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Historical update: the layout on stdio changed in Mbed Crypto 1.1.0

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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Gilles Peskine 2022-06-30 12:16:50 +02:00
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docs/architecture/mbed-crypto-storage-specification.md
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@ -171,7 +171,21 @@ Tags: mbedcrypto-1.1.0
Released in early June 2019. <br>
Integrated in Mbed OS 5.13.
Identical to [1.0.0](#mbed-crypto-1.0.0) except for some changes in the key file format.
Changes since [1.0.0](#mbed-crypto-1.0.0):
* The stdio backend for storage has been replaced by an implementation of [PSA ITS over stdio](#file-namespace-on-stdio-for-1.1.0).
* [Some changes in the key file format](#key-file-format-for-1.1.0).
### File namespace on stdio for 1.1.0
Assumption: C stdio, allowing names containing lowercase letters, digits and underscores, of length up to 23.
An undocumented build-time configuration value `PSA_ITS_STORAGE_PREFIX` allows storing the key files in a directory other than the current directory. This value is simply prepended to the file name (so it must end with a directory separator to put the keys in a different directory).
* `PSA_ITS_STORAGE_PREFIX "tempfile.psa_its"`: used as a temporary file. Must be writable. May be overwritten or deleted if present.
* `sprintf(PSA_ITS_STORAGE_PREFIX "%016llx.psa_its", key_id)`: a key or non-key file. The `key_id` in the name is the 64-bit file identifier, which is the [key identifier](#key-names-for-mbed-tls-2.25.0) for a key file or some reserved identifier for a non-key file (currently: only the [nonvolatile random seed](#nonvolatile-random-seed-file-format-for-1.0.0)). The contents of the file are:
* Magic header (8 bytes): `"PSA\0ITS\0"`
* File contents.
### Key file format for 1.1.0