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Clarify that the key backup MAC is implemented incorrectly (#1712)

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* Clarify that the key backup MAC is implemented incorrectly

Due to a bug in libolm, all implementations of the
m.megolm_backup.v1.curve25519-aes-sha2 key backup algorithm incorrectly
pass an empty string through HMAC-SHA-256 to generate the `mac` property
of the `session_data`.

It was intended for the entire raw encrypted data to be passed through
HMAC-SHA-256, but the issue was caught too late in the process, and thus
we are stuck with this until a new key backup algorithm is introduced.

This commit clarifies the real-world behavior of all current
implementations.

Signed-off-by: Sumner Evans <sumner@beeper.com>
This commit is contained in:
Sumner Evans 2024-01-16 12:11:44 -07:00 • committed by GitHub
parent 1d35e7aac6
commit 9a5cacda90
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Clarify that the key backup MAC is implemented incorrectly and does not pass the ciphertext through HMAC-SHA-256.