Handle backup secret gossip

[BillCarsonFr]

Fixes element-hq/element-web#26288
Second step will be to automatically restore after the secret is received (in a following PR)

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[florianduros]

LGTM

[richvdh]

can we use this in isKeyBackupTrusted? I think we just need

const backupMatchesSavedPrivateKey = backupKeys?.decryptionKey !== undefined && backupInfoMatchesBackupDecruptionKey(info, backupKeys.decryptionKey);

[BillCarsonFr]

not anymore because as per previous comment we move it to a standalone function

[richvdh]

It's not a big deal, but I don't understand why that makes any difference.

Here's what I'm suggesting, in full:

    /**
     * Determine if a key backup can be trusted.
     *
     * @param info - key backup info dict from {@link MatrixClient#getKeyBackupVersion}.
     */
    public async isKeyBackupTrusted(info: KeyBackupInfo): Promise<BackupTrustInfo> {
        const signatureVerification: SignatureVerification = await this.olmMachine.verifyBackup(info);
        const backupKeys: RustSdkCryptoJs.BackupKeys = await this.olmMachine.getBackupKeys();
        const decryptionKey = backupKeys?.decryptionKey;
        const backupMatchesSavedPrivateKey = !!decryptionKey && backupInfoMatchesBackupDecryptionKey(info, decryptionKey);
        return {
            matchesDecryptionKey: backupMatchesSavedPrivateKey,
            trusted: signatureVerification.trusted(),
        };
    }

does that not work?

[BillCarsonFr]

Didn't know about the !! trick.
Yes it should work, looks better, updated.

[richvdh]

LGTM apart from the two suggestions I have made