Sharing is caring: Attestable and Trusted Workflows out of Distrustful Components

Trusted Execution Environments (TEEs) protect private memory and attest initial code and configuration, but neither constrain nor attest shared channels between TEEs or with the untrusted host. Confidentiality thus depends on the functional correctness of all in-TEE code accessing these channels: a single misuse can leak data and compromise a pipeline. This assumption is increasingly untenable for agentic applications that generate code at runtime and cloud pipelines of mutually distrustful components. We present MICA, a confidential-computing firmware extension that decouples confidentiality from trust in in-TEE code. MICA lets tenants define, restrict, and attest channel constraints between TEEs and with the untrusted host, including transitive constraints spanning a pipeline. It extends attestation to groups of communicating TEEs, letting external parties verify the confidentiality properties induced by these constraints across a pipeline. We implement MICA on Arm CCA by extending the Realm Management Monitor with a small policy language, requiring no hardware changes and minor QEMU and KVM changes. Our evaluation demonstrates support for confidential agentic applications and pipeline computations without trusting the functional correctness of in-TEE code. MICA increases the TCB by 7% and incurs infrequent setup costs: 2-4% boot-time overhead, approximately 1.5 s for policy upload, and 246-598 ms for group attestation. Once established, Realm communication is direct, uses protected memory, and requires no MICA intervention or software encryption, yielding up to 14.5 times lower cross-TEE message latency than encrypted communication over untrusted memory.