This paper is from 2016. Signal's UX has improved massively in many ways since this was published.
Signal has been having multiple issues due to the poor usability of their identity management lately[2].
[1] Safety number updates https://signal.org/blog/safety-number-updates/
[2]
A Third Party Breached The Intercept’s Signal Tip Line and Has Been Soliciting Whistleblowers https://www.dropsitenews.com/p/intercept-signal-tip-line-bre...
Twilio Incident: What Signal Users Need to Know https://support.signal.org/hc/en-us/articles/4850133017242-T...
Russian State-Backed Hackers Intensify Attacks on Signal Messenger Accounts https://thecyberexpress.com/signal-attacks-russian-fackers-t...
End to End Encrypted Messaging in the News: An Editorial Usability Case Study (my article) https://articles.59.ca/doku.php?id=em:sg