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Add a subsection with driver interface terminology

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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Gilles Peskine 2020-09-07 10:28:38 +02:00
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docs/proposed/psa-conditional-inclusion-c.md
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@ -5,7 +5,7 @@ This document is a proposed interface for deciding at build time which cryptogra
This is currently a proposal for Mbed TLS. It is not currently on track for standardization in PSA.
Time-stamp: "2020/09/07 07:41:23 GMT"
Time-stamp: "2020/09/07 08:27:32 GMT"
## Introduction
@ -15,11 +15,15 @@ The [PSA Cryptography API specification](https://armmbed.github.io/mbed-crypto/p
The present document proposes a way for an application using the PSA cryptography interface to declare which mechanisms it requires.
### Current situation
### Conditional inclusion of legacy cryptography modules
Mbed TLS offers a way to select which cryptographic mechanisms are included in a build through its configuration file (`config.h`). This mechanism is based on two main sets of symbols: `MBEDTLS_xxx_C` controls the availability of the mechanism to the application, and `MBEDTLS_xxx_ALT` controls the availability of an alternative implementation, so the software implementation is only included if `MBEDTLS_xxx_C` is defined but not `MBEDTLS_xxx_ALT`.
This is difficult to adapt to the PSA interface for several reasons. The `MBEDTLS_xxx_ALT` symbols are somewhat inconsistent, and in particular do not work well for asymmetric cryptography. For example, many parts of the ECC code have no `MBEDTLS_xxx_ALT` symbol, so a platform with ECC acceleration that can perform all ECDSA and ECDH operations in the accelerator would still embark the `bignum` module and large parts of the `ecp_curves`, `ecp` and `ecdsa` modules. Also the availability of a transparent driver for a mechanism does not translate directly to `MBEDTLS_xxx` symbols.
### PSA evolution
In the PSA cryptography interface, the **core** (built-in implementations of cryptographic mechanisms) can be augmented with drivers. **Transparent drivers** replace the built-in implementation of a cryptographic mechanism (or, with **fallback**, the built-in implementation is tried if the driver only has partial support for the mechanism). **Opaque drivers** implement cryptographic mechanisms on keys which are stored in a separate domain such as a secure element, for which the core only does key management and dispatch using wrapped key blobs or key identifiers.
The current model is difficult to adapt to the PSA interface for several reasons. The `MBEDTLS_xxx_ALT` symbols are somewhat inconsistent, and in particular do not work well for asymmetric cryptography. For example, many parts of the ECC code have no `MBEDTLS_xxx_ALT` symbol, so a platform with ECC acceleration that can perform all ECDSA and ECDH operations in the accelerator would still embark the `bignum` module and large parts of the `ecp_curves`, `ecp` and `ecdsa` modules. Also the availability of a transparent driver for a mechanism does not translate directly to `MBEDTLS_xxx` symbols.
### Requirements