Internet DRAFT - draft-ietf-tls-external-psk-guidance
draft-ietf-tls-external-psk-guidance
tls R. Housley
Internet-Draft Vigil Security
Intended status: Informational J. Hoyland
Expires: 8 August 2022 Cloudflare Ltd.
M. Sethi
Ericsson
C.A. Wood
Cloudflare
4 February 2022
Guidance for External PSK Usage in TLS
draft-ietf-tls-external-psk-guidance-06
Abstract
This document provides usage guidance for external Pre-Shared Keys
(PSKs) in Transport Layer Security (TLS) 1.3 as defined in RFC 8446.
It lists TLS security properties provided by PSKs under certain
assumptions, and then demonstrates how violations of these
assumptions lead to attacks. Advice for applications to help meet
these assumptions is provided. This document also discusses PSK use
cases and provisioning processes. Finally, it lists the privacy and
security properties that are not provided by TLS 1.3 when external
PSKs are used.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/tlswg/external-psk-design-team.
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."
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This Internet-Draft will expire on 8 August 2022.
Copyright Notice
Copyright (c) 2022 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
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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. PSK Security Properties . . . . . . . . . . . . . . . . . . . 4
4.1. Shared PSKs . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. PSK Entropy . . . . . . . . . . . . . . . . . . . . . . . 5
5. External PSKs in Practice . . . . . . . . . . . . . . . . . . 6
5.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Provisioning Examples . . . . . . . . . . . . . . . . . . 7
5.3. Provisioning Constraints . . . . . . . . . . . . . . . . 8
6. Recommendations for External PSK Usage . . . . . . . . . . . 8
6.1. Stack Interfaces . . . . . . . . . . . . . . . . . . . . 9
6.1.1. PSK Identity Encoding and Comparison . . . . . . . . 10
6.1.2. PSK Identity Collisions . . . . . . . . . . . . . . . 10
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This document provides guidance on the use of external Pre-Shared
Keys (PSKs) in Transport Layer Security (TLS) 1.3 [RFC8446]. This
guidance also applies to Datagram TLS (DTLS) 1.3
[I-D.ietf-tls-dtls13] and Compact TLS 1.3 [I-D.ietf-tls-ctls]. For
readability, this document uses the term TLS to refer to all such
versions.
External PSKs are symmetric secret keys provided to the TLS protocol
implementation as external inputs. External PSKs are provisioned
out-of-band.
This document lists TLS security properties provided by PSKs under
certain assumptions and demonstrates how violations of these
assumptions lead to attacks. This document discusses PSK use cases,
provisioning processes, and TLS stack implementation support in the
context of these assumptions. This document also provides advice for
applications in various use cases to help meet these assumptions.
There are many resources that provide guidance for password
generation and verification aimed towards improving security.
However, there is no such equivalent for external Pre-Shared Keys
(PSKs) in TLS. This document aims to reduce that gap.
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.
3. Notation
For purposes of this document, a "logical node" is a computing
presence that other parties can interact with via the TLS protocol.
A logical node could potentially be realized with multiple physical
instances operating under common administrative control, e.g., a
server farm. An "endpoint" is a client or server participating in a
connection.
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4. PSK Security Properties
The use of a previously established PSK allows TLS nodes to
authenticate the endpoint identities. It also offers other benefits,
including resistance to attacks in presence of quantum computers; see
Section 4.2 for related discussion. However, these keys do not
provide privacy protection of endpoint identities, nor do they
provide non-repudiation (one endpoint in a connection can deny the
conversation); see Section 7 for related discussion.
PSK authentication security implicitly assumes one fundamental
property: each PSK is known to exactly one client and one server, and
that these never switch roles. If this assumption is violated, then
the security properties of TLS are severely weakened as discussed
below.
4.1. Shared PSKs
As discussed in Section 5.1, to demonstrate their attack, [AASS19]
describes scenarios where multiple clients or multiple servers share
a PSK. If this is done naively by having all members share a common
key, then TLS authenticates only group membership, and the security
of the overall system is inherently rather brittle. There are a
number of obvious weaknesses here:
1. Any group member can impersonate any other group member.
2. If PSK is combined with a fresh ephemeral key exchange, then
compromise of a group member that knows the resulting shared
secret will enable the attacker to passively read (and actively
modify) traffic.
3. If PSK is not combined with fresh ephemeral key exchange, then
compromise of any group member allows the attacker to passively
read (and actively modify) all traffic, including reading past
traffic.
Additionally, a malicious non-member can reroute handshakes between
honest group members to connect them in unintended ways, as described
below. Note that a partial mitigation against this class of attack
is available: each group member includes the SNI extension [RFC6066]
and terminates the connection on mismatch between the presented SNI
value and the receiving member's known identity. See [Selfie] for
details.
To illustrate the rerouting attack, consider three peers, A, B, and
C, who all know the PSK. The attack proceeds as follows:
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1. A sends a ClientHello to B.
2. The attacker intercepts the message and redirects it to C.
3. C responds with a second flight (ServerHello, ...) to A.
4. A sends a Finished message to B. A has completed the handshake,
ostensibly with B.
5. The attacker redirects the Finished message to C. C has
completed the handshake with A.
In this attack, peer authentication is not provided. Also, if C
supports a weaker set of cipher suites than B, cryptographic
algorithm downgrade attacks might be possible. This rerouting is a
type of identity misbinding attack [Krawczyk][Sethi]. Selfie attack
[Selfie] is a special case of the rerouting attack against a group
member that can act both as TLS server and client. In the Selfie
attack, a malicious non-member reroutes a connection from the client
to the server on the same endpoint.
Finally, in addition to these weaknesses, sharing a PSK across nodes
may negatively affect deployments. For example, revocation of
individual group members is not possible without establishing a new
PSK for all of the non-revoked members.
4.2. PSK Entropy
Entropy properties of external PSKs may also affect TLS security
properties. For example, if a high entropy PSK is used, then PSK-
only key establishment modes provide expected security properties for
TLS, including establishing the same session keys between peers,
secrecy of session keys, peer authentication, and downgrade
protection. See [RFC8446], Appendix E.1 for an explanation of these
properties. However, these modes lack forward security. Forward
security may be achieved by using a PSK-DH mode, or, alternatively,
by using PSKs with short lifetimes.
In contrast, if a low entropy PSK is used, then PSK-only key
establishment modes are subject to passive exhaustive search attacks
which will reveal the traffic keys. PSK-DH modes are subject to
active attacks in which the attacker impersonates one side. The
exhaustive search phase of these attacks can be mounted offline if
the attacker captures a single handshake using the PSK, but those
attacks will not lead to compromise of the traffic keys for that
connection because those also depend on the Diffie-Hellman (DH)
exchange. Low entropy keys are only secure against active attack if
a password-authenticated key exchange (PAKE) is used with TLS. The
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Crypto Forum Research Group (CFRG) is currently working on specifying
recommended PAKEs (see [I-D.irtf-cfrg-cpace] and
[I-D.irtf-cfrg-opaque], for the symmetric and asymmetric cases,
respectively).
5. External PSKs in Practice
PSK ciphersuites were first specified for TLS in 2005. PSKs are now
an integral part of the TLS version 1.3 specification [RFC8446]. TLS
1.3 also uses PSKs for session resumption. It distinguishes these
resumption PSKs from external PSKs which have been provisioned out-
of-band. This section describes known use cases and provisioning
processes for external PSKs with TLS.
5.1. Use Cases
This section lists some example use-cases where pair-wise external
PSKs, i.e., external PSKs that are shared between only one server and
one client, have been used for authentication in TLS. There was no
attempt to prioritize the examples in any particular order.
* Device-to-device communication with out-of-band synchronized keys.
PSKs provisioned out-of-band for communicating with known
identities, wherein the identity to use is discovered via a
different online protocol.
* Intra-data-center communication. Machine-to-machine communication
within a single data center or point-of-presence (PoP) may use
externally provisioned PSKs, primarily for the purposes of
supporting TLS connections with early data; see Section 8 for
considerations when using early data with external PSKs.
* Certificateless server-to-server communication. Machine-to-
machine communication may use externally provisioned PSKs,
primarily for the purposes of establishing TLS connections without
requiring the overhead of provisioning and managing PKI
certificates.
* Internet of Things (IoT) and devices with limited computational
capabilities. [RFC7925] defines TLS and DTLS profiles for
resource-constrained devices and suggests the use of PSK
ciphersuites for compliant devices. The Open Mobile Alliance
Lightweight Machine to Machine Technical Specification [LwM2M]
states that LwM2M servers MUST support the PSK mode of DTLS.
* Securing RADIUS [RFC2865] with TLS. PSK ciphersuites are optional
for this use case, as specified in [RFC6614].
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* 3GPP server to user equipment authentication. The Generic
Authentication Architecture (GAA) defined by 3GGP mentions that
TLS-PSK ciphersuites can be used between server and user equipment
for authentication [GAA].
* Smart Cards. The electronic German ID (eID) card supports
authentication of a card holder to online services with TLS-PSK
[SmartCard].
* Quantum resistance. Some deployments may use PSKs (or combine
them with certificate-based authentication as described in
[RFC8773]) because of the protection they provide against quantum
computers.
There are also use cases where PSKs are shared between more than two
entities. Some examples below (as noted by Akhmetzyanova et al.
[AASS19]):
* Group chats. In this use-case, group participants may be
provisioned an external PSK out-of-band for establishing
authenticated connections with other members of the group.
* Internet of Things (IoT) and devices with limited computational
capabilities. Many PSK provisioning examples are possible in this
use-case. For example, in a given setting, IoT devices may all
share the same PSK and use it to communicate with a central server
(one key for n devices), have their own key for communicating with
a central server (n keys for n devices), or have pairwise keys for
communicating with each other (n^2 keys for n devices).
5.2. Provisioning Examples
The exact provisioning process depends on the system requirements and
threat model. Whenever possible, avoid sharing a PSK between nodes;
however, sharing a PSK among several nodes is sometimes unavoidable.
When PSK sharing happens, other accommodations SHOULD be used as
discussed in Section 6.
Examples of PSK provisioning processes are included below.
* Many industrial protocols assume that PSKs are distributed and
assigned manually via one of the following approaches: typing the
PSK into the devices, or using a Trust On First Use (TOFU)
approach with a device completely unprotected before the first
login did take place. Many devices have very limited UI. For
example, they may only have a numeric keypad or even fewer
buttons. When the TOFU approach is not suitable, entering the key
would require typing it on a constrained UI.
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* Some devices provision PSKs via an out-of-band, cloud-based
syncing protocol.
* Some secrets may be baked into hardware or software device
components. Moreover, when this is done at manufacturing time,
secrets may be printed on labels or included in a Bill of
Materials for ease of scanning or import.
5.3. Provisioning Constraints
PSK provisioning systems are often constrained in application-
specific ways. For example, although one goal of provisioning is to
ensure that each pair of nodes has a unique key pair, some systems do
not want to distribute pair-wise shared keys to achieve this. As
another example, some systems require the provisioning process to
embed application-specific information in either PSKs or their
identities. Identities may sometimes need to be routable, as is
currently under discussion for EAP-TLS-PSK
[I-D.mattsson-emu-eap-tls-psk].
6. Recommendations for External PSK Usage
Recommended requirements for applications using external PSKs are as
follows:
1. Each PSK SHOULD be derived from at least 128 bits of entropy,
MUST be at least 128 bits long, and SHOULD be combined with an
ephemeral key exchange, e.g., by using the "psk_dhe_ke" Pre-
Shared Key Exchange Mode in TLS 1.3, for forward secrecy. As
discussed in Section 4, low entropy PSKs, i.e., those derived
from less than 128 bits of entropy, are subject to attack and
SHOULD be avoided. If only low-entropy keys are available, then
key establishment mechanisms such as Password Authenticated Key
Exchange (PAKE) that mitigate the risk of offline dictionary
attacks SHOULD be employed. Note that no such mechanisms have
yet been standardised, and further that these mechanisms will not
necessarily follow the same architecture as the process for
incorporating external PSKs described in
[I-D.ietf-tls-external-psk-importer].
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2. Unless other accommodations are made to mitigate the risks of
PSKs known to a group, each PSK MUST be restricted in its use to
at most two logical nodes: one logical node in a TLS client role
and one logical node in a TLS server role. (The two logical
nodes MAY be the same, in different roles.) Two acceptable
accommodations are described in
[I-D.ietf-tls-external-psk-importer]: (1) exchanging client and
server identifiers over the TLS connection after the handshake,
and (2) incorporating identifiers for both the client and the
server into the context string for an external PSK importer.
3. Nodes SHOULD use external PSK importers
[I-D.ietf-tls-external-psk-importer] when configuring PSKs for a
client-server pair when applicable. Importers make provisioning
external PSKs easier and less error prone by deriving a unique,
imported PSK from the external PSK for each key derivation
function a node supports. See the Security Considerations in
[I-D.ietf-tls-external-psk-importer] for more information.
4. Where possible the main PSK (that which is fed into the importer)
SHOULD be deleted after the imported keys have been generated.
This prevents an attacker from bootstrapping a compromise of one
node into the ability to attack connections between any node;
otherwise the attacker can recover the main key and then re-run
the importer itself.
6.1. Stack Interfaces
Most major TLS implementations support external PSKs. Stacks
supporting external PSKs provide interfaces that applications may use
when configuring PSKs for individual connections. Details about some
existing stacks at the time of writing are below.
* OpenSSL and BoringSSL: Applications can specify support for
external PSKs via distinct ciphersuites in TLS 1.2 and below.
They also then configure callbacks that are invoked for PSK
selection during the handshake. These callbacks must provide a
PSK identity and key. The exact format of the callback depends on
the negotiated TLS protocol version, with new callback functions
added specifically to OpenSSL for TLS 1.3 [RFC8446] PSK support.
The PSK length is validated to be between [1, 256] bytes. The PSK
identity may be up to 128 bytes long.
* mbedTLS: Client applications configure PSKs before creating a
connection by providing the PSK identity and value inline.
Servers must implement callbacks similar to that of OpenSSL. Both
PSK identity and key lengths may be between [1, 16] bytes long.
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* gnuTLS: Applications configure PSK values, either as raw byte
strings or hexadecimal strings. The PSK identity and key size are
not validated.
* wolfSSL: Applications configure PSKs with callbacks similar to
OpenSSL.
6.1.1. PSK Identity Encoding and Comparison
Section 5.1 of [RFC4279] mandates that the PSK identity should be
first converted to a character string and then encoded to octets
using UTF-8. This was done to avoid interoperability problems
(especially when the identity is configured by human users). On the
other hand, [RFC7925] advises implementations against assuming any
structured format for PSK identities and recommends byte-by-byte
comparison for any operation. When PSK identities are configured
manually it is important to be aware that due to encoding issues
visually identical strings may, in fact, differ.
TLS version 1.3 [RFC8446] follows the same practice of specifying the
PSK identity as a sequence of opaque bytes (shown as opaque
identity<1..2^16-1> in the specification) that thus is compared on a
byte-by-byte basis. [RFC8446] also requires that the PSK identities
are at least 1 byte and at the most 65535 bytes in length. Although
[RFC8446] does not place strict requirements on the format of PSK
identities, we do however note that the format of PSK identities can
vary depending on the deployment:
* The PSK identity MAY be a user configured string when used in
protocols like Extensible Authentication Protocol (EAP) [RFC3748].
gnuTLS for example treats PSK identities as usernames.
* PSK identities MAY have a domain name suffix for roaming and
federation. In applications and settings where the domain name
suffix is privacy sensitive, this practice is NOT RECOMMENDED.
* Deployments should take care that the length of the PSK identity
is sufficient to avoid collisions.
6.1.2. PSK Identity Collisions
It is possible, though unlikely, that an external PSK identity may
clash with a resumption PSK identity. The TLS stack implementation
and sequencing of PSK callbacks influences the application's behavior
when identity collisions occur. When a server receives a PSK
identity in a TLS 1.3 ClientHello, some TLS stacks execute the
application's registered callback function before checking the
stack's internal session resumption cache. This means that if a PSK
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identity collision occurs, the application's external PSK usage will
typically take precedence over the internal session resumption path.
Since resumption PSK identities are assigned by the TLS stack
implementation, it is RECOMMENDED that these identifiers be assigned
in a manner that lets resumption PSKs be distinguished from external
PSKs to avoid concerns with collisions altogether.
7. Privacy Considerations
PSK privacy properties are orthogonal to security properties
described in Section 4. TLS does little to keep PSK identity
information private. For example, an adversary learns information
about the external PSK or its identifier by virtue of the identifier
appearing in cleartext in a ClientHello. As a result, a passive
adversary can link two or more connections together that use the same
external PSK on the wire. Depending on the PSK identity, a passive
attacker may also be able to identify the device, person, or
enterprise running the TLS client or TLS server. An active attacker
can also use the PSK identity to suppress handshakes or application
data from a specific device by blocking, delaying, or rate-limiting
traffic. Techniques for mitigating these risks require further
analysis and are out of scope for this document.
In addition to linkability in the network, external PSKs are
intrinsically linkable by PSK receivers. Specifically, servers can
link successive connections that use the same external PSK together.
Preventing this type of linkability is out of scope.
8. Security Considerations
Security considerations are provided throughout this document. It
bears repeating that there are concerns related to the use of
external PSKs regarding proper identification of TLS 1.3 endpoints
and additional risks when external PSKs are known to a group.
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It is NOT RECOMMENDED to share the same PSK between more than one
client and server. However, as discussed in Section 5.1, there are
application scenarios that may rely on sharing the same PSK among
multiple nodes. [I-D.ietf-tls-external-psk-importer] helps in
mitigating rerouting and Selfie style reflection attacks when the PSK
is shared among multiple nodes. This is achieved by correctly using
the node identifiers in the ImportedIdentity.context construct
specified in [I-D.ietf-tls-external-psk-importer]. One solution
would be for each endpoint to select one globally unique identifier
and use it in all PSK handshakes. The unique identifier can, for
example, be one of its MAC addresses, a 32-byte random number, or its
Universally Unique IDentifier (UUID) [RFC4122]. Note that such
persistent, global identifiers have privacy implications; see
Section 7.
Each endpoint SHOULD know the identifier of the other endpoint with
which it wants to connect and SHOULD compare it with the other
endpoint's identifier used in ImportedIdentity.context. It is
however important to remember that endpoints sharing the same group
PSK can always impersonate each other.
Considerations for external PSK usage extend beyond proper
identification. When early data is used with an external PSK, the
random value in the ClientHello is the only source of entropy that
contributes to key diversity between sessions. As a result, when an
external PSK is used more than one time, the random number source on
the client has a significant role in the protection of the early
data.
9. IANA Considerations
This document makes no IANA requests.
10. References
10.1. Normative References
[I-D.ietf-tls-external-psk-importer]
Benjamin, D. and C. A. Wood, "Importing External PSKs for
TLS", Work in Progress, Internet-Draft, draft-ietf-tls-
external-psk-importer-06, 3 December 2020,
<https://www.ietf.org/archive/id/draft-ietf-tls-external-
psk-importer-06.txt>.
[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>.
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[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>.
10.2. Informative References
[AASS19] Akhmetzyanova, L., Alekseev, E., Smyshlyaeva, E., and A.
Sokolov, "Continuing to reflect on TLS 1.3 with external
PSK", 2019, <https://eprint.iacr.org/2019/421.pdf>.
[GAA] "TR33.919 version 12.0.0 Release 12", n.d.,
<https://www.etsi.org/deliver/
etsi_tr/133900_133999/133919/12.00.00_60/
tr_133919v120000p.pdf>.
[I-D.ietf-tls-ctls]
Rescorla, E., Barnes, R., and H. Tschofenig, "Compact TLS
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
ctls-04, 25 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-tls-ctls-
04.txt>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-43, 30 April 2021,
<https://www.ietf.org/archive/id/draft-ietf-tls-
dtls13-43.txt>.
[I-D.irtf-cfrg-cpace]
Abdalla, M., Haase, B., and J. Hesse, "CPace, a balanced
composable PAKE", Work in Progress, Internet-Draft, draft-
irtf-cfrg-cpace-05, 14 January 2022,
<https://www.ietf.org/archive/id/draft-irtf-cfrg-cpace-
05.txt>.
[I-D.irtf-cfrg-opaque]
Bourdrez, D., Krawczyk, H., Lewi, K., and C. A. Wood, "The
OPAQUE Asymmetric PAKE Protocol", Work in Progress,
Internet-Draft, draft-irtf-cfrg-opaque-07, 25 October
2021, <https://www.ietf.org/archive/id/draft-irtf-cfrg-
opaque-07.txt>.
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[I-D.mattsson-emu-eap-tls-psk]
Mattsson, J. P., Sethi, M., Aura, T., and O. Friel, "EAP-
TLS with PSK Authentication (EAP-TLS-PSK)", Work in
Progress, Internet-Draft, draft-mattsson-emu-eap-tls-psk-
00, 9 March 2020, <https://www.ietf.org/archive/id/draft-
mattsson-emu-eap-tls-psk-00.txt>.
[Krawczyk] Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and Its Use in the IKE
Protocols", Annual International Cryptology Conference.
Springer, Berlin, Heidelberg , 2003,
<https://link.springer.com/content/
pdf/10.1007/978-3-540-45146-4_24.pdf>.
[LwM2M] "Lightweight Machine to Machine Technical Specification",
n.d.,
<http://www.openmobilealliance.org/release/LightweightM2M/
V1_0-20170208-A/OMA-TS-LightweightM2M-
V1_0-20170208-A.pdf>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000,
<https://www.rfc-editor.org/info/rfc2865>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<https://www.rfc-editor.org/info/rfc3748>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/info/rfc4279>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
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[RFC6614] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS",
RFC 6614, DOI 10.17487/RFC6614, May 2012,
<https://www.rfc-editor.org/info/rfc6614>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.
[RFC8773] Housley, R., "TLS 1.3 Extension for Certificate-Based
Authentication with an External Pre-Shared Key", RFC 8773,
DOI 10.17487/RFC8773, March 2020,
<https://www.rfc-editor.org/info/rfc8773>.
[Selfie] Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3
with PSK", 2019, <https://eprint.iacr.org/2019/347.pdf>.
[Sethi] Sethi, M., Peltonen, A., and T. Aura, "Misbinding Attacks
on Secure Device Pairing and Bootstrapping", Proceedings
of the 2019 ACM Asia Conference on Computer and
Communications Security , 2019,
<https://arxiv.org/pdf/1902.07550>.
[SmartCard]
"Technical Guideline TR-03112-7 eCard-API-Framework -
Protocols", 2015, <https://www.bsi.bund.de/SharedDocs/Down
loads/DE/BSI/Publikationen/TechnischeRichtlinien/TR03112/
TR-03112-api_teil7.pdf?__blob=publicationFile&v=1>.
Appendix A. Acknowledgements
This document is the output of the TLS External PSK Design Team,
comprised of the following members: Benjamin Beurdouche, Björn Haase,
Christopher Wood, Colm MacCarthaigh, Eric Rescorla, Jonathan Hoyland,
Martin Thomson, Mohamad Badra, Mohit Sethi, Oleg Pekar, Owen Friel,
and Russ Housley.
This document was improved by a high quality reviews by Ben Kaduk and
John Mattsson.
Authors' Addresses
Russ Housley
Vigil Security
Email: housley@vigilsec.com
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Jonathan Hoyland
Cloudflare Ltd.
Email: jonathan.hoyland@gmail.com
Mohit Sethi
Ericsson
Email: mohit@piuha.net
Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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