You can do things like require the service to verify that the court order is valid before they gain the capability to decrypt a subset of messages that the court order allows them to see. There doesn't have to be a skeleton key.
What is the mathematical formula for a valid court order? How does it look different from a court order signed by a judge held at gunpoint? How does it look different from a court order signed by a dictator's minion? How does it prevent someone from tricking a judge into signing an order to decrypt message 2421425241 instead of 2421475241? What is stopping the service from accepting invalid court orders? What is stopping the service from just decrypting everything for convenience?
>What is the mathematical formula for a valid court order?
This is a political / social issue. A human has to decide.
>How does it look different from a court order signed by a judge held at gunpoint?
It doesn't. You could similarly hold developers at gunpoint to push malicous updates too or at users to give them their messages. Putting people at gunpoint is illegal.
>How does it look different from a court order signed by a dictator's minion?
Judges could have their own private key so it would be noticeable that the minions private key would not be trusted.
>How does it prevent someone from tricking a judge into signing an order to decrypt message 2421425241 instead of 2421475241?
It doesn't. This is solved by laws that make such an action illegal to do
>What is stopping the service from accepting invalid court orders?
They want to protect the privacy of their users so they should reject invalid court orders.
>What is stopping the service from just decrypting everything for convenience?
They would not have the keys LE has. So it could be setup such that LE's keys are required to decrypt something.
Yes, but the original pegasus was limited to folks that had whatsapp installed. If you mandate le has a backdoor to everyone, then everyone is vulnerable. There is no safety in software selection.
Note: this comment is just about what is technically possible, not what should actually be done.
Actually those issues could be addressed by using a sufficiently large threshold system to protect each message's decryption key, so that in order to decrypt a message a large number of people have to agree that it is justified.
You could make it for example so that a message can only be decrypted if it is approved by judges in 3 different federal circuits, with at least 2 judges in each of those 3 circuits approving, and also approved by at least 2 outside civil rights groups chosen from {ACLU, EFF, Amnesty International}, and INTERPOL.
If that's not good enough we could also make it so that for each message the threshold system includes 5000 random citizens with smartphones and at least 3000 of them must agree.
Because network effects drive what chat apps people use. If people they want to talk to don't use that obscure app then they probably won't use it either.
