HTTP M. Bishop
Internet-Draft Akamai
Intended status: Standards Track N. Sullivan
Expires: April 26, 2019 Cloudflare
M. Thomson
Mozilla
October 23, 2018
Secondary Certificate Authentication in HTTP/2
draft-ietf-httpbis-http2-secondary-certs-03
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 to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at http://httpwg.github.io/
[2]; source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/secondary-certs [3].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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 April 26, 2019.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Server Certificate Authentication . . . . . . . . . . . . 3
1.2. Client Certificate Authentication . . . . . . . . . . . . 4
1.2.1. HTTP/1.1 Using TLS 1.2 and Earlier . . . . . . . . . 5
1.2.2. HTTP/1.1 Using TLS 1.3 . . . . . . . . . . . . . . . 6
1.2.3. HTTP/2 . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. HTTP-Layer Certificate Authentication . . . . . . . . . . 7
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
2. Discovering Additional Certificates at the HTTP/2 Layer . . . 8
2.1. Indicating Support for HTTP-Layer Certificate
Authentication . . . . . . . . . . . . . . . . . . . . . 8
2.2. Making Certificates or Requests Available . . . . . . . . 9
2.3. Requiring Certificate Authentication . . . . . . . . . . 10
2.3.1. Requiring Additional Server Certificates . . . . . . 10
2.3.2. Requiring Additional Client Certificates . . . . . . 11
3. Certificates Frames for HTTP/2 . . . . . . . . . . . . . . . 12
3.1. The CERTIFICATE_NEEDED Frame . . . . . . . . . . . . . . 12
3.2. The USE_CERTIFICATE Frame . . . . . . . . . . . . . . . . 14
3.3. The CERTIFICATE_REQUEST Frame . . . . . . . . . . . . . . 15
3.3.1. Exported Authenticator Request Characteristics . . . 16
3.4. The CERTIFICATE Frame . . . . . . . . . . . . . . . . . . 16
3.4.1. Exported Authenticator Characteristics . . . . . . . 17
4. Indicating Failures During HTTP-Layer Certificate
Authentication . . . . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5.1. Impersonation . . . . . . . . . . . . . . . . . . . . . . 19
5.2. Fingerprinting . . . . . . . . . . . . . . . . . . . . . 19
5.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 19
5.4. Confusion About State . . . . . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
6.1. HTTP/2 SETTINGS_HTTP_CERT_AUTH Setting . . . . . . . . . 20
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6.2. New HTTP/2 Frames . . . . . . . . . . . . . . . . . . . . 21
6.3. New HTTP/2 Error Codes . . . . . . . . . . . . . . . . . 21
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Normative References . . . . . . . . . . . . . . . . . . 21
7.2. Informative References . . . . . . . . . . . . . . . . . 22
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23
A.1. Since draft-ietf-httpbis-http2-secondary-certs-01: . . . 23
A.2. Since draft-ietf-httpbis-http2-secondary-certs-00: . . . 23
A.3. Since draft-bishop-httpbis-http2-additional-certs-05: . . 23
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
HTTP clients need to know that the content they receive on a
connection comes from the origin that they intended to retrieve in
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], [I-D.ietf-tls-tls13]) 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.
1.1. Server Certificate Authentication
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.
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[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.
1.2. Client Certificate Authentication
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.
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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.
1.2.1. HTTP/1.1 Using TLS 1.2 and Earlier
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.
Figure 1 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
Figure 1: 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 Figure 1). 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).
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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).
1.2.2. HTTP/1.1 Using TLS 1.3
TLS 1.3 [I-D.ietf-tls-tls13] 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. Figure 2
shows the simpler exchange this enables.
Client Server
-- (HTTP) GET /protected ------------------->
<---------------- (TLS) CertificateRequest --
-- (TLS) Certificate, CertificateVerify,
Finished ----------------------->
<--------------------------- (HTTP) 200 OK --
Figure 2: 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.
1.2.3. HTTP/2
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
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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.
1.3. HTTP-Layer Certificate Authentication
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 [I-D.ietf-tls-tls13], 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.
Section 2 describes how the feature is employed, defining means to
detect support in peers (Section 2.1), make certificates and requests
available (Section 2.2), and indicate when streams are blocked
waiting on an appropriate certificate (Section 2.3). Section 3
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defines the required frame types, which parallel the TLS 1.3 message
exchange. Finally, Section 4 defines new error types which can be
used to notify peers when the exchange has not been successful.
1.4. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Discovering Additional Certificates at the HTTP/2 Layer
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 Section 3.4) 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.
2.1. Indicating Support for HTTP-Layer Certificate Authentication
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.
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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 [I-D.ietf-tls-tls13]
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:
o Label:
* "EXPORTER HTTP CERTIFICATE client" for clients
* "EXPORTER HTTP CERTIFICATE server" for servers
o Context: Empty
o 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.
2.2. Making Certificates or Requests Available
When both peers have advertised support for HTTP-layer certificates
as in Section 2.1, 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.
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Client Server
<------------------ (stream 0) CERTIFICATE --
...
-- (stream N) GET /from-new-origin --------->
<----------------------- (stream N) 200 OK --
Figure 3: 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 --
Figure 4: 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.
2.3. Requiring Certificate Authentication
2.3.1. Requiring Additional Server Certificates
As defined in [RFC7540], when a client finds that a 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
Section 2.1), 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
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[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 --
Figure 5: 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.
2.3.2. Requiring Additional Client Certificates
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 Section 2.2 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 --
Figure 6: 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
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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).
3. Certificates Frames for HTTP/2
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 |
+---------+ +---------+
Figure 7: 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.
3.1. The CERTIFICATE_NEEDED Frame
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
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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) |
+-------------------------------+
Figure 8: 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".
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3.2. The USE_CERTIFICATE Frame
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)] |
+-------------------------------+
Figure 9: 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
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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.
Section 4 defines certificate-related error codes which might be
applicable.
3.3. The CERTIFICATE_REQUEST Frame
The "CERTIFICATE_REQUEST" frame (id=0xFRAME-TBD2) provides a 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 (?) ...
+---------------------------------------------------------------+
Figure 10: CERTIFICATE_REQUEST frame payload
The frame contains the following fields:
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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 Section 3.4.1 for more details on the input to this API.
3.3.1. Exported Authenticator Request Characteristics
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.
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.
3.4. The CERTIFICATE Frame
The "CERTIFICATE" frame (id=0xFRAME-TBD3) provides a 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.
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) | Authenticator Fragment (*)...
+---------------------------------------------------------------+
Figure 11: CERTIFICATE frame payload
The frame contains the following fields:
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Cert-ID: "Cert-ID" is a 16-bit opaque identifier used to correlate
other certificate- related frames with this exported authenticator
fragment.
Authenticator Fragment: A portion of the opaque data returned from
the TLS connection exported authenticator "authenticate" API. See
Section 3.4.1 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".
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".
3.4.1. Exported Authenticator Characteristics
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:
o Using the "get context" API, retrieve the
"certificate_request_context" used to generate the authenticator,
if any.
o Verify that the "certificate_request_context" is either empty
(clients only) or contains the Request-ID of a previously-sent
"CERTIFICATE_REQUEST" frame.
o Use the "validate" API to confirm the validity of the
authenticator with regard to the generated request (if any).
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Once the authenticator is accepted, the endpoint can perform any
other checks for the acceptability of the certificate itself.
4. Indicating Failures During HTTP-Layer Certificate Authentication
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.
5. Security Considerations
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.
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5.1. 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. As recommended in [RFC8336], 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
[I-D.ietf-tls-exported-authenticator], it 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 collocated 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.
5.2. Fingerprinting
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.
5.3. Denial of Service
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
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implementation MAY instead disable HTTP-layer certificates for the
current connection instead.
5.4. 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 "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.
6. IANA Considerations
This draft adds entries in three registries.
The HTTP/2 "SETTINGS_HTTP_CERT_AUTH" setting is registered in
Section 6.1. Four frame types are registered in Section 6.2. Six
error codes are registered in Section 6.3.
6.1. HTTP/2 SETTINGS_HTTP_CERT_AUTH Setting
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.
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6.2. New HTTP/2 Frames
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 | Section 3.1 |
| | | |
| CERTIFICATE_REQUEST | 0xFRAME-TBD2 | Section 3.3 |
| | | |
| CERTIFICATE | 0xFRAME-TBD3 | Section 3.4 |
| | | |
| USE_CERTIFICATE | 0xFRAME-TBD4 | Section 3.2 |
+---------------------+--------------+---------------+
6.3. New HTTP/2 Error Codes
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 | Section 4 |
| | | |
| UNSUPPORTED_CERTIFICATE | 0xERROR-TBD2 | Section 4 |
| | | |
| CERTIFICATE_REVOKED | 0xERROR-TBD3 | Section 4 |
| | | |
| CERTIFICATE_EXPIRED | 0xERROR-TBD4 | Section 4 |
| | | |
| CERTIFICATE_GENERAL | 0xERROR-TBD5 | Section 4 |
| | | |
| CERTIFICATE_OVERUSED | 0xERROR-TBD6 | Section 4 |
+-------------------------+--------------+---------------+
7. References
7.1. Normative References
[I-D.ietf-tls-exported-authenticator]
Sullivan, N., "Exported Authenticators in TLS", draft-
ietf-tls-exported-authenticator-07 (work in progress),
June 2018.
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[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>.
[RFC8336] Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
RFC 8336, DOI 10.17487/RFC8336, March 2018,
<https://www.rfc-editor.org/info/rfc8336>.
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7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/secondary-certs
Appendix A. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
A.1. Since draft-ietf-httpbis-http2-secondary-certs-01:
o Clients can send "CERTIFICATE_NEEDED" for stream 0 rather than
speculatively reserving a stream for an origin.
o Use SETTINGS to disable when a TLS-terminating proxy is present
(#617,#651)
A.2. Since draft-ietf-httpbis-http2-secondary-certs-00:
o All frames sent on stream zero; replaced "AUTOMATIC_USE" on
"CERTIFICATE" with "UNSOLICITED" on "USE_CERTIFICATE". (#482,#566)
o Use Exported Requests from the TLS Exported Authenticators draft;
eliminate facilities for expressing certificate requirements in
"CERTIFICATE_REQUEST" frame. (#481)
A.3. Since draft-bishop-httpbis-http2-additional-certs-05:
o Adopted as draft-ietf-httpbis-http2-secondary-certs
Acknowledgements
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.
Authors' Addresses
Mike Bishop
Akamai
Email: mbishop@evequefou.be
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Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
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