Disconnected edge CA key custody

An edge CA is allowed to sign while its host cannot reach the control plane, so its private key is the dangerous part. The safe default puts that key in a TPM2 object that cannot be exported. The agent persists only a JSON handle containing the provider name, opaque object ID, algorithm, and public key.

TPM2 default

Generate generation 1 of the segment key and its CSR:

trstctl-agent \
  --edge-csr \
  --edge-tenant 77777777-7777-7777-7777-777777777777 \
  --edge-segment 88888888-8888-8888-8888-888888888888 \
  --edge-cn bunker-01-edge-ca \
  --edge-key-provider tpm2 \
  --edge-key-generation 1 \
  --edge-key-handle-out /var/lib/trstctl/edge-ca.keyref.json \
  --edge-csr-out /var/lib/trstctl/edge-ca.csr

--edge-tpm-path can name /dev/tpmrm0, /dev/tpm0, or a swtpm Unix socket. Owner/key authorization values come only from --edge-tpm-owner-auth-file and --edge-tpm-key-auth-file; they are read as bytes and wiped after the device opens. Repeating the same tenant, segment, and generation reconciles the same persistent object after a lost response. Use a new generation label when intentionally rotating the edge CA.

The command prints the CSR as base64 and the exact attestation challenge. TPM attestation tooling must produce a WebAuthn TPM credential over that challenge for the CSR key. The mint request uses key_provider: "tpm2". The control plane checks that the attested public-key digest equals the CSR public-key digest, so an unrelated software key cannot borrow the host TPM's proof.

After the control plane returns the delegated certificate chain:

trstctl-agent \
  --edge-issue \
  --edge-key-provider tpm2 \
  --edge-ca-cert /var/lib/trstctl/edge-ca.crt \
  --edge-ca-key-handle /var/lib/trstctl/edge-ca.keyref.json \
  --edge-issue-cn db.edge.example.test \
  --edge-cert-out /var/lib/trstctl/db.crt \
  --edge-leaf-key-out /var/lib/trstctl/db.key \
  --edge-journal /var/lib/trstctl/edge-journal.json

The agent verifies that the certificate public key equals the handle public key before asking the TPM to sign. A missing TPM, unknown handle, changed provider, or mismatched certificate refuses issuance. None triggers a software fallback.

PKCS#11 alternative

Set --edge-key-provider pkcs11 and configure --edge-pkcs11-module, --edge-pkcs11-token, and --edge-pkcs11-pin-file for both CSR creation and issuance. The shipping module creates persistent RSA-2048 private objects with CKA_SENSITIVE=true and CKA_EXTRACTABLE=false. The segment policy must explicitly include pkcs11 in allowed_key_providers.

The default static artifact has CGO disabled and therefore refuses this lane. Build the agent with CGO enabled on the token host so it can load the vendor's native PKCS#11 module; a static artifact never substitutes software custody.

The host TPM attestation still authenticates the host and binds the request to the CSR, but it cannot attest a different PKCS#11 object. Evidence therefore says host_attested_operator_claim. It does not claim that the TPM proved token custody. Independent token inspection is the deployment proof that the object is non-extractable.

Exportable software exception

Software custody needs two independent choices:

1. The segment policy includes software in allowed_key_providers.
2. Both CSR creation and issuance specify --edge-key-provider software --edge-allow-software-key.

The agent then uses --edge-key-out / --edge-ca-key for a mode-0600 PKCS#8 PEM file. The control plane records key_storage=file, key_exportable=true, and custody_assurance=host_attested_software_exception. The Sovereign/edge panel shows that warning. A host TPM proof authenticates the request but never changes the software key's exportable classification.

Segment policy shape

An enabled policy is still default-off unless it exists, pins at least one TPM attestation root, and carries DNS name constraints. Custody is an additional closed allowlist:

{
  "enabled": true,
  "attestation_roots_pem": ["-----BEGIN CERTIFICATE-----..."],
  "permitted_dns_domains": ["edge.example.test"],
  "excluded_dns_domains": ["blocked.edge.example.test"],
  "allowed_key_providers": ["tpm2"]
}

Omitting allowed_key_providers means exactly ["tpm2"]. Unknown providers, duplicate values, and unapproved PKCS#11/software mint requests fail closed.