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

* 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>

changelogs/client_server/newsfragments/1712.clarification

@@ -0,0 +1 @@
+Clarify that the key backup MAC is implemented incorrectly and does not pass the ciphertext through HMAC-SHA-256.

content/client-server-api/modules/end_to_end_encryption.md

@@ -1364,10 +1364,18 @@ The `session_data` field in the backups is constructed as follows:
     PKCS\#7 padding. This encrypted data, encoded using unpadded base64,
     becomes the `ciphertext` property of the `session_data`.
 
-5.  Pass the raw encrypted data (prior to base64 encoding) through
-    HMAC-SHA-256 using the MAC key generated above. The first 8 bytes of
-    the resulting MAC are base64-encoded, and become the `mac` property
-    of the `session_data`.
+5.  Pass an empty string through HMAC-SHA-256 using the MAC key generated above.
+    The first 8 bytes of the resulting MAC are base64-encoded, and become the
+    `mac` property of the `session_data`.
+
+{{% boxes/warning %}}
+Step 5 was intended to pass the raw encrypted data, but due to a bug in libolm,
+all implementations have since passed an empty string instead.
+
+Future versions of the spec will fix this problem. See
+[MSC4048](https://github.com/matrix-org/matrix-spec-proposals/pull/4048) for a
+potential new key backup algorithm version that would fix this issue.
+{{% /boxes/warning %}}
 
 {{% definition path="api/client-server/definitions/key_backup_session_data" %}}