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Elaborate on psa_destroy_key requirements

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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Gilles Peskine 2023-08-07 16:29:19 +02:00
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docs/architecture/psa-thread-safety.md
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@ -71,6 +71,13 @@ We may want to go directly to a more sophisticated approach because when a syste
As noted above in [“Correctness out of the box”](#correctness-out-of-the-box), when a key is destroyed, it's ok if `psa_destroy_key` allows copies of the key to live until ongoing operations using the key return. In the long term, it would be good to guarantee that `psa_destroy_key` wipes all copies of the key material. As noted above in [“Correctness out of the box”](#correctness-out-of-the-box), when a key is destroyed, it's ok if `psa_destroy_key` allows copies of the key to live until ongoing operations using the key return. In the long term, it would be good to guarantee that `psa_destroy_key` wipes all copies of the key material.
#### Summary of guarantees when `psa_destroy_key` returns
* The key identifier doesn't exist. Rationale: this is a functional requirement for persistent keys: the caller can immediately create a new key with the same identifier.
* The resources from the key have been freed. Rationale: in a low-resource condition, this may be necessary for the caller to re-create a similar key, which should be possible.
* The call must not block indefinitely, and in particular cannot wait for an event that is triggered by application code such as calling an abort function. Rationale: this may not strictly be a functional requirement, but it is an expectation `psa_destroy_key` does not block forever due to another thread, which could potentially be another process on a multi-process system.
* In the long term, no copy of the key material exists. Rationale: this is a security requirement. We do not have this requirement yet, but we need to document this as a security weakness, and we would like to become compliant.
## Resources to protect ## Resources to protect
Analysis of the behavior of the PSA key store as of Mbed TLS 9202ba37b19d3ea25c8451fd8597fce69eaa6867. Analysis of the behavior of the PSA key store as of Mbed TLS 9202ba37b19d3ea25c8451fd8597fce69eaa6867.