HTTP Working Group B. Campbell
Internet-Draft Ping Identity
Intended status: Informational M. Bishop, Ed.
Expires: September 1, 2023 Akamai
February 28, 2023
Client-Cert HTTP Header Field
draft-ietf-httpbis-client-cert-field-05
Abstract
This document describes HTTP extension header fields that allow a TLS
terminating reverse proxy to convey the client certificate
information of a mutually authenticated TLS connection to the origin
server in a common and predictable manner.
About This Document
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This Internet-Draft will expire on September 1, 2023.
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1. Introduction
A fairly common deployment pattern for HTTPS applications is to have
the origin HTTP application servers sit behind a reverse proxy that
terminates TLS connections from clients. The proxy is accessible to
the internet and dispatches client requests to the appropriate origin
server within a private or protected network. The origin servers are
not directly accessible by clients and are only reachable through the
reverse proxy. The backend details of this type of deployment are
typically opaque to clients who make requests to the proxy server and
see responses as though they originated from the proxy server itself.
Although HTTPS is also usually employed between the proxy and the
origin server, the TLS connection that the client establishes for
HTTPS is only between itself and the reverse proxy server.
The deployment pattern is found in a number of varieties such as
n-tier architectures, content delivery networks, application load
balancing services, and ingress controllers.
Although not exceedingly prevalent, TLS client certificate
authentication is sometimes employed and in such cases the origin
server often requires information about the client certificate for
its application logic. Such logic might include access control
decisions, audit logging, and binding issued tokens or cookies to a
certificate, and the respective validation of such bindings. The
specific details from the certificate needed also vary with the
application requirements. In order for these types of application
deployments to work in practice, the reverse proxy needs to convey
information about the client certificate to the origin application
server. At the time of writing, a common way this information is
conveyed is by using non-standard fields to carry the certificate (in
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some encoding) or individual parts thereof in the HTTP request that
is dispatched to the origin server. This solution works but
interoperability between independently developed components can be
cumbersome or even impossible depending on the implementation choices
respectively made (like what field names are used or are
configurable, which parts of the certificate are exposed, or how the
certificate is encoded). A well-known predictable approach to this
commonly occurring functionality could improve and simplify
interoperability between independent implementations.
The scope of this document is to describe existing practice while
codifying specific details sufficient to facilitate improved and
lower-touch interoperability. As such, this document describes two
HTTP header fields, "Client-Cert" and "Client-Cert-Chain", which a
TLS terminating reverse proxy (TTRP) adds to requests sent to the
backend origin servers. The "Client-Cert" field value contains the
end-entity client certificate from the mutually authenticated TLS
connection between the originating client and the TTRP. Optionally,
the "Client-Cert-Chain" field value contains the certificate chain
used for validation of the end-entity certificate. This enables the
backend origin server to utilize the client certificate information
in its application logic. While there may be additional proxies or
hops between the TTRP and the origin server (potentially even with
mutually authenticated TLS connections between them), the scope of
the "Client-Cert" header field is intentionally limited to exposing
to the origin server the certificate that was presented by the
originating client in its connection to the TTRP.
1.1. Requirements Notation and Conventions
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.
1.2. Terminology and Applicability
This document uses the following terminology from Section 3 of
[STRUCTURED-FIELDS] to specify syntax and parsing: List and Byte
Sequence.
Phrases like TLS client certificate authentication or mutually
authenticated TLS are used throughout this document to refer to the
process whereby, in addition to the normal TLS server authentication
with a certificate, a client presents its X.509 certificate [RFC5280]
and proves possession of the corresponding private key to a server
when negotiating a TLS connection or the resumption of such a
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connection. In contemporary versions of TLS [TLS] [TLS1.2] this
requires that the client send the Certificate and CertificateVerify
messages during the handshake and for the server to verify the
CertificateVerify and Finished messages.
HTTP/2 restricts TLS 1.2 renegotiation (Section 9.2.1 of [RFC9113])
and prohibits TLS 1.3 post-handshake authentication (Section 9.2.3 of
[RFC9113]). However, they are sometimes used to implement reactive
client certificate authentication in HTTP/1.1 [RFC9112] where the
server decides whether to request a client certificate based on the
HTTP request. HTTP application data sent on such a connection after
receipt and verification of the client certificate is also mutually
authenticated and thus suitable for the mechanisms described in this
document. With post-handshake authentication there is also the
possibility, though unlikely in practice, of multiple certificates
and certificate chains from the client on a connection, in which case
only the certificate and chain of the last post-handshake
authentication are to be utilized for the header fields described
herein.
2. HTTP Header Fields and Processing Rules
This document designates the following headers, defined further in
Section 2.2 and Section 2.3 respectively, to carry the client
certificate information of a mutually authenticated TLS connection.
The headers convey the information from the reverse proxy to the
origin server.
Client-Cert: The end-entity certificate used by the client in the
TLS handshake with the reverse proxy.
Client-Cert-Chain: The certificate chain used for validation of the
end-entity certificate provided by the client in the TLS handshake
with the reverse proxy.
2.1. Encoding
The headers in this document encode certificates as Byte Sequences
(Section 3.3.5 of [STRUCTURED-FIELDS]) where the value of the binary
data is a DER encoded [ITU.X690.1994] X.509 certificate [RFC5280].
In effect, this means that the binary DER certificate is encoded
using base64 (without line breaks, spaces, or other characters
outside the base64 alphabet) and delimited with colons on either
side.
Note that certificates are often stored encoded in a textual format,
such as the one described in Section 5.1 of [RFC7468], which is
already nearly compatible with a Byte Sequence; if so, it will be
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sufficient to replace "---(BEGIN|END) CERTIFICATE---" with ":" and
remove line breaks in order to generate an appropriate item.
2.2. Client-Cert HTTP Header Field
In the context of a TLS terminating reverse proxy deployment, the
proxy makes the TLS client certificate available to the backend
application with the Client-Cert HTTP header field. This field
contains the end-entity certificate used by the client in the TLS
handshake.
Client-Cert is a Byte Sequence with the value of the header encoded
as described in Section 2.1.
The "Client-Cert" header field is only for use in HTTP requests and
MUST NOT be used in HTTP responses. It is a singleton header field
value as defined in Section 5.5 of [HTTP], which MUST NOT have a list
of values or occur multiple times in a request.
Figure 2 in Appendix A has an example of the "Client-Cert" header
field.
2.3. Client-Cert-Chain HTTP Header Field
In the context of a TLS terminating reverse proxy deployment, the
proxy MAY make the certificate chain available to the backend
application with the Client-Cert-Chain HTTP header field.
Client-Cert-Chain is a List (Section 3.3.1 of [STRUCTURED-FIELDS]).
Each item in the list MUST be a Byte Sequence encoded as described in
Section 2.1. The order is the same as the ordering in TLS (such as
described in Section 4.4.2 of [TLS]).
Client-Cert-Chain MUST NOT appear unless Client-Cert is also present,
and it does not itself include the end-entity certificate that is
already present in Client-Cert. The root certificate MAY be omitted
from Client-Cert-Chain, provided that the target origin server is
known to possess the omitted trust anchor.
The "Client-Cert-Chain" header field is only for use in HTTP requests
and MUST NOT be used in HTTP responses. It MAY have a list of values
or occur multiple times in a request. For header compression
purposes, it might be advantageous to split lists into multiple
instances.
Figure 3 in Appendix A has an example of the "Client-Cert-Chain"
header field.
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2.4. Processing Rules
This section outlines the applicable processing rules for a TLS
terminating reverse proxy (TTRP) that has negotiated a mutually
authenticated TLS connection to convey the client certificate from
that connection to the backend origin servers. Use of the technique
is to be a configuration or deployment option and the processing
rules described herein are for servers operating with that option
enabled.
A TTRP negotiates the use of a mutually authenticated TLS connection
with the client, such as is described in [TLS] or [TLS1.2], and
validates the client certificate per its policy and trusted
certificate authorities. Each HTTP request on the underlying TLS
connection is dispatched to the origin server with the following
modifications:
1. The client certificate is placed in the "Client-Cert" header
field of the dispatched request, as described in Section 2.2.
2. If so configured, the validation chain of the client certificate
is placed in the "Client-Cert-Chain" header field of the request,
as described in Section 2.3.
3. Any occurrence of the "Client-Cert" or "Client-Cert-Chain" header
fields in the original incoming request MUST be removed or
overwritten before forwarding the request. An incoming request
that has a "Client-Cert" or "Client-Cert-Chain" header field MAY
be rejected with an HTTP 400 response.
Requests made over a TLS connection where the use of client
certificate authentication was not negotiated MUST be sanitized by
removing any and all occurrences of the "Client-Cert" and "Client-
Cert-Chain" header fields prior to dispatching the request to the
backend server.
Backend origin servers may then use the "Client-Cert" header field of
the request to determine if the connection from the client to the
TTRP was mutually authenticated and, if so, the certificate thereby
presented by the client. Access control decisions based on the
client certificate (or lack thereof) can be conveyed by selecting
response content as appropriate or with an HTTP 403 response, if the
certificate is deemed unacceptable for the given context. Note that
TLS clients that rely on error indications at the TLS layer for an
unacceptable certificate will not receive those signals.
When the value of the "Client-Cert" request header field is used to
select a response (e.g., the response content is access-controlled),
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the response MUST either be uncacheable (e.g., by sending "Cache-
Control: no-store") or be designated for selective reuse only for
subsequent requests with the same "Client-Cert" header value by
sending a "Vary: Client-Cert" response header. If a TTRP encounters
a response with a "client-cert" field name in the "Vary" header
field, it SHOULD prevent the user agent from caching the response by
transforming the value of the "Vary" response header field to "*".
Forward proxies and other intermediaries MUST NOT add the "Client-
Cert" or "Client-Cert-Chain" header fields to requests, or modify an
existing "Client-Cert" or "Client-Cert-Chain" header field.
Similarly, clients MUST NOT employ the "Client-Cert" or "Client-Cert-
Chain" header field in requests.
3. Deployment Considerations
3.1. Header Field Compression
If the connection between the TTRP and origin is capable of field
compression (e.g., HPACK [HPACK] or QPACK [QPACK]), and the TTRP
multiplexes more than one client's requests into that connection, the
size and variation of "Client-Cert" and "Client-Cert-Chain" field
values can reduce compression efficiency significantly. An origin
could mitigate the efficiency loss by increasing the size of the
dynamic table. If the TTRP determines that the origin dynamic table
is not sufficiently large, it may find it beneficial to always send
the field value as a literal, rather than entering it into the table.
3.2. Message Header Size
A server in receipt of a larger message header than it is willing to
handle can send an HTTP 431 (Request Header Fields Too Large) status
code per Section 5 of [RFC6585]. Due to the typical size of the
field values containing certificate data, recipients may need to be
configured to allow for a larger maximum header size. An
intermediary generating client certificate header fields on
connections that allow for advertising the maximum acceptable header
size (e.g., HTTP/2 [RFC9113] or HTTP/3 [RFC9114]) should account for
the additional size of the header of the requests it sends vs.
requests it receives by advertising a value to its clients that is
sufficiently smaller so as to allow for the addition of certificate
data.
3.3. TLS Session Resumption
Some TLS implementations do not retain client certificate information
when resuming. Providing inconsistent values of Client-Cert and
Client-Cert-Chain when resuming might lead to errors, so
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implementations that are unable to provide these values SHOULD either
disable resumption for connections with client certificates or
initially omit a "Client-Cert" or "Client-Cert-Chain" field if it
might not be available after resuming.
4. Security Considerations
The header fields described herein enable a TTRP and backend or
origin server to function together as though, from the client's
perspective, they are a single logical server-side deployment of
HTTPS over a mutually authenticated TLS connection. Use of the
header fields outside that intended use case, however, may undermine
the protections afforded by TLS client certificate authentication.
Therefore, steps such as those described below need to be taken to
prevent unintended use, both in sending the header field and in
relying on its value.
Producing and consuming the "Client-Cert" and "Client-Cert-Chain"
header fields SHOULD be configurable options, respectively, in a TTRP
and backend server (or individual application in that server). The
default configuration for both should be to not use the header
fields, thus requiring an "opt-in" to the functionality.
In order to prevent field injection, backend servers MUST only accept
the "Client-Cert" and "Client-Cert-Chain" header fields from a
trusted TTRP (or other proxy in a trusted path from the TTRP). A
TTRP MUST sanitize the incoming request before forwarding it on by
removing or overwriting any existing instances of the fields.
Otherwise, arbitrary clients can control the field values as seen and
used by the backend server. It is important to note that neglecting
to prevent field injection does not "fail safe" in that the nominal
functionality will still work as expected even when malicious actions
are possible. As such, extra care is recommended in ensuring that
proper field sanitation is in place.
The communication between a TTRP and backend server needs to be
secured against eavesdropping and modification by unintended parties.
The configuration options and request sanitization are necessary
functionality of the respective servers. The other requirements can
be met in a number of ways, which will vary based on specific
deployments. The communication between a TTRP and backend or origin
server, for example, might be authenticated in some way with the
insertion and consumption of the "Client-Cert" and "Client-Cert-
Chain" header fields occurring only on that connection. Appendix B.3
of [HTTPSIG] gives one example of this with an application of HTTP
Message Signatures. Alternatively, the network topology might
dictate a private network such that the backend application is only
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able to accept requests from the TTRP and the proxy can only make
requests to that server. Other deployments that meet the
requirements set forth herein are also possible.
5. IANA Considerations
5.1. HTTP Field Name Registrations
Please register the following entries in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" defined by HTTP Semantics
[HTTP]:
o Field name: Client-Cert
o Status: permanent
o Specification document: Section 2 of [this document]
o Field name: Client-Cert-Chain
o Status: permanent
o Specification document: Section 2 of [this document]
6. References
6.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
.
[ITU.X690.1994]
International Telecommunications Union, "Information
Technology - ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
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[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,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
.
6.2. Informative References
[HPACK] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
.
[HTTPSIG] Backman, A., Richer, J., and M. Sporny, "HTTP Message
Signatures", draft-ietf-httpbis-message-signatures-16
(work in progress), February 2023.
[QPACK] Krasic, C., Bishop, M., and A. Frindell, Ed., "QPACK:
Field Compression for HTTP/3", RFC 9204,
DOI 10.17487/RFC9204, June 2022,
.
[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
.
[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, .
[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", RFC 8705,
DOI 10.17487/RFC8705, February 2020,
.
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[RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, .
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
.
[RFC9114] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, .
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
.
[TLS1.2] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
.
6.3. URIs
[1] https://datatracker.ietf.org/meeting/106/materials/slides-106-
secdispatch-securing-protocols-between-proxies-and-backend-http-
servers-00
[2] https://datatracker.ietf.org/meeting/106/materials/minutes-
106-secdispatch
Appendix A. Example
In a hypothetical example where a TLS client presents the client and
intermediate certificate from Figure 1 when establishing a mutually
authenticated TLS connection with the TTRP, the proxy would send the
"Client-Cert" field shown in Figure 2 to the backend. Note that line
breaks and extra spaces have been added to the field value in
Figure 2 and Figure 3 for display and formatting purposes only.
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-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
Figure 1: Certificate Chain (with client certificate first)
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Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKDBJ
MZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTAeFw0
yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFkwEwYHKoZ
Izj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmckC8vdgJ1p5Be
5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDVR0TBAIwADAfBgN
VHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf8EBAMCBsAwEwYDVR0
lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV4YW1wbGUuY29tMAoGCCq
GSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6bMjeSkC3dFCOOB8TAiEAx/k
HSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
Figure 2: Header Field in HTTP Request to Origin Server
If the proxy were configured to also include the certificate chain,
it would also include the "Client-Cert-Chain" header field. Note
that while the following example does illustrate the TTRP inserting
the root certificate, many deployments will opt to omit the trust
anchor.
Client-Cert-Chain: :MIIB5jCCAYugAwIBAgIBFjAKBggqhkjOPQQDAjBWMQsw
CQYDVQQGEwJVUzEbMBkGA1UECgwSTGV0J3MgQXV0aGVudGljYXRlMSowKAYDVQQ
DDCFMZXQncyBBdXRoZW50aWNhdGUgUm9vdCBBdXRob3JpdHkwHhcNMjAwMTE0Mj
EzMjMwWhcNMzAwMTExMjEzMjMwWjA6MRswGQYDVQQKDBJMZXQncyBBdXRoZW50a
WNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTBZMBMGByqGSM49AgEG
CCqGSM49AwEHA0IABJf+aA54RC5pyLAR5yfXVYmNpgd+CGUTDp2KOGhc0gK91zx
hHesEYkdXkpS2UN8Kati+yHtWCV3kkhCngGyv7RqjZjBkMB0GA1UdDgQWBBRm3W
jLa38lbEYCuiCPct0ZaSED2DAfBgNVHSMEGDAWgBTEA2Q6eecKu9g9yb5glbkhh
VINGDASBgNVHRMBAf8ECDAGAQH/AgEAMA4GA1UdDwEB/wQEAwIBhjAKBggqhkjO
PQQDAgNJADBGAiEA5pLvaFwRRkxomIAtDIwg9D7gC1xzxBl4r28EzmSO1pcCIQC
JUShpSXO9HDIQMUgH69fNDEMHXD3RRX5gP7kuu2KGMg==:, :MIICBjCCAaygAw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:
Figure 3: Certificate Chain in HTTP Request to Origin Server
Appendix B. Select Design Considerations
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Internet-Draft Client-Cert Header February 2023
B.1. Field Injection
This document requires that the TTRP sanitize the fields of the
incoming request by removing or overwriting any existing instances of
the "Client-Cert" and "Client-Cert-Chain" header fields before
dispatching that request to the backend application. Otherwise, a
client could inject its own values that would appear to the backend
to have come from the TTRP. Although numerous other methods of
detecting/preventing field injection are possible, such as the use of
a unique secret value as part of the field name or value or the
application of a signature, HMAC, or AEAD, there is no common general
mechanism. The potential problem of client field injection is not at
all unique to the functionality of this document, and it would
therefore be inappropriate for this document to define a one-off
solution. In the absence of a generic common solution existing
currently, stripping/sanitizing the fields is the de facto means of
protecting against field injection in practice. Sanitizing the
fields is sufficient when properly implemented and is a normative
requirement of Section 4.
B.2. The Forwarded HTTP Extension
The "Forwarded" HTTP header field defined in [RFC7239] allows proxy
components to disclose information lost in the proxying process. The
TLS client certificate information of concern to this document could
have been communicated with an extension parameter to the "Forwarded"
field; however, doing so would have had some disadvantages that this
document endeavored to avoid. The "Forwarded" field syntax allows
for information about a full chain of proxied HTTP requests, whereas
the "Client-Cert" and "Client-Cert-Chain" header fields of this
document are concerned only with conveying information about the
certificate presented by the originating client on the TLS connection
to the TTRP (which appears as the server from that client's
perspective) to backend applications. The multi-hop syntax of the
"Forwarded" field is expressive but also more complicated, which
would make processing it more cumbersome, and more importantly, make
properly sanitizing its content as required by Section 4 to prevent
field injection considerably more difficult and error-prone. Thus,
this document opted for a flatter and more straightforward structure.
B.3. The Whole Certificate and Certificate Chain
Different applications will have varying requirements about what
information from the client certificate is needed, such as the
subject and/or issuer distinguished name, subject alternative
name(s), serial number, subject public key info, fingerprint, etc.
Furthermore, some applications, such as [RFC8705], make use of the
entire certificate. In order to accommodate the latter and ensure
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Internet-Draft Client-Cert Header February 2023
wide applicability by not trying to cherry-pick particular
certificate information, this document opted to pass the full,
encoded certificate as the value of the "Client-Cert" field.
The validation of the client certificate and chain of the mutually
authenticated TLS connection is typically performed by the TTRP
during the handshake. With the responsibility of certificate
validation falling on the TTRP, the end-entity certificate is
oftentimes sufficient for the needs of the origin server. The
separate "Client-Cert-Chain" field can convey the certificate chain
for origin server deployments that require this additional
information.
Appendix C. Acknowledgements
The authors would like to thank the following individuals who've
contributed in various ways ranging from just being generally
supportive of bringing forth the document to providing specific
feedback or content:
o Evan Anderson
o Annabelle Backman
o Alan Frindell
o Rory Hewitt
o Fredrik Jeansson
o Benjamin Kaduk
o Torsten Lodderstedt
o Kathleen Moriarty
o Mark Nottingham
o Erik Nygren
o Mike Ounsworth
o Lucas Pardue
o Matt Peterson
o Eric Rescorla
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Internet-Draft Client-Cert Header February 2023
o Justin Richer
o Michael Richardson
o Joe Salowey
o Rich Salz
o Mohit Sethi
o Rifaat Shekh-Yusef
o Travis Spencer
o Nick Sullivan
o Willy Tarreau
o Martin Thomson
o Peter Wu
o Hans Zandbelt
Appendix D. Document History
To be removed by the RFC Editor before publication as an RFC
draft-ietf-httpbis-client-cert-field-05
o Correct a couple references
o Updates from Genart Last Call review
o Incorporate AD review feedback
o Editorial updates
draft-ietf-httpbis-client-cert-field-04
o Updates, fixes, and clarifications from WGLC feedback
draft-ietf-httpbis-client-cert-field-03
o State that the certificate chain is in the same order as it
appears in TLS rather than copying the language from TLS
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Internet-Draft Client-Cert Header February 2023
o Update references for HTTP Semantics, HTTP/3, and QPACK to point
to the now RFCs 9110/9114/9204
o HTTP Semantics now a normative ref
o Mention that origin server access control decisions can be
conveyed by selecting response content or with a 403
draft-ietf-httpbis-client-cert-field-02
o Add a note about cert retention on TLS session resumption
o Say to use only the last one in the case of multiple post-
handshake client cert authentications
draft-ietf-httpbis-client-cert-field-01
o Use RFC 8941 Structured Field Values for HTTP
o Introduce a separate header that can convey the certificate chain
o Add considerations on header compression and size
o Describe interaction with caching
o Fill out IANA Considerations with HTTP field name registrations
o Discuss renegotiation
draft-ietf-httpbis-client-cert-field-00
o Initial WG revision
o Mike Bishop added as co-editor
draft-bdc-something-something-certificate-05
o Change intended status of the draft to Informational
o Editorial updates and (hopefully) clarifications
draft-bdc-something-something-certificate-04
o Update reference from draft-ietf-oauth-mtls to RFC8705
draft-bdc-something-something-certificate-03
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Internet-Draft Client-Cert Header February 2023
o Expanded further discussion notes to capture some of the feedback
in and around the presentation of the draft in SECDISPATCH at IETF
107 and add those who've provided such feedback to the
acknowledgements
draft-bdc-something-something-certificate-02
o Editorial tweaks + further discussion notes
draft-bdc-something-something-certificate-01
o Use the RFC v3 Format or die trying
draft-bdc-something-something-certificate-00
o Initial draft after a time constrained and rushed secdispatch
presentation [1] at IETF 106 in Singapore with the recommendation
to write up a draft (at the end of the minutes [2]) and some folks
expressing interest despite the rather poor presentation
Authors' Addresses
Brian Campbell
Ping Identity
Email: bcampbell@pingidentity.com
Mike Bishop (editor)
Akamai
Email: mbishop@evequefou.be
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