website/docs/protocol/custom_extension.md

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Custom Extension

The attestation signed by the Notary can be extended with custom extensions to support application specific requirements. Both the Prover and Notary can be configured to include custom data, which can later be verified by the third-party Verifier. The Notary can also be configured to validate any extensions requested by the Prover.

Use Cases

• Prover includes their public key to bind it to their identity.
• Notary includes their TEE (trusted execution environment) attestation to prove code integrity.
• Prover includes a nullifier to prevent reuse of the attestation.

Example

The following modifies the attestation example to include the Prover's public key as a custom extension.

Prover

The attestation prover is modified as follows.

...
    
let builder = RequestConfig::builder();

builder.extension(Extension {
    id: b"prover_public_key".to_vec(),
    value: b"PUBLIC_KEY_PEM".to_vec(),
});

let request_config = builder.build()?;

...

Note that Extension's id and value are of Vec<u8>, which means one is free to choose their encoding format.

Notary

The notary server is run with allow_extensions turned on.

NS_NOTARIZATION__ALLOW_EXTENSIONS=true cargo run --release

Note that the notary server currently doesn't support adding its own extension, or performing custom validations on extensions from the Prover. To do that, the notary server needs to be modified by using the relevant APIs outlined in the API docs.

Verifier

The attestation verifier is modified as follows.

...

let PresentationOutput {
    server_name,
    connection_info,
    transcript,
    mut extensions, // Optionally, verify any custom extensions from prover/notary.
    ..
} = presentation.verify(&crypto_provider).unwrap();

let Extension { id, value } = extensions.pop().unwrap();
// Check the prover's public key.
if id.as_slice() == b"prover_public_key" {
    let public_key_pem = String::from_utf8(value).unwrap();
    ...
}

...

The Verifier can now be confident that the attestation is binded to the identity associated with this public key.