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Accept key backups as usable if they're signed with the master cross-signing key #1492

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Accept key backups as usable if they're signed with the master cross-signing key #1492

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8142b65
Accept key backups as usable if they're signed with the master cross-…
bradtgmurray Jun 7, 2022
606ee86
Add a changelog entry
bradtgmurray Jun 7, 2022
71256d0
Only accept MSK signatures, update unit test to validate
bradtgmurray Jul 25, 2022
43c107e
Rename to deviceIdOrCrossSigningKey
bradtgmurray Jul 25, 2022
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30 changes: 18 additions & 12 deletions MatrixSDK/Crypto/KeyBackup/MXKeyBackup.m
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Original file line number Diff line number Diff line change
Expand Up @@ -1139,18 +1139,18 @@ - (MXKeyBackupVersionTrust *)trustForKeyBackupVersionFromCryptoQueue:(MXKeyBacku
for (NSString *keyId in mySigs)
{
// XXX: is this how we're supposed to get the device id?
NSString *deviceId;
NSString *deviceIdOrCrossSigningKey;
NSArray<NSString *> *components = [keyId componentsSeparatedByString:@":"];
if (components.count == 2)
{
deviceId = components[1];
deviceIdOrCrossSigningKey = components[1];
}

if (deviceId)
if (deviceIdOrCrossSigningKey)
{
BOOL valid = NO;

MXDeviceInfo *device = [self->crypto.deviceList storedDevice:myUserId deviceId:deviceId];
MXDeviceInfo *device = [self->crypto.deviceList storedDevice:myUserId deviceId:deviceIdOrCrossSigningKey];
if (device)
{
NSError *error;
Expand All @@ -1162,27 +1162,33 @@ - (MXKeyBackupVersionTrust *)trustForKeyBackupVersionFromCryptoQueue:(MXKeyBacku
}

MXKeyBackupVersionTrustSignature *signature = [MXKeyBackupVersionTrustSignature new];
signature.deviceId = deviceId;
signature.deviceId = device.deviceId;
signature.device = device;
signature.valid = valid;
[signatures addObject:signature];
}
else // Try interpreting it as the MSK public key
else if ([deviceIdOrCrossSigningKey isEqualToString:crypto.crossSigning.myUserCrossSigningKeys.masterKeys.keys])
{
NSError *error;
BOOL valid = [crypto.crossSigning.crossSigningTools pkVerifyObject:authData.JSONDictionary userId:myUserId publicKey:deviceId error:&error];
BOOL valid = [crypto.crossSigning.crossSigningTools pkVerifyObject:authData.JSONDictionary userId:myUserId publicKey:deviceIdOrCrossSigningKey error:&error];

if (!valid)
{
MXLogDebug(@"[MXKeyBackup] trustForKeyBackupVersion: Signature with unknown key %@", deviceId);
MXLogDebug(@"[MXKeyBackup] trustForKeyBackupVersion: Signature with cross-signing master key is invalid");
}
else
{
MXKeyBackupVersionTrustSignature *signature = [MXKeyBackupVersionTrustSignature new];
signature.keys = deviceId;
signature.valid = valid;
[signatures addObject:signature];
keyBackupVersionTrust.usable = YES;
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@Anderas Anderas Jun 8, 2022 •
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The existing parent else branch does not ensure that the deviceId is indeed a master key (it only mentions it in a comment), as opposed to other type of key. Could you please change the else // Try interpreting it as the MSK public key into something like else if ([deviceId isEqualToString:masterKey])? The master key can be obtained for instance as crypto.crossSigning.myUserCrossSigningKeys.masterKeys.keys. See also Android implementation for reference.

If the comparison check fails, we can log that as another issue.

Additionally it may be worth renaming the deviceId local variable into deviceIdOrCrossSigningKey, otherwise the name can be misleading

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Only a month and a half later I've finally addressed this feedback, thanks @Anderas !

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Thanks for making the change. I see that we will now be adding a signature object even if the master key verification did not succeed, but it will be set as invalid (plus this copies js / android behaviour) so it sounds good to me

}

MXKeyBackupVersionTrustSignature *signature = [MXKeyBackupVersionTrustSignature new];
signature.keys = deviceIdOrCrossSigningKey;
signature.valid = valid;
[signatures addObject:signature];
}
else
{
MXLogDebug(@"[MXKeyBackup] trustForKeyBackupVersion: Signature with unknown key %@", deviceIdOrCrossSigningKey);
}
}
}
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1 change: 1 addition & 0 deletions changelog.d/pr-1492.bugfix
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@@ -0,0 +1 @@
Accept key backups as usable if they're signed with the master cross-signing key. Contributed by Brad @ Beeper