Guidance for External PSK Usage in TLS

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Table of Contents

1. Introduction

This document provides usage guidance for external Pre-Shared Keys (PSKs) in TLS. It lists TLS security properties provided by PSKs under certain assumptions and demonstrates how violations of these assumptions lead to attacks. This document also discusses PSK use cases, provisioning processes, and TLS stack implementation support in the context of these assumptions. It provides advice for applications in various use cases to help meet these assumptions.¶

The guidance provided in this document is applicable across TLS [RFC8446], DTLS [I-D.ietf-tls-dtls13], and Constrained TLS [I-D.rescorla-tls-ctls].¶

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.¶

4. PSK Security Properties

The external PSK authentication mechanism in TLS 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 in Section 6, there are use cases where it is desirable for multiple clients or multiple servers to share a PSK. If this is done naively by having all members share a common key, then TLS only authenticates the entire group, 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 a group member is compromised, then the attacker can impersonate any group member (this follows from property (1)).¶
3. If PSK without DH is used, then compromise of any group member allows the attacker to passively read all traffic.¶

In addition to these, a malicious non-member can reroute handshakes between honest group members to connect them in unintended ways, as detailed below.¶

Let the group of peers who know the key be A, B, and C. The attack proceeds as follows:¶

1. A sends a ClientHello to B.¶
2. The attacker intercepts the message and redirects it to C.¶
3. C responds with a 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, ostensibly with A.¶

This attack violates the peer authentication property, and if C supports a weaker set of cipher suites than B, this attack also violates the downgrade protection property. 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.¶

Entropy properties of external PSKs may also affect TLS security properties. In particular, if a high entropy PSK is used, then PSK-only key establishment modes are secure against both active and passive attack. However, they lack forward security. Forward security may be achieved by using a PSK-DH mode.¶

In contrast, if a low entropy PSK is used, then PSK-only key establishment modes are subject to passive exhaustive search passive 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 PAKE is used with TLS.¶

5. Privacy Properties

PSK privacy properties are orthogonal to security properties described in Section 4. Traditionally, 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 it 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. Applications should take precautions when using external PSKs to mitigate these risks.¶

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, as PSKs are explicitly designed to support mutual authentication.¶

6. External PSK Use Cases and Provisioning Processes

Pre-shared Key (PSK) ciphersuites were first specified for TLS in 2005. Now, PSK is 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 (OOB). Below, we list some example use-cases where pair-wise external PSKs with TLS have been used for authentication.¶

• 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 PoP may use externally provisioned PSKs, primarily for the purposes of supporting TLS connections with fast open (0-RTT data).¶
• 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.¶
• Use of PSK ciphersuites are optional when securing RADIUS [RFC2865] with TLS as specified in [RFC6614].¶
• The Generic Authentication Architecture (GAA) defined by 3GGP mentions that TLS-PSK can be used between a 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].¶

There are also use cases where PSKs are shared between more than two entities. Some examples below (as noted by Akhmetzyanova et al.[Akhmetzyanova]):¶

• Group chats. In this use-case, the membership of a group is confirmed by the possession of a PSK distributed out-of-band to the group participants. Members can then establish peer-to-peer connections with each other using the external PSK. It is important to note that any node of the group can behave as a TLS client or server.¶
• Internet of Things (IoT). In this use-case, resource-constrained IoT devices act as TLS clients and share the same PSK. The devices use this PSK for quickly establishing connections with a central server. Such a scheme ensures that the client IoT devices are legitimate members of the system. To perform rare system specific operations that require a higher security level, the central server can request resource-intensive client authentication with the usage of a certificate after successfully establishing the connection with a PSK.¶

7. Recommendations for External PSK Usage

Applications MUST use external PSKs that adhere to the following requirements:¶

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 a DH exchange 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 these mechanisms do not necessarily follow the same architecture as the ordinary process for incorporating EPSKs described in this draft.¶
2. Unless other accommodations are made, 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 EPSK importer.¶
3. Nodes SHOULD use external PSK importers [I-D.ietf-tls-external-psk-importer] when configuring PSKs for a pair of TLS client and server.¶
4. Where possible the master PSK (that which is fed into the importer) SHOULD be deleted after the imported keys have been generated. This protects an attacker from bootstrapping a compromise of one node into the ability to attack connections between any node; otherwise the attacker can recover the master key and then re-run the importer itself.¶

8. Security Considerations

It is NOT RECOMMENDED to share the same PSK between more than one client and server. However, as discussed in Section 6, 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]. It is RECOMMENDED that each endpoint selects one globally unique identifier and uses 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]. Each endpoint SHOULD know the identifier of the other endpoint with which its 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.¶

9. IANA Considerations

This document makes no IANA requests.¶

Appendix A. Acknowledgements

This document is the output of the TLS External PSK Design Team, comprised of the following members: Benjamin Beurdouche, Bjoern Haase, Christopher Wood, Colm MacCarthaigh, Eric Rescorla, Jonathan Hoyland, Martin Thomson, Mohamad Badra, Mohit Sethi, Oleg Pekar, Owen Friel, and Russ Housley.¶

Authors' Addresses