TCPM M. Piraux
Internet-Draft O. Bonaventure
Intended status: Experimental T. Wirtgen
Expires: 25 April 2024 UCLouvain & WELRI
23 October 2023
Opportunistic TCP-AO with TLS
draft-piraux-tcp-ao-tls-00
Abstract
This document specifies an opportunistic mode for TCP-AO. In this
mode, the TCP connection starts with a well-known authentication key
which is later replaced by a secure key derived from a TLS handshake.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://obonaventure.github.io/draft-tcp-ao-tls/draft-piraux-tcp-ao-
tls.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-piraux-tcp-ao-tls/.
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This Internet-Draft will expire on 25 April 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
2.1. Notational conventions . . . . . . . . . . . . . . . . . 3
3. An overview of Opportunistic TCP-AO . . . . . . . . . . . . . 3
4. Opportunistic TCP-AO . . . . . . . . . . . . . . . . . . . . 4
4.1. The TCPAO TLS Extension . . . . . . . . . . . . . . . . . 4
4.2. The initial MKT . . . . . . . . . . . . . . . . . . . . . 5
4.3. Derivation of the secure TCP AO MKT . . . . . . . . . . . 6
4.4. Current limitations . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Change log . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Normative References . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The TCP Authentication Option (TCP-AO) [RFC5925] provides integrity
protection for long-lived TCP connections. It assumes that the
communicating hosts share a Master Key Tuple (MKT). This MKT is used
to derive traffic keys to authenticate the TCP packets exchanged by
the two hosts. TCP-AO supports different authentication algorithms
[RFC5926].
TCP-AO protects the integrity of all the packets exchanged during a
TCP connection, including the SYNs. Such a protection is important
for some specific services, but many applications would benefit from
the integrity protection offered by TCP-AO, notably against RST
attacks that can happen later in the connection. Unfortunately, from
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a deployment viewpoint, for many applications that use long-lived TCP
connections, having an existing MKT on the client and the server
before establishing a connection is a severe limitation.
This document proposes a way to derive a MKT from the TLS secure
handshake [RFC8446]. Before the TLS handshake completes, this
document defines default keys which offer a limited protection to the
first TCP packets of the connection. These default keys are then
replaced by secure keys to protect the integrity of subsequent
packets past the TLS handshake. This prevents packet injection
attacks that could result in the failure of the TLS connection.
This document is organised as follows. We provide a brief overview
of Opportunistic TCP-AO in section Section 3. Then section Section 4
discusses the required changes to TCP-AO and TLS.
2. Conventions and Definitions
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.
2.1. Notational conventions
This document uses network byte order (that is, big endian) values.
Fields are placed starting from the high-order bits of each byte.
3. An overview of Opportunistic TCP-AO
In a nutshell, an opportunistic TCP-AO connection starts like a TCP-
AO connection, i.e. the SYNs and all subsequent packets are
authenticated, but using a MKT with a default key specified in this
document. After the TLS secure handshake, the communicating hosts
derive secure keys and update their MKT with these secure keys.
Thus, the beginning of the connection is not protected against packet
modifications and packet injection attacks. The real protection only
starts once the TLS handshake finishes. The TCP packets carrying the
TLS Finished message, and all subsequent TLS records, are protected
with the secure traffic keys derived from the TLS handshake.
Figure Figure 1 illustrates the establishment of an opportunistic
TCP-AO connection. The client sends a SYN packet using the default
MKT defined in this document. The TCP-AO option in the SYN packet
indicates the use of this default MKT. The server validates the TCP-
AO option and replies with an integrity protected SYN+ACK. The
client confirms the establishment of the TCP-AO connection with an
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ACK and sends a TLS ClientHello. This ClientHello contains the AO
Extension defined in this document. This extension specifies the
authentication algorithms that the client wishes to use for the
connection and whether TCP options should be protected or not. At
this point the server can derive the TLS keys and the TCP-AO keys.
The server replies with TLS ServerHello and TLS EncryptedExtensions
messages that are sent in packets using the default TCP-AO MKT. It
installs the new key in its TCP-AO MKT. Upon reception of these
messages, the client can derive the TLS and TCP-AO keys. It installs
the TCP-AO keys in its MKT and sends the Finished message protected
with the new MKT. All the packets exchanged after the Finished are
protected using the MKT derived from the secure TLS handshake.
Client Server
| SYN (KeyID=0, RNextID=0) |
|------------------------------------------>|
| SYN+ACK (KeyID=0, RNextID=0) |
|<------------------------------------------|
| ACK, TLS ClientHello + AO |
| (KeyID=0, RNextID=0) |
|------------------------------------------>|
| TLS ServerHello, TLS EncryptedExtensions |
| (KeyID=0, RNextID=x) |
|<------------------------------------------|
| [TLS Finished] |
| (KeyID=x, RNextID=y) |
|------------------------------------------>|
| [TLS records] |
| (KeyID=y, RNextID=x) |
|<----------------------------------------->|
Figure 1: Starting an opportunistic TCP-AO connection with TLS.
The messages between brackets are authenticated using the TCP-AO
MKT derived from the TLS handshake.
4. Opportunistic TCP-AO
4.1. The TCPAO TLS Extension
This document specifies one TLS extension to support the
opportunistic utilization of TCP-AO with keys derived from the TLS
secure handshake. The "tcp_ao" extension can only be placed in the
ClientHello message.
enum {
tcp_ao(TBD),
(65535)
} ExtensionType;
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The format for the "tcp_ao" extension is defined by:
enum {
tcp_option_protection_disabled(0),
tcp_option_protection_enabled(1),
(255)
} TCPAOOptionProt;
enum {
HMAC-SHA-1-96(0),
AES-128-CMAC-96(1),
(255)
} TCPAOAuth;
enum {
KDF_HMAC_SHA1(0),
KDF_AES_128_CMAC(1),
(255)
} TCPAOKDF;
struct {
TCPAOOptionProt prot;
TCPAOAuth auth;
TCPAOKDF kdf;
} TCPAO;
The TCPAOOptionProt indicates whether the client requests integrity
protection for the TCP options or not. The TCPAOAuth specifies the
authentication algorithm defined in [RFC5926] that will be used to
protect the packets starting from the transmission of the Finished
message. The TCPAOKDF specifies the key derivation function defined
in [RFC5926] and requested by the client to derive the keys from the
TLS Master Key.
4.2. The initial MKT
To support the establishment of opportunistics TCP-AO connections,
the client and the server must be configured with a default MKT.
This default MKT is used to authenticate the packets until the
derivation of the secure MKT from the TLS Master Key. This document
defines the following default MKT:
* TCP connection identifier: selected by the TCP stack.
* TCP option flag. The default MKT assumes that TCP options are not
included in the MAC calculation.
* The current values for the SendID and RecvID are set to 0.
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* The Master key is set to 0x1cebb1ff.
* The default key derivation function is KDF_HMAC_SHA1.
* The default message authentication code is HMAC-SHA-1-96.
Given that the TCP-AO KeyID is a local field and has no global
meaning, hosts have no guarantee that a KeyID of 0 will be
unequivocally recognised as designating the default MKT specified in
this document. Section 7.5.1 of [RFC5925] indicates that hosts
receiving SYN segments with TCP-AO enabled and no matching MKT should
remove the option and accept them. A client initiating a TCP
connection in the opportunistic mode of TCP-AO MUST check that the
server accepted the use of TCP-AO in this mode by replying using the
default MKT before deriving a secure MKT as described in this
document.
4.3. Derivation of the secure TCP AO MKT
The Master key for the MKT to protect the TCP packets after the
transmission of the Finished messages shall be derived from the TLS
Master secret using:
Derive-Secret(Master Secret, "tcpao", ClientHello...server Finished)
= tcp_ao_secret
The traffic keys used by the client and the server can then be
derived from this Master key using the procedures defined in
[RFC5925] and [RFC5926].
The client and the server also need to decide on the key identifier
to use after having sent (for the server) or received (for the
client) the Finished message. This is a local decision of each host
provided that they select a different key identifier than the default
KeyID of 0.
4.4. Current limitations
This version of the document does not specifiy how to do key updates
for the MKT. It is left for later versions of this document to fill
this gap. One way would be to derive a new tcp_ao_secret from the
previous tcp_ao_secret and use a new KeyID. However, this could
expose the key update event to on-path attackers. Further guidance
is required on the severity of this issue and how it could be
mitigated.
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Later versions of this document will also specify the interactions
between this mode of enabling TCP-AO and other TLS mechanisms, such
as using pre-shared keys and 0-RTT data, as well as other TCP
extensions, such as TCP Fast Open.
{::comment}
5. Security Considerations
TCP-AO provides a protection against the injection of TCP RST. This
can impact legitimate connectionless resets, e.g. when an endpoint
loses the required state to send TCP-AO segments. Section 7.7 of
[RFC5925] provides recommendations to mitigate this effect.
Using TCP-AO with TLS can also inhibits the triggering of the
"bad_record_mac" alert that abruptly closes the TLS session when a
decryption error occurs. For instance, injected packets will fail
the TCP-AO authentication and be ignored by the receiver instead.
This also prevents sessionless resets at the TLS level, and similar
recommendations as in Section 7.7 of [RFC5925] can apply.
6. IANA Considerations
IANA is requested to create a new "Opportunistic TCP-AO with TLS"
heading for the new registries defined in this section. New
registrations under this heading follow the "Specification Required"
policy of [RFC8126].
IANA is requested to add the following entries to the existing "TLS
ExtensionType Values" registry.
+=======+================+=========+=============+===============+
| Value | Extension Name | TLS 1.3 | Recommended | Reference |
+=======+================+=========+=============+===============+
| TBD | tcp_ao | CH | N | This document |
+-------+----------------+---------+-------------+---------------+
Table 1
Note that "Recommended" is set to N as this extension is intended for
uses as described in this document.
IANA is requested to create a new registry "Authentication
Algorithms" under the "Opportunistic TCP-AO with TLS" heading.
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The registry governs an 8-bit space. Entries in this registry must
include a "Algorithm name" field containing a short mnemonic for the
algorithm. Its initial content is presented in Section 4.1 in the
TCPAOAuth enum. The registry has a "Reference" column. It is set to
[RFC5926] for the two initial algorithms.
IANA is requested to create a new registry "Key Derivation Functions"
under the "Opportunistic TCP-AO with TLS" heading.
The registry governs an 8-bit space. Entries in this registry must
include a "Key Derivation Function name" field containing a short
mnemonic for the function. Its initial content is presented in
Section 4.1 in the TCPAOKDF enum. The registry has a "Reference"
column. It is set to [RFC5926] for the two initial functions.
Acknowledgments
The authors thank Dimitri Safonov for the TCP-AO implementation in
Linux.
Change log
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/rfc/rfc2119>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/rfc/rfc5925>.
[RFC5926] Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms
for the TCP Authentication Option (TCP-AO)", RFC 5926,
DOI 10.17487/RFC5926, June 2010,
<https://www.rfc-editor.org/rfc/rfc5926>.
[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/rfc/rfc8126>.
[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/rfc/rfc8174>.
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[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/rfc/rfc8446>.
Authors' Addresses
Maxime Piraux
UCLouvain & WELRI
Email: maxime.piraux@uclouvain.be
Olivier Bonaventure
UCLouvain & WELRI
Email: olivier.bonaventure@uclouvain.be
Thomas Wirtgen
UCLouvain & WELRI
Email: thomas.wirtgen@uclouvain.be
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