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docs: Explain revocation of unused but compromised keys

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Explain the revocation of unused but compromised keys.

docs: Apply updates from code review

Add some nitpicks from AdityaHPatwardhan.

Co-authored-by: Aditya Patwardhan <aditya.patwardhan@espressif.com>
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Lucas Dekker 2024-09-12 16:17:04 +02:00 committed by Lucas
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docs/en/security/secure-boot-v2.rst
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@ -610,6 +610,7 @@ Secure Boot Best Practices
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* Keys are processed in a linear order, i.e., key #0, key #1, key #2.
* When a key is revoked, all remaining unrevoked keys can still be used to sign applications. For instance, if key #1 is revoked, keys such as key #0 and key #2 will remain valid for signing the application.
* Applications should be signed with only one key at a time, to minimize the exposure of unused private keys.
* The bootloader can be signed with multiple keys from the factory.
@ -634,6 +635,11 @@ Secure Boot Best Practices
* A similar approach can also be used to physically re-flash with a new key. For physical re-flashing, the bootloader content can also be changed at the same time.
.. note::
It may be necessary to revoke a key that isn't currently being used.
For example, if the active application is signed with key #0, but key #1 becomes compromised, you should revoke key #1 by using the above approach. The new OTA update should continue to be signed with key #0, and the API `esp_ota_revoke_secure_boot_public_key(SECURE_BOOT_PUBLIC_KEY_INDEX_[N])` can be used to revoke the key #N (N would be 1 in this case). After revoking, all remaining unrevoked keys can still be used to sign future applications.
.. _secure-boot-v2-aggressive-key-revocation: