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Random driver: make initial_entropy_size mandatory

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If a random driver has a built-in entropy source and doesn't need an
external entropy source, make the driver author declare this
explicitly, rather than it being a less secure default.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2020-11-23 12:30:15 +01:00
parent ee914f34fa
commit 6a530e8d26
1 changed files with 2 additions and 2 deletions
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4
docs/proposed/psa-driver-interface.md
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@ -486,9 +486,9 @@ This operation family requires the following type, entry points and parameters (
* Type `"random_context_t"`: the type of a random generation context.
* `"init_random"` (entry point, optional): if this function is present, [the core calls it once](#random-generator-initialization) after allocating a `"random_context_t"` object.
* `"add_entropy"` (entry point, optional): the core calls this function to [inject entropy](#entropy-injection). This entry point is optional if the driver is for a peripheral that includes an entropy source of its own, however [random generator drivers without entropy injection](#random-generator-drivers-without-entropy-injection) have limited portability since they can only be used on platforms with no other entropy source.
* `"add_entropy"` (entry point, optional): the core calls this function to [inject entropy](#entropy-injection). This entry point is optional if the driver is for a peripheral that includes an entropy source of its own, however [random generator drivers without entropy injection](#random-generator-drivers-without-entropy-injection) have limited portability since they can only be used on platforms with no other entropy source. This entry point is mandatory if `"initial_entropy_size"` is nonzero.
* `"get_random"` (entry point, mandatory): the core calls this function whenever it needs to [obtain random data](#the-get_random-entry-point).
* `"initial_entropy_size"` (integer, optional): the minimum number of bytes of entropy that the core must supply before the driver can output random data. This can be `0` if the driver is for a peripheral that includes an entropy source of its own. If omitted, the value is `0`.
* `"initial_entropy_size"` (integer, mandatory): the minimum number of bytes of entropy that the core must supply before the driver can output random data. This can be `0` if the driver is for a peripheral that includes an entropy source of its own.
* `"reseed_entropy_size"` (integer, optional): the minimum number of bytes of entropy that the core must supply when the driver runs out of entropy. This value is also a hint for the size to supply if the core makes additional calls to `"add_entropy"`, for example to enforce prediction resistance. If omitted, the core chooses a value which is at least the expected security strength of the device.
Random generation is not parametrized by an algorithm. The choice of algorithm is up to the driver.