Internet DRAFT - draft-bishop-httpbis-http2-additional-certs
draft-bishop-httpbis-http2-additional-certs
HTTP M. Bishop
Internet-Draft
Intended status: Standards Track N. Sullivan
Expires: May 3, 2018 Cloudflare
M. Thomson
Mozilla
October 30, 2017
Secondary Certificate Authentication in HTTP/2
draft-bishop-httpbis-http2-additional-certs-05
Abstract
TLS provides fundamental mutual authentication services for HTTP,
supporting up to one server certificate and up to one client
certificate associated to the session to prove client and server
identities as necessary. This draft provides mechanisms for
providing additional such certificates at the HTTP layer when these
constraints are not sufficient.
Many HTTP servers host content from several origins. HTTP/2
[RFC7540] 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 [I-D.ietf-tls-tls13] handshake.
In many cases, servers will wish to maintain separate certificates
for different origins but still desire the benefits of a shared HTTP
connection. Similarly, servers may require clients to present
authentication, but have different requirements based on the content
the client is attempting to access.
This document describes how TLS exported authenticators
[I-D.ietf-tls-exported-authenticator] can be used to provide proof of
ownership of additional certificates to the HTTP layer to support
both scenarios.
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 http://datatracker.ietf.org/drafts/current/.
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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 May 3, 2018.
Copyright Notice
Copyright (c) 2017 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
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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 previous . . . . . . . . . 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 . . . . . . . . 8
2.3. Requiring certificate authentication . . . . . . . . . . 9
3. Certificates Frames for HTTP/2 . . . . . . . . . . . . . . . 11
3.1. The CERTIFICATE_NEEDED frame . . . . . . . . . . . . . . 11
3.2. The USE_CERTIFICATE Frame . . . . . . . . . . . . . . . . 12
3.3. The CERTIFICATE_REQUEST Frame . . . . . . . . . . . . . . 13
3.4. The CERTIFICATE Frame . . . . . . . . . . . . . . . . . . 14
3.4.1. Exported Authenticator Characteristics . . . . . . . 15
4. Indicating failures during HTTP-Layer Certificate
Authentication . . . . . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
5.1. Impersonation . . . . . . . . . . . . . . . . . . . . . . 16
5.2. Fingerprinting . . . . . . . . . . . . . . . . . . . . . 17
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5.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 17
5.4. Confusion About State . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6.1. HTTP/2 SETTINGS_HTTP_CERT_AUTH Setting . . . . . . . . . 18
6.2. New HTTP/2 Frames . . . . . . . . . . . . . . . . . . . . 18
6.3. New HTTP/2 Error Codes . . . . . . . . . . . . . . . . . 19
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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 X.509 certificates 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.
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.
[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.
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[I-D.ietf-httpbis-origin-frame] 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.
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.
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1.2.1. HTTP/1.1 using TLS 1.2 and previous
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] accomodates 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).
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).
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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
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
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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 the on-stream frames incorporate them 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. An authenticator message can be constructed by either
the client or the server given an established TLS connection, a
certificate, and a corresponding private key. Exported
Authenticators use the message structures from section 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. These pre-supplied elements are then available for
automatic use (in some situations) or for reference by individual
streams.
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
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.
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1.4. Terminology
RFC 2119 [RFC2119] defines the terms "MUST", "MUST NOT", "SHOULD" and
"MAY".
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 single
"CERTIFICATE" frame (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. If the certificate is marked as "AUTOMATIC_USE", the
certificate may be used by the recipient to authorize any current or
future request. Otherwise, the recipient requests the required
certificate on each stream, but the previously-supplied certificates
are available for reference without having to resend them.
Likewise, the details of a request are sent on stream zero and stored
by the recipient. These details will be referenced by subsequent
"CERTIFICATE_NEEDED" frames.
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.
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 1.
2.2. Making certificates or requests available
When a peer has advertised support for HTTP-layer certificates as in
Section 2.1, either party can supply additional certificates into the
connection at any time. These certificates then become available for
the peer to consider when deciding whether a connection is suitable
to transport a particular request.
Available certificates which have the "AUTOMATIC_USE" flag set MAY be
used by the recipient without further notice. This means that
clients or servers which predict a certificate will be required could
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pre-supply the certificate without being asked. Regardless of
whether "AUTOMATIC_USE" is set, these certificates are available for
reference by future "USE_CERTIFICATE" frames.
Client Server
<-------- (stream 0) CERTIFICATE (AU flag) --
...
-- (stream N) GET /from-new-origin --------->
<----------------------- (stream N) 200 OK --
Figure 3: Proactive Server Certificate
Client Server
-- (stream 0) CERTIFICATE (AU flag) -------->
-- (streams 1,3) GET /protected ------------>
<-------------------- (streams 1,3) 200 OK --
Figure 4: Proactive Client Certificate
Likewise, either party can supply a "CERTIFICATE_REQUEST" that
outlines parameters of a certificate they might request in the
future. It is important to note that this does not currently request
such a certificate, but makes the contents of the request available
for reference by a future "CERTIFICATE_NEEDED" frame.
2.3. Requiring certificate authentication
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
[I-D.ietf-httpbis-origin-frame]) and advertised support for HTTP-
layer certificates (see Section 2.1), it MAY send a
"CERTIFICATE_NEEDED" frame on the stream it will use to make the
request. (If the request parameters have not already been made
available using a "CERTIFICATE_REQUEST" frame, the client will need
to send the "CERTIFICATE_REQUEST" in order to generate the
"CERTIFICATE_NEEDED" frame.) The stream represents a pending request
to that origin which is blocked until a valid certificate is
processed.
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The request is blocked until the server has responded with a
"USE_CERTIFICATE" frame pointing to a certificate for that origin.
If the certificate is already available, the server SHOULD
immediately respond with the appropriate "USE_CERTIFICATE" frame.
(If the certificate has not already been transmitted, the server will
need to make the certificate available as described in Section 2.2
before completing the exchange.)
If the server does not have the desired certificate, it MUST respond
with an empty "USE_CERTIFICATE" frame. 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 N) CERTIFICATE_NEEDED ----------->
<------------------ (stream 0) CERTIFICATE --
<-------------- (stream N) USE_CERTIFICATE --
-- (stream N) GET /from-new-origin --------->
<----------------------- (stream N) 200 OK --
Figure 5: Client-Requested Certificate
Likewise, on each stream where certificate authentication is
required, the server sends a "CERTIFICATE_NEEDED" frame, which the
client answers with a "USE_CERTIFICATE" frame indicating the
certificate to use. 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 N) CERTIFICATE_NEEDED --
-- (stream 0) CERTIFICATE ------------------>
-- (stream N) USE_CERTIFICATE -------------->
<----------------------- (stream N) 200 OK --
Figure 6: Reactive Certificate Authentication
A server SHOULD provide certificates for an origin before pushing
resources from it or supplying content referencing the origin. If a
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client receives a "PUSH_PROMISE" referencing an origin for which it
has not yet received the server's certificate, the client MUST verify
the server's possession of an appropriate certificate by sending a
"CERTIFICATE_NEEDED" frame on the pushed stream to inform the server
that progress is blocked until the request is satisfied. The client
MUST NOT use the pushed resource until an appropriate certificate has
been received and validated.
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 on 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.
"Request-ID" and "Cert-ID" are sender-local, and the use of the same
value by the other peer does not imply any correlation between their
frames. These values MUST be unique per sender over the lifetime of
the connection.
3.1. The CERTIFICATE_NEEDED frame
The "CERTIFICATE_NEEDED" frame (0xFRAME-TBD1) is sent to indicate
that the HTTP request on the current stream is blocked pending
certificate authentication. The frame includes 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.
The "CERTIFICATE_NEEDED" frame contains 2 octets, which is 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 on 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
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required certificate). To reduce the risk of client confusion,
servers SHOULD NOT have multiple outstanding "CERTIFICATE_NEEDED"
frames on the same stream at any given time.
Clients MUST NOT send multiple "CERTIFICATE_NEEDED" frames on the
same stream.
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 be sent on stream zero, and
MUST NOT be sent on a stream in the "half-closed (local)" state
[RFC7540]. A client that receives a "CERTIFICATE_NEEDED" frame on a
stream which is not in a valid state SHOULD treat this as a stream
error of type "PROTOCOL_ERROR".
3.2. The USE_CERTIFICATE Frame
The "USE_CERTIFICATE" frame (0xFRAME-TBD4) is sent in response to a
"CERTIFICATE_NEEDED" frame to indicate which certificate is being
used to satisfy the requirement.
A "USE_CERTIFICATE" frame with no payload 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.
Otherwise, the "USE_CERTIFICATE" frame contains 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.
The "USE_CERTIFICATE" frame MUST NOT be sent on stream zero or a
stream on which a "CERTIFICATE_NEEDED" frame has not been received.
Receipt of a "USE_CERTIFICATE" frame in these circumstances SHOULD be
treated as a stream error of type "PROTOCOL_ERROR". Each
"USE_CERTIFICATE" frame should reference a preceding "CERTIFICATE"
frame. Receipt of a "USE_CERTIFICATE" frame before the necessary
frames have been received on stream zero MUST also result in a stream
error of type "PROTOCOL_ERROR".
The referenced certificate chain MUST conform to the requirements
expressed in the "CERTIFICATE_REQUEST" to the best of the sender's
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ability. Specifically, if the "CERTIFICATE_REQUEST" contained a non-
empty "Cert-Extensions" element, the end-entity certificate MUST
match with regard to the extensions recognized by the sender.
If these requirements are not satisfied, the recipient 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
TLS 1.3 defines the "CertificateRequest" message, which prompts the
client to provide a certificate which conforms to certain properties
specified by the server. This draft defines the
"CERTIFICATE_REQUEST" frame (0xFRAME-TBD2), which uses the same set
of extensions to specify a desired certificate, but can be sent over
any TLS version and can be sent by either peer.
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) | Extension-Count (16) |
+-------------------------------+-------------------------------+
| Extensions(?) ...
+---------------------------------------------------------------+
Figure 7: 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.
Extension-Count and Extensions: A list of certificate selection
criteria, represented in a series of "Extension" structures (see
[I-D.ietf-tls-tls13] section 4.2). This criteria MUST be used in
certificate selection as described in [I-D.ietf-tls-tls13]. The
number of "Extension" structures is given by the 16-bit
"Extension-Count" field, which MAY be zero.
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Some extensions used for certificate selection allow multiple values
(e.g. oid_filters on Extended Key Usage). If the sender has
included a non-empty Extensions list, the certificate MUST match all
criteria specified by extensions the recipient recognizes. However,
the recipient MUST ignore and skip any unrecognized certificate
selection extensions.
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:
AUTOMATIC_USE (0x01): Indicates that the certificate can be used
automatically on future requests.
TO_BE_CONTINUED (0x02): 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 8: CERTIFICATE frame payload
The "Exported Authenticator Fragment" field contains 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.
This opaque data 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".
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If the "AUTOMATIC_USE" flag is set, the recipient MAY omit sending
"CERTIFICATE_NEEDED" frames on future streams which would require a
similar certificate and use the referenced certificate for
authentication without further notice to the holder. This behavior
is optional, and receipt of a "CERTIFICATE_NEEDED" frame does not
imply that previously-presented certificates were unacceptable, even
if "AUTOMATIC_USE" was set. Servers MUST set the "AUTOMATIC_USE"
flag when sending a "CERTIFICATE" frame. A server MUST NOT send
certificates for origins which it is not prepared to service on the
current connection.
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 certificate,
supporting information about the certificate (OCSP, SCT, etc.), and
an optional "certificate_request_context". When generating exported
authenticators for use with this extension, the
"certificate_request_context" MUST be the two-octet Cert-ID.
Upon receipt of a completed authenticator, an endpoint MUST check
that:
o the "validate" API confirms the validity of the authenticator
itself
o the "certificate_request_context" matches the Cert-ID of the
frame(s) in which it was received
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
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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
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.
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
[I-D.ietf-httpbis-origin-frame], clients opting not to consult DNS
ought to employ some alternative means to increase confidence that
the certificate is legitimate.
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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 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.
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
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
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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.
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.
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+---------------------+--------------+---------------+
| 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
Five 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 |
+-------------------------+--------------+---------------+
7. Acknowledgements
Eric Rescorla pointed out several failings in an earlier revision.
Andrei Popov contributed to the TLS considerations.
8. References
8.1. Normative References
[I-D.ietf-tls-exported-authenticator]
Sullivan, N., "Exported Authenticators in TLS", draft-
ietf-tls-exported-authenticator-03 (work in progress),
July 2017.
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[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
[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>.
[RFC2459] Housley, R., Ford, W., Polk, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 2459, DOI 10.17487/RFC2459, January 1999,
<https://www.rfc-editor.org/info/rfc2459>.
[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>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[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>.
[X690] ITU-T, "Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ISO ISO/IEC 8825-1:2002, 2002,
<http://www.itu.int/ITU-T/studygroups/com17/languages/
X.690-0207.pdf>.
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8.2. Informative References
[I-D.ietf-httpbis-origin-frame]
Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
draft-ietf-httpbis-origin-frame-04 (work in progress),
August 2017.
[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>.
Authors' Addresses
Mike Bishop
Email: mbishop@evequefou.be
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
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