TLS supplemental data: TLS Handshake Message for Supplemental Data
A TLS handshake message that carries extra application data, such as authorization hints, alongside the normal handshake.
Santesson, S., "TLS Handshake Message for Supplemental Data", RFC 4680, DOI 10.17487/RFC4680, October 2006, <https://www.rfc-editor.org/info/rfc4680>.
@misc{rfc4680,
series = {Request for Comments},
number = 4680,
howpublished = {RFC 4680},
publisher = {RFC Editor},
doi = {10.17487/RFC4680},
url = {https://www.rfc-editor.org/info/rfc4680},
author = {S. Santesson},
title = {{TLS Handshake Message for Supplemental Data}},
pagetotal = 9,
year = 2006,
month = oct,
} RFC text
Contents
Network Working Group S. Santesson
Request for Comments: 4680 Microsoft
Updates: 4346 September 2006
Category: Standards Track
TLS Handshake Message for Supplemental Data
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification defines a TLS handshake message for exchange of
supplemental application data. TLS hello message extensions are used
to determine which supplemental data types are supported by both the
TLS client and the TLS server. Then, the supplemental data handshake
message is used to exchange the data. Other documents will define
the syntax of these extensions and the syntax of the associated
supplemental data types.
1. Introduction
Recent standards activities have proposed different mechanisms for
transmitting supplemental application data in the TLS handshake
message. For example, recent proposals transfer data that is not
processed by the TLS protocol itself, but assist the TLS-protected
application in the authentication and authorization decisions. One
proposal transfers user name hints for locating credentials, and
another proposal transfers attribute certificates and Security
Assertions Markup Language (SAML) assertions for authorization
checks.
In order to avoid definition of multiple handshake messages, one for
each new type of application-specific supplemental data, this
specification defines a new handshake message type that bundles
together all data objects that are to be delivered to the TLS-
protected application and sends them in a single handshake message.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [N1].
The syntax for the supplemental_data handshake message is defined
using the TLS Presentation Language, which is specified in Section 4
of [N2].
2. Supplemental Data Handshake Message
The new supplemental_data handshake message type is defined to
accommodate communication of supplemental data objects as agreed
during the exchange of extensions in the client and server hello
messages. See RFC 2246 (TLS 1.0) [N2] and RFC 4346 (TLS 1.1) [N3]
for other handshake message types.
Information provided in a supplemental data object MUST be intended
to be used exclusively by applications and protocols above the TLS
protocol layer. Any such data MUST NOT need to be processed by the
TLS protocol.
enum {
supplemental_data(23), (255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* octets in message */
select (HandshakeType) {
case supplemental_data: SupplementalData;
} body;
} Handshake;
struct {
SupplementalDataEntry supp_data<1..2^24-1>;
} SupplementalData;
struct {
SupplementalDataType supp_data_type;
uint16 supp_data_length;
select(SupplementalDataType) { }
} SupplementalDataEntry;
enum {
(65535)
} SupplementalDataType;
supp_data_length
This field is the length (in bytes) of the data selected by
SupplementalDataType.
The client MUST NOT send more than one SupplementalData handshake
message, and the server MUST NOT send more than one SupplementalData
handshake message. Receiving more than one SupplementalData
handshake message results in a fatal error, and the receiver MUST
close the connection with a fatal unexpected_message alert.
If present, the SupplementalData handshake message MUST contain a
non-empty SupplementalDataEntry structure carrying data associated
with at least one defined SupplementalDataType. An explicit
agreement that governs presence of any supplemental data MUST be
concluded between client and server for each SupplementalDataType
using the TLS extensions [N4] in the client and server hello
messages. Receiving an unexpected SupplementalData handshake message
results in a fatal error, and the receiver MUST close the connection
with a fatal unexpected_message alert.
Other documents will define specific SupplementalDataTypes and their
associated data syntax and processing. These same specifications
must also specify the client and server hello message extensions that
are used to negotiate the support for the specified supplemental data
type. This document simply specifies the TLS Handshake Protocol
message that will carry the supplemental data objects.
Different situations require the transfer of supplemental data from
the client to the server, require the transfer of supplemental data
from the server to the client, or both ways. All three situations
are fully supported.
3. Message Flow
The SupplementalData handshake message, if exchanged, MUST be sent as
the first handshake message as illustrated in Figure 1 below.
Client Server
ClientHello (with extensions) -------->
ServerHello(with extensions)
SupplementalData*
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
SupplementalData*
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
* Indicates optional or situation-dependent messages.
Figure 1. Message Flow with SupplementalData
4. Security Considerations
Each SupplementalDataType included in the handshake message defined
in this specification introduces its own unique set of security
properties and related considerations. Security considerations must
therefore be defined in each document that defines a supplemental
data type.
In some cases, the SupplementalData information may be sensitive.
The double handshake technique can be used to provide protection for
the SupplementalData information. Figure 2 illustrates the double
handshake, where the initial handshake does not include any
extensions, but it does result in protected communications. Then, a
second handshake that includes the SupplementalData information is
performed using the protected communications. In Figure 2, the
number on the right side indicates the amount of protection for the
TLS message on that line. A zero (0) indicates that there is no
communication protection; a one (1) indicates that protection is
provided by the first TLS session; and a two (2) indicates that
protection is provided by both TLS sessions.
The placement of the SupplementalData message in the TLS Handshake
results in the server providing its SupplementalData information
before the client is authenticated. In many situations, servers will
not want to provide authorization information until the client is
authenticated. The double handshake illustrated in Figure 2 provides
a technique to ensure that the parties are mutually authenticated
before either party provides SupplementalData information.
Client Server
ClientHello (no extensions) --------> |0
ServerHello (no extensions) |0
Certificate* |0
ServerKeyExchange* |0
CertificateRequest* |0
<-------- ServerHelloDone |0
Certificate* |0
ClientKeyExchange |0
CertificateVerify* |0
[ChangeCipherSpec] |0
Finished --------> |1
[ChangeCipherSpec] |0
<-------- Finished |1
ClientHello (w/ extensions) --------> |1
ServerHello (w/ extensions) |1
SupplementalData* |1
Certificate* |1
ServerKeyExchange* |1
CertificateRequest* |1
<-------- ServerHelloDone |1
SupplementalData* |1
Certificate* |1
ClientKeyExchange |1
CertificateVerify* |1
[ChangeCipherSpec] |1
Finished --------> |2
[ChangeCipherSpec] |1
<-------- Finished |2
Application Data <-------> Application Data |2
* Indicates optional or situation-dependent messages.
Figure 2. Double Handshake to Protect Supplemental Data
5. IANA Considerations
IANA has taken the following actions:
1) Created an entry, supplemental_data(23), in the existing registry
for HandshakeType (defined in RFC 2246 [N2]).
2) Established a registry for TLS Supplemental Data Formats
(SupplementalDataType). Values in the inclusive range 0-16385
(decimal) are assigned via RFC 2434 [N5] Standards Action. Values
from the inclusive range 16386-65279 (decimal) are assigned via
RFC 2434 IETF Consensus. Values from the inclusive range
65280-65535 (decimal) are reserved for RFC 2434 Private Use.
6. Normative References
[N1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[N2] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[N3] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[N4] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
T. Wright, "Transport Layer Security (TLS) Extensions", RFC
4366, April 2006.
[N5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
7. Acknowledgements
The fundamental architectural idea for the supplemental data
handshake message was provided by Russ Housley and Eric Rescorla.
Author's Address
Stefan Santesson
Microsoft
Finlandsgatan 30
164 93 KISTA
Sweden
EMail: stefans@microsoft.com
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AI ELI5
On this page
§1The Need for Supplemental Data
Recent standards activity had proposed ways of sending supplemental application data in the TLS handshake. This is data that TLS itself does not process, but that helps the TLS-protected application make authentication and authorization decisions. One proposal sends user name hints for locating credentials. Another sends attribute certificates and SAML assertions for authorization checks.
To avoid defining one handshake message per kind of data, this RFC defines a single new handshake message that bundles all data objects destined for the application. It is specified in the TLS Presentation Language of RFC 2246.
Imagine a handshake where you also want to hand over a note for the person you are meeting. Instead of inventing a new kind of note every time, there is one standard envelope that can hold any of them.
§2The supplemental_data Message
The new supplemental_data handshake message has type value 23. Its body is a SupplementalData structure holding a list of SupplementalDataEntry items (between 1 and 2^24-1 bytes). Each entry has a supp_data_type, a 16-bit supp_data_length giving the length in bytes of the data, and the data itself, whose format depends on the type.
The RFC defines no actual types: the SupplementalDataType enum is empty apart from its maximum, (65535). Other documents define specific types, their syntax and processing, and the hello extensions used to negotiate them.
Information in an entry MUST be intended exclusively for applications and protocols above TLS, and TLS MUST NOT need to process it. If the message is present it MUST contain a non-empty entry for at least one defined type.
The envelope has a label saying what kind of note is inside, how long it is, and then the note. This RFC makes the envelope but leaves the list of note types blank for others to fill in.
§2.1Agreeing in Advance
Supplemental data must never arrive as a surprise. For each SupplementalDataType, client and server MUST first reach an explicit agreement using TLS extensions in the client and server hello messages.
Each side may send at most one SupplementalData message. Receiving more than one, or receiving one that was not agreed, is a fatal error: the receiver MUST close the connection with a fatal unexpected_message alert. Data can flow client to server, server to client, or both ways.
Before anyone hands over a note, both sides say in their greetings what kinds of notes they can handle. A surprise note gets the call hung up.
§3Message Flow
If it is used, the SupplementalData message MUST be sent as the first handshake message after the hello messages, as shown in the RFC's Figure 1. The server sends it right after its ServerHello, before Certificate. The client sends it before its own Certificate. Both are marked as optional or situation-dependent.
The note goes first in line, right after the hellos, before the identity cards are shown.
- ClientHello (with extensions)
- ServerHello (with extensions)
- SupplementalData*First message after the hellos.
- Certificate*, ServerKeyExchange*, CertificateRequest*, ServerHelloDone
- SupplementalData*Sent before the client Certificate.
- Certificate*, ClientKeyExchange, CertificateVerify*, [ChangeCipherSpec], Finished
- [ChangeCipherSpec], Finished
§4Security Considerations
Each supplemental data type brings its own security properties, so the document defining the type must discuss them. This RFC adds one general concern: because of where the message sits, the server provides its supplemental data before the client is authenticated, and in many cases a server will not want to share authorization information with an unauthenticated client.
The RFC's answer is the double handshake. A first handshake without extensions sets up protected communication. A second handshake, carried inside that protection and including the extensions and SupplementalData, then runs after both parties are authenticated. The messages of the second handshake are protected by both TLS sessions.
If the note is private, first make a safe tunnel with a plain handshake, then do the real handshake with the note inside the tunnel, so nobody outside can read it and both sides already know who they are talking to.
- First handshake, no extensionsEnds with Finished messages; later traffic is protected by session one.
- ClientHello (with extensions)Second handshake, inside the first session.
- ServerHello (with extensions), SupplementalData*
- SupplementalData*
- FinishedApplication Data is then protected by both sessions.
§5IANA Registries
IANA added an entry supplemental_data(23) to the existing HandshakeType registry from RFC 2246. It also created a new registry for TLS Supplemental Data Formats.
In that registry, values 0 to 16385 are assigned by Standards Action, values 16386 to 65279 by IETF Consensus, and values 65280 to 65535 are reserved for Private Use, following RFC 2434.
A central list keeps track of the note types so two different groups do not pick the same number. Some number ranges are strict, and some are left for private use.