Secondary Certificate Authentication of HTTP Servers

1.1. Server Certificate Authentication

Section 9.1.1 of [H2] and Section 3.3 of [H3] describe how connections may be used to make requests from multiple origins as long as the server is authoritative for both. A server is considered authoritative for an origin if DNS resolves the origin to the IP address of the server and (for TLS) if the certificate presented by the server contains the origin in the Subject Alternative Names field.¶

[ALTSVC] enables a step of abstraction from the DNS resolution. If both hosts have provided an Alternative Service at hostnames which resolve to the IP address of the server, they are considered authoritative just as if DNS resolved the origin itself to that address. However, the server's one TLS certificate is still required to contain the name of each origin in question.¶

[ORIGIN] relaxes the requirement to perform the DNS lookup if already connected to a server with an appropriate certificate which claims support for a particular origin.¶

Servers which host many origins often would prefer to have separate certificates for some sets of origins. This may be for ease of certificate management (the ability to separately revoke or renew them), due to different sources of certificates (a CDN acting on behalf of multiple origins), or other factors which might drive this administrative decision. Clients connecting to such origins cannot currently reuse connections, even if both client and server would prefer to do so.¶

Because the TLS SNI extension is exchanged in the clear, clients might also prefer to retrieve certificates inside the encrypted context. When this information is sensitive, it might be advantageous to request a general-purpose certificate or anonymous ciphersuite at the TLS layer, while acquiring the "real" certificate in HTTP after the connection is established.¶

1.2. TLS Exported Authenticators

TLS Exported Authenticators [EXPORTED-AUTH] are structured messages that can be exported by either party of a TLS connection and validated by the other party. Given an established TLS connection, an authenticator message can be constructed proving possession of a certificate and a corresponding private key. The mechanisms that this draft defines are primarily focused on the server's ability to generate TLS Exported Authenticators.¶

Each Authenticator is computed using a Handshake Context and Finished MAC Key derived from the TLS session. The Handshake Context is identical for both parties of the TLS connection, while the Finished MAC Key is dependent on whether the Authenticator is created by the client or the server.¶

Successfully verified Authenticators result in certificate chains, with verified possession of the corresponding private key, which can be supplied into a collection of available certificates. Likewise, descriptions of desired certificates can also be supplied into these collections.¶

1.3. HTTP-Layer Certificate Authentication

This draft defines HTTP/2 and HTTP/3 CERTIFICATE frames (Section 5) to carry the relevant certificate messages, enabling certificate-based authentication of servers independent of TLS version. This mechanism can be implemented at the HTTP layer without breaking the existing interface between HTTP and applications above it.¶

TLS Exported Authenticators [EXPORTED-AUTH] allow the opportunity for an HTTP/2 and HTTP/3 servers to send certificate frames which can be used to prove the servers authenticity for multiple origins.¶

This draft additionally defines SETTINGS parameters for HTTP/2 and HTTP/3 (Section 4) that allow the client and server to indicate support for HTTP-Layer certificate authentication.¶

3.1. Indicating Support for HTTP-Layer Certificate Authentication

The SETTINGS_HTTP_SERVER_CERT_AUTH parameters for HTTP/2 and HTTP/3 are defined in Section 4 so that clients and servers can indicate support for secondary certificate authentication of servers.¶

HTTP/2 and HTTP/3 endpoints who wish to indicate support for HTTP-Layer certificate authentication MUST send a SETTINGS_HTTP_SERVER_CERT_AUTH parameter set to "1" in their SETTINGS frame. Endpoints MUST NOT use any of the authentication functionality described in this draft unless the parameter has been negotiated by both sides.¶

Endpoints MUST NOT send a SETTINGS_HTTP_SERVER_CERT_AUTH parameter with a value of 0 after previously sending a value of 1.¶

SETTINGS_HTTP_SERVER_CERT_AUTH indicates that servers are able to offer additional certificates to demonstrate control over other origin hostnames, and that clients are able to make requests for hostnames received in a TLS Exported Authenticator that the server sends.¶

3.2. Making Certificates or Requests Available

When both peers have advertised support for HTTP-layer certificates in a given direction as in Section 3.1, the indicated endpoint can supply additional certificates into the connection at any time. That is, if both endpoints have sent SETTINGS_HTTP_SERVER_CERT_AUTH and validated the value received from the peer, the server may send certificates unprompted, at any time.¶

This does mean that if a server knows it supports secondary certificate authentication, and it receives SETTINGS_HTTP_SERVER_CERT_AUTH from the client, that it can enqueue certificates immediately following the received SETTINGS frame.¶

Certificates supplied by servers can be considered by clients without further action by the server. A server SHOULD NOT send certificates which do not cover origins which it is prepared to service on the current connection, and should NOT send them if the client has not indicated support with SETTINGS_HTTP_SERVER_CERT_AUTH.¶

A client MUST NOT send certificates to the server. The server SHOULD close the connection upon receipt of a CERTIFICATE frame from a client.¶

Client                                        Server
   <-- (stream 0 / control stream) CERTIFICATE --
   ...
   -- (stream N) GET /from-new-origin ---------->
   <----------------------- (stream N) 200 OK ---

A server MAY send a CERTIFICATE immediately after sending its SETTINGS. However, it MAY also send certificates at any time later. For example, a proxy might discover that a client is interested in an origin that it can reverse proxy at the time that a client sends a CONNECT request. It can then send certificates for those origins to allow for TLS-terminated reverse proxying to those origins for the remainder of the connection lifetime. Figure 2 illustrates this behavior.¶

Client                                        Server
   -- (stream N) CONNECT /to-new-origin -------->
   <-- (stream 0 / control stream) CERTIFICATE --
   ...
   -- (stream M) GET /to-new-origin ------------>
   <--- (stream M, direct from server) 200 OK ---

4.1. The SETTINGS_HTTP_SERVER_CERT_AUTH HTTP/2 SETTINGS Parameter{#http2-setting}

This document adds a new HTTP/2 SETTINGS(0xTBD) parameter to those defined by Section 6.5.2 of [H2].¶

The new parameter name is SETTINGS_HTTP_SERVER_CERT_AUTH. The value of the parameter MUST be 0 or 1.¶

The usage of this parameter is described in Section 3.1.¶

4.2. The SETTINGS_HTTP_SERVER_CERT_AUTH HTTP/3 SETTINGS Parameter{#http3-setting}

This document adds a new HTTP/3 SETTINGS(0xTBD) parameter to those defined by Section 7.2.4.1 of [H3].¶

The new parameter name is SETTINGS_HTTP_SERVER_CERT_AUTH. The value of the parameter MUST be 0 or 1.¶

The usage of this parameter is described in Section 3.1.¶

5.1. HTTP/2 CERTIFICATE frame

A CERTIFICATE frame in HTTP/2 (type=0xTBD) carrries a TLS Exported authenticator that clients can use to authenticate secondary origins from a sending server.¶

The CERTIFICATE frame MUST be sent on stream 0. A CERTIFICATE frame received on any other stream MUST not be used for server authentication.¶

CERTIFICATE Frame {
  Length (24),
  Type (8) = 0xTBD,

  Unused Flags (8),

  Reserved (1),
  Stream Identifier (31) = 0,

  Authenticator (..),
}

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier fields are described in Section 4 of [H2].¶

The CERTIFICATE frame does not define any flags.¶

The authenticator field is a portion of the opaque data returned from the TLS connection exported authenticator authenticate API. See Section 5.3 for more details on the input to this API.¶

The CERTIFICATE frame applies to the connection, not a specific stream. An endpoint MUST treat a CERTIFICATE frame with a stream identifier other than 0x00 as a connection error.¶

5.2. HTTP/3 CERTIFICATE frame

A CERTIFICATE frame in HTTP/3 (type=0xTBD) carrries a TLS Exported authenticator that clients can use to authenticate secondary origins from a sending server.¶

The CERTIFICATE frame MUST be sent on the control stream. A CERTIFICATE frame received on any other stream MUST not be used for server authentication.¶

CERTIFICATE Frame {
  Type (i) = 0xTBD,
  Length (i),
  Authenticator (...),
}

The Type and Length fields are described in Section 7.1 of [H3].¶

The authenticator field is a portion of the opaque data returned from the TLS connection exported authenticator authenticate API. See Section 5.3 for more details on the input to this API.¶

The CERTIFICATE frame applies to the connection, not a specific stream. An endpoint MUST treat a CERTIFICATE frame received on any stream other than the control stream as a connection error.¶

5.3. Exported Authenticator Characteristics

The Exported Authenticator API defined in [EXPORTED-AUTH] takes as input a request, a set of certificates, and supporting information about the certificate (OCSP, SCT, etc.). The result is an opaque token which is used when generating the CERTIFICATE frame.¶

Upon receipt of a CERTIFICATE frame, an endpoint which has negotiated support for secondary certfiicates MUST perform the following steps to validate the token it contains:¶

• Using the get context API, retrieve the certificate_request_context used to generate the authenticator, if any.¶
• Use the validate API to confirm the validity of the authenticator with regard to the generated request, if any.¶

If the authenticator cannot be validated, this SHOULD be treated as a connection error.¶

Once the authenticator is accepted, the endpoint can perform any other checks for the acceptability of the certificate itself.¶

6.1. Misbehavior

This category of errors could indicate a peer failing to follow requirements in this document or might indicate that the connection is not fully secure. These errors are fatal to stream or connection, as appropriate.¶

CERTIFICATE_UNREADABLE (0xERROR-TBD):

An exported authenticator could not be validated.¶

6.2. Invalid Certificates

Unacceptable certificates (expired, revoked, or insufficient to satisfy the request) are not treated as stream or connection errors. This is typically not an indication of a protocol failure. Servers SHOULD process requests with the indicated certificate, likely resulting in a "4XX"-series status code in the response. Clients SHOULD establish a new connection in an attempt to reach an authoritative server.¶

7.1. Impersonation

This mechanism could increase the impact of a key compromise. Rather than needing to subvert DNS or IP routing in order to use a compromised certificate, a malicious server now only needs a client to connect to some HTTPS site under its control in order to present the compromised certificate. Clients SHOULD consult DNS for hostnames presented in secondary certificates if they would have done so for the same hostname if it were present in the primary certificate.¶

As recommended in [ORIGIN], clients opting not to consult DNS ought to employ some alternative means to increase confidence that the certificate is legitimate.¶

As noted in the Security Considerations of [EXPORTED-AUTH], it is difficult to formally prove that an endpoint is jointly authoritative over multiple certificates, rather than individually authoritative on each certificate. As a result, clients MUST NOT assume that because one origin was previously colocated with another, those origins will be reachable via the same endpoints in the future. Clients MUST NOT consider previous secondary certificates to be validated after TLS session resumption. However, clients MAY proactively query for previously-presented secondary certificates.¶

7.2. Fingerprinting

This draft defines a mechanism which could be used to probe servers for origins they support, but it opens no new attack that was not already possible by making repeat TLS connections with different SNI values.¶

Servers can also learn information about clients using this mechanism. The hostnames a user agent finds interesting and retrieves certificates for might indicate origins the user has previously accessed.¶

7.3. Persistence of Service

CNAME records in the DNS are frequently used to delegate authority for an origin to a third-party provider. This delegation can be changed without notice, even to the third-party provider, simply by modifying the CNAME record in question.¶

After the owner of the domain has redirected traffic elsewhere by changing the CNAME, new connections will not arrive for that origin, but connections which are properly directed to this provider for other origins would continue to claim control of this origin (via Secondary Certificates). This is proper behavior based on the third-party provider's configuration, but would likely not be what is intended by the owner of the origin.¶

This is not an issue which can be mitigated by the protocol, but something about which third-party providers SHOULD educate their customers before using the features described in this document.¶

7.4. Confusion About State

Implementations need to be aware of the potential for confusion about the state of a connection. The presence or absence of a validated certificate can change during the processing of a request, potentially multiple times, as CERTIFICATE frames are received. A client that uses certificate authentication needs to be prepared to reevaluate the authorization state of a request as the set of certificates changes.¶

Behavior for TLS-Terminated reverse proxies is also worth considering. If a server which situationally reverse-proxies wishes for the client to view a request made prior to receipt of certificates as TLS-Terminated, or wishes for the client to start a new tunnel alternatively, this draft does not currently define formal mechanisms to facilitate that intention.¶

8.1. Frame Types

This specification registers the following entry in the "HTTP/2 Frame Type" registry defined in [H2]:¶

Code: : TBD¶

Frame Type: : CERTIFICATE¶

Reference: : This document¶

This specification registers the following entry in the "HTTP/3 Frame Types" registry established by [H3]:¶

Value: : TBD¶

Frame Type: : CERTIFICATE¶

Status: : permanent¶

Reference: : This document¶

Change Controller: : IETF¶

Contact: : ietf-http-wg@w3.org¶

8.2. Settings Parameters

This specification registers the following entry in the "HTTP/2 Settings" registry defined in [H2]:¶

Code: : TBD¶

Name: : SETTINGS_HTTP_SERVER_CERT_AUTH¶

Initial Value: : 0¶

Reference: : This document¶

This specification registers the following entry in the "HTTP/3 Settings" registry defined in [H3]:¶

Code: : TBD¶

Name: : SETTINGS_HTTP_SERVER_CERT_AUTH¶

Default: : 0¶

Reference: : This document¶

Change Controller: : IETF¶

Contact: : ietf-http-wg@w3.org¶