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Updating CCF PDO/TP Documentation with details about recent API changes
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used to set PDO contract enclave attestation policy. The API changes are
part of PR 467 (https://github.com/hyperledger-labs/private-data-objects/pull/467/files#)

This PR updates the subsection `CCF TP TEE attestation verification policy` contained
within ledgers/ccf/README.md to reflect the new APIs introduced in PR 467.

Signed-off-by: Prakash Narayana Moorthy <[email protected]>
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prakashngit committed Mar 4, 2024
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make test
```

## CCF TP TEE attestation policy
We briefly describe the attestation verification policy implemented by CCF TP.
- CCF Governance consortium registers attestation policy after TP is deployed.
- Currently, TP supports two policies:
- No attestation verification: This policy is used while using PDO enclaves in SGX SIM mode.
- Check EPID attestation verification reports generated by IAS: This policy is used while using PDO enclaves in SGX HW mode.
- One of the two policies must necessarily have been registered by the CCF consortium before any PDO enclave can be registered. There is no default policy.
CCF TP provides a member-rpc that can be used for registering one of the above two policies. For the second policy above, the consortium
gets to specify expected MREnclave, basename and IAS public key via the member-rpc.
- The policy (including expected value of MREnclave) can be changed anytime by the CCF Governance consortium, subject to voting rules of the consortium.
## CCF TP TEE attestation verification policy
CCF TP provides two APIs to be used by the CCF Governance consortium to register attestation verification policy that must be satisfied by PDO contract enclaves.

1. The first API `set_attestation_check_flag` is invoked as part of the TP start up scripts to specify whether PDO runs in SGX `HW` mode or SGX `SIM` mode. The flag can be set only once. There is no default value for the flag, and hence must be set explicitly before the TP can accept any `register_encalve` transactions.

2. The second API `set_expected_sgx_measurements` is used whenever the `set_attestation_check_flag` specifies that PDO runs in SGX `HW` mode. In this case, the second API is used to the specify expected `MREnclave` value, and additionally `basename` and the `ias_public_key`. Note that PDO currently supports SGX `HW mode` with EPID attestation. The expected SGX measurements can be updated via the second API, subject to voting rules of the consortium.

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