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doc: secure boot: Fix relative reference to key generation step

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Angus Gratton 2019-12-31 17:19:16 +11:00 committed by Angus Gratton
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docs/en/security/secure-boot.rst
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@ -117,7 +117,7 @@ Configuration "Secure Boot: One-Time Flash" is the recommended configuration for
However, an alternative mode :ref:`Secure Boot: Reflashable <CONFIG_SECURE_BOOTLOADER_MODE>` is also available. This mode allows you to supply a binary key file that is used for the secure bootloader key. As you have the key file, you can generate new bootloader images and secure boot digests for them. However, an alternative mode :ref:`Secure Boot: Reflashable <CONFIG_SECURE_BOOTLOADER_MODE>` is also available. This mode allows you to supply a binary key file that is used for the secure bootloader key. As you have the key file, you can generate new bootloader images and secure boot digests for them.
In the esp-idf build process, this 256-bit key file is derived from the app signing key generated during the generate_signing_key step above. The private key's SHA-256 digest is used as the secure bootloader key (as-is for Coding Scheme None, or truncate to 192 bytes for 3/4 Encoding). This is a convenience so you only need to generate/protect a single private key. In the esp-idf build process, this 256-bit key file is derived from the ECDSA app signing key generated by the user (see the :ref:`secure-boot-generate-key` step below). This private key's SHA-256 digest is used as the secure bootloader key in efuse (as-is for Coding Scheme None, or truncate to 192 bytes for 3/4 Encoding). This is a convenience so you only need to generate/protect a single private key.
.. note:: Although it's possible, we strongly recommend not generating one secure boot key and flashing it to every device in a production environment. The "One-Time Flash" option is recommended for production environments. .. note:: Although it's possible, we strongly recommend not generating one secure boot key and flashing it to every device in a production environment. The "One-Time Flash" option is recommended for production environments.