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Add some design notes about multipart operation structures

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Gilles Peskine 2019-08-13 15:00:57 +02:00
parent 30e13eb1c6
commit b4e73e9747
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include/psa/crypto_struct.h
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@ -12,6 +12,26 @@
* In implementations with isolation between the application and the
* cryptography module, it is expected that the front-end and the back-end
* would have different versions of this file.
*
* <h3>Design notes about multipart operation structures</h3>
*
* Each multipart operation structure contains a `psa_algorithm_t alg`
* field which indicates which specific algorithm the structure is for.
* When the structure is not in use, `alg` is 0. Most of the structure
* consists of a union which is discriminated by `alg`.
*
* Note that when `alg` is 0, the content of other fields is undefined.
* In particular, it is not guaranteed that a freshly-initialized structure
* is all-zero: we initialize structures to something like `{0, 0}`, which
* is only guaranteed to initializes the first member of the union;
* GCC and Clang initialize the whole structure to 0 (at the time of writing),
* but MSVC and CompCert don't.
*
* In Mbed Crypto, multipart operation structures live independently from
* the key. This allows Mbed Crypto to free the key objects when destroying
* a key slot. If a multipart operation needs to remember the key after
* the setup function returns, the operation structure needs to contain a
* copy of the key.
*/
/*
* Copyright (C) 2018, ARM Limited, All Rights Reserved