HF ACARS is now dead, replaced by SATCOM.
Anyone can scan the ACRS messages, which isn't a good thing IMHO. My last airline had an incident where a PDA was found in a seatback, evidently left from a previous flight. The crew ACARSed ops to say they found it. Some idiot spotter was scanning ACARS, saw the message and though it would be a joke to send a bomb phone hoax to the company saying a bomb was on the flight hidden in a PDA. Flight diverted mid Atlantic.
Idiot is still probably in prison. He got a long stretch!