The underlying weakness has to do with alerts rather than cryptography. Although they share the same underlying encryption, the Signal app isn’t vulnerable to the same attack. If the Signal client detects a new key, it will block the message rather than risk sending it insecurely. WhatsApp will send that message anyway. Since the key alert isn’t on by default, most users would have no idea.
It’s a controversial choice, but WhatsApp has good reasons for wanting a looser policy. Hard security is hard, as anyone who’s forgotten their PGP password can attest. Key irregularities happen, and each app has different policies on how to respond. Reached by The Guardian, WhatsApp pointed to users who change devices or SIM cards, the most common source of key irregularities. If WhatsApp followed the same rules as Signal, any message sent with an unverified key would simply be dropped. Signal users are happy to accept that as the price of stronger security, but with over a billion users across the world, WhatsApp is playing to a much larger crowd. Most of those users aren’t aware of WhatsApp’s encryption at all. Smoothing over those irregularties made the app itself simpler and more reliable, at the cost of one specific security measure. It’s easy to criticize that decision, and many have — but you don’t need to invoke a government conspiracy to explain it.
A multitude of secure messaging applications are vulnerable to keys being changed at the server level without the end-user being notified. This theoretically opens a way for state security agencies to ‘break into’ secured communications channels but, to date, we don’t have any evidence of a company in the Western or Western-affiliated world engaging in such behaviours.
There are laws that require some types of communications to be interceptable. Mobile communications carried by telecommunications carriers in Canada must be interceptable, and VoIP along with most other kinds of voice communications that are transmitted by equivalent carriers are subject to interception in the United States. There are not, however, similar demands currently placed on companies that provide chat or other next-generation communications system.
While there are not currently laws mandating either interception or decryption of chat or next-generation communications it remains plausible that laws will be introduced to compel this kind of functionality. It’s that possibility that makes how encryption keys are managed so important: as politicians smell that there is even the possibility of demanding decrypted communications the potential for such interception laws increases dramatically. Such laws would formalize and calcify vulnerabilities into the communications that we use everyday, to the effect of not just ensuring that domestic authorities could always potentially be listening, but foreign and unauthorized parties as well.