Threema::Receive::File Bytesize never matches on iOS
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Ok, so we're running into one of those wonderful errors.
This line here errors out on iOS but does not fail on Android. On iOS the download.bytesize is consistently larger than structure['s'] by 16 (bytes).
Here are our devices:
| OS | Threema | Result |
|---|---|---|
| Android 10 | v4.5 | Success |
| iOS 14.3 | 4.6.5 (2604) | Error |
| iOS 12.5.1 | 4.6.5 (2605) | Error |
The error is the same on mobile app or a connected Threema web app.
When I do a git blame on the line, the commit message tells me Initial commit.
See here: https://github.com/thorsteneckel/threema/blame/master/lib/threema/receive/file.rb#L19
There is a test case for the behaviour. The test case description, however, does not explain why the check is necessary: https://github.com/thorsteneckel/threema/blob/master/spec/threema/receive/file_spec.rb#L70
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Does this ring a bell?
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Can you explain us, what might happen if this if-clause wouldn't be there? Man-in-the-middle because the file size of the incoming message is used as some sort of signature?
@mattwr18 had a quick look into the iOS and Android implementations. There is something about padding of 16 bytes but it's not 100% clear (the "authenticator"? - why is it different for Android/iOS?) to me what it is and what's the best way how the gem threema should treat the file size difference.
https://github.com/threema-ch/threema-ios/blob/main/NaClCrypto/NaClCrypto.h#L26