PSA interruptible asymmetric operations: basic design

[gilles-peskine-arm]

The goal of this task is to write a basic design of a PSA interruptible operations API. At this stage, some minor questions may be left unanswered.

Normally a cryptographic operation is implemented as a function. The function returns when the calculation is finished. Interruptible operations (which Mbed TLS calls “restartable”) are operations that return after a short period of time, whether the calculation is finished or not. If the calculation is unfinished, its state is saved in a context, and the caller can resume the operation by passing the context to the crypto library again. This is intended for very constrainted embedded environments that do not have preemption.

This API should support long-running operations, meaning asymmetric cryptography: signature, key agreement, asymmetric encryption. (Mbed TLS will not implement the asymmetric encryption interface, but I think it should be there for completeness, and it's only a very minor design effort.)

For the first phase, we're doing just sign-hash, tracked in #6061.

[hanno-becker]

This is intended for very constrainted embedded environments that do not have preemption.

(1) It may also be needed in high performance systems code which runs with interrupts masked.

(2) [Entirely Unrelated] It may be worth considering if the design of such an API can be consolidated with the design of an API for 'streaming' processing, with the user passing or receiving one piece of key material a time (this may be hidden in the 'restart' context). While the use cases are different, the interfaces seem very similar, so maybe something can be done. Such streaming API matters for the support of very large public-key schemes (esp. from PQC) on highly constrained devices, see e.g. https://eprint.iacr.org/2015/1042.pdf MPS already has support for such streaming crypto, so it would be nice for PSA to support it, too.

[gilles-peskine-arm]

We've implemented interruptible sign-hash. The rest of the API design will be tracked in the PSA API repository. So I am closing this issue here.