If the closed source E2EE app is sending encrypted messages but leaking bits of the key in how those messages are padded, for example, how would you detect that

If the closed source E2EE app is sending encrypted messages but leaking bits of the key in how those messages are padded, for example, how would you detect that? As for the threat model of open source E2EE apps auto-updating to an insecure version, you're right that this needs some extra defences. One way would be to use a binary-transparency log[0] to make sure that the open source project had publicly committed to a specific binary at least 24 hours in advance of pushing the auto-update. This system relies on there being auditors out there who would raise the alarm if a malicious update was released, or a hash was included in the log for which there was no corresponding (reproducibly buildable) source code. [0] https://wiki.mozilla.org/Security/Binary_Transparency https://wiki.mozilla.org/Security/Binary_Transparency