init_cert_verify | - | | s_hand_and_app_secret | - | | s_new_ticket | - | | c_binder_key | M | | c_init_early_secret | M | | c_init_hand_secret | * | | c_hand_secret | - | | c_app_secret | - | | c_cert_verify | - | | c_register_ticket | - | | c_post_hand | - | +-----------------------+----------+ M indicates the extension is mandatory - indictaes the extension MUST NOT be provided * indicates the extension MAY be provided
Figure 6: psk extension presence per LURK exchange
The extension data is defined as follows:
PskIdentity psk_id; //RFC8446 section 4.2.11
When the psk extension is provided in LURK exchange that is not permitted an invalid_extension error SHOULD be returned.
Upon receiving this extension in the permitted LURK exchange the CS checks the PSK is available. In case the PSK is not available, an invalid_psk error is returned. If the PSK is not provided, a default PSK is generated as described in [RFC8446] section 7.1. If the default PSK is not allowed then an invalid_psk is returned.
The freshness_function provides perfect forward secrecy (PFS) and is used by the LURK client on the TLS client to generate the ClientHello.random or by the LURK client on the TLS server to generate the ServerHello.random. When these randoms are provided to the CS, the freshness_function MUST be provided as well.
Table Figure 7 lists the LURK exchange that MUST include the freshness function extension as well as those where the extension may be provided.
+-----------------------+------------+ | LURK exchange | Presence | +-----------------------+------------+ | s_init_early_secret | - | | s_init_cert_verify | M | | s_hand_and_app_secret | M | | s_new_ticket | - | | c_binder_key | - | | c_init_early_secret | M | | c_init_hand_secret | M | | c_hand_secret | - | | c_app_secret | - | | c_cert_verify | - | | c_register_ticket | - | | c_post_hand | - | +-----------------------+------------+ * indicates the extension MAY be provided M indicates the extension is mandatory - indictaes the extension MUST NOT be provided
Figure 7: freshness_funct extension presence per LURK exchange
The extension data is defined as follows:
FreshnessFunct freshness_funct; // {{I-D.mglt-lurk-tls12}} section 4.1
If the CS does not support the freshness_funct, an invalid_freshness_funct error is returned.
Perfect forward secrecy is implemented in a similar manner as with the TLS 1.2 extension described in [I-D.mglt-lurk-tls12] section 4.1.1. As ServerHello.random in TLS 1.3 do not include time, it is not considered here. In addition, we use a specific context related to TLS 1.3.
As a result, the ServerHello.random is generated as follows on the TLS server.
ServerHello.random = freshness_funct( server_random + "tls13 pfs srv" );
The ClientHello.random is generated as follows on the TLS client side:
ClientHello.random = freshness_funct( server_random + "tls13 pfs clt" );
Perfect forward secrecy applies to the ServerHello.random on the TLS server and on the ClientHello.random on the TLS client. As a result, PFS is provided on the TLS server as long as the ServerHello is part of the Handshake Context. Similarly PFS is provided on the TLS client as long as ClientHello is part of the Handshake Context. On the TLS server, s_init_early_secret exchange do not have the ServerHello so this exchange is not protected by PFS later exchanges are. On the TLS client side, c_binder_key does not have any Handshake Context so this exchange is not protected by PFS. Later exchanges are.
A LURK client and the CS are likely to establish a LURK session. A session is mandatory to be set when a given TLS session requires multiple interactions between the Lurk client and the CS. Stateless interactions MAY be possible with ECDHE authentication without session resumption. Other configuration MAY also use other configurations to determine the session, such as a TCP session for example, in which case multiplexing LURK sessions over a common transport layer is not possible.
The LURK client indicates its willing to set a session as well as the session ID that should be used by the CS by inserting a Session ID Extension. The LURK client MAY only insert the Extension in a LURK message that initiates a session.
Upon receiving the Session ID Extension in an unexpected message, the CS MUST return an invalid_extension error. Upon receiving the Session ID Extension in an expected LURK message, The CS MAY ignored the received extension indicating thus to the LURK client that no session ID are needed. The LURK client MUST NOT insert a session ID in the following messages. If the CS agrees on setting a LURK session, the CS will return the Extension back with the expected Session ID use for the communication between the LURK client and the CS. When a session ID has been agreed all remaining exchange contain the session ID provided by the peer.
The CS MAY require a session ID being agreed between the LURK client and the CS. When the LURK client does not include the Session ID Extension, the CS MUST respond with an invalid_session_id error.
The policy to have session ID on LURK message is a policy that applies to the CS and LURK client cannot have different policies. When session ID are enabled, the CS expect every LURK message to have a session ID except for the initiating messages.
+-----------------------+----------+ | LURK exchange | Presence | +-----------------------+----------+ | s_init_early_secret | * | | s_init_cert_verify | * | | s_hand_and_app_secret | - | | s_new_ticket | - | | c_binder_key | - | | c_init_early_secret | * | | c_init_hand_secret | * | | c_hand_secret | | | c_app_secret | - | | c_cert_verify | - | | c_register_ticket | - | | c_post_hand | - | +-----------------------+----------+ * indicates the extension MAY be provided - indicates the extension MUST NOT be provided
Figure 8: Presence of the Session ID Extension in the various LURK exchanges
The extension data is defined as follows:
uint32 session_id
The session ID agreement leads to the definition of the following structure that will be embedded into any non initiating LURK exchange.
struct{
select( session_id_agreed ){
case True:
unint32 session_id
case False:
}
} SessionID
session_id_agreed: indicates the LURK client and the CS have agreed on using session ID as well as the respective session ID value to use.
session_id can take the following values: 1. session_id_cs: the session ID provided by the CS to the LURK client in the Session ID extension. This is the value the LURK client MUST use in any subsequent exchange. That value will be used by the CS to associate its internal context to the session. 2. session_id_client: the session ID provided by the LURK client to the CS in the Session ID extension. This is the value the CS MUST use in any subsequent exchange. That value will be used by the LURK client to associate its internal context to the session.
The signature requires the signature scheme (sig_algo), the designated private key (key_id), as well as sufficient context to generate the necessary data to be signed. In our case the necessary context is provided by the LURKCertificate, assuming the CS will have the necessary Handshake Context. The latest may be provided in a combination of a secret request.
key_id is processed as described in [I-D.mglt-lurk-tls12] section 4.1. If the CS does not support the KeyPairIdType an invalid_key_id_type is returned. If the CS does not recognize the key, an invalid_key_id error is returned.
sig_algo designates the signature algorithm scheme, and it is defined in {{!RFC8446} section 4.2.3. When the CS does not support the signature scheme an invalid_signature_scheme error is returned.
The certificate is a public data that may repeat over multiple distinct TLS handshakes. To limit the load of unnecessary information being transmitted multiple times, the LURKCertificate enables to carry the index of the Certificate structure rather than the structure itself. When the lurk_certificate_type is set to sha256_32, the index of the Certificate structure is sent. The current specification generates the index using sha256_32, that is the first 32 bits of the hash of the Certificate structure using SHA256 as the hashing function. When lurk_certificate_type is set to X509 or RawPublicKey the full Certificate structure is expected. When the CS does not support the certificate_type, an invalid_certificate_type error is returned. When the Certificate structure does not match the private key, an invalid_certificate error is returned.
Signing operations are described in [RFC8446] section 4.4.3. The context string is derived from the role and the type of the LURK exchange as described below. The Handshake Context is taken from the key schedule context.
+--------------------+-------------------------------------+ | type | context | +--------------------+-------------------------------------+ | s_init_cert_verify | "TLS 1.3, server CertificateVerify" | | c_cert_verify | "TLS 1.3, client CertificateVerify" | +--------------------+-------------------------------------+
The CS computes the signature as described in [RFC8446] section 4.4.3. and returns signature in SigningResponse. When the CS does not have the necessary Handshake Context, context or is unable to proceeds to the signing operation, an invalid_certificate_verify error is returned.
The structure is represented below:
enum { X509(0), RawPublicKey(1),
sha256_32(128) (255)}; LURK13CertificateType
struct {
LURK13CertificateType certificate_type;
select (lurk_certificate_type) {
case sha256_32:
uint32 hash_cert;
case X509, RawPublicKey:
Certificate certificate; // RFC8446 section 4.4.2
};
} LURK13Certificate;
struct {
KeyPairId key_id; // draft-mglt-lurk-tls12 section 4.1
SignatureScheme sig_algo; //RFC8446 section 4.2.3.
LURKCertificate certificate;
} SigningRequest;
struct {
opaque signature<0..2^16-1>; //RFC8446 section 4.4.3.
} SigningResponse;
This section describes the LURK exchanges that are performed on the TLS server. The state diagram is provided in section Section 10.2
A TLS server MAY receive a ClientHello that proposes PSK or PSK-ECDHE authentication via the pre_shared_key and psk_key_exchange_modes extensions. Depending on its policies, the TLS server MAY decide to proceed to such authentication. It chooses a PSK identity so the LURK client initiates a key schedule context (ks_ctx) that will manage the session with the CS. This session is initiated with a s_init_early_secret exchange.
The binder_key MUST be requested, since it is used to validate the PSK.
The TLS client MAY indicate support for early application data via the early_data extension. Depending on the TLS server policies, it MAY accept early data and request the client_early_traffic_secret.
The TLS server MAY have specific policies and request early_exporter_master_secret.
Upon receiving an s_init_early_secret request, the CS proceeds the SecretRequest as described in section Section 4.4.
The CS MUST check pre_shared_key and psk_key_exchange_modes extensions are present in the ClientHello. If these extensions are not present, a invalid_handshake error SHOULD be returned.
The CS MUST ignore the client_early_traffic_secret if early_data extension is not found in the ClientHello. The Cryptographic Service MAY ignore the request for client_early_traffic_secret, in any case. The CS MAY ignored the request for early_exporter_master_secret.
struct{
SecretRequest secret_request
} InitEarlySecretRequest
struct{
SecretResponse secret_response
} InitEarlySecretResponse
secret_request: The structure associated to the secret request defined in section Section 4.4
secret_response: The structure associated to the secret request defined in section Section 4.4.
A TLS server MAY receive a ClientHello that proposes ECDHE authentication with a key_share extension. Depending on its policies, the TLS server MAY decide to proceed to such authentication and indicate it to the LURK client so it initiates a key schedule context (ks_ctx) that will manage the session with the CS. This session is initiated with a s_init_cert_verify exchange.
The Cryptographic MUST ensure the ServerHello has selected the ECDHE authentication that is a key_share extension is present and no pre_shared_key extension is present. If these conditions are not met, a invalid_handshake error SHOULD be returned.
In order to provide generate the client_application_traffic_secret_0 and server_application_traffic_secret_0, the CS generates the server Finished. This value is computed to avoid multiple round trips. This value is not returned to the LURK client and needs to be computed again by the TLS server.
After the exchange is completed, the TLS server is able to build and return the ServeHello and complete the TLS handshake.
If the CS has been configured not to handle session resumption. The session is finished and ks_ctx SHOULD be deleted and some implementations MAY NOT create the ks_ctx.
struct{
SecretRequest secret_request
SigingRequest signing_request
}InitCertVerifyRequest
struct{
SecretResponse secret_response
SigingResponse signing_response
}InitCertVerifyResponse
secret_request and secret_response are defined in section Section 5.1.
The s_hand_and_app_secret is necessary to complete the ServerHello and always follows an s_init_early_secret LURK exchange. Such sequence is guaranteed by the session_id and cookie mechanism. In case of unknown session_id or an unexpected cookie value, an invalid_request error SHOULD be returned.
The LURK client MUST ensure that PSK or PSK-ECDHE authentication has been selected via the presence of the pre_shared_key extension in the the ServerHello. In addition, the selected identity MUST be the one provided in the psk extension of the previous s_init_early_secret exchange.
The LURK client MAY request the exporter_master_secret depending on its policies. The CS MAY ignore the request based on its policies.
Similarly to the s_init_cert_verify, if session resumption is not provided by the CS, the LURK session ends after this exchange and ks_ctx SHOULD be removed.
struct{
SessionID session_id_cs
SecretRequest secret_request
} HandAndAppRequest
struct{
SessionID session_id_client
SecretResponse secret_response
} HandAndAppResponse
new_session ticket handles session resumption. It enables to retrieve NewSessionTickets that will be forwarded to the TLS client by the TLS server to be used later when session resumption is used. It also provides the ability to delegate the session resumption authentication from the CS to the TLS server. In fact, if the LURK client requests and receives the resumption_master_secret it is able to emit on its own NewSessionTicket. As a result s_new_ticket LURK exchanges are only initiated if the TLS server expects to perform session resumption and the CS responds only if if session_resumption is enabled. If session resumption is not enabled, the Cryptographic MAY have ended the LURK session and the s_new_ticket will be ignored or responded with a invalid_request error.
The CS MAY responds with a resumption_master_secret based on its policies.
The LURK client MAY perform multiple s_new_ticket exchanges before the session between the LURK client and the CS is in a finished state with ks_ctx deleted.
struct {
SessionID session_id_cs
uint8 ticket_nbr;
unint16 key_request;
Handshake handshake_context<0..2^32> //RFC8446 section 4.
} NewTicketRequest;
struct {
SessionID session_id_client
Secrets secrets
NewSessionTicket ticket_list<0..2^16-1>; //RFC8446 section 4.6.1.
} NewTicketResponse;
crypto_service_session_id, crypto_service_cookie, lurk_client_session_id, and lurk_client_cookie are defined in section Section 5.3. key_request is defined in section Section 4.1.
ticket_nbr: designates the requested number of NewSessionTicket. In the case of delegation this number MAY be set to zero. The CS MAY responds with less tickets when the value is too high.
This section describes the LURK exchanges that are performed on the TLS server. The state diagram is provided in section Section 10.1
The c_binder_key LURK exchange is initiates a LURK session when the TLS client is willing to propose a PSK for PSK or PSK-ECDHE authentication.
The handshake_context is empty as the ClientHello is under construction.
When a LURK client proposes multiple PSK, multiple binder_keys are requested.
The c_binder_key is equivalent to a secret request LURK exchange and there is no creation of a ks_ctx.
``` struct{ SecretRequest secret_request } BinderKeyRequest
struct{ SecretResponse secret_response } BinderKeyResposne ```
c_init_early_secret on the TLS client side works similarly as the s_init_early_secret LURK exchange on the TLS server as described in section Section 5.1. One key difference is that the c_binder_key is not requested during that LURK exchange, as a result, this LURK exchange MAY be omited even when PSK or PSK-ECDHE authentication has been chosen by the TLS client. The c_init_early_secret will only be performed in the case of 0-RTT handshake or when early exporters are required.
The c_hand_secret is performed after an c_init_early_secret LURK exchange. This exchange is performed in the case of an PSK or PSK-ECDHE authentication and coherence with the Handshake Context MUST be checked by the LURK client as well as by the CS as described in Section 6.2 and section Section 5.3.
The structures of the c_hand_secret follow those of the s_hand_and_app_secret described in section Section 5.3.
Coherence between with the Handshake Context and the authentication ECDHE versus PSK or PSK-ECDHE) is performed as described in section Section 6.2 and section Section 5.3. The LURK client and the CS MUST ensure such coherence. A Signing sub exchange MUST only be performed when ECDHE authentication has been selected which is determined by the presence of a key_share extension as well as the absence of a pre_shared_key extension in the ServerHello.
Only the client_handshake_traffic_secret_0 and server_handshake_traffic_secret_0 secrets MAY be requested.
struct{
SecretRequest secret_request
select (handshake_context.ecdhe_selected){
case :
SigningRequest signing_request
};
}InitHandshakeRequest
struct{
SecretResponse secret_response
select (handshake_context.ecdhe_selected){
case :
SigningResponse signing_response
};
}InitHandsahkeResponse
The c_app_secret LURK exchange is performed when no TLS client authentication has been requested, i.e. CertificateRequest message is not provided in the flight of the ServerHello. The LURK client and the CS MUST ensure no CertificateRequest is present in the Handshake Context.
Only the client_application_traffic_secret_0 and server_application_traffic_secret_0 secrets MAY be requested.
The structure follows the one of the c_hand_secret described in section Section 6.3.
After the c_app_secret LURK exchange, unless the TLS client supports session resumption or post_handshake, the LURK session is finished. The support for post_handshake by the TLS client is indicated by the post_handshake_auth extension.
The c_cert_verify LURK exchange is performed when TLS client authentication has been requested by the TLS server. When performed, the LURK client and the CS MUST check the presence of a CertificateRequest structure in the Handshake Context. When not present, a invalid_handshake error SHOULD be returned.
After the c_app_secret LURK exchange, unless the TLS client supports session resumption or post_handshake, the LURK session is finished. The support for post_handshake by the TLS client is indicated by the post_handshake_auth extension.
The CertVerifyRequest and CertVerifyResponse structures are used for this LURK exchange.
struct{
SessionID session_id_cs
InitCertVerifyRequest cert_request
}CertVerifyRequest
struct{
SessionID session_id_client
InitCertVerifyRequest cert_response
}CertVerifyResponse
The c_register_ticket is only used when the TLS client intend to perform session resumption. This LURK exchange has three functions. First, it is used to register the handshake in order to provide the full TLS handshake. Such information will be necessary to generate the PSK value during the future session resumptions. Second, the LURK client MAY provide one or multiple NewSessionTickets. These tickets will be helpful for the session resumption to bind the PSK value to some identities. Third, the LURK client MAY retrieve the resumption_master_secret when session resumption is being delegated by the CS to the TLS client.
The first c_register_ticket MUST carry the TLS handshake and future c_register_ticket LURK exchange MUST have a handshake_context of zero length. If these conditions are not met, the CS SHOULD return a invalid_handshake error.
The first c_register_ticket MAY request the session_resumption_master. Next register_new_session MUST not request that secret.If these conditions are not met, a invalid_key_request error is returned.
The ticket_list MAY have zero NewSessionTickets for the first register_new_Session_ticket. Next LURK exchanges MUST have at least one NewSessionTickets.
struct {
SessionID session_id_cs
Handshake handshake_context<0..2^32>; //RFC8446 section 4.
NewSessionTicket ticket_list<0..2^16-1>; //RFC8446 section 4.6.1.
uint16 key_request;
} RegisterTicketRequest;
struct {
SessionID session_id_client
} RegisterTicketResponse;
crypto_service_session_id, crypto_service_cookie, lurk_client_session_id, and lurk_client_cookie are defined in section Section 5.3. handshake_contex is defined in section Section 4.3. NewSessionTicket is defined in [RFC8466] section 4.6.1. key_request is defined in Section 4.1.
The c_post_hand LURK exchange is performed in order to the client to authenticate after the TLS handshake has complete. The TLS client MUST NOT proceed to this exchange if post handshake support has not been announced in the ClientHello with the post_handshake_auth extension. When such extension is not found the CS MUST return a invalid_handshake error.
struct {
SessionID session_id_cs
Handshake handshake_context<0..2^32>; //RFC8446 section 4.
int16 app_n;
} PostHandRequest;
struct {
SessionID session_id_client
} PostHandResponse;
handshake_context is defined in section Section 4.3
app_n: describes the number of iteration of the session keys.
Security credentials as per say are the private key used to sign the CertificateVerify when ECDHE authentication is performed as well as the PSK when PSK or PSK-ECDHE authentication is used.
The protection of these credentials means that someone gaining access to the CS MUST NOT be able to use that access from anything else than the authentication of an TLS being established. In other way, it MUST NOT leverage this for: * any operations outside the scope of TLS session establishment. * any operations on past established TLS sessions * any operations on future TLS sessions * any operations on establishing TLS sessions by another LURK client.
The CS outputs are limited to secrets as well as NewSessionTickets. The design of TLS 1.3 make these output of limited use outside the scope of TLS 1.3. Signature are signing data specific to TLS 1.3 that makes the signature facility of limited interest outside the scope of TLS 1.3. NewSessionTicket are only useful in a context of TLS 1.3 authentication.
ECDHE and PSK-ECDHE provides perfect forward secrecy which prevents past session to be decrypted as long as the secret keys that generated teh ECDHE share secret are deleted after every TLS handshake. PSK authentication does not provide perfect forward secrecy and authentication relies on the PSK remaining sercet. The Cryptographic Service does not reveal the PSK and instead limits its disclosure to secrets that are generated from the PSK and hard to be reversed.
Future session may be impacted if an attacker is able to authenticate a future session based on what it learns from a current session. ECDHE authentication relies on cryptographic signature and an ongoing TLS handshake. The robustness of the signature depends on the signature scheme and the unpredictability of the TLS Handshake. PSK authentication relies on not revealing the PSK. The CS does not reveal the PSK. TLS 1.3 has been designed so secrets generated do not disclose the PSK as a result, secrets provided by the Cryptographic do not reveal the PSK. NewSessionTicket reveals the identity (ticket) of a PSK. NewSessionTickets.ticket are expected to be public data. It value is bound to the knowledge of the PSK. The Cryptographic does not output any material that could help generate a PSK - the PSK itself or the resumption_master_secret. In addition, the Cryptographic only generates NewSessionTickets for the LURK client that initiates the key schedule with CS with a specific way to generate ctx_id. This prevents the leak of NewSessionTickets to an attacker gaining access to a given CS.
If an the attacker get the NewSessionTicket, as well as access to the CS of the TLS client it will be possible to proceed to the establishment of a TLS session based on the PSK. In this case, the CS cannot make the distinction between the legitimate TLS client and teh attacker. This corresponds to the case where the TLS client is corrupted.
Note that when access to the CS on the TLS server side, a similar attack may be performed. However the limitation to a single re-use of the NewSessionTicket prevents the TLS server to proceed to the authentication.
Attacks related to other TLS sessions are hard by design of TLS 1.3 that ensure a close binding between the TLS Handshake and the generated secrets. In addition communications between the LURK client and the CS cannot be derived from an observed TLS handshake (freshness function). This makes attacks on other TLS sessions unlikely.
The state diagram sums up the LURK exchanges. The notations used are defined below:
LURK exchange indicates a LURK exchange is stated by the LURK client or is received by the CS —-> (resp. <—-) indicates a TLS message is received (resp. received). These indication are informative to illustrates the TLS state machine.
CAPITAL LETTER indicates potential configuration parameters or policy applied by the LURK client or the CS. The following have been considered:
Note that SESSION_RESUMPTION, POST_HANDSAHKE_AUTH are mostly informative and the current specification does not mandate to have such configuration parameters. By default, these SHOULD be enabled.
Other potential configuration could be proposed for configuring LURK client or CS policies. These have not been represented in the state diagram and the specification does not mandate to have these parameters implemented.
The analysis of the TLS Handshake Context enables to set some variables that can be used by the LURK client to determine which LURK exchange to proceed as well as by the CS to determine which secret MAY be responded. The following variables used are:
psk_proposed: The TLS Client is proposing PSK authentication by including a pre_shared_key and a psk_key_exchange_mode extensions in the ClientHello.
dhe_proposed: The received or to be formed ClientHello contains a key_share extensions.
psk_accepted: The chosen authentication method is pSK or PSK-ECDHE which is indicated via the pre_shared_key extension in the ServerHello.
0rtt_proposed: Indicates the TLS client supports early data which is indicated by the early_data extension in the ClientHello.
post_handshake_proposed: indicates the TLS client supports post handshake authentication which is indicated by the presence of a post_handshake_auth extension in the ClientHello.
finished: indicates that the LURK client or the CS has determined the session shoudl be closed an ks_ctx are deleted.
The CS contains three databases:
CTX_ID_DB: database that contains the valid ctx_id of type opaque.
PSK_DB: contains the list of PSKs, with associated parameters such as Hash function. This database includes the session resumption tickets.
Key_DB: contains the asymetric signing keys with supported signing algorithms.
TLS Client Policy for authentication
PSK, PSK-ECDHE ECDHE
| |
| |
v |
psk ---> +--------------------+ |
| c_binder_key | |
+--------------------+ |
EARLY_EXPORTER, 0-RTT | |
v | |
/------------------------\ NO |
\------------------------/----+ |
YES v | |
+---------------------+ | |
| c_init_early_secret | | |
+---------------------+ | |
ClientHello | | |
<---- +<-----------------+--------+
ServerHello YES v
----> +-------------------------------------+
| c_init_hand_secret or c_hand_secret |
+-------------------------------------+
|
/--------------------\ NO
| CertificateRequest |------+
\--------------------/ |
YES v v
+-------------------+------------------+
| c_cert_verify | c_app_secret |
+-------------------+------------------+
client Finished | |
<---- +-----------+---------+
|
+--------------------------------------+
| LURK client post handshake exchanges |
+--------------------------------------+
The LURK client post handshake diagram is represented below:
POST_HANDSHAKE_AUTH |
v v
/-------------------------\ NO
| post_hand_auth_proposed |------+
\-------------------------/ |
YES v |
+-----------------------------+ |
| c_register_tickets | |
| (empty NewSessionTickets) | |
+-----------------------------+ |
| |
+<-----------------+
|
+<-----------------------------------------------+
| |
+------------------------------+ |
SESSION_RESUMPTION | POST_HANDSHAKE_AUTH | |
client Finished | | CertificateRequest | | |
NewSessionTickets| | | v v |
| v v | /-------------------------\NO |
| /-------------\ NO +---> | post_hand_auth_proposed |--+ |
+----> \-------------/---------+ \-------------------------/ | |
YES v | YES v | |
+-----------------------------+ | +-------------------------+ | |
| c_register_ticket | | | c_post_hand | | |
+-----------------------------+ | +-------------------------+ | |
v v v | |
+-----------------+----------+---+----------------+ |
v |
/--------------------\ NO |
| finished |----------+
\--------------------/
YES v
+-------------------------+
| LURK exchanges Finished |
+-------------------------+
TLS13Request
|
/---------------------------\NO /-------------------------------\NO
| type is c_init_early_secret|-->| type is c_init_hand_secret |-+
\---------------------------/ \-------------------------------/ |
| | +------------------+
| +------------+ |
| v |
| /-------------------\NO /----------------\NO
| | psk_selected |-+ | session,cookie | +-------+
| \------------------ / | | consistent |---| ERROR |
| YES | | \----------------/ +-------+
+----------------+ | |
PSK, PSK-ECDHE | | ECHDE |
v +-------------+ |
/-------------------\NO +-------+ | |
| psk_key in PSK_DB |---| ERROR | | |
\-------------------/ +-------+ | |
+-------------------------+ |
| |
+-------------+ |
| Init ks_ctx | |
+-------------+ |
| |
+---------------------------+
|
v
+---------------------------+
| process the request |
| update CTX_DB, PSK_DB |
+---------------------------+
TLS Server Policy for authentication
received PSK, PSK-ECDHE, ECDHE
ClientHello | |
----> v v
psk ---->+----------------------+ +----------------------+
| Init ks_ctx | | Init ks_ctx |
+----------------------+ +----------------------+
v |
+---------------------+ |
| s_init_early_secret | |
+---------------------+ |
| |
to be formed YES v v
ServerHello +--------------------------+ +-------------------------+
----> | s_hand_and_app_secret | | s_init_cert_verify |
+--------------------------+ +-------------------------+
| |
+---------------------------+
|
v
+--------------------------------------+
| LURK client post handshake exchanges |
+--------------------------------------+
TLS13Request
|
/---------------------------\NO /---------------------------\NO
|type is s_init_early_secret|-->| type is s_init_cert_verify |-+
\---------------------------/ \---------------------------/ |
PSK, | +--------------------+
PSK-ECDHE v |
/-------------------\NO +-------+ /----------------\NO
| psk_key in PSK_DB |---| ERROR | | session,cookie | +-------+
\-------------------/ +-------+ | consistent |---| ERROR |
| \----------------/ +-------+
v |
+-------------+ |
| Init ks_ctx | |
+-------------+ |
| |
+-----------------------------+
|
v
+---------------------------+
| process the request |
| update CTX_DB, PSK_DB |
+---------------------------+
This section is non normative. It illustrates the use of LURK in various configurations.
The TLS client may propose multiple ways to authenticate the server (ECDHE, PSK or PSK-ECDHE). The TLS server may chose one of those, and this choice is reflected by the LURK client on the TLS server. In other words, this decision is out of scope of the CS.
The derivation of the secrets is detailed in {{!RFC8446)) section 7.1. Secrets are derived using Transcript-Hash and HKDF, PSK and ECDHE secrets as well as some Handshake Context.
The Hash function: When PSK or PSK-ECDHE authentication is selected, the Hash function is a parameter associated to the PSK. When ECDHE, the hash function is defined by the cipher suite algorithm negotiated. Such algorithm is defined in the cipher_suite extension provided in the ServerHello which is provided by the LURK client in the first request when ECDHE authentication is selected.
PSK secret: When PSK or PSK-ECDHE authentication is selected, the PSK is the PSK value identified by the identity. When ECDHE authentication is selected, the PSK takes a default value of string of Hash.length bytes set to zeros.
ECDHE secret: When PSK or PSK-ECDHE authentication is selected, the ECDHE secret takes the default value of a string of Hash.length bytes set to zeros. The Hash is always known as a parameter associated to the selected PSK. When ECDHE authentication is selected, the ECDHE secret is generated from the secret key (ephemeral_sercet) provided by the LURK client and the counter part public key in the key_share extension. When the LURK client is on the TLS client, the public key is provided in the ServerHello. When the LURK client is on the TLS Server, the public key is provided in the ClientHello. When ECDHE secret is needed, ClientHello…ServerHello is always provided to the CS.
Handshake Context: is a subset of Handshake messages that are necessary to generated the requested secrets. The various Handshake Contexts are summarized below:
+------------------------------------+--------------------------------+ | Key Schedule secret or key | Handshake Context | +---------------------------------------------------------------------+ | binder_key | None | | client_early_traffic_secret | ClientHello | | early_exporter_master_secret | ClientHello | | client_handshake_traffic_secret | ClientHello...ServerHello | | server_handshake_traffic_secret | ClientHello...ServerHello | | client_application_traffic_secret_0 | ClientHello...server Finished | | server_application_traffic_secret_0 | ClientHello...server Finished | | exporter_master_secret | ClientHello...server Finished | | resumption_master_secret | ClientHello...client Finished | +---------------------------------------------------------------------+
The CS has always the Hash function, the PSK and ECDHE secrets and the only remaining parameter is the Handshake Context. The remaining sections will only focus on checking the Handshake Context available to the CS is sufficient to perform the key schedule.
When ECDHE authentication is selected both for the TLS server or the TLS client, a CertificateVerify structure is generated as described in [RFC8446] section 4.4.3.. CertificateVerify consists in a signature over a context that includes the output of Transcript-Hash(Handshake Context, Certificate) as well as a context string. Both Handshake Context and context string depends on the Mode which is set to server in this case via the configuration of the LURK server. Similarly to the key schedule, the Hash function is defined by the PSK or the ServerHello. The values for the Handshake Context are represented below:
+-----------+-------------------------+-----------------------------+ | Mode | Handshake Context | Base Key | +-----------+-------------------------+-----------------------------+ | Server | ClientHello ... later | server_handshake_traffic_ | | | of EncryptedExtensions/ | secret | | | CertificateRequest | | | | | | | Client | ClientHello ... later | client_handshake_traffic_ | | | of server | secret | | | Finished/EndOfEarlyData | | | | | | | Post- | ClientHello ... client | client_application_traffic_ | | Handshake | Finished + | secret_N | | | CertificateRequest | | +-----------+-------------------------+-----------------------------+
When ECDHE authentication is selected, the CS generates a Finished message, which is a MAC over the value Transcript-Hash(Handshake Context, Certificate, CertificateVerify) using a MAC key derived from the Base Key. As a result, the same Base Key and Handshake Context are required for its computation describe din [RFC8466] section 4.4.4..
This example illustrates the case of a TLS handshake where the TLS server is authenticated using ECDHE only, that is not PSK or PSK-ECDHE authentication is provided and so session resumption is provided either.
The TLS client does not provides any PSK and omits the pre_shared_key as well as the psk_key_exchange_mode extensions. Note that omitting the psk_key_exchange_mode extension prevents the TLS client to perform further session resumption.
The TLS client does not need any interaction with the Cryptographic Service to generate and send the ClientHello message to the TLS server.
TLS Client TLS Server
Key ^ ClientHello
Exch | + key_share
v + signature_algorithms --------->
Upon receiving the ClientHello, the TLS server determines the TLS client requests an ECDHE authentication. The TLS server initiates a LURK session to provide ECDHE authentication as represented below:
TLS Client TLS Server
ServerHello ^ Key
+ key_share | Exch
{EncryptedExtensions} ^ Server
{CertificateRequest*} v Params
{Certificate} ^
{CertificateVerify} | Auth
{Finished} v
<-------- [Application Data*]
The LURK Client on the TLS server initiates a s_init_cert_verify to retrieves the necessary secrets to finish the exchange and request the generation of the signature (certificate_verify) carried by the CertificateVerify TLS structure.
The s_init_cert_verify request uses a InitCertVerifyRequest structure which is composed of two substructures: A SecretRequest structure (secret_request) is in charge of requesting the necessary secrets to decrypt and encrypt the TLS handshake as well as the applications carried over the TLS session. Finally a SigningRequest substructure (signing_request) is used to request the certificate_verify payload.
The secret_request carries the requested secrets as well as the necessary parameters to generate the secrets. In our case, the requested secrets are the handshake secrets (h_c, h_s) as well as the application secrets (a_c, a_s). This corresponds to the most expected use cases, though other use case may require different secrets to be requested. Theses requests are indicated in the key_request. The necessary Handshake Context is provided through handshake_context which is set to ClientHello … EncryptedExtensions. The ECDHE shared secret is provided in this example via the ephemeral extension. In our case, the secret key is provided directly thought other means may be used. In particularly providing the secret key implies the dhe parameters have been generated outside the CS. The freshness function is provided through the freshness extension.
The signing_request provides the key_id that identifies the private key used to generate the signature, the algorithm use dto generate the signature (sig_algo) as well as the certificate. The certificate carries information to generate the Certificate structure of the ServerHello, and may not be the complete certificate chain but only an index.
TLS Server
Lurk Client CS
InitCertVerifyRequest
secret_request
key_request = h_c, h_s, a_c, a_s
handshake_context = ClientHello ... EncryptedExtensions
ext
ephemeral = dhe_secret
freshness
session_id
signing_request
key_id,
sig_algo
certificate
-------->
InitCertVerifyResponse
secret_response
keys
ext
session_id
signing_response
certificate_verify
<---------
Upon receiving the InitCertificateRequest, the CS initiates a context associated to the newly created LURK session.
The secrets are generated from the TLS 1.3 key schedule describe din [RFC8446] and requires as input PSK, ECDHE as well as some context handshake.
The CS determine that ECDHE without specific PSK is used from the ClientHello and associated extensions. As a result, the default PSK value is used. The ECDHE share secret is derived, in our case from the dhe_secret of the TLS server and the public dhe value provided by the ClientHello shared_key extension.
The CS reads the freshness extension and generates the handshake_context that will be used further.
The necessary Handshake Context to generate the handshake secrets is ClientHello…ServerHello which is provided by the handshake_context. The CS uses the freshness function provided in the freshness extension to derive the appropriated server.random.
The generation of the CertificateVerify is described in [RFC8446] section 4.4.3. and consists in a signature over a context that includes the output of Transcript-Hash(Handshake Context, Certificate) as well as a context string. Both Handshake Context and context string depends on the Mode which is set to server in this case via the configuration of the LURK server.
The necessary Handshake Context to generate the CertificateVerify is ClientHello … later of EncryptedExtensions / CertificateRequest. In our case, this is exactly handshake_context, that is ClientHello … EncryptedExtensions. The Certificate payload is generated from the information provided in the certificate extension.
Once the certificate_verify value has been defined, the LURK server generates the server Finished message in order to have the necessary Handshake Context ClientHello…server Finished to generate the application secrets.
The LURK server returns the requested keys, the certificate_verify in a InitCertVerifyResponse structure. This structure is composed of the two substructures: SecretResponse that contains the secrets and SigningResponse that contains the certificate_verify.
The TLS server can complete the ServerHello response, that is proceed to the encryption and generates the Finished message.
As session resumption is not provided, the LURK server goes into a finished state and delete the ks_ctx. The special case described in this session does not use LURK session and as such may be stateless.
Upon receiving the ServerHello message, the TLS client retrieve the handshake and application secrets to decrypt the messages received from server as well as to encrypt its own messages and application data as represented below:
TLS Client TLS Server
{Finished} -------->
[Application Data] <-------> [Application Data]
To retrieves these secrets, the TLS client proceeds successively to an c_init_hand_secret LURK exchange followed by a c_app_secret LURK exchange.
The c_init_hand_secret exchange is composed of one substructure: (secret_request) to request the secrets. Optionally, a SigningRequest (signing_request) when the TLS server requests the TLS client to authenticate itself. The indication of a request for TLS client authentication is performed by the TLS server by providing a CertificateRequest message associated to the ServerHello. We consider that such request has not been provided here so the SigingRequest structure is not present.
The secret_request specifies the secrets requested via the key_request. In our case only the handshake secrets are requested (h_c, h_s). In this example the ECDHE share secret is provided via the ephemeral extension. In this case the ECDHE secrets have been generated by the TLS client, and the TLS client chooses to provide the ephemeral secret (dhe_secret) to the CS via the ephemeral extension. The TLS client also provides the freshness function via the freshness extension so the handshake_context can be appropriately be interpreted. The handshake context is provided via the handshake_context and is set to ClientHello … ServerHello.
Note that if the TLS client would have like the CS to generate the ECDHE public and private keys, the generation of the keys would have been made before the ClientHello is sent, that is in our case during a c_init_early_secret LURK exchange. If that had been the case a c_hand_secret LURK exchange would have followed and not a c_init_hand_secret exchange.
TLS Client
Lurk Client Cryptographic Service
InitHandshakeSecretRequest
secret_request
key_request = h_c, h_s
handshake_context = ClientHello ... ServerHello
ext
ephemeral = dhe_secret
freshness
session_id
------->
InitHandshakeSecretResponse
secret_response
ext
session_id
<-------- keys
TLS Client
Lurk Client Cryptographic Service
AppSecretRequest
session_id
cookie
secret_Request
key_request
handshake_context
------->
AppSecretResponse
session_id
cookie
secret_response
<-------- keys
Upon receiving the InitHandshakeSecretRequest, the servers initiates a LURK session context (ks_ctx) and initiates a key schedule. The key schedule requires PSK, ECDHE as well as Handshake Context to be complete. As no pre_shared_key and psk_key exchange_modes are found in the ClientHello the CS determines that ECDHE is used for the authentication. The PSK is set to its default value. The ECHDE shared secret is generated from the ephemeral extension as well as the public value provided in the ClientHello. The CS takes the freshness function and generates the appropriated handshake context. The necessary Handshake Context to generate handshake secrets is ClientHello…ServerHello which is provided by the handshake_context.
The handshake secrets are returned in the secret_response to the TLS client. The TLS client decrypt the encrypted extensions and messages of the ServerHello exchange.
As no CertificateREquest appears, the LURK client initiates an app_secret LURK exchange decrypt and encrypt application data while finishing the TLS handshake.
The AppSecretRequest structure uses session_id and cookies as agreed in the previous c_init_hand_secret exchange. The AppSecretRequest embeds a SecretRequest sub structure. The application secrets requested are indicated by the key_request (a_s, a_s). The Handshake Context (handshake_context) is set to server EncryptedExtensions … server Finished.
Upon receiving the AppSecretRequest, the CS checks the session_id. The CS has now the ClientHello … server Finished which enables it to compute the application secrets.
As no session resumption is provided, the CS and the LURK client goes into a finished state and delete their ks_ctx.
This scenario considers that the TLS server is authenticated using ECDHE only in the first time and that further TLS handshake use the session resumption mechanism. The first TLS Handshake is very similar as the previous one. The only difference is that psk_key_exchange_mode extension is added to the ClientHello. However, as no PSK identity is provided, the Full exchange is performed as described in section Section 10.4.
The only change is that session resumption is activated, and thus LURK client and LURK servers do not go in a finished state and close the LURK session after the exchanges are completed. Instead further exchanges are expected. Typically, on the TLS server side new_Session_ticket exchanges are expected while registered_session_ticket are expected on the client side.
When session resumption is performed, a new LURK session is initiated.
The Full TLS Handshake use ECDHE authentication. It is very similar to the logic described in section Section 10.4. The TLS handshake is specified below for convenience.
TLS Client TLS Server
Key ^ ClientHello
Exch | + key_share
| + psk_key_exchange_mode
v + signature_algorithms --------->
ServerHello ^ Key
+ key_share | Exch
{EncryptedExtensions} Server Param
{Certificate} ^
{CertificateVerify} | Auth
{Finished} v
<-------- [Application Data*]
{Finished} -------->
[Application Data] <-------> [Application Data]
As session resumption has been activated by the psk_key_exchange_mode, the TLS Server is expected to provide the TLS client NewSessionTickets as mentioned below:
TLS Client TLS Server
<-------- [NewSessionTicket]
The LURK client and LURK server on the TLS server does not go into a finished state. Instead, the LURK client continues the LURK session with a NewTicketRequest to enable the CS to generate the resumption_master_secret necessary to generate the PSK and generate a NewTicketSession. ticket_nbr indicates the number of NewSessionTickets and handshake_context is set to earlier of client Certificate client CertificateVerify … client Finished. As we do not consider TLS client authentication, the handshake_context is set to client Finished as represented below.
TLS Server
Lurk Client Cryptographic Service
NewTicketRequest
session_id
cookie
ticket_nbr
handshake_context=client Finished -------->
NewTicketResponse
session_id
cookie
<--------- tickets
The necessary Handshake Context to generate the resumption_master_secret is ClientHello…client Finished. From the InitCerificateVerify the context_handshake was set to ClientHello…server Finished. The additional handshake_context enables the CS to generate the NewSessionTickets.
Note that the LURK client on the TLS server may send multiple NewTicketRequest. Future request have an empty handshake_context.
Upon receiving the NewTicketRequest, the LURK server checks the session_id and cookie. It then generates the resumption_master_secret, NewSessionTickets. NewSessionTickets are stored into the PSK_DB under NewSessionTicket.ticket. Note that PSK is associated with the authentication mode as well as the Hash function negotiated for the cipher suite. The CS responds with NewSessionTickets that are then transmitted back to the TLS client. The TLS server is ready for session resumption.
Similarly, the LURK client on the TLS client will have to provide sufficient information to the CS the necessary PSK can be generated in case of session resumption. This includes the remaining Handshake Context to generate the resumption_master_secret as well as NewSessionTickets provided by the TLS server. The LURK client uses the c_register_ticket exchange.
Note that the LURK client may provide the handshake_context with an empty list of NewSessionTickets, and later provide the NewSessionTickets as they are provided by the TLS server. The Handshake Context only needs to be provided for the first RegisterTicketRequest.
TLS Client
Lurk Client Cryptographic Service
NewTicketRequest
session_id
cookie
handshake_context=client Finished
ticket_list -------->
NewTicketResponse
session_id
cookie
<--------- tickets
Both TLS client and TLS Servers are ready for further session resumption. On both side the CS stores the PSK in a database designated as PSK_DB. Each PSK is associated to a Hash function as well as authentication modes. Each PSK is designated by an identity. The identity may be a label, but in our case the identity is derived from the NewSessionTicket.ticket.
Session resumption is initiated by the TLS client. Session resumption is based on PSK authentication and different PSK may be proposed by the TLS client. The TLS handshake is presented below.
TLS Client TLS Server
ClientHello
+ key_share
+ psk_key_exchange_mode
+ pre_shared_key -------->
ServerHello
+ pre_shared_key
+ key_share
{EncryptedExtensions}
{Finished}
<-------- [Application Data*]
The TLS client may propose to the TLS Server multiple PSKs. Each of these PSKs is associated a PskBindersEntry defined in [RFC8446] section 4.2.11.2. PskBindersEntry is computed similarly to the Finished message using the binder_key and the partial ClientHello.
The TLS server is expected to pick a single PSK and validate the binder. In case the binder does not validate the TLS Handshake is aborted. As a result, only one binder_key is expected to be requested by the TLS server as opposed to the TLS client.
In this example we assume the psk_key_exchange_mode indicated by the TLS client supports PSK-ECDHE as well as PSK authentication. The presence of a pre_shared_key and a key_share extension in the ServerHello inidcates that PSK-ECDHE has been selected.
To compute binders, the TLS Client needs to request the binder_key associated to each proposed PSK. These binder_keys are retrieved to the CS using the BinderKeyRequest. The key_request is set to binder_key, and the PSK_id extension indicates the PSK's identity (PSKIdentity.identity or NewSessionTicket.ticket). No Handsahke Context is needed and handshake_context is empty.
TLS Client
Lurk Client Cryptographic Service
BinderKeyRequest
key_request=binder_key
handshake_context=""
ext
PSK_id
BinderKeyResponse
<--------- key
Upon receiving the BinderKeyRequest, the CS checks the psk is in the PSK_DB and returns the binder_key.
With the binder keys, the TLS Client is able to send it ClientHello message.
We assume in this example that the ECDHE secrets is generated by the TLS client and not the Cryptographic service. As a result, the TLS client does not need an extra exchange to request the necessary parameters to derive the key_shared extension.
The TLS server is expected to select a PSK, check the associated binder and proceed further. If the binder fails, it is not expected to proceed to another PSK, as a result, the TLS server is expected to initiates a single LURK session.
The binder_key is requested by the TLS server via and s_init_early_secret LURK exchange. The InitEarlySecretRequest structure is composed of a SecretRequest structure (secret_request).
In our case, only the binder_key is requested so key_request is set to binder_key only. Similarly, to the TLS client, the handshake_context is not needed to generate the binder_key. However, the EarlySecret exchange requires the ClientHello to be provided so early secrets may be computed in the same round during 0-RTT handshake. The chosen PSK is indicated in the PSK_id extension and the freshness function is indicated in the freshness extension.
TLS Server
Lurk Client Cryptographic Service
InitEarlySecretRequest
secret_Request
key_request=binder_key
handshake_context=ClientHello
ext
freshenss
PSK_id
session_id
InitEarlySecretResponse
secret_response
<--------- key
ext
session_id
To complete to the ServerHello exchange, the TLS server needs the handshake and application secrets. These secrets are requested via an s_hand_and_app_secret LURK exchange. The HandshakeAndAppSecretRequest is composed of SecretRequest structure. The key_request is set to handshake (h_c, h_s) and application secrets (a_s, a_c). The Handshake Context (handshake_context) is set to ServerHello … EncryptedExtensions as their is no authentication of the TLS client. Finally, the ephemeral ECDHE is provided or requested via the ephemeral extension. In our case, we assume the ephemeral secrets is generated by the tLS client is provided to the CS.
The necessary Handshake Context to generate the handshake secrets is ClientHello … ServerHello, so the CS can generate the handshake secrets. The necessary Handshake Context to generate the application secrets is ClientHello … server Finished. So the CS needs to generate the Finished message before as in the case of the InitCerificateVerify exchange detailed in Section 10.5.1.
TLS Server
Lurk Client Cryptographic Service
HandshakeAndAppRequest
session_id
secret_request
key_request = h_c, h_s, a_c, a_s
handshake_context = ServerHello ... EncryptedExtensions
ext
ephemeral = dhe_secret -------->
HandshakeAndAppResponse
session_id
secret_response
keys
<---------
The CS returns the necessary secret to the TLS server to complete the ServerHello response.
The remaining of the TLS handshake is proceeded similarly as described in the Full Handshake in section Section 10.5.
The 0-RTT Handshake is a PSK or PSK-ECDHE authentication that enables the TLS client to provide application data during the first round trip. The main differences to the PSK PSK-ECDHE authentication described in the case of session resumption is that:
TLS Client TLS Server
ClientHello
+ early_data
+ key_share*
+ psk_key_exchange_modes
+ pre_shared_key
(Application Data*) -------->
ServerHello
+ pre_shared_key
+ key_share*
{EncryptedExtensions}
+ early_data
{Finished}
<-------- [Application Data*]
(EndOfEarlyData)
{Finished} -------->
[Application Data] <-------> [Application Data]
With 0-RTT handshake, the TLS client builds binders as in session resumption described in section Section 10.5.4. The binder_key is retrieved for each proposed PSK with a BinderKeyRequest. When early application data is sent it is encrypted using the client_early_traffic_secret. This secret is retrieved using the c_init_early_secret LURK exchange.
The InitEarlySecretRequest is composed of a SecretRequest (secret_request) substructure. The TLS Client sets the key_request to client_early_traffic_secret (e_s). The handshake is set to ClientHello. The PSK is indicated via the the PSK_id extension, the freshness function is indicated via the freshness extension. If the TLS client is willing to have the ECDHE keys generated by the CS an ephemeral extension MAY be added also.
When multiple PSK are proposed by the TLS client, the first proposed PSK is used to encrypt the application data.
TLS Client
Lurk Client Cryptographic Service
InitEarlySecretRequest
secret_request
key_request=e_s
handshake_context=ClientHello
ex
PSK_id
fresness
session_id
InitEarlySecretResponse
secret_response
<--------- keys=e_s
ext
session_id
Upon receiving the InitEarlySecretRequest, the CS generates the client_early_traffic_secret.
The TLS client is able to send its ClientHello with associated binders and application data.
If the TLS server accepts the early data. It proceeds as described in session resumption described in section Section 10.5.4. In addition to the binder_key, the TLS server also request the client_early_traffic_secret to decrypt the early data as well as to proceed to the ServerHello exchange.
The TLS client proceeds as described in handshake based on ECDHE, PSK or PSK-ECDHE authentications described in Section 10.4 and Section 10.5. The main difference is that upon requesting handshake and application secrets, using an HandAndAppRequest the TLS client will not provide the ClientHello as part as the handshake_context. The Client as already been provided during the EarlySercret exchange.
TLS client authentication can be performed during the Full TLS handshake or after the TLS handshake as a post handshake authentication. In both cases, the TLS client authentication is initiated by the TLS server sending a CertificateRequest. The authentication is performed via a CertificateVerify message generated by the TLS client but such verification does not involve the CS on the TLS server.
The ServerHello MAY carry a CertificateRequest encrypted with the handshake sercets.
Upon receiving the ServerHello response, the TLS client decrypts the ServerHello response. If a CertificateRequest message is found, the TLS Client requests the Cryptographic to compute the CertificateVerify in addition to the application secrets via a certificate_verify LURK exchange. The CertVerifyRequest is composed of a Secret Request structure and a SigningRequest structure.
The key_request is set to the application secrets (a_c, a_s) and the handshake_context is set to server EncryptedExtensions … later of server Finished/EndOfEarlyData. As the request follows a (BinderKey, EarlySecret, HandshakeSecret) or HandshakeSecret the Handshake Context on the CS now becomes: ClientHello … later of server Finished/EndOfEarlyData which is the Handshake Context required to generate the CertificateVerify on the TLS client side and includes the Handshake Context required to generate the application secrets (ClientHello…server Finished).
TLS Client
Lurk Client Cryptographic Service
CertVerifyRequest
session_id
secret_request
key_request
handshake_context = EncryptedExtensions ...
later of server Finished/EndOfEarlyData
signing_request
CertVerifyResponse
session_id
secret_response
keys
signing_response
<--------- certificate_verify
Upon receiving the CertificateRequest, the CS checks the session_id and cookie.
When post-handshake is enabled by the TLS client, the TLS client may receive at any time after the handshake a CertificateRequest message. When post handshake is enabled by the TLS client, as soon as the client Finished message has been sent, the TLS client sends a RegisteredNewSessionTicketRequest with an empty NewSessionTicket to register the remaining Handshake Context to the CS. ctx_id is set to opaque, handshake_context is set to earlier of client Certificate client CertificateVerify … client Finished.
Upon receiving the RegisteredNewSessionTicketsRequest the Cryptographic is aware of the full Handshake Context. It updates ks_ctx.next_request to c_post_hand or c_register_ticket.
TLS Client
Lurk Client Cryptographic Service
RegisteredNewSessionTicketRequest
session_id
handshake_context
ticket_list (empty)
<--------- RegisteredNewSessionTicketResponse
session_id
cookie
When the TLS client receives a CertificateRequest message from the TLS server, the TLS client sends a PostHandshakeRequest to the Cryptographic Service to generate certificate_verify. The handshake_context is set to CertificateRequest. The index N of the client_application_traffic_N key is provided as well as the Cryptographic so it can generate the appropriated key.
TLS Client
Lurk Client Cryptographic Service
PostHandshakeRequest
session_id
handshake_context=CertificateRequest
app_n=N
PostHandshakeResponse
session_id
<--------- certificate_verify
Upon receiving the PostHandshakeRequest the CS checks session_id and cookie. The necessary Handshake Context to generate the certificate_verify is ClientHello … client Finished + CertificateRequest. Once the PostHandshakeResponse. Next requests expected are c_post_hand or c_register_ticket.
| [I-D.mglt-lurk-tls12] | Migault, D. and I. Boureanu, "LURK Extension version 1 for (D)TLS 1.2 Authentication", Internet-Draft draft-mglt-lurk-tls12-02, January 2020. |
| [RFC8446] | Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018. |
| [RFC8466] | Wen, B., Fioccola, G., Xie, C. and L. Jalil, "A YANG Data Model for Layer 2 Virtual Private Network (L2VPN) Service Delivery", RFC 8466, DOI 10.17487/RFC8466, October 2018. |
| [I-D.mglt-lurk-lurk] | Migault, D., "LURK Protocol version 1", Internet-Draft draft-mglt-lurk-lurk-00, February 2018. |
| [I-D.mglt-lurk-tls-use-cases] | Migault, D., Ma, K., Salz, R., Mishra, S. and O. Dios, "LURK TLS/DTLS Use Cases", Internet-Draft draft-mglt-lurk-tls-use-cases-02, June 2016. |