Telegram is known as a privacy-focused secure messaging app because it markets itself that way. However, it is often criticized by security experts, privacy advocates, and people with common sense who can understand why its claims about being privacy-friendly don't make sense. In this brief article, I'll show you all
As long as the keys are handled via a closed source app and server system, e2ee is potentially broken.
Even if you generated the key, keep the private part locally and submitted only the public part to your communication partner, you can never be sure that the intransparent app does keep your private key private.
With WhatsApp I’m quite sure that they somehow can retrieve the private key. Certain events point to that. But I see no reason to consider signal or telegram any more trustworthy - they are all prone to governmental influence.
And as open source and closed app infrastructure are incompatible, I would not handle anything important on an Android or Apple device.