Since I have no control over iMessage, I would not consider it safe. It could easily be spying on me without my knowledge.
If Apple actually cared about security they'd implement an open protocol that is provably secure. Imagine if they supported something like Matrix. But that's clearly not their primary concern here. It's just a convenient excuse to maintain their walled garden.
A correctly implemented end-to-end encrypted protocol would be safe for all participating clients.
The only way to break that security is by copying messages outside the protocol in the app itself.
Neither of us knows whether iMessage or Beeper Mini does this. To bring up the possibility is to criticize both apps equally.
As long as the clients are closed source, this is a circular argument. The client itself is a vector. Not just for a good E2E implementation but for the 3rd party company to not outright steal everyone’s messages, create a backdoor, etc. You have to be willing to trust every client used in the thread.
If we must be willing to distrust one closed source client, then we ought to distrust both.
That said, I don't personally trust either of them. When it comes to matters of security, I prefer open protocols which can be proven to be secure over pinky swears from companies.
The level of trust I currently give in Beeper is that identity verification happened such that someone could potentially be prosecuted for abuses after-the-fact.
They have not built up a reputation, and in the face of potential scams or privacy abuses their reputation may not be as valuable as the user information they can gain access to.
Small incidents can cause significant reputation harm to Apple, and those equate to billions of dollars lost in corporate value.
Even the recent notification monitoring announcement harms their reputation, where the government itself mandated non-transparency. (For this reason, I somewhat expect they are trying to design an oblivious notification system, where role separation prevents a single intermediary from knowing both where a notification is from and where it is going to.)
You just brought up a better word: "liability". I'll go one step further: "attack surface".
When it comes to security in software, we don't need to work with many unknowns. The unknowns we do work with are the attack surface. By presenting a greater domain of unknown behavior, closed source software effectively presents me (the user) a larger attack surface. Sure, I could trust that the extra attack surface is actually covered; but I can't know. With open source, I don't have to trust, because I can know instead.
If I am to choose between open and closed source software, then I am choosing between knowledge and trust. That is a completely different position than choosing between closed and closed: trust vs. trust. So long as any securely-designed open-source messaging app exists, iMessage is at a disadvantage in end-user security. Even if Apple can know for certain that iMessage's attack surface is not larger than an open-source alternative, we the users can't. Closed source software will always present a higher demand for trust.
Generally fair assumption. There's been some research (both positive and negative) around their E2EE claims, though AFAIK much of what's known about iMessage's E2EE guts has been learned through unofficial means. I think that for the vast majority of users, iMessage is probably safe enough.
As a user, you have the agency to choose a messenger app that better suits your privacy/convenience balance, though in fairness, I think even among users who care about privacy, many don't know how to judge privacy features and implementation details well.
Like others in this thread, I personally recommend Signal. It's widely available, easily usable, has been audited and researched a fair bit, and though it doesn't have a self-hosted option, it does have white papers out about its protocol which IMO are worth a read.
It does not.
Is signal better than iMessage? Probably. Should we ask for them to be better than they are? Yes.
If there is some recent revelation that makes phone numbers all of a sudden a secure, portable and censorship-resistant identifier please link me that.
Until then I'd prefer to not have my private communication determined by telephone companies that often have not cared for either security, censorship or privacy. Regardless of signals e2e encryption having my access to the network determined by a telephone company is not the right way to go.
For a nominally privacy focused app, for them to literally alert people to my new Signal account I'd gotten to securely message someone violated all trust I had in them. What's to stop someone from just adding a Contact for every single valid phone number on their phone and then getting an alert for any time anyone makes a Signal account? I may as well just use Facebook then.
I would trust iMessage about 95% less if I had written, or even implemented, the protocols myself, and I consider myself a pretty good developer.
I do not need to have had a hand in developing any of this. It's not my expertise and, like you, I'd feel more comfortable having it developed by the experts.