Convincing people to use apps such as Signal is hard work and most can’t be convinced. But with those you manage to convince, do you feel happy to talk to them on Signal?
The problem is these people use Signal on Android/IOS which can’t be trusted and IOS has recently been in the news for having a backdoor. And it has also been revealed that american feds are able to read everyone’s push notifications and they do this as mass surveillance.
So not only do you have to convince people to use Signal which is an incredibly difficult challenge. You also have to convince them to go into settings to disable message and sender being included in the push notifications. And then there’s the big question is the Android and IOS operating systems are doing mass surveillance anyway. And many people find it taking a lot of effort to type on the phone so they install Signal on the computer which is a mac or Windows OS.
So I don’t think I feel comfortable sending messages in Signal but it’s better than Whatsapp.
These were some thoughts to get the discussion started and set the context.
Yes, compromising the key exchange would be one attack. But that’s not technically breaking the encryption, that’s just stealing the key. To do that, you need control of the client - which is a thousand times easier when it’s impossible to check the source code of the software it’s running. Otherwise, your only option is to break the encryption (i.e. discover the key) and that is gonna be very hard indeed because, unlike logins that humans use, the “password” is always completely random and very strong.
Telegram has open source client software, but it uses its own in-house encryption algorithm, which is not an industry standard. Some people think it might therefore be easier to compromise. But in any case, as you say, Telegram doesn’t even have encryption enabled by default.
The better reason not to use Telegram is because it’s a shady company with no obvious business model and therefore has an incentive to do bad things.