RATS P. Yang
Internet-Draft M. Chen
Intended status: Standards Track Li. Su
Expires: April 25, 2022 China Mobile
October 22, 2021
Use TEE Identification in EAP-TLS
draft-chen-rats-tee-identification-03
Abstract
In security considerations, identities of devices like
certifications, private keys should be protected and cannot be
exposed in public in plaintext during whole lifecycle. When using
these identities to make authentications a unified architecture to
prevent identity information leakage is needed. This document
creates a secure and trusted TEE authentication architecture to
authenticate a device's identity based on EAP-TLS and TEE. In this
architecture, certificate and handshake keys which are used for EAP-
TLS 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 authentication could be used in LAN or WLAN
scenarios.
Status of This Memo
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This Internet-Draft will expire on April 25, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Threat Model and Motivation . . . . . . . . . . . . . . . . . 4
3.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 4
3.2. motivation . . . . . . . . . . . . . . . . . . . . . . . 5
4. Architecture Overview . . . . . . . . . . . . . . . . . . . . 6
4.1. Middle Layer Message . . . . . . . . . . . . . . . . . . 7
4.2. information pre-stored in TEE . . . . . . . . . . . . . . 8
4.3. key derivation process in TEE . . . . . . . . . . . . . . 8
4.4. Mutual Authentication Procedure . . . . . . . . . . . . . 9
4.5. Ticket Establishment . . . . . . . . . . . . . . . . . . 11
4.6. Resumption . . . . . . . . . . . . . . . . . . . . . . . 11
4.7. Termination . . . . . . . . . . . . . . . . . . . . . . . 11
4.8. Hello Retry Request . . . . . . . . . . . . . . . . . . . 11
5. Use Scenarios . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 12
9. Normative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Mutual authentication is an important way to implement identity
authentication, it refers to both sides of the authentication
procedure to verify each other's identity. Typical examples of
mutual authentication are EAP-TLS and EAP-AKA, and EAP-AKA has been
used in telecommunication networks for subscriber's network access
authentication.
However, mutual authentication protocol itself is not sufficient to
provide a trusted authentication procedure. The storage and
execution of identity related to authentication also need to be
protected. That's also why SIM (Subscriber Identity Module) is
introduced in telecommunication network to enhance EAP-AKA. SIM
provides a trusted execution environment, which could be used to
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store and execute identity and key derivation about EAP-AKA protocol.
And that's one of the reasons why applications could treat the
telecommunication network as a trusted network, and secret
information like verification code about account of bank could be
transferred in this network.
In IoT devices or other Internet-access devices, there is still no
unified and trusted mechanism that can authenticate a device's
identity in a trusted way in LAN or WLAN. So this document tries to
use EAP-TLS and TEE (Trusted Execution Environment) to create a
secure and trusted architecture to standardize the trusted
authentication in LAN or WLAN.
EAP-TLS1.3 protocol is defined in RFC 8446[RFC8446], which is treated
as a security method that can provide client-server mutual
authentication. Usually the authentication server is highly
protected and monitored by operators. So the server could be treated
as a trust party. But client is more likely to be vulnerable due to
the lack of sufficient security mechanisms. TEE used in client could
make sure that any code within that environment cannot be tampered
with, and that any data used by such code cannot be read or tampered
with by any code outside that environment.
The primary goal of this document is to provide a trusted
authentication architecture which uses EAP-TLS as the essential
authentication protocol and TEE as the security shelter to store and
execute private key derivations and establish encrypted communication
channel. The specific method is to add a middle layer in REE and TEE
to exchange data in the form of EAP-TLS.
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.
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backend authenticator server: A backend authentication server is an
entity that provides an authentication service to an
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.
EAP-TLS: Extensible Authentication Protocol-Transport Layer.
EAP-AKA: Extensible Authentication Protocol-Authentication.
SIM: Subscriber Identity Module.
DevID: Initial Device Identifier.
3. Threat Model and Motivation
3.1. Threat Model
When executing authentication process in devices, the threat model
could be treated as shown in figure 1. In this figure, there are
three possible statuses that attackers could manipulate the
authentication information: authentication in rest, authentication in
process, and authentication in translation.
+--------------+----Attacker
| | |
+--------+--------------+----+ |
| REE | | | |
| +-----v---------+ | | | +------------+
| | TEE/Identity +----v----+------v--+ Auth center|
| +---------------+ | +------------+
| |
+----------------------------+
Figure 1: threat model of authentication
(1) Authentication in rest. In order to protect identity information
in rest from stealing and tampering, methods like DevID or encrypted
storage could be used.
(2) Authentication in process. In this status the authentication
information needs to be calculated and prepared by the device to
transfer to authentication center. When a device is preparing to
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authenticate, some processes need to be executed. For example, key
derivation from the chosen ID, encrypted channel establishment
between device and Auth Center. When designing an authentication
protocol or method, the device is always treated as an atomic point
and is secure with no doubt. However, any device is composed of
component like hardware, firmware, and operating system. When
processing the Identification information in REE environment, the ID
may be compromised.
(3) Authentication in translation. This status is also easy to
deploy in network protocols like TLS. After exchanging handshake
keys, it is hard to crack information during translation.
3.2. motivation
In IETF and other organizations, there are lots of authentication
methods and identity systems like DevID, X509.3, X802.1, etc. These
procedures are used in different scenario and different network
layers. When compared to the threat model, the protection of
authentication in process is still undetermined.
For example, DevID could prevent the attack of status 1 in threat
model. When composed with EAP-TLS, the composition can prevent
attack from both status 1 and 3 in threat model. But about the
attack for status 2 in threat model, there is no direct solution.
There are two possible methods to mitigate this potential risk of
status 2 of threat model. The first one is to put all the
authentication process and relevant software and network stacks in
TEE environment, as shown in figure 2. This method requires large
scale TEE and is hard to deploy in resource restricted devices.
+------------+ Trusted +------------+
|Device |authentication| |
| <--------------> Auth center|
| TEE | | |
+------------+ +------------+
Figure 2: solution 1: all authentication is under TEE
In some cases, TEE is restricted resource and cannot cover all the
authentication procedure. Like TPM and other TEE chips for IoT
devices, only very limited computing capability is available. When
pursuing concise architecture of authentication that consumes minimal
TEE, the second solution needs to be discovered.
The second solution builds a strict and limited middle layer
interface to communicate between TEE and REE. The TEE will only in
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charge of the generation of encrypted and unreusable identities for
authentication. The REE could in charge of the other process of
authentication. This design could address the authentication in
process threat model. Before going into the detail of TEE
authentication architecture, there are three key features about the
middle layer need to be point out. (1) Transportation tunnel about
authentication is built between TEE and authentication server. (2)The
TEE should verify server ID first, otherwise never expose client
identity. (3) Nonce and handshake key are used to encrypt ID to
prevent replay attack.
These three features could make sure:
Even the REE of the device is compromised like the memory is dumped,
the attackers cannot get any plain text of authentication
information.
The encrypted message could be stolen, but cannot be reused because
of random nonce produced by TEE and server.
Attackers cannot use a fake server identity to defraud a client's
real identity. Because the private key of server's certificate never
exposed and the signature with nonce cannot be imitated.
The usage of trusted authentication is extensive. Trusted
authentication could be used in factory, campus, transportation
sector, etc, in where devices need to get access to network by
identifying their identity. Also, devices also need to know the
network's identity. In fact, anywhere that needs to identify
devices' Identification by LAN or WLAN could use this architecture.
4. Architecture Overview
This architecture brings 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.
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+------------------------------------------------------+
| +----------------------------+ REE |
| | TEE | |
| | +---------------+ +------+ | +-------------------+ | +-------+
| | | certificates| | | | |---------+ | | | |
| | +---------------+ | inner| | || | | | | |
| | +---------------+ |middle<---->external| EAP-TLS| | |EAP-TLS|
| | | key | | layer| | ||middle | Client <-----> server|
| | | derivation | | | | ||layer | | | | |
| | +---------------+ +------+ | |---------+ | | | |
| +----------------------------+ |-------------------+ | +-------+
+------------------------------------------------------+
Figure 3: architecture of middle layer
In figure 3, 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.
4.1. 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.
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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
4.2. 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 with 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.
4.3. key derivation process in TEE
Key derivation process MUST be executed in TEE.
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4.4. Mutual Authentication Procedure
Figure 4 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.
EAP-TLS Peer
+--------------------+
+-----+ +-----+ +-------+
| TEE | | REE | |EAP|TLS|
+--+--+ +--+--+ |server |
| | +-------+
| | | EAP-Request/
| <-----------------------+ Identity
| EAP-Response/ |
| Identity(privacy-friendly) |
| Recommend random hex +------->
| | |
| | EAP-Request/
| <----------------+ EAP-Type=EAP-TLS
| | (TLS Start)
| Middle layer |
<----------------+ Message 1 |
| | |
Middle layer | |
Message 2 +--------------> |
| | |
| EAP-Response/ |
| EAP-Type=EAP-TLS+----------->
| (TLS ClientHello) |
| | EAP-Request/
| | EAP-Type=EAP-TLS
| | (TLS ServerHello,
| <----------------+ TLS EncryptedExtensions,
| | TLS CertificateRequest,
| | TLS Certificate,
| | TLS CertificateVerify,
| | TLS Finished)
| Middle Layer |
<---------------+Message 3 |
| | |
Middle Layer | |
Message 4 +----------------> |
| | |
| EAP-Response/ |
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| EAP-Type=EAP-TLS |
| (TLS Certificate, |
| TLS CertificateVerify, |
| TLS Finished) +------------>
| | |
| | EAP-Request/
| | EAP-Type=EAP-TLS
| | (TLS Application
| <---------------+ Data 0x00)
| | |
| Middle Layer |
<--------------+ Message 5 |
| | |
Middle Layer | |
Message 6 +---------------> |
| | |
| EAP-Response/ |
| EAP-Type=EAP-TLS+------------->
| | |
| <------------------+EAP-Success
| | |
| | |
Figure 4: 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, encrypted TLS Client Certificate, TLS CertificateVerify
and TLS Finished message will be included.
Message 5 is the encrypted application data 0x00, which will be sent
to IML to decode.
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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.
4.5. 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.
4.6. 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.
4.7. 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.
4.8. 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.
5. Use Scenarios
Like SIM/eSIM is for 4G/5G to authentication, TEE authentication
could be used in WLAN and LAN for devices that need to gain access to
the network. TEE authentication could be used as complementary of
RATs and DevID. In scenarios like manufacturing industry and some
IoT scenes, to recognize a device's identity may be important, the
specific use scenarios are shown below.
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Scenario 1, TEE authentication could be used in situations like
industrial Internet, in where machines need to have their identity
checked for property management or network access.
Scenario 2, TEE authentication could be used in situations like
campus or factory, in where devices need to authenticate to gain
access to network.
Scenario 3, TEE authentication could be used in situations in where
identities of devices need to be traced with trust.
6. Security Considerations
This document uses the concept of TEE, which could be considered as a
trusted anchor in device that cannot be tampered. The REE of a
device cannot be fully trusted or it may be tampered by attackers.
The middle layer has two parts: the IML and the EML. Even though the
messages conveyed between IML and EML are already encrypted, TEE
cannot guarantee the integrity of the message in REE. In another
word, DoS attack against REE cannot be prevented. For example, EAP-
AKA in 5G involves two components: UE and ME. UE includes SIM and
represents authentication procedure, ME represents establishing
communication between mobile phone and base station. If an attacker
invades into the ME and tampered the communication 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.
7. IANA Considerations
No new item needs to register in IANA.
8. Acknowledgement
TBD
9. Normative References
[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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