TLS & RA: Is Crypto Binding Needed For Secure Attestation?

In the realm of secure communications and remote attestation, the question of whether cryptographic binding between TLS (Transport Layer Security) and RA (Remote Attestation) is necessary frequently arises. This discussion is crucial for ensuring the security and trustworthiness of remote systems. This article will delve into what cryptographic binding means, why it is important, and how it can be achieved, while also addressing key questions for consideration.

Understanding Cryptographic Binding

Cryptographic binding is a critical security mechanism that links a secure channel session with a remote attestation session. This link gives the relying party (RP) confidence that the entity establishing the secure channel is indeed the attester. When the attester possesses a WebPKI-based identifier, the RP can confidently trust that the attestation credential (AC) accurately represents the entity owning that identifier. This is paramount in establishing trust and security in various applications and systems.

The essence of cryptographic binding lies in creating an unbreakable link between the secure channel and the attestation process. This ensures that the relying party can verify that the attestation credential truly belongs to the entity communicating through the secure channel. Without this binding, the entire process becomes vulnerable to attacks, undermining the security guarantees that remote attestation aims to provide. This secure connection safeguards against potential threats and ensures the integrity of the communication channel.

Implementing cryptographic binding involves intricate technical considerations. It requires carefully designed protocols and mechanisms to ensure that the link between the secure channel and the attestation process cannot be forged or manipulated. This often entails using cryptographic techniques such as key derivation, hashing, and digital signatures to create a secure and verifiable binding. By employing these methods, the system can confidently assert the identity and trustworthiness of the remote entity, fostering a secure environment for communication and data exchange.

The significance of cryptographic binding extends to various domains, including cloud computing, IoT devices, and secure enclaves. In cloud computing, it enables secure attestation of virtual machines and containers, ensuring that only authorized and trusted entities can access sensitive resources. In IoT devices, it facilitates secure bootstrapping and authentication, preventing unauthorized devices from infiltrating the network. In secure enclaves, it provides a means to verify the integrity and authenticity of the code running within the enclave, safeguarding against malicious attacks and data breaches. Through these applications, cryptographic binding plays a pivotal role in enhancing the security and trustworthiness of diverse systems and applications.

The Importance of Strong Binding

Strong binding is essential to prevent man-in-the-middle (MitM) attacks, where a malicious entity intercepts and relays communications between two parties. Without robust binding, an attacker could simply forward an AC from a legitimate TEE (Trusted Execution Environment) as their own. The absence of strong binding compromises the provenance of the AC, rendering its contents untrustworthy. Relying solely on nonces, for instance, is insufficient because they can be easily forwarded between separate sessions.

Imagine a scenario where a device claims to be a trusted entity based on an attestation credential. Without strong binding, an attacker could intercept this claim and present a valid AC stolen from a legitimate device. The relying party, unaware of the deception, would grant access to sensitive resources, believing they are communicating with the trusted entity. This breach of trust could lead to severe consequences, such as data theft, system compromise, and financial losses. Strong binding acts as a shield, preventing such attacks by ensuring that the attestation credential is inextricably linked to the actual device presenting it.

To achieve strong binding, it is necessary to employ cryptographic techniques that establish an unforgeable connection between the attestation credential and the entity presenting it. This may involve using digital signatures, key derivation functions, or other cryptographic primitives to create a secure binding that cannot be easily broken. The relying party can then verify this binding to ensure that the attestation credential is authentic and that the entity presenting it is indeed the legitimate owner. By implementing strong binding, the system can effectively defend against MitM attacks and maintain the integrity and trustworthiness of the attestation process.

The importance of strong binding cannot be overstated in today's increasingly interconnected world. As more and more devices and systems rely on remote attestation to establish trust, the need for robust security measures becomes paramount. Strong binding is a cornerstone of this security, providing assurance that the attestation credential is valid and that the entity presenting it is who they claim to be. By prioritizing strong binding, organizations can protect their systems from malicious attacks, maintain the confidentiality of their data, and foster a secure environment for communication and collaboration.

Achieving Cryptographic Binding: Methods and Considerations

The methods to achieve cryptographic binding vary, and the decision of whether to specify a particular mechanism in a draft is complex. It is often best to allow protocol designers the flexibility to ensure MitM attacks are not possible. However, here are some examples in the context of TLS:

Exporting Keying Material

Exporting keying material from the TLS session and including it in the AC is one approach. The RP can verify this binding to confirm the provenance. This method provides a strong guarantee but may be complex to implement, potentially requiring changes to the key schedule. The process of exporting keying material involves extracting a portion of the cryptographic keys used to establish the TLS session. This material is then incorporated into the attestation credential, creating a cryptographic link between the secure channel and the attestation process. When the relying party receives the attestation credential, it can verify the exported keying material against the TLS session to ensure that the credential is valid and that the entity presenting it is indeed the legitimate owner. This approach provides a high level of security but may require modifications to the TLS protocol and the attestation framework.

While exporting keying material offers a strong guarantee, it also presents some challenges. Implementing this method may require significant changes to the key schedule of the TLS protocol, which could impact compatibility with existing systems. Additionally, the process of extracting and incorporating keying material into the attestation credential may add complexity to the attestation process, potentially increasing the overhead and latency. Therefore, it is important to carefully consider the trade-offs between security and performance when deciding whether to implement this approach.

Binding to WebPKI Certificate

Binding the AC to the attester's webPKI certificate (e.g., by hashing the cert or the public key) is another option. The RP can verify that the AC was issued for an entity who owns that cert, which CertificateVerify proves is the peer. This is simpler to implement and does not involve invasive changes to the secure channel protocol. However, it only works if the webPKI-certified key has never left the TEE/attester; if provisioned at runtime, the guarantee vanishes. The process of binding the AC to the webPKI certificate involves creating a cryptographic link between the attestation credential and the public key certificate associated with the attester. This can be achieved by hashing the certificate or the public key and including the hash in the attestation credential. When the relying party receives the attestation credential, it can verify the hash against the certificate to ensure that the credential is valid and that the entity presenting it is indeed the legitimate owner.

One of the advantages of this approach is that it does not require significant changes to the secure channel protocol. However, it is important to ensure that the webPKI-certified key has never left the TEE/attester. If the key was provisioned at runtime, the guarantee vanishes because the key owner could just use the same webPKI credential from an insecure context. Therefore, it is important to carefully manage the lifecycle of the webPKI certificate to ensure that it remains secure and trustworthy.

Binding WebPKI Certificate to Attesting Platform

Binding the webPKI certificate to the (unique) attesting platform allows the RP to verify that the platform identifier from the webPKI cert corresponds to whatever is in the AC. This is similar to the above but only possible if the attester has a unique identity. By linking the webPKI certificate to the unique attesting platform, the relying party gains assurance that the attestation credential is not only valid but also tied to a specific, identifiable device. This approach adds an extra layer of security by ensuring that the attestation credential cannot be easily transferred or reused on different platforms.

However, this method relies on the attester having a unique identity, which may not always be the case. In situations where the attester lacks a distinct identifier, this approach may not be feasible. Additionally, similar to the previous method, it is essential to maintain the security and integrity of the webPKI certificate to prevent unauthorized access or manipulation.

Key Questions for the Working Group

• Is cryptographic binding truly necessary for all use cases? While it appears critical, are there scenarios where it might be less important? The necessity of cryptographic binding hinges on the specific security requirements of the application. In high-security scenarios, such as those involving sensitive data or critical infrastructure, cryptographic binding is essential to prevent man-in-the-middle attacks and ensure the integrity of the attestation process. However, in lower-risk scenarios, the overhead and complexity of cryptographic binding may outweigh the benefits. Therefore, it is important to carefully assess the security requirements of each application to determine whether cryptographic binding is necessary. Factors to consider include the value of the data being protected, the potential impact of a security breach, and the cost and complexity of implementing cryptographic binding.

• How detailed should the draft be in describing cryptographic binding? Does a lower-level wording limit the scope of the charter? The level of detail in the draft should strike a balance between providing sufficient guidance to protocol designers and avoiding unnecessary constraints on innovation. A lower-level wording may limit the scope of the charter if it restricts the types of cryptographic binding mechanisms that can be used. However, a higher-level wording may lack the specificity needed to ensure that cryptographic binding is implemented correctly. Therefore, it is important to carefully consider the level of detail in the draft to ensure that it provides adequate guidance without unduly restricting the design space.

• Should the property be phrased negatively, such as