Internet DRAFT - draft-nir-tls-tlsflags
draft-nir-tls-tlsflags
TLS Y. Nir
Internet-Draft DellEMC
Intended status: Standards Track July 22, 2019
Expires: January 23, 2020
A Flags Extension for TLS 1.3
draft-nir-tls-tlsflags-02
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
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This Internet-Draft will expire on January 23, 2020.
Copyright Notice
Copyright (c) 2019 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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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements and Other Notation . . . . . . . . . . . . . 2
2. The tls_flags Extension . . . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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:
o An extension that allows the server to tell the client that cross-
SNI resumption is allowed: [I-D.sy-tls-resumption-group].
o 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].
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.
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,
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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 an unbounded number
of features.
struct {
uint8 flags<0..31>;
} FlagExtensions;
The FlagExtensions field 8 flags with each octet, and its length is
the minimal length that allows it to encode all of the present flags.
For example, if we want to encode only flag number zero, the
FlagExtension field will be 1 octet long, that is encoded as follows:
10000000
If we want to encode flags 1 and 5, the field will still be 1 octet
long:
01000100
If we want to encode flags 3, 5, and 23, the field will have to be 3
octets long:
00010100 00000000 00000001
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
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is the high-order bit of the first octet as the flags field is
transmitted.
A client that supports this extension SHALL send this extension with
the flags field having bits set only for those extensions that it
intends to set. If it does not wish to set any such flags in the
ClientHello message, then this extension MUST NOT be sent.
A server that supports this extension and also supports at least one
of the flag-type features that use this extension and that were
declared by the ClientHello extension SHALL send this extension with
the intersection of the flags it supports with the flags declared by
the client. The intersection operation MAY be implemented as a
bitwise AND. The server may need to send two tls_flags extensions,
one in the ServerHello and the other in the EncryptedExtensions
message. It is up to the document for the specific feature to
determine whether support should be acknowledged in the ServerHello
or the EncryptedExtensions message.
3. IANA Considerations
IANA is requested to assign a new value from the TLS ExtensionType
Values registry:
o The Extension Name should be tls_flags
o The TLS 1.3 value should be CH,SH,EE
o The Recommended value should be Y
o 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:
o Value, which is a number between 0 and 63. All potential values
are available for assignment.
o Flag Name, which is a string
o 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.
o Recommended, which is a Y/N value determined in the document
defining the optional feature.
o Reference, which is a link to the document defining this flag.
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The policy for this shall be "Specification Required" as described in
[RFC8126].
4. 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.
5. 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.
6. References
6.1. 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>.
6.2. Informative References
[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", draft-
ietf-tls-tls13-cert-with-extern-psk-02 (work in progress),
May 2019.
[I-D.sy-tls-resumption-group]
Sy, E., "TLS Resumption across Server Name Indications for
TLS 1.3", draft-sy-tls-resumption-group-00 (work in
progress), March 2019.
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[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
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
DellEMC
9 Andrei Sakharov St
Haifa 3190500
Israel
Email: ynir.ietf@gmail.com
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