Trustworthy Enrollment of Secure Credentials

Introduction Success of a technology is ultimately measured by its adoption. The RATS Architecture requires that RATS Relying Parties understand Attestation Results expressed using standards such as EAT and AR4SI, execute Appraisal Policy for Attestation Results, and have trust in Verifiers. Additionally, there is an unstated assumption present in the RATS Architecture that a change in Evidence may lead to a change in either the Attestation Results or Appraisal Policy for Attestation Results. This requirement may pose a significant adoption blocker. One key requirement for successful deployment of Remote Attestation-capable workloads is minimal blast radius. When a workload is moved from a legacy to a remotely attestable (e.g. Trusted Execution) environment, including Intel SGX, AMD SEV-SNP, ARM TrustZone, that workload can use Remote Attestation to obtain a stable and trustworthy Identity Document while its clients and servers do not notice anything different. For that, a mechanism is required by means of which a Credential Broker, a Key Broker, or a Credential Authority takes on the role of RATS Relying Party. This provides an intermediation between Attestation Results, expressed using formats such as EAT and AR4SI, and the RATS-Unaware Relying Parties whose authentication and authorization policies may precede the introduction of Remotely Attestable Workloads and remain static for long periods of time. For the RATS-Unaware Relying Parties, these adoption barriers are eliminated, as these RUPs are capable of authenticating their clients utilizing appropriate Identity Documents. This includes shared symmetric keys (bearer tokens), credentials including PKIX certificates , JWTs , or WIMSE WITs . In this world, the Attester uses Remote Attestation to obtain from the RATS Relying Party a key, token or credential that is compatible with the RUP. This document details an architecture by which legacy Identity Document Identity Document issuance mechanisms are replaced with identical Identity Documents issued, but with the additional prerequisite of successful Remote Attestation of the workloads in question.

Assumptions about Workload Immutability While updates and upgrades to the workload and the RATS Unaware Party to add a Remote Attestation capability are not possible in this environment, some changes to the Attesting environment are required in order to do anything. The assumption is that the workload may be a compiled object or container provided by a third party. Or the workload may be in a language not easily changed or upgraded with new capabilities. At an extreme example, it could be an ancient COBOL program compiled into a WASM object, perhaps connected to the network via virtual paper-tape and virtual printer interfaces. Further, such a system may require extensive and significant review by an authority before changes to the core algorithm can be made. These workloads run in a virtual machine (VM with unique kernel), or in a containerized environment (common kernel). They never run on bare hardware, and there is a hypervisor and/or orchestration environment that arranges the workload and any needed configurations. However, it is assumed that some the following changes can be made:

network connections use mutual TLS, and the origin of the keypair used for client authentication can be changed or configured by an operator
the TLS code, while built-in to the application, can be configured to use a Secure Element or TPM as the source for the private key. Current TLS stacks such as OpenSSL can be configured to use engines or providers to do asymmetric operations, and providers exist that talk to a TPM for all private key operations.
in the case of bearer token authentication, that the token can be configured external to the code
that other components or configurations can be added to the execution environment by the operator
that the orchestration environment can be extended with new capabilities without affecting the workload itself

Hostile Regulator A motivating factor in this work is that there are workloads that are mandated to operate in specific geographies under inspection by a local authority. The inspection process by the regulator may include agents that must run within the secured environment, where it may examine inputs and outputs to the workload. These agents do not have the full trust of the workload owners or RATS Unaware Party. The trustworthiness of the workload is not absolute (no trust ever is), however there is a need to provide assurance that only the regulator's agent is present, and no additional malware has been introduced. (For instance, the agent may have exploits known to additional parties, not yet revealed or fixed by the regulator)

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 when, and only when, they appear in all capitals, as shown here. This document uses terms and concepts defined by the WIMSE and RATS architectures, as well as the terms defined by the Trustworthy Workload Identity Special Interest Group at the Confidential Computing Consortium. For a complete glossary, see , & . The definitions of terms like Trustworthy Workload Identity and Workload Credential match those specified by the TWI SIG Definitions .

Broker:: an entity that deals out pre-existing keys or credential. Constrast to a Credential Authority which mints new credentials.
RUP:: The RATS Unaware Party (RUP). A target service that interacts with many clients based upon credentials provided. This is sometimes called the Collaborating Party.
Workload:: defines 'Workload' as "an instance of software executing for a specific purpose". Here we restrict that definition to the portions of the deployed software and its configuration that are subject to Remote Attestation.
Workload Duration:: the lifespan of the workload. While some workloads can be very long lived, but many workloads are created for a brief period of time, often added on demand to support rising demand, and persisting for only minutes to a small fraction of a day.
Workload Owner:: the entity that manages a workload, arranging to provision it with appropriate Workload Credentials before Workload is launched
Workload Credential:: an ephemeral identity document containing an identity and a number of additional claims, that can be short-lived or long-lived, and that is used to access a service
Proof of possession credential:: this is a credential, such as a JWT, that contains no other identity or authorization claims. It is trusted by the RUP due to local policy.
Collaborating Party:: see RUP.
Verifier:: an entity performing the role of Attestation Verification, as documented in

Overview of Mechanism A newly created workload connects to the Credential Broker to obtain a set of credentials to be used to perform it's functions. Within this connection, Evidence is transferred to the Credential Broker to demonstrate the workloads' trusthworthiness. The Credential Broker is acting as a RATS Relying Party, the workload is the Attester. The Credential Broker contacts (using the background check model), a Verifier that it trusts in order to evaluate the Evidence, obtaining an Attestation Result. Figure extends the architecture to show how the workload and credential broker take on the roles of Attester and Relying Party. If the Attestation Result is acceptable, then the Credential Broker provides the set of credentials that the workload needs to accomplish it's task.

Credential Enrollment Architecture | Verifier +------. | | '-------------------------' | | | | | | Evidence Attestation | | | Results | | | | | | v v .-----+----. .-------------------. | Attester | | Relying Party | |----------|------------enrollment--------->|-------------------| | workload | | credential broker | '-----+----' '-------------------' \ \ authentication .---------------------. `----work-data---------->| RATS Unaware | | Relying Party | | Collaborating Party | '---------------------' RATS-Unaware ^ Authentication | Policy | .--------------+-. | RATS-Unaware | | Relying Party | | Owner | '----------------' ]]>

Types of Credentials There are three kinds of credentials that can be involved. Some workloads might use a few of each, possibly with each one being used with a different RATS Unaware Party.

The Credential Broker is also an Identity Provider (IdP), and acts as an Registration Authority (possibly including the Certification Authority). It issues new credentials in the form of PKIX certificates to each trustworthy workload.
The Credential Broker is a respository for a credential issued by another Identity Provider (IdP). The Credential Broker has both the private key (encrypted) and the certificate, and it discloses these to trustworthy workloads by returning them in a unique encryption, bound to the workload identity.
The Credential Broker is a resposity for a bearer token issued by a Resource Owner, or a Workload Identity Tokens (WITs) defined in Section 3.1 of . The Credential Broker discloses this to trustworthy workloads by returning them in a unique encryption, bound to the workload identity.

The use of a shared assymetric private key is unorthodox. This architecture is justified by the need to rapidly scale the number of workers and to recover from hardware or network failures. The alternatives is that external Identity Providers would need to be willing to respond to spikes of hundreds of credential requests within a small period of time. This would look like a denial of service attack, and it may also require additional human authorization for each. The patterns of communication shown in figure are designed specifically such that as few modifications are required to the workload, and no changes to the Collaborating Party are required.

Deployment of Credentials Workloads are expected to include a (virtual) Trusted Platform Module (TPM) (or equivalent) by which they will collect and sign Evidence to be used in the Remote Attestation process. The credential that will be shared by the Credential Broker will be encrypted to a key involved in the Remote Attestation process. The most natural mechanism is to encrypt to a key that is available only to the TPM. The credential are then decrypted by the TPM, and the keypair can then be made available to the workload, while never permitting the workload to ever see the key. In this way, a workload that be designed to do mutual TLS using a client-certificate, and for which the location of the private key can be configured to be in a TPM, can be adapted to the mechanism described in this document without any significant change to the workload itself.

Details of protocol As there are three major types of credentials that may be used, it is not unreasonable that they may get provisioned in different ways, using different protocols.

Use of Enrollment over Secure Transport (EST) EST () describes a mechanism to enroll with a certification authority using a TLS secured HTTP based protocol.

Credential Broker as Identity Provider EST is used by the hypervisor (or container orchestrator) to connect to the Identity Provider. The EST protocol is extended to include transmission of Evidence from the Attester to the Identity Provider. This Identity Provider acts as a RATS Relying Party, in Background-Check mode. The Evidence is passed to an appropriately trusted Verifier, and evaluated. Based upon the Attestation Results, the Identity Provide then allows the hypervisor to use the EST /simpleenroll mechanism to provide a CSR, and retrieve an appropriate certificate. The private key for the certificate can be generated within a TPM, never to leave. The hypervisor then inserts the certificate into an appropriate place for inline transmission by mutual TLS. There are three ways to handle the Evidence:

via a new, Remote Attestation extension to EST
using extensions to the CSR itself
within TLS itself, using for instance, , or whichever protocol the SEAT WG standardizes

Credential Broker as Secure Repository EST is used by the hypervisor (or container orchestrator) to connect to the Secure Repository The EST protocol is extended to include transmission of Evidence from the Attester to the Secure Repository. The EST /serverkeygen mechanism is used. The server does not generate a fresh key, but rather retrieves the keypair (private key and certificate) from the store. This is encrypted back to the client using one of the mechanisms described in RFC7030. (TBD: This needs more detail, particularly for the mTLS used in the EST) As before, there are three possible ways to transmit the Evidence:

via a new, Remote Attestation extension to EST
using extensions to the CSR itself. The serverkeygen mechanism still sends a CSR, with a fake public key.
within TLS itself, using for instance, , or whichever protocol the SEAT WG standardizes

Credential Broker as short-term Bearer Token issuer EST is not appropriate for this use case. Another protocol will be required. TBD.

Security Considerations All communications between entities (Workload to Credential Authority, Workload to Verifier etc) MUST be secured using mutually authenticated, confidential, and integrity-protected channels (e.g., TLS). In addition to the considerations herein, Verifier, which is a central point of anchor for Trustworthy Workload Identifer MUST follow the security guidance detailed in the "Security and Privacy considerations" as detailed in the RATS Architecture Section and Section of . The credential key MUST always be stored securely at all time, for example in a secure element of the underlying platform running the Workload. There is a risk that a live Workload Migration may render some of the claims about the Workload invalid (e.g., live-migrating a Workload between Germany and France may incorrectly preserve the "Country=Germany" claim, but correctly preserve the "Region=Europe" claim).

Privacy Considerations Remote Attestation of a Workload requires exchange of attestation related messages, for example, Evidence and Attestation Results. This can potentially leak sensitive information about the Workload. Confidentiality: Encryption could be used to prevent unauthorised parties from accessing sensitive information from Evidence or Attestation Results. This is crucial in multi-tenant environments. The Credential Key to be released to a Workload MUST always be encrypted to avoid potential leakage to unauthorised actors.

IANA Considerations This document has no IANA actions (yet).