RTMP is an Adobe Proprietary Protocol and stands for Real Time Messaging Protocol.
RTMPS is its encrypted transport version.
RTMP sent over a secure socket layer (SSL). SSL is a protocol that enables secure TCP/IP connections. The default port is 443 (instead of 1935 for RTMP)
Digicast is using sha256 algortihm with 2048 bit key length
Originally it was developed by Macromedia Inc. which was later purchased by Adobe; if you still remember Macromedia you can guess how old RTMPS as a Protocol is. Let’s face it: the Adobe Flash Player has been in use for long and is still used widely, though it’s Legacy nowadays if you go and ask most CDN Vendors for example.
RTMPS still used by the live streaming entertainment industry (Facebook Live, Youtube Live, Twitch etc) and also by the corporate webcast industry.
It could be replaced by webRTC which is open-source but RTMPS is currently much easier to manipulate.
Also RTMPS is a TCP-based protocol which brings convenience and resilience to poor network conditions (as in remote events scenarios)