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Reword the explanation of fallback

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Add the rationale which I'd accidentally omitted.

No intended meaning change.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2020-08-06 22:47:39 +02:00
parent 85b3e13cd0
commit 15319454e6
1 changed files with 13 additions and 4 deletions
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17
docs/proposed/psa-driver-interface.md
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@ -5,7 +5,7 @@ This document describes an interface for cryptoprocessor drivers in the PSA cryp
This specification is work in progress and should be considered to be in a beta stage. There is ongoing work to implement this interface in Mbed TLS, which is the reference implementation of the PSA Cryptography API. At this stage, Arm does not expect major changes, but minor changes are expected based on experience from the first implementation and on external feedback.
Time-stamp: "2020/08/06 20:13:43 GMT"
Time-stamp: "2020/08/06 20:48:44 GMT"
## Introduction
@ -88,7 +88,7 @@ A capability is a JSON object containing the following properties:
* `"key_types"` (optional, list of strings). Each element is a [key type specification](#key-type-specifications). If specified, the core will invoke this capability of the driver only for operations involving a key with one of the specified key types. If omitted, the core will invoke this capability of the driver for all applicable key types.
* `"key_sizes"` (optional, list of integers). If specified, the core will invoke this capability of the driver only for operations involving a key with one of the specified key sizes. If omitted, the core will invoke this capability of the driver for all applicable key sizes. Key sizes are expressed in bits.
* `"names"` (optional, object). A mapping from entry point names described by the `"functions"` property, to the name of the C function in the driver that implements the corresponding function. If a function is not listed here, name of the driver function that implements it is the driver's prefix followed by an underscore (`_`) followed by the function name. If this property is omitted, it is equivalent to an empty object (so each entry point *suffix* is implemented by a function called *prefix*`_`*suffix*).
* `"fallback"` (optional for transparent drivers, not permitted for opaque drivers, boolean). If present and true, the driver may return `PSA_ERROR_NOT_SUPPORTED`, in which case the core should call another driver or use built-in code to perform this operation. If absent or false, the core should not include built-in code to perform this particular cryptographic mechanism. See the section “[Fallback](#fallback)” for more information.
* `"fallback"` (optional for transparent drivers, not permitted for opaque drivers, boolean). If present and true, the driver may return `PSA_ERROR_NOT_SUPPORTED`, in which case the core should call another driver or use built-in code to perform this operation. If absent or false, the driver is expected to fully support the mechanisms described by this capabilit. See the section “[Fallback](#fallback)” for more information.
Example: the following capability declares that the driver can perform deterministic ECDSA signatures using SHA-256 or SHA-384 with a SECP256R1 or SECP384R1 private key (with either hash being possible in combination with either curve). If the prefix of this driver is `"acme"`, the function that performs the signature is called `acme_sign_hash`.
```
@ -306,9 +306,14 @@ Transparent drivers are not involved when importing, exporting, copying or destr
### Fallback
If a transparent driver entry point is part of a capability which has a true `"fallback"` property and returns `PSA_ERROR_NOT_SUPPORTED`, the built-in software implementation will be called instead. Any other value (`PSA_SUCCESS` or a different error code) is returned to the application.
Sometimes cryptographic accelerators only support certain cryptographic mechanisms partially. The capability description language allows specifying some restrictions, including restrictions on key sizes, but it cannot cover all the possibilities that may arise in practice. Furthermore, it may be desirable to deploy the same binary image on different devices, only some of which have a cryptographic accelerators.
For these purposes, a transparent driver can declare that it only supports a [capability](#driver-description-capability) partially, by setting the capability's `"fallback"` property to true.
If there are multiple available transparent drivers, the core tries them in turn until one is declared without a true `"fallback"` property or returns a status other than `PSA_ERROR_NOT_SUPPORTED`. The order in which the drivers are called is unspecified and may be different for different entry points.
If a transparent driver entry point is part of a capability which has a true `"fallback"` property and returns `PSA_ERROR_NOT_SUPPORTED`, the core will call the next transparent driver that supports the mechanism, if there is one. If all the available driver have fallback enabled and return `PSA_ERROR_NOT_SUPPORTED`, the core will perform the operation using built-in code.
As soon as a driver returns any value other than `PSA_ERROR_NOT_SUPPORTED` (`PSA_SUCCESS` or a different error code), this value is returned to the application, without attempting to call any other driver or built-in code.
The order in which the drivers are called is unspecified and may be different for different entry points.
If a transparent driver entry point is part of a capability where the `"fallback"` property is false or omitted, the core should not include any other code for this capability, whether built in or in another transparent driver.
@ -587,6 +592,10 @@ How does a driver author decide which location values to use? It should be possi
Can the driver assembly process generate distinct location values as needed? This can be convenient, but it's also risky: if you upgrade a device, you need the location values to be the same between builds.
#### Multiple transparent drivers
When multiple transparent drivers implement the same mechanism, which one is called? The first one? The last one? Unspecified? Or is this an error (excluding capabilities with fallback enabled)?
### Driver function interfaces
#### Driver function parameter conventions