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Enclave constraints

Constraints

How do you know the EnclaveInstanceInfo you've got is for the enclave you really intend to interact with? In normal client/server programming you connect to a host using some sort of identity, like a domain name or IP address. TLS is used to ensure the server that picks up is the rightful owner of the domain name you intended to connect to. In enclave programming the location of the enclave might not matter much because the host is untrusted. Instead, you have to verify what is running, rather than where it's running.

Note

The domain name of the server can still be important in some applications, in which case you should still use TLS in addition to enclave constraints.

One way to do this is by inspecting the properties on the EnclaveInstanceInfo object and hard-coding some logic. That works fine, but testing an EnclaveInstanceInfo is a common pattern in enclave programming, so we provide an API to do it for you.

The EnclaveConstraint class takes an EnclaveInstanceInfo and performs some matching against it. A constraint object can be built in code, or it can be loaded from a small domain specific language encoded as a one-line string. The string form is helpful if you anticipate frequent upgrades that should be whitelisted or other frequent changes to the acceptable enclave, as it can be easily put into a configuration file, JSON, XML or command line flags.

The constraint lets you specify:

  1. Acceptable code hashes (measurements)
  2. Acceptable signing public keys
  3. The minimum revocation level
  4. The product ID
  5. The security level of the instance: SECURE, STALE, INSECURE
  6. The maximum age of the attestation in ISO-8601 duration format

If you specify a signing public key then you must also specify the product ID, otherwise if the organisation that created the enclave makes a second different kind of enclave in the future, a malicious host might connect you with the wrong one. If the input/output commands are similar then a confusion attack could be opened up. That's why you must always specify the product ID.

The simplest possible string-form constraint looks like this:

C:F86798C4B12BE12073B87C3F57E66BCE7A541EE3D0DDA4FE8853471139C9393F

It says "accept exactly one program, with that measurement hash". In this case the value came from the output of the build process as shown above. This is useful when you neither trust the author nor the host of the enclave, and want to audit the source code and then reproduce the build.

Often that's too rigid. We trust the developer of the enclave, just not the host. In that case we'll accept any enclave signed by the developer's public key. We can express that by listing code signing key hashes, like this:

S:5124CA3A9C8241A3C0A51A1909197786401D2B79FA9FF849F2AA798A942165D3 PROD:1

When constraining to a signing key we must also specify the product ID, because a key can be used to sign more than one product.

As you can see from the above code, the enclave constraint is passed into the client via a command line flag:

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--constraint="S:5124CA3A9C8241A3C0A51A1909197786401D2B79FA9FF849F2AA798A942165D3 PROD:1 SEC:INSECURE"

Tip

Replace the signing key in the snippet above with the enclave signer hash that was printed when you built the enclave.

The above constraint says that any enclave (even if run in simulation mode) signed by this hash of a code signing key with product ID of 1 is acceptable. Obviously in a real app, you would remove the part that says SEC:INSECURE, but it's convenient to have this whilst developing.

EnclaveConstraint.check compares the enclave's EnclaveInstanceInfo against the constraint and throws an InvalidEnclaveException) if it doesn't match. This check is done automatically by EnclaveClient.start when using EnclaveClient.

If needed, more than one key hash could be added to the list of enclave constraints (e.g. if simulation and debug modes use a distinct key from release mode). The enclave is accepted if one key hash matches.

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// Two distinct signing key hashes can be accepted.
S:5124CA3A9C8241A3C0A51A1909197786401D2B79FA9FF849F2AA798A942165D3 S:01280A6F7EAC8799C5CFDB1F11FF34BC9AE9A5BC7A7F7F54C77475F445897E3B PROD:1 SEC:INSECURE

If you are building an enclave in mock mode then the enclave reports it is using a signing key hash consisting of all zeros. If you want to allow a mock enclave to pass the constraint check then you need to include this dummy signing key in your constraint:

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// The zero dummy hash can be accepted.
S:5124CA3A9C8241A3C0A51A1909197786401D2B79FA9FF849F2AA798A942165D3 S:0000000000000000000000000000000000000000000000000000000000000000 PROD:1 SEC:INSECURE

It is also possible to specify a maximum age for the attestation using the EXPIRE keyword:

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S:5124CA3A9C8241A3C0A51A1909197786401D2B79FA9FF849F2AA798A942165D3 PROD:1 SEC:INSECURE EXPIRE:P6M2W5D

When specified, this will cause the check to fail if the timestamp within the attestation object indicates an age older than the specified duration. If no period is specified then no expiry check will be applied. The age string uses the ISO-8601 duration format. The above example is enforcing a maximum age of 6 months, 2 weeks and 5 days.