Adding operations to Parsec

This mini how-to will cover how to add operations to the Parsec service and Rust client. The steps for adding a new provider operation will differ between providers, this guide will cover Mbed Crypto. The steps for adding operations to Parsec, Rust interface, and Rust client should be the same for all backend providers.

Operation specification


Create the specification page for the Parsec Book, under Operations (can also be found in the menu on the left of all pages). Include operation details such as parameters, results, errors specific to the operation, a description, and a link to the protobuf contract (which will be created further on). Also update all API tables on the Parsec Operations Coverage page to reflect the new addition (create any API tables if they are missing).

The opcode used should be the next available one. To find this, go to parsec-interface-rs/src/requests/ and find the largest in-use opcode.


Create the protobuf contracts in parsec-operations. These are the contracts that client libraries use to communicate with the Parsec service.

Rust Interface


Create the Rust interfaces for the operation. These interfaces go between protobuf and the Parsec service on the service side, and the Rust client library and protobuf on the client side. Take care when deciding on datatypes. Sensitive data should be wrapped in either Zeroizing or Secret (choose depending on the sensitivity of the data. Favour Zeroizing unless there is a clear requirement for Secret) to ensure they are wiped from memory when dropped and should not have any output for debugging. The operation definitions are stored in src/operations/.

A validate method can be added to allow easy validation of the properties of the operations, to check if there are any conflicting options, or combinations of options that are invalid. Tests must be added for each operation with a validate method, to check it catches all erroneous combinations of operation, and passes valid combinations.

A converter then needs to be added to src/operations_protobuf. This will convert to and from the protobuf contract and the Rust interface. There should be four methods per operation; two for going to and from the operation struct, and two for going to and from the result struct. Again, tests need to be added to ensure all conversions happen correctly.

Add your protobuf contracts to the include_protobuf_as_module block in src/operations_protobuf/ If your Rust operation and/or result interfaces do not contain any sensitive information, add them to the empty_clear_message! block. Otherwise, implement ClearProtoMessage for the interface/s that contain sensitive information and zeroize any of the sensitive fields.

In src/requests/, add the opcode that was specified in the parsec-book for the operation. Finally, add the relevant NativeOperation and NativeResult entries, along with the mappings to the opcode from the NativeOperation and NativeResult to the Opcode. Then, add From implementations to NativeOperation and NativeResult.

Finally, add the mod declaration in src/operations_protobuf/, along with the entries in body_to_operation, operation_to_body, body_to_result and result_to_body.

Parsec Rust client


Add the user facing methods to src/core/ These are what users interacting with Parsec using Rust will use as the entrypoint. They are also what Parsec’s e2e test suit uses to test Parsec, which is why this step comes before extending Parsec. Add the relevant tests to src/core/testing/

Other clients (e.g. Go)

The procedure for adding operations to another client library should be similar. We encourage that all clients should be updated to allow the new operation is as many languages as possible.



Locate the parts of the PSA API in Mbed Crypto that will need to be used during the operation you are adding. This includes all constants, datatypes, macros and functions. Note: Mbed Crypto does not yet fully conform to the PSA API 1.0 specification, so there may be inconsistencies between the PSA documentation and the macros and functions that Mbed Crypto exposes.

Starting in psa-crypto-sys, constants are added to Inline functions and macros require a shim. Add the inline function and macro definitions into shim.h, shim.c and When adding entries to, take note of where you place unsafe. If the PSA API specification states that under any circumstance, an unspecified result can be returned, then mark the entire fn definition as unsafe. Otherwise, if the return values are always specified, wrap the call to the shim method as unsafe. Regular functions are added to pub use psa_crypto_binding::{ … }.


Define a Rust native interface for the operation you are adding in src/operations, along with any Rust native types you need in src/types. Generally, any helper macro functions are placed in the impl block of the Rust native type they are for.

The interface will work between the Rust native datatypes given to it and the C bindings in psa-crypto-sys. It is important to ensure that the Rust interface handles all possible situations (e.g. closing a key-handle on an error) to ensure it is a safe Rust native interface to the operation.

At this point, you will now have a safe Rust interface for the PSA operation you added.


Add the new operation to the correct provider (in this case, Mbed Crypto) as a psa_xxx_internal method. The operation method should take the user inputs, arrange them in a way that psa-crypto accepts, and provide any extra logic or storage if required (e.g. an output buffer). The external implementation is to be added to the provider’s file, which outputs a trace entry and passes the call back to the internal implementation.

A default implementation is added to src/providers/ that is used when a provider does not support a particular operation. It outputs a trace entry and returns an error, stating that the operation is not supported. The external implementation in the provider’s overrides this.

Add the NativeOperation mapping in src/back/ execute_request function so Parsec can call the correct operation method when it receives a request.

Finally, add end to end (e2e) testing for the new operation. Operations are added to e2e_tests/src/ to save from code duplication in the tests. This also makes it quick and easy to write tests, and to see exactly what operations a test is performing. Test should be very thorough, taking the operation through valid and invalid inputs, checking that the proper output is returned. If possible, external data and crates should be used during testing. E.g. if testing the cryptographic operation, generate keys and an encrypted message externally and test that Parsec can correctly decrypt the message. (Note: if crates are added solely for testing purposes, add them to [dev-dependencies] in Cargo.toml)