Session is a cross-platform end-to-end encrypted instant messaging application. It is developed by The Oxen Project under the non-profit Oxen Privacy Tech Foundation. It uses a decentralized network for transmission. Users can send one-to-one and group messages, which can include files, voice notes, images and videos. Taken from Wikipedia.
Terms of Service, Privacy Policy
While not exactly using the Tor network, messages are routed through 3 different nodes in the Oxen Service Node Network, making it essentially Tor functionality in all but name.
If you look closely at the Github page you'll find Session is somehow listed as a fork of Signal. Hmmmm.
Backed by the Australian non-profit Oxen Privacy Tech Foundation. Pretty sweet.
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Analysis: I was honestly surprised at the score Session managed to get. I've heard a lot of good things about this but damn this blew me away. Session is a somewhat solid choice when it comes to private messaging. Them not collecting any data of sorts pretty much makes you anonymous when using the service. That being said, it does make it harder for your friends to find you so I wouldn't suggest using this for daily use. Session also uses their own onion functionality except instead of Tor nodes they use their own. They claim these nodes do not log your IP and cannot see what you're doing or who you're messaging, but they admit your IP will be visible to your initial seed node. If you really think you need to hide your IP, you can use a VPN if you want to. As we all know, this type of connection can be really slow so add that to another reason why you shouldn't use it for daily use. But if you really need anonymity and privacy with the person you intend to communicate to, then Session might be the right tool for that.
Update: It has come to my attention that Session has very weak encryption compared to the original app its based on (Signal). Read more about it here and here. As such I wouldn't recommend it over Signal or SimpleX (SPT coming soon), but it still has a larger SPT score than most services so it stays on the recommended list. Please do reconsider using it in the first place though. If I had a choice between a padlocked box and a safe for secure storage I would go with the safe.