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Use TEE Identification in EAP-TLS
draft-chen-rats-tee-identification-01

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Document	Type	This is an older version of an Internet-Draft whose latest revision state is "Expired".
	Authors	Penglin Yang , Meiling Chen , Li Su
	Last updated	2021-06-02 (Latest revision 2021-05-28)
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draft-chen-rats-tee-identification-01

RATS P. Yang
Internet-Draft M. Chen
Intended status: Standards Track Li. Su
Expires: December 5, 2021 China Mobile
June 03, 2021

Use TEE Identification in EAP-TLS
draft-chen-rats-tee-identification-01

Abstract

In security considerations, identity of a device should be protected
and cannot be exposed in public in plaintext. The storage and
execution of identity in device also need to be protected during the
lifecycle. Based on this purpose, this document specifies the
architecture of TEE identification based on EAP-TLS. In this
architecture, certificate protection and handshake keys generation
which are used for EAP-TLS authentication will be executed in TEE.
Communication establishment with EAP-TLS Server will be executed in
REE. A middle layer is introduced to communicate between TEE and REE
to compose the original function of EAP-TLS Client.

TEE identification based on EAP-TLS could be used in different
network layers to implement identity authentication.

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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

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 December 5, 2021.

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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 and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architecture Overview . . . . . . . . . . . . . . . . . . . . 4
3.1. Middle Layer Message . . . . . . . . . . . . . . . . . . 5
3.2. information pre-stored in TEE . . . . . . . . . . . . . . 5
3.3. key derivation process in TEE . . . . . . . . . . . . . . 6
3.4. Mutual Authentication Procedure . . . . . . . . . . . . . 6
3.5. Ticket Establishment . . . . . . . . . . . . . . . . . . 8
3.6. Resumption . . . . . . . . . . . . . . . . . . . . . . . 8
3.7. Termination . . . . . . . . . . . . . . . . . . . . . . . 8
3.8. Hello Retry Request . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9

1. Introduction

In security considerations, identity of a device should be protected
and cannot be exposed in public in plaintext. The storage and
execution of identity in device also need to be protected during the
lifecycle. Even though the authentication protocol like EAP-TLS,
802.1X can guarantee the procedure of communication is security but
they are all built by the assumption that the device and the
implementation of procedure is trusted. In fact, security is the
result of multi-layer composition which could affect the security
both in protocol level and equipment level. Based on these
considerations, there is still no a unified and trusted mechanism
that can attest a remote device's identity in a trusted way in
Internet. So this document tries to use TEE and EAP-TLS to create a
secure and trusted procedure to attest a device's identity.

In this document, TEE (Trusted Execution Environment) described in
draft-ietf-teep-architecture-14 will be involved. This environment
emphasizes that any code within that environment cannot be tampered

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with, and that any data used by such code cannot be read or tampered
with by any code outside that environment. On the contrary, REE
(Rich Execution Environment) is an environment that code and data in
that environment may be tampered with. Need to mention that in
device TEE is scarce resource which can only involve security
critical processes inside.

EAP-TLS1.3 protocol, defined in RFC RFC 5216[RFC5216] which is
recommended by IETF because of its swift and security features. This
protocol is treated as a security method that can provide client-
server mutual authentication. In this protocol there is an
assumption that both client and server are trusted or uncompromised
by any attacker. Usually the server of authentication is highly
protected and surveilled by operators, this means that the server
could be considered as a trust party. But client especially IoT
device is more likely to be vulnerable due to the lack of sufficient
security mechanisms.

The primary goal of this document is to provide a remote identity
attestation method which uses EAP-TLS as the essential authentication
protocol and TEE as the security shelter to store and execute the
certificate and private key derivations. The specific method is to
add a middle layer in REE and TEE to exchange data in the form of
EAP-TLS. In application scenarios, this method could be used in
transport layer authentication, application layer authentication and
other scenarios.

2. Terminology

The readers should be familiar with the terms defined in.

In addition, this document makes use of the following terms:

TEE: Trust Execution Environment.

REE: Rich Execution Environment.

ML: Middle Layer.

IML: Inner Middle Layer.

EML: External Middle Layer.

peer: The entity that responds to the authenticator.

backend authenticator server: A backend authentication server is an
entity that provides an authentication service to an

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authenticator. When used, this server typically executes EAP
methods for the authenticator.

EAP server: The entity that terminates the EAP authentication method
with the peer. In the case where no backend authentication server
is used, the EAP server is part of the authenticator. In the case
where the authenticator operates in pass-through mode, the EAP
server is located on the backend authentication server.

3. Architecture Overview

This architecture will bring in a Middle Layer which is implemented
in TEE and REE to translate information between TEE and REE. The
structure of this Middle Layer is shown below

+------------------------------------------------------+
| +----------------------------+ REE |
| | TEE | |
| | +---------------+ +------+ | +-------------------+ | +-------+
| | | certificates| | | | |---------+ | | | |
| | +---------------+ | inner| | || | | | | |
| | +---------------+ |middle<---->external| EAP-TLS| | |EAP-TLS|
| | | key | | layer| | ||middle | Client <-----> server|
| | | derivation | | | | ||layer | | | | |
| | +---------------+ +------+ | |---------+ | | | |
| +----------------------------+ |-------------------+ | +-------+
+------------------------------------------------------+

Figure 1: architecture of middle layer

In figure 1, the middle layer is separated in two parts: Inner Middle
Layer (IML) and External Middle Layer (EML). The IML is responsible
for

a. Key derivation b. Response to EML about EAP-TLS encryption and
decryption relevant message.

In this document, the EML could be set as a part of EAP-TLS Client
function which is responsible for:

a. Communicate with EAP-TLS Server b. Request encryption and
decryption relevant messages from IML.

The communication mechanism between IML and EML should follow the
specific trust computing architecture like Intel Enclave and
TrustZone which is out of this document's scope.

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3.1. Middle Layer Message

The message transmitted between IML and EML will follow the format of
TLS1.3, but not all TLS1.3 [RFC8446]message will be transmitted. The
IML only accept message relevant to encryption and decryption. The
structure of Middle Layer Message is shown below.

enum{
Random;
keyshareExtension;
PreSharedKeyExchange
CertificateList
CertificateVerify
Finished
NewSessionTicket
ApplicationData
Alert
}ParameterType

Struct{
bool request//true:request; false response. If it's request message, then the payload of message should be set as zero.
ParameterType type
uint24 length
select(type){
case Random randomValue
case KeyshareExtension keyshareextensionValue
case PreSharedKeyExchange value;
case CertificateList
case CertificateVerify
case Finished
case NewSessionTicket
case ApplicationData
case Alert
}
}MiddleLayerMessage

3.2. information pre-stored in TEE

(1) Certificate that complies with X509.3. If using EAP-TLS as the
authentication protocol, then the ID of the TEE enabled device is the
certificate complies X509.3. In this document, the certificate of
this device is the only item that needs to be stored in TEE before
the process of EAP-TLS starts. And regarding to how to get this
certificate or update this certificate is out of scope. The
certificate will never be allowed to be exposed outside the TEE in
plaintext.

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3.3. key derivation process in TEE

Key derivation process MUST be executed in TEE.

3.4. Mutual Authentication Procedure

Figure 2 illustrates the steps of TEE identification based on EAP-
TLS. From the view of EAP-TLS Server there are no changes of the
procedure. All the changes are in the EAP-TLS Peer side. This
document defines 6 middle layer messages from message 1 to message 6
which will be used for different purpose to communicate between IML
and EML. The specific steps are shown in bullets below.

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Figure 2: Mutual TEE Identification based on EAP-TLS

In order to complete ClientHello Message, the Key_Share Extension
message is needed. This message involves the key derivation function
which Must be executed in TEE. So the Middle Layer Message 1 is
KeyShareExtension request from EML to IML.

Middle Layer Message 2 from IML responses to message1 and returns the
KeyShareExtension response to EML.

Middle Layer Message 3 includes plaintext ServerHello message and
encrypted Server Params and Auth. Since EML does not carry the
relevant private key which is derived from KeyShareExtension, it will
transfer this message to IML to decode. Message 3 also includes the
entire handshake context which will be used to create
CertificateVerify and Finished context.

In Message 4, IML retains the KeyShareExtension, and other message
context will be transferred to EML as plaintext. Message 4 also
contains context the HMAC of (finished_key, Transcript-Hash(Handshake

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Context, Certificate, CertificateVerify)), which can only be
generated by IML.

Message 5 is the encrypted application data 0x00, which will be sent
to IML to decode.

After decrypted the message 5, the plaintext will be packed in
message 6 and sent to EML. Then EML will make the determination if
the authentication procedure is finished.

3.5. Ticket Establishment

If the NewSessionTicket context is sent by EAP-TLS Server, it will be
packed in the middle of Server's TLS Finished message and TLS
Application Data 0x00 message. This context will be included in
message 5 by EML and conveyed to IML. After received message 5, IML
will decrypt and retain this ticket establishment context for
resumption.

3.6. Resumption

After the Client has received a NewSessionTicket message from the
EAP-TLS Server, the Client can use PSK mode to connect with EAP-TLS
Server. This action happens in TLS ClientHello message, in which the
Pre-shared-key extension will be used. Need to notice that the
action of resumption is deployed by EAP-TLS Client. EAP-TLS Client
determines if it will use NewSessionTicket to rebuild connection with
EAP-TLS Server. If do so, the message 1 will include the type of
NewSeesionTIcket request to IML. After received this request
message, IML will generate the Pre-shared key extension in Message2
for EMLREE to generate ClientHello Message.

3.7. Termination

TLS Error Alert could be sent both by EAP-TLS Server and Client. If
sent by Server, the message will be transferred to IML by EML to
decrypt. And the IML will notify EML in message 4 or 6. If the TLS
Error Alert message is sent by IML, it will be generate in message4,
which will be directly transferred to EML.

3.8. Hello Retry Request

This message happens after the EAP-TLS Server received ClientHello.
Since the negotiation is not successful, the Hello Retry Request
message will be sent in plaintext to EAP-TLS Client.

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4. Security Considerations

This document used the concept of TEE, which can be considered as a
trusted anchor in device that cannot be tampered. But the REE of a
device cannot be fully trusted or it may be tampered by attackers.
The middle layer has two parts: the inner middle layer and the
external middle layer. Even though the message conveyed between IML
and EML is already encrypted, TEE cannot guarantee the integrity
message from EML and trust the behavior of EAP-TLS Client. As a
result this architecture can make sure that crucial information like
certificate or identity cannot be obtained by illegal parties, but
cannot deny DOS attack. In fact unless there is a trust channel that
directly connects between TEE and EAP-TLS Server, otherwise the DOS
attack cannot be prevented. For example, in the SUCI-SUPI
architecture in 5G system the ME is in charge of establishing
communications between USIM and AMF. If an attacker invaded into the
ME and tampered the SUCI message, a DOS attack will be implemented.

The other aspects of the security considerations will follow TLS1.3,
EAP-TLS, and RATs draft--ietf-rats-architecture.

5. IANA Considerations

TBD

6. Acknowledgement

TBD

7. Normative References

[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <https://www.rfc-editor.org/info/rfc5216>.

[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>.

Authors' Addresses

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Penglin Yang
China Mobile
32, Xuanwumen West
BeiJing, BeiJing 100053
China

Email:
yangpenglin@chinamobile.com

Meiling Chen
China Mobile
32, Xuanwumen West
BeiJing, BeiJing 100053
China

Email:
chenmeiling@chinamobile.com

Li Su
China Mobile

32, Xuanwumen West

BeiJing

100053

China

Email:
suli@chinamobile.com

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Use TEE Identification in EAP-TLS
draft-chen-rats-tee-identification-01