title: Secondary Certificate Authentication in HTTP/2 abbrev: Secondary Cert Auth in HTTP/2 docname: draft-ietf-httpbis-http2-secondary-certs-latest date: {DATE} category: std
ipr: trust200902 area: General workgroup: HTTP keyword: Internet-Draft
stand_alone: yes pi: [toc, sortrefs, symrefs]
name: Mike Bishop
organization: Akamai
email: [email protected]
-
name: Nick Sullivan organization: Cloudflare email: [email protected]
-
name: Martin Thomson organization: Mozilla email: [email protected]
normative: RFC5246: RFC6066: RFC7230: RFC7540: RFC8446: I-D.ietf-tls-exported-authenticator:
informative: RFC7838:
--- abstract
A use of TLS Exported Authenticators is described which enables HTTP/2 clients and servers to offer additional certificate-based credentials after the connection is established. The means by which these credentials are used with requests is defined.
--- note_Note_to_Readers
Discussion of this draft takes place on the HTTP working group mailing list ([email protected]), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.
Working Group information can be found at http://httpwg.github.io/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/secondary-certs.
--- middle
HTTP clients need to know that the content they receive on a connection comes from the origin that they intended to retrieve it from. The traditional form of server authentication in HTTP has been in the form of a single X.509 certificate provided during the TLS ([RFC5246], [RFC8446]) handshake.
Many existing HTTP [RFC7230] servers also have authentication requirements for the resources they serve. Of the bountiful authentication options available for authenticating HTTP requests, client certificates present a unique challenge for resource-specific authentication requirements because of the interaction with the underlying TLS layer.
TLS 1.2 [RFC5246] supports one server and one client certificate on a connection. These certificates may contain multiple identities, but only one certificate may be provided.
Many HTTP servers host content from several origins. HTTP/2 permits clients to reuse an existing HTTP connection to a server provided that the secondary origin is also in the certificate provided during the TLS handshake. In many cases, servers choose to maintain separate certificates for different origins but still desire the benefits of a shared HTTP connection.
Section 9.1.1 of [RFC7540] describes 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.
[RFC7838] 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.
{{?RFC8336}} 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.
For servers that wish to use client certificates to authenticate users, they might request client authentication during or immediately after the TLS handshake. However, if not all users or resources need certificate-based authentication, a request for a certificate has the unfortunate consequence of triggering the client to seek a certificate, possibly requiring user interaction, network traffic, or other time-consuming activities. During this time, the connection is stalled in many implementations. Such a request can result in a poor experience, particularly when sent to a client that does not expect the request.
The TLS 1.3 CertificateRequest can be used by servers to give clients hints about which certificate to offer. Servers that rely on certificate-based authentication might request different certificates for different resources. Such a server cannot use contextual information about the resource to construct an appropriate TLS CertificateRequest message during the initial handshake.
Consequently, client certificates are requested at connection establishment time only in cases where all clients are expected or required to have a single certificate that is used for all resources. Many other uses for client certificates are reactive, that is, certificates are requested in response to the client making a request.
In HTTP/1.1, a server that relies on client authentication for a subset of users or resources does not request a certificate when the connection is established. Instead, it only requests a client certificate when a request is made to a resource that requires a certificate. TLS 1.2 [RFC5246] accommodates this by permitting the server to request a new TLS handshake, in which the server will request the client's certificate.
{{ex-http11}} shows the server initiating a TLS-layer renegotiation in response to receiving an HTTP/1.1 request to a protected resource.
Client Server
-- (HTTP) GET /protected -------------------> *1
<---------------------- (TLS) HelloRequest -- *2
-- (TLS) ClientHello ----------------------->
<------------------ (TLS) ServerHello, ... --
<---------------- (TLS) CertificateRequest -- *3
-- (TLS) ..., Certificate ------------------> *4
-- (TLS) Finished -------------------------->
<-------------------------- (TLS) Finished --
<--------------------------- (HTTP) 200 OK -- *5
{: #ex-http11 title="HTTP/1.1 reactive certificate authentication with TLS 1.2"}
In this example, the server receives a request for a protected resource (at *1 on {{ex-http11}}). Upon performing an authorization check, the server determines that the request requires authentication using a client certificate and that no such certificate has been provided.
The server initiates TLS renegotiation by sending a TLS HelloRequest (at *2). The client then initiates a TLS handshake. Note that some TLS messages are elided from the figure for the sake of brevity.
The critical messages for this example are the server requesting a certificate with a TLS CertificateRequest (*3); this request might use information about the request or resource. The client then provides a certificate and proof of possession of the private key in Certificate and CertificateVerify messages (*4).
When the handshake completes, the server performs any authorization checks a second time. With the client certificate available, it then authorizes the request and provides a response (*5).
TLS 1.3 [RFC8446] introduces a new client authentication mechanism that allows for clients to authenticate after the handshake has been completed. For the purposes of authenticating an HTTP request, this is functionally equivalent to renegotiation. {{ex-tls13}} shows the simpler exchange this enables.
Client Server
-- (HTTP) GET /protected ------------------->
<---------------- (TLS) CertificateRequest --
-- (TLS) Certificate, CertificateVerify,
Finished ----------------------->
<--------------------------- (HTTP) 200 OK --
{: #ex-tls13 title="HTTP/1.1 reactive certificate authentication with TLS 1.3"}
TLS 1.3 does not support renegotiation, instead supporting direct client authentication. In contrast to the TLS 1.2 example, in TLS 1.3, a server can simply request a certificate.
An important part of the HTTP/1.1 exchange is that the client is able to easily identify the request that caused the TLS renegotiation. The client is able to assume that the next unanswered request on the connection is responsible. The HTTP stack in the client is then able to direct the certificate request to the application or component that initiated that request. This ensures that the application has the right contextual information for processing the request.
In HTTP/2, a client can have multiple outstanding requests. Without some sort of correlation information, a client is unable to identify which request caused the server to request a certificate.
Thus, the minimum necessary mechanism to support reactive certificate authentication in HTTP/2 is an identifier that can be use to correlate an HTTP request with a request for a certificate. Since streams are used for individual requests, correlation with a stream is sufficient.
[RFC7540] prohibits renegotiation after any application data has been sent. This completely blocks reactive certificate authentication in HTTP/2 using TLS 1.2. If this restriction were relaxed by an extension or update to HTTP/2, such an identifier could be added to TLS 1.2 by means of an extension to TLS. Unfortunately, many TLS 1.2 implementations do not permit application data to continue during a renegotiation. This is problematic for a multiplexed protocol like HTTP/2.
This draft defines HTTP/2 frames to carry the relevant certificate messages, enabling certificate-based authentication of both clients and 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.
This could be done in a naive manner by replicating the TLS messages as HTTP/2 frames on each stream. However, this would create needless redundancy between streams and require frequent expensive signing operations. Instead, TLS Exported Authenticators [I-D.ietf-tls-exported-authenticator] are exchanged on stream zero and other frames incorporate them to particular requests by reference as needed.
TLS Exported Authenticators 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, a request can be constructed which describes the desired certificate and an authenticator message can be constructed proving possession of a certificate and a corresponding private key. Both requests and authenticators can be generated by either the client or the server. Exported Authenticators use the message structures from Sections 4.3.2 and 4.4 of [RFC8446], but different parameters.
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 be supplied into these collections.
{{discovery}} describes how the feature is employed, defining means to detect support in peers ({{setting}}), make certificates and requests available ({{cert-available}}), and indicate when streams are blocked waiting on an appropriate certificate ({{cert-challenge}}). {{certs-http2}} defines the required frame types, which parallel the TLS 1.3 message exchange. Finally, {{errors}} defines new error types which can be used to notify peers when the exchange has not been successful.
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.
A certificate chain with proof of possession of the private key corresponding to
the end-entity certificate is sent as a sequence of CERTIFICATE frames (see
{{http-cert}}) on stream zero. Once the holder of a certificate has sent the
chain and proof, this certificate chain is cached by the recipient and available
for future use. Clients can proactively indicate the certificate they intend to
use on each request using an unsolicited USE_CERTIFICATE frame, if desired.
The previously-supplied certificates are available for reference without having
to resend them.
Otherwise, the server uses a CERTIFICATE_REQUEST frame to describe a class of
certificates on stream zero, then uses CERTIFICATE_NEEDED frames to associate
these with individual requests. The client responds with a USE_CERTIFICATE
frame indicating the certificate which should be used to satisfy the request.
Data sent by each peer is correlated by the ID given in each frame. This ID is
unrelated to values used by the other peer, even if each uses the same ID in
certain cases. USE_CERTIFICATE frames indicate whether they are sent
proactively or are in response to a CERTIFICATE_NEEDED frame.
Clients and servers that will accept requests for HTTP-layer certificate
authentication indicate this using the HTTP/2 SETTINGS_HTTP_CERT_AUTH
(0xSETTING-TBD) setting.
The initial value for the SETTINGS_HTTP_CERT_AUTH setting is 0, indicating
that the peer does not support HTTP-layer certificate authentication. If a peer
does support HTTP-layer certificate authentication, the value is non-zero.
In order to ensure that the TLS connection is direct to the server, rather than via a TLS-terminating proxy, each side will separately compute and confirm the value of this setting. The setting is derived from a TLS exporter (see Section 7.5 of [RFC8446] and {{?RFC5705}} for more details on exporters). Clients MUST NOT use an early exporter during their 0-RTT flight, but MUST send an updated SETTINGS frame using a regular exporter after the TLS handshake completes.
The exporter is constructed with the following input:
- Label:
- "EXPORTER HTTP CERTIFICATE client" for clients
- "EXPORTER HTTP CERTIFICATE server" for servers
- Context: Empty
- Length: Four bytes
The resulting exporter is converted to a setting value as:
(Exporter & 0x3fffffff) | 0x80000000
That is, the most significant bit will always be set, regardless of the value of the exporter. Each endpoint will compute the expected value from their peer. If the setting is not received, or if the value received is not the expected value, the frames defined in this document SHOULD NOT be sent.
When both peers have advertised support for HTTP-layer certificates as in
{{setting}}, either party can supply additional certificates into the connection
at any time. This means that clients or servers which predict a certificate will
be required could supply the certificate before being asked. These
certificates are available for reference by future USE_CERTIFICATE frames.
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, but MAY use the ORIGIN frame {{?RFC8336}} to indicate that not all covered origins will be served.
Client Server
<------------------ (stream 0) CERTIFICATE --
...
-- (stream N) GET /from-new-origin --------->
<----------------------- (stream N) 200 OK --
{: #ex-http2-server-proactive title="Proactive server authentication"}
Client Server
-- (stream 0) CERTIFICATE ------------------>
-- (stream 0) USE_CERTIFICATE (S=1) -------->
-- (stream 0) USE_CERTIFICATE (S=3) -------->
-- (streams 1,3) GET /protected ------------>
<-------------------- (streams 1,3) 200 OK --
{: #ex-http2-client-proactive title="Proactive client authentication"}
Likewise, either party can supply a CERTIFICATE_REQUEST that outlines
parameters of a certificate they might request in the future. Upon receipt of a
CERTIFICATE_REQUEST, endpoints SHOULD provide a corresponding certificate in
anticipation of a request shortly being blocked. Clients MAY wait for a
CERTIFICATE_NEEDED frame to assist in associating the certificate request with
a particular HTTP transaction.
As defined in [RFC7540], when a client finds that an https:// origin (or Alternative Service [RFC7838]) to which it needs to make a request has the same IP address as a server to which it is already connected, it MAY check whether the TLS certificate provided contains the new origin as well, and if so, reuse the connection.
If the TLS certificate does not contain the new origin, but the server has
claimed support for that origin (with an ORIGIN frame, see {{?RFC8336}}) and
advertised support for HTTP-layer certificates (see {{setting}}), the client MAY
send a CERTIFICATE_REQUEST frame describing the desired origin. The client
then sends a CERTIFICATE_NEEDED frame for stream zero referencing the request,
indicating that the connection cannot be used for that origin until the
certificate is provided.
If the server does not have the desired certificate, it MUST send an Empty
Authenticator, as described in Section 5 of
[I-D.ietf-tls-exported-authenticator], in a CERTIFICATE frame in response to
the request, followed by a USE_CERTIFICATE frame for stream zero which
references the Empty Authenticator. In this case, or if the server has not
advertised support for HTTP-layer certificates, the client MUST NOT send any
requests for resources in that origin on the current connection.
Client Server
<----------------------- (stream 0) ORIGIN --
-- (stream 0) CERTIFICATE_REQUEST ---------->
-- (stream 0) CERTIFICATE_NEEDED (S=0) ----->
<------------------ (stream 0) CERTIFICATE --
<-------- (stream 0) USE_CERTIFICATE (S=0) --
-- (stream N) GET /from-new-origin --------->
<----------------------- (stream N) 200 OK --
{: #ex-http2-server-requested title="Client-requested certificate"}
If a client receives a PUSH_PROMISE referencing an origin for which it has not
yet received the server's certificate, this is a fatal connection error (see
section 8.2 of [RFC7540]). To avoid this, servers MUST supply the associated
certificates before pushing resources from a different origin.
Likewise, the server sends a CERTIFICATE_NEEDED frame for each stream where
certificate authentication is required. The client answers with a
USE_CERTIFICATE frame indicating the certificate to use on that stream. If the
request parameters or the responding certificate are not already available, they
will need to be sent as described in {{cert-available}} as part of this
exchange.
Client Server
<---------- (stream 0) CERTIFICATE_REQUEST --
...
-- (stream N) GET /protected --------------->
<----- (stream 0) CERTIFICATE_NEEDED (S=N) --
-- (stream 0) CERTIFICATE ------------------>
-- (stream 0) USE_CERTIFICATE (S=N) -------->
<----------------------- (stream N) 200 OK --
{: #ex-http2-client-requested title="Reactive certificate authentication"}
If the client does not have the desired certificate, it instead sends an Empty
Authenticator, as described in Section 5 of
[I-D.ietf-tls-exported-authenticator], in a CERTIFICATE frame in response to
the request, followed by a USE_CERTIFICATE frame which references the Empty
Authenticator. In this case, or if the client has not advertised support for
HTTP-layer certificates, the server processes the request based solely on the
certificate provided during the TLS handshake, if any. This might result in an
error response via HTTP, such as a status code 403 (Not Authorized).
The CERTIFICATE_REQUEST and CERTIFICATE_NEEDED frames are correlated by
their Request-ID field. Subsequent CERTIFICATE_NEEDED frames with the same
Request-ID value MAY be sent for other streams where the sender is expecting a
certificate with the same parameters.
The CERTIFICATE, and USE_CERTIFICATE frames are correlated by their
Cert-ID field. Subsequent USE_CERTIFICATE frames with the same Cert-ID MAY
be sent in response to other CERTIFICATE_NEEDED frames and refer to the same
certificate.
CERTIFICATE_NEEDED and USE_CERTIFICATE frames are correlated by the Stream
ID they reference. Unsolicited USE_CERTIFICATE frames are not responses to
CERTIFICATE_NEEDED frames; otherwise, each USE_CERTIFICATE frame for a
stream is considered to respond to a CERTIFICATE_NEEDED frame for the same
stream in sequence.
+---------+ +---------+
| REQUEST | | CERT |
+---------+ +---------+
| |
| Request-ID | Cert-ID
| |
v v
+---------+ Stream ID +---------+
| NEEDED |---------->| USE |
+---------+ +---------+
{: #frame-relationships title="Frame correlation"}
Request-ID and Cert-ID are independent and sender-local. The use of the
same value by the other peer or in the other context does not imply any
correlation between these frames. These values MUST be unique per sender for
each space over the lifetime of the connection.
The CERTIFICATE_NEEDED frame (0xFRAME-TBD1) is sent on stream zero to indicate
that the HTTP request on the indicated stream is blocked pending certificate
authentication. The frame includes stream ID and a request identifier which can
be used to correlate the stream with a previous CERTIFICATE_REQUEST frame sent
on stream zero. The CERTIFICATE_REQUEST describes the certificate the sender
requires to make progress on the stream in question.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
|R| Stream ID (31) |
+-------------------------------+-------------------------------+
| Request-ID (16) |
+-------------------------------+
{: #fig-cert-needed title="CERTIFICATE_NEEDED frame payload"}
The CERTIFICATE_NEEDED frame contains 6 octets. The first four octets
indicate the Stream ID of the affected stream. The following two octets are the
authentication request identifier, Request-ID. A peer that receives a
CERTIFICATE_NEEDED of any other length MUST treat this as a stream error of
type PROTOCOL_ERROR. Frames with identical request identifiers refer to the
same CERTIFICATE_REQUEST.
A server MAY send multiple CERTIFICATE_NEEDED frames for the same stream. If a
server requires that a client provide multiple certificates before authorizing a
single request, each required certificate MUST be indicated with a separate
CERTIFICATE_NEEDED frame, each of which MUST have a different request
identifier (referencing different CERTIFICATE_REQUEST frames describing each
required certificate). To reduce the risk of client confusion, servers SHOULD
NOT have multiple outstanding CERTIFICATE_NEEDED frames for the same stream at
any given time.
Clients MUST only send multiple CERTIFICATE_NEEDED frames for stream zero.
Multiple CERTIFICATE_NEEDED frames on any other stream MUST be considered
a stream error of type PROTOCOL_ERROR.
The CERTIFICATE_NEEDED frame MUST NOT be sent to a peer which has not
advertised support for HTTP-layer certificate authentication.
The CERTIFICATE_NEEDED frame MUST NOT reference a stream in the "half-closed
(local)" or "closed" states [RFC7540]. A client that receives a
CERTIFICATE_NEEDED frame for a stream which is not in a valid state SHOULD
treat this as a stream error of type PROTOCOL_ERROR.
The USE_CERTIFICATE frame (0xFRAME-TBD4) is sent on stream zero to indicate
which certificate is being used on a particular request stream.
The USE_CERTIFICATE frame defines a single flag:
UNSOLICITED (0x01):
: Indicates that no CERTIFICATE_NEEDED frame has yet been received for this
stream.
The payload of the USE_CERTIFICATE frame is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
|R| Stream ID (31) |
+-------------------------------+-------------------------------+
| [Cert-ID (16)] |
+-------------------------------+
{: #fig-use-cert title="USE_CERTIFICATE frame payload"}
The first four octets indicate the Stream ID of the affected stream. The
following two octets, if present, contain the two-octet Cert-ID of the
certificate the sender wishes to use. This MUST be the ID of a certificate for
which proof of possession has been presented in a CERTIFICATE frame.
Recipients of a USE_CERTIFICATE frame of any other length MUST treat this as a
stream error of type PROTOCOL_ERROR. Frames with identical certificate
identifiers refer to the same certificate chain.
A USE_CERTIFICATE frame which omits the Cert-ID refers to the certificate
provided at the TLS layer, if any. If no certificate was provided at the TLS
layer, the stream should be processed with no authentication, likely returning
an authentication-related error at the HTTP level (e.g. 403) for servers or
routing the request to a new connection for clients.
The UNSOLICITED flag MAY be set by clients on the first USE_CERTIFICATE
frame referring to a given stream. This permits a client to proactively indicate
which certificate should be used when processing a new request. When such an
unsolicited indication refers to a request that has not yet been received,
servers SHOULD cache the indication briefly in anticipation of the request.
Receipt of more than one unsolicited USE_CERTIFICATE frames or an unsolicited
USE_CERTIFICATE frame which is not the first in reference to a given stream
MUST be treated as a stream error of type CERTIFICATE_OVERUSED.
Each USE_CERTIFICATE frame which is not marked as unsolicited is considered to
respond in order to the CERTIFICATE_NEEDED frames for the same stream. If a
USE_CERTIFICATE frame is received for which a CERTIFICATE_NEEDED frame has
not been sent, this MUST be treated as a stream error of type
CERTIFICATE_OVERUSED.
Receipt of a USE_CERTIFICATE frame with an unknown Cert-ID MUST result in a
stream error of type PROTOCOL_ERROR.
The referenced certificate chain needs to conform to the requirements expressed
in the CERTIFICATE_REQUEST to the best of the sender's ability, or the
recipient is likely to reject it as unsuitable despite properly validating the
authenticator. If the recipient considers the certificate unsuitable, it MAY at
its discretion either return an error at the HTTP semantic layer, or respond
with a stream error {{RFC7540}} on any stream where the certificate is used.
{{errors}} defines certificate-related error codes which might be applicable.
The CERTIFICATE_REQUEST frame (id=0xFRAME-TBD2) provides an exported
authenticator request message from the TLS layer that specifies a desired
certificate. This describes the certificate the sender wishes to have
presented.
The CERTIFICATE_REQUEST frame SHOULD NOT be sent to a peer which has not
advertised support for HTTP-layer certificate authentication.
The CERTIFICATE_REQUEST frame MUST be sent on stream zero. A
CERTIFICATE_REQUEST frame received on any other stream MUST be rejected with a
stream error of type PROTOCOL_ERROR.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-------------------------------+-------------------------------+
| Request-ID (16) | Request (?) ...
+---------------------------------------------------------------+
{: #fig-cert-request title="CERTIFICATE_REQUEST frame payload"}
The frame contains the following fields:
Request-ID:
: Request-ID is a 16-bit opaque identifier used to correlate subsequent
certificate-related frames with this request. The identifier MUST be unique
in the session for the sender.
Request:
: An exported authenticator request, generated using the request API described
in [I-D.ietf-tls-exported-authenticator]. See {{exp-auth}} for more details on
the input to this API.
The Exported Authenticator request API defined in
[I-D.ietf-tls-exported-authenticator] takes as input a set of desired
certificate characteristics and a certificate_request_context, which needs to
be unpredictable. When generating exported authenticators for use with this
extension, the certificate_request_context MUST contain both the two-octet
Request-ID as well as at least 96 bits of additional entropy.
Upon receipt of a CERTIFICATE_REQUEST frame, the recipient MUST verify that
the first two octets of the authenticator's certificate_request_context
matches the Request-ID presented in the frame.
The TLS library on the authenticating peer will provide mechanisms to select an
appropriate certificate to respond to the transported request. TLS libraries on
servers MUST be able to recognize the server_name extension ([RFC6066]) at a
minimum. Clients MUST always specify the desired origin using this extension,
though other extensions MAY also be included.
The CERTIFICATE frame (id=0xFRAME-TBD3) provides an exported authenticator
message from the TLS layer that provides a chain of certificates, associated
extensions and proves possession of the private key corresponding to the
end-entity certificate.
The CERTIFICATE frame defines two flags:
TO_BE_CONTINUED (0x01): : Indicates that the exported authenticator spans more than one frame.
UNSOLICITED (0x02): : Indicates that the exported authenticator does not contain a Request-ID.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-------------------------------+-------------------------------+
| Cert-ID (16) | Request-ID (16) |
+-------------------------------+-------------------------------+
| Authenticator Fragment (*) ...
+---------------------------------------------------------------+
{: #fig-proof-frame title="CERTIFICATE frame payload"}
The frame contains the following fields:
Cert-ID:
: Cert-ID is a 16-bit opaque identifier used to correlate other certificate-
related frames with this exported authenticator fragment.
Request-ID:
: Request-ID is an optional 16-bit opaque identifier used to correlate this
exported authenticator with the request which triggered it, if any. This field
is present only if the UNSOLICITED flag is not set.
Authenticator Fragment:
: A portion of the opaque data returned from the TLS connection exported
authenticator authenticate API. See {{exp-auth}} for more details on the
input to this API.
An exported authenticator is transported in zero or more CERTIFICATE frames
with the TO_BE_CONTINUED flag set, followed by one CERTIFICATE frame with
the TO_BE_CONTINUED flag unset. Each of these frames contains the same
Cert-ID field, permitting them to be associated with each other. Receipt of
any CERTIFICATE frame with the same Cert-ID following the receipt of a
CERTIFICATE frame with TO_BE_CONTINUED unset MUST be treated as a connection
error of type PROTOCOL_ERROR.
If the UNSOLICITED flag is not set, the CERTIFICATE frame also contains
a Request-ID indicating the certificate request which caused this exported
authenticator to be generated. The value of this flag and the contents
of the Request-ID field MUST NOT differ between frames with the same
Cert-ID.
Upon receiving a complete series of CERTIFICATE frames, the receiver may
validate the Exported Authenticator value by using the exported authenticator
API. This returns either an error indicating that the message was invalid, or
the certificate chain and extensions used to create the message.
The CERTIFICATE frame MUST be sent on stream zero. A CERTIFICATE frame
received on any other stream MUST be rejected with a stream error of type
PROTOCOL_ERROR.
The Exported Authenticator API defined in [I-D.ietf-tls-exported-authenticator]
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 MUST perform the following
steps to validate the token it contains:
- Verify that either the
UNSOLICITEDflag is set (clients only) or that the Request-ID field contains the Request-ID of a previously-sentCERTIFICATE_REQUESTframe. - Using the
get contextAPI, retrieve thecertificate_request_contextused to generate the authenticator, if any. Verify that thecertificate_request_contextbegins with the supplied Request-ID, if any. - Use the
validateAPI to confirm the validity of the authenticator with regard to the generated request (if any).
Once the authenticator is accepted, the endpoint can perform any other checks for the acceptability of the certificate itself. Clients MUST NOT accept any end-entity certificate from an exported authenticator which does not contain the Required Domain extension; see {{extension}} and {{impersonation}}.
Because this draft permits certificates to be exchanged at the HTTP framing layer instead of the TLS layer, several certificate-related errors which are defined at the TLS layer might now occur at the HTTP framing layer. In this section, those errors are restated and added to the HTTP/2 error code registry.
BAD_CERTIFICATE (0xERROR-TBD1): : A certificate was corrupt, contained signatures that did not verify correctly, etc.
UNSUPPORTED_CERTIFICATE (0xERROR-TBD2): : A certificate was of an unsupported type or did not contain required extensions
CERTIFICATE_REVOKED (0xERROR-TBD3): : A certificate was revoked by its signer
CERTIFICATE_EXPIRED (0xERROR-TBD4): : A certificate has expired or is not currently valid
CERTIFICATE_GENERAL (0xERROR-TBD5): : Any other certificate-related error
CERTIFICATE_OVERUSED (0xERROR-TBD6): : More certificates were used on a request than were requested
As described in [RFC7540], implementations MAY choose to treat a stream error as
a connection error at any time. Of particular note, a stream error cannot occur
on stream 0, which means that implementations cannot send non-session errors in
response to CERTIFICATE_REQUEST, and CERTIFICATE frames. Implementations
which do not wish to terminate the connection MAY either send relevant errors on
any stream which references the failing certificate in question or process the
requests as unauthenticated and provide error information at the HTTP semantic
layer.
The Required Domain extension allows certificates to limit their use with Secondary Certificate Authentication. A client MUST verify that the server has proven ownership of the indicated identity before accepting the limited certificate over Secondary Certificate Authentication.
The identity in this extension is a restriction asserted by the requester of the certificate and is not verified by the CA. Conforming CAs SHOULD mark the requiredDomain extension as non-critical. Conforming CAs MUST require the presence of a CAA record {{!RFC6844}} prior to issuing a certificate with this extension. Because a Required Domain value of "*" has a much higher risk of reuse if compromised, conforming Certificate Authorities are encouraged to require more extensive verification prior to issuing such a certificate.
The required domain is represented as a GeneralName, as specified in Section 4.2.1.6 of {{!RFC5280}}. Unlike the subject field, conforming CAs MUST NOT issue certificates with a requiredDomain extension containing empty GeneralName fields. Clients that encounter such a certificate when processing a certification path MUST consider the certificate invalid.
The wildcard character "" MAY be used to represent that any previously authenticated identity is acceptable. This character MUST be the entirety of the name if used and MUST have a type of "dNSName". (That is, "" is acceptable, but ".com" and "w.example.com" are not).
id-ce-requiredDomain OBJECT IDENTIFIER ::= { id-ce TBD1 }
RequiredDomain ::= GeneralName
This mechanism defines an alternate way to obtain server and client certificates other than in the initial TLS handshake. While the signature of exported authenticator values is expected to be equally secure, it is important to recognize that a vulnerability in this code path is at least equal to a vulnerability in the TLS handshake.
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. As recommended in {{?RFC8336}}, clients opting not to consult DNS ought to employ some alternative means to increase confidence that the certificate is legitimate.
One such means is the Required Domain certificate extension defined in {extension}. Clients MUST require that server certificates presented via this mechanism contain the Required Domain extension and require that a certificate previously accepted on the connection (including the certificate presented in TLS) lists the Required Domain in the Subject field or the Subject Alternative Name extension.
As noted in the Security Considerations of [I-D.ietf-tls-exported-authenticator], 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.
This draft defines a mechanism which could be used to probe servers for origins
they support, but opens no new attack versus making repeat TLS connections with
different SNI values. Servers SHOULD impose similar denial-of-service
mitigations (e.g. request rate limits) to CERTIFICATE_REQUEST frames as to new
TLS connections.
While the extensions in the CERTIFICATE_REQUEST frame permit the sender to
enumerate the acceptable Certificate Authorities for the requested certificate,
it might not be prudent (either for security or data consumption) to include the
full list of trusted Certificate Authorities in every request. Senders,
particularly clients, SHOULD send only the extensions that narrowly specify
which certificates would be acceptable.
Failure to provide a certificate on a stream after receiving
CERTIFICATE_NEEDED blocks processing, and SHOULD be subject to standard
timeouts used to guard against unresponsive peers.
Validating a multitude of signatures can be computationally expensive, while generating an invalid signature is computationally cheap. Implementations will require checks for attacks from this direction. Invalid exported authenticators SHOULD be treated as a session error, to avoid further attacks from the peer, though an implementation MAY instead disable HTTP-layer certificates for the current connection instead.
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 ORIGIN frame and 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.
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
USE_CERTIFICATE frames are received. A server that uses certificate
authentication needs to be prepared to reevaluate the authorization state of a
request as the set of certificates changes.
Client implementations need to carefully consider the impact of setting the
AUTOMATIC_USE flag. This flag is a performance optimization, permitting the
client to avoid a round-trip on each request where the server checks for
certificate authentication. However, once this flag has been sent, the client
has zero knowledge about whether the server will use the referenced cert for any
future request, or even for an existing request which has not yet completed.
Clients MUST NOT set this flag on any certificate which is not appropriate for
currently-in-flight requests, and MUST NOT make any future requests on the same
connection which they are not willing to have associated with the provided
certificate.
This draft adds entries in three registries.
The HTTP/2 SETTINGS_HTTP_CERT_AUTH setting is registered in {{iana-setting}}.
Four frame types are registered in {{iana-frame}}. Six error codes are
registered in {{iana-errors}}.
The SETTINGS_HTTP_CERT_AUTH setting is registered in the "HTTP/2 Settings" registry established in [RFC7540].
Name: : SETTINGS_HTTP_CERT_AUTH
Code: : 0xSETTING-TBD
Initial Value: : 0
Specification: : This document.
Four new frame types are registered in the "HTTP/2 Frame Types" registry established in [RFC7540]. The entries in the following table are registered by this document.
|---------------------|--------------|-------------------------|
| Frame Type | Code | Specification |
|---|---|---|
| CERTIFICATE_NEEDED | 0xFRAME-TBD1 | {{http-cert-needed}} |
| CERTIFICATE_REQUEST | 0xFRAME-TBD2 | {{http-cert-request}} |
| CERTIFICATE | 0xFRAME-TBD3 | {{http-cert}} |
| USE_CERTIFICATE | 0xFRAME-TBD4 | {{http-use-certificate}} |
| --------------------- | -------------- | ------------------------- |
Six new error codes are registered in the "HTTP/2 Error Code" registry established in [RFC7540]. The entries in the following table are registered by this document.
|-------------------------|--------------|-------------------------|
| Name | Code | Specification |
|---|---|---|
| BAD_CERTIFICATE | 0xERROR-TBD1 | {{errors}} |
| UNSUPPORTED_CERTIFICATE | 0xERROR-TBD2 | {{errors}} |
| CERTIFICATE_REVOKED | 0xERROR-TBD3 | {{errors}} |
| CERTIFICATE_EXPIRED | 0xERROR-TBD4 | {{errors}} |
| CERTIFICATE_GENERAL | 0xERROR-TBD5 | {{errors}} |
| CERTIFICATE_OVERUSED | 0xERROR-TBD6 | {{errors}} |
| ------------------------- | -------------- | ------------------------- |
--- back
RFC Editor's Note: Please remove this section prior to publication of a final version of this document.
CERTIFICATE_REQUESTframes contain the Request-ID, which MUST be checked against thecertificate_request_contextof the Exported Authenticator RequestCERTIFICATEframes contain the Request-ID to which they respond, unless the UNSOLICITED flag is set- The Required Domain extension is defined for certificates, which must be present for certificates presented by servers
Editorial updates only.
- Clients can send
CERTIFICATE_NEEDEDfor stream 0 rather than speculatively reserving a stream for an origin. - Use SETTINGS to disable when a TLS-terminating proxy is present (#617,#651)
- All frames sent on stream zero; replaced
AUTOMATIC_USEonCERTIFICATEwithUNSOLICITEDonUSE_CERTIFICATE. (#482,#566) - Use Exported Requests from the TLS Exported Authenticators draft; eliminate
facilities for expressing certificate requirements in
CERTIFICATE_REQUESTframe. (#481)
- Adopted as draft-ietf-httpbis-http2-secondary-certs
{:numbered="false"} Eric Rescorla pointed out several failings in an earlier revision. Andrei Popov contributed to the TLS considerations.
A substantial portion of Mike's work on this draft was supported by Microsoft during his employment there.