The message can't be intercepted in transit, since we are talking about spyware, I assume they get it from the device, hard to defend against that if they have access to your process' memory space.
Even if you had to input your private key every time you wanted to read or send a message, having malware in your phone voids practically any form of encryption, because it has to be decrypted eventually to be used.
not at all. there is no encryption that can save you when one of the legitimate participants is somehow compromised. doesn't even need to be a sophisticated device compromise, literal shoulder surfing does that too.
Thank you for that link. Your original comment implied that Signal's threat model should have included an attacker-controlled end. The only way to do that is to make decryption impossible by anyone, including the intended recipient. A labyrinthine way to do that would be to substitute the symmetric-encryption algorithm with a hash algorithm, which of course destroys the plaintext, but does accomplish the goal of obfuscating it in transit, at rest, and forever.
This entire thread should be annihilated, but since you mentioned being pedantic...
You're correct that a pure encryption algorithm doesn't use hashing. But real-world encryption systems will include an HMAC to detect whether messages were altered in transit. HMACs do use hash functions.