Internet DRAFT - draft-ietf-tls-tlsflags
draft-ietf-tls-tlsflags
TLS Y. Nir
Internet-Draft Dell Technologies
Intended status: Standards Track 23 July 2023
Expires: 24 January 2024
A Flags Extension for TLS 1.3
draft-ietf-tls-tlsflags-12
Abstract
A number of extensions are proposed in the TLS working group that
carry no interesting information except the 1-bit indication that a
certain optional feature is supported. Such extensions take 4 octets
each. This document defines a flags extension that can provide such
indications at an average marginal cost of 1 bit each. More
precisely, it provides as many flag extensions as needed at 4 + the
order of the last set bit divided by 8.
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
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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 24 January 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements and Other Notation . . . . . . . . . . . . . 3
2. The tls_flags Extension . . . . . . . . . . . . . . . . . . . 3
3. Rules for The Flags Extension . . . . . . . . . . . . . . . . 4
3.1. Interaction with the 0-RTT Handshake . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Since the publication of TLS 1.3 ([RFC8446]) there have been several
proposals for extensions to this protocol, where the presence of the
content-free extension in both the ClientHello and either the
ServerHello or EncryptedExtensions indicates nothing except either
support for the optional feature or an intent to use the optional
feature. Examples:
* An extension that allows the server to tell the client that cross-
SNI resumption is allowed: [I-D.sy-tls-resumption-group].
* An extension that is used to negotiate support for authentication
using both certificates and external PSKs:
[I-D.ietf-tls-tls13-cert-with-extern-psk].
* The post_handshake_auth extension from the TLS 1.3 base document
indicates that the client is willing to perform post-handshake
authentication.
This document proposes a single extension called tls_flags that can
enumerate such flag extensions and allowing both client and server to
indicate support for optional features in a concise way.
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None of the current proposed extensions allow for indication of
support in ServerHello (SH), EncryptedExtensions (EE), Certificate
(CT), or HelloRetryRequest (HRR) without first being indicated in
ClientHello (CH). Similarly, none of the current proposed extensions
allow for an indication of support in the client-side Certificate
(CT) message without first being indicated in the server's
CertificateRequest (CR) message. This restriction is enforced by the
rules in Section 3.
1.1. Requirements and Other Notation
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 BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
The term "flag extension" is used to denote an extension where the
extension_data field is always zero-length in a particular context,
and the presence of the extension denotes either support for some
feature or the intent to use that feature.
The term "flag-type feature" denotes an options TLS 1.3 feature the
support for which is negotiated using a flag extension, whether that
flag extension is its own extension or a value in the extension
defined in this document.
2. The tls_flags Extension
This document defines the following extension code point:
enum {
...
tls_flags(TBD),
(65535)
} ExtensionType;
This document also defines the data for this extension as a variable-
length bit string, allowing for the encoding of up to 2040 features.
struct {
opaque flags<1..255>;
} FlagExtensions;
The FlagExtensions field contains 8 flags in each octet. The length
of the extension is the minimal length that allows it to encode all
of the present flags. Within each octet, the bits are packed such
that the first bit is the least significant bit and the eighth bit is
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the most significant. Using zero-based indexing, the first octet
holds flags 0-7, the second octet holds bits 8-15 and so on. For
example, if we want to encode only flag number zero, the
FlagExtension field will be 1 octet long, that is encoded as follows:
00000001
If we want to encode flags 1 and 5, the field will still be 1 octet
long:
00100010
If we want to encode flags 3, 5, and 23, the field will have to be 3
octets long:
00101000 00000000 10000000
An implementation that receives an all-zero value for this extension
or a value that contains trailing zero bytes MUST generate a fatal
illegal_parameter alert.
Note that this document does not define any particular bits for this
string. That is left to the protocol documents such as the ones in
the examples from the previous section. Such documents will have to
define which bit to set to show support, and the order of the bits
within the bit string shall be enumerated in network order: bit zero
is the high-order bit of the first octet as the flags field is
transmitted.
3. Rules for The Flags Extension
Any TLS implementation that intends to propose or indicate support
for a flag extension SHALL send this extension with the relevant bits
set. It MUST NOT send this extension empty -- with a length of zero.
This specification does not require every flag extension to be
acknowledged. Acknowledging a flag extension is typically needed to
inform the peer proposing the extension that the other side
understands and supports the extension, but some extensions do not
require this acknowledgement.
For a flag that does require a response, the only proper response is
the same flag in a flags extension. This extension MUST NOT be used
to specify extensions where the response is a proper extension with
content.
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A flag proposed by the client in ClientHello (CH) that requires
acknowledgement SHOULD be acknowledged in either ServerHello (SH), in
EncryptedExtensions (EE), in Certificate (CT), or in
HelloRetryRequest (HRR) as the corresponding flag document specifies.
Similarly, a flag proposed by the server in the CertificateRequest
(CR) message that requires acknowledgement SHOULD be acknowledged in
the client's Certificate (CT) message. A flag proposed by the server
in the NewSessionTicket (NST) message is never acknowledged as there
is not client-side response message.
Multiple flags can be proposed or acknowledged in the same extension.
In all of the above cases, a flag MUST NOT be acknowledged in SH, EE,
CT, or HRR without first having been proposed in CH or CR.
Unsolicited flags may appear only in CH, CR, and NST. And endpoint
that receives an unsolicited flag in another message (HRR, SH, EE, or
CT) MUST generate a fatal illegal_parameter alert.
A client that supports this extension and at least one flag extension
SHALL send this extension with the flags field having bits set only
for those extensions that it intends to set. It MUST NOT send this
extension with a length of zero.
An implementation that receives an invalid tls_flags extension MUST
terminate the TLS handshake with a fatal illegal_parameter alert.
3.1. Interaction with the 0-RTT Handshake
The 0-RTT handshake, defined in section 2.3 of [RFC8446], has a
ClientHello message, a ServerHello message, and an
EncryptedExtensions message. Those can include the tls_flags
extension just as they can in a regular handshake.
Future flag extensions MUST define their interaction with 0-RTT, just
as other extensions are required to.
4. IANA Considerations
IANA is requested to assign a new value from the TLS ExtensionType
Values registry:
* The Extension Name should be tls_flags
* The TLS 1.3 value should be CH,SH,HRR,EE,CR,CT,NST
* The DTLS-Only value should be N
* The Recommended value should be Y
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* The Reference should be this document
IANA is also requested to create a new registry under the TLS
namespace with name "TLS Flags" and the following fields:
* Value, which is a number between 0 and 2039. All potential values
are available for assignment.
* Flag Name, which is a string
* Message, which like the "TLS 1.3" field in the ExtensionType
registry contains the abbreviations of the messages that may
contain the flag: CH, SH, EE, etc.
* Recommended, which is a Y/N value determined in the document
defining the optional feature.
* Reference, which is a link to the document defining this flag.
The policy for this shall be "Specification Required" as described in
Section 4.6 of [RFC8126] with the exception of flags numbered from
0-15, which follow the "Standards Action" policy (Section 4.9 of
[RFC8126]). Designated expert(s) are advised to follow the advice in
Section 17 of [RFC8447] when reviewing registration requests.
The initial contents of the registry shall be one entry, as follows:
* Value shall be 8
* Flag Name shall be resumption_across_names
* Message shall be NST
* Recommended shall be set to no (N)
* The reference shall the the RFC-to-be
[I-D.ietf-tls-cross-sni-resumption].
5. Security Considerations
The extension described in this document provides a more concise way
to express data that could otherwise be expressed in individual
extensions. It does not send in the clear any information that would
otherwise be sent encrypted, nor vice versa. For this reason this
extension is neutral as far as security is concerned.
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Extension authors should be aware that acknowledging flags in a
tls_flags extension of the ServerHello and HelloRetryRequest messages
expose this response to passive observers. Unless there is a special
reason to place the response in the ServerHello, most flags should go
in other (encrypted) messages.
6. Acknowledgements
The idea for writing this was expressed at the mic during the TLS
session at IETF 104 by Eric Rescorla.
The current bitwise formatting was suggested on the mailing list by
Nikos Mavrogiannopoulos.
Improvement to the encoding were suggested by Ilari Liusvaara, who
also asked for a better explanation of the semantics of missing
extensions.
Useful comments received from Martin Thomson, including the
suggestion to eliminate the option to have the server send
unsolicited flag types and the rules for where unsolicited flags can
appear.
7. References
7.1. Normative References
[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>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
7.2. Informative References
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[I-D.ietf-tls-cross-sni-resumption]
Vasiliev, V., "Transport Layer Security (TLS) Resumption
across Server Names", Work in Progress, Internet-Draft,
draft-ietf-tls-cross-sni-resumption-02, 5 December 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
cross-sni-resumption-02>.
[I-D.ietf-tls-tls13-cert-with-extern-psk]
Housley, R., "TLS 1.3 Extension for Certificate-Based
Authentication with an External Pre-Shared Key", Work in
Progress, Internet-Draft, draft-ietf-tls-tls13-cert-with-
extern-psk-07, 23 December 2019,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
tls13-cert-with-extern-psk-07>.
[I-D.sy-tls-resumption-group]
Sy, E., "TLS Resumption across Server Name Indications for
TLS 1.3", Work in Progress, Internet-Draft, draft-sy-tls-
resumption-group-00, 1 March 2019,
<https://datatracker.ietf.org/doc/html/draft-sy-tls-
resumption-group-00>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Change Log
RFC EDITOR: PLEASE REMOVE THIS SECTION AS IT IS ONLY MEANT TO AID THE
WORKING GROUP IN TRACKING CHANGES TO THIS DOCUMENT.
draft-ietf-tls-tlsflags-02 set the maximum number of flags to 2048,
and added guidance for the IANA experts.
draft-ietf-tls-tlsflags-01 allows server-only flags and allows the
client to send an empty extension. Also modified the packing order
of the bits.
draft-ietf-tls-tlsflags-00 had the same text as draft-nir-tls-
tlsflags-02, and was re-submitted as a working group document
following the adoption call.
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Version -02 replaced the fixed 64-bit string with an unlimited
bitstring, where only the necessary octets are encoded.
Version -01 replaced the enumeration of 8-bit values with a 64-bit
bitstring.
Version -00 was a quickly-thrown-together draft with the list of
supported features encoded as an array of 8-bit values.
Author's Address
Yoav Nir
Dell Technologies
9 Andrei Sakharov St
Haifa 3190500
Israel
Email: ynir.ietf@gmail.com
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