Sessions developers dropped Signal’s Perfect Forward Secrecy (PFS) and deniability [0] security features. Personally I would not trust a product that drops an end-user security feature for the sake of making the developer’s life easier [1] .

Using existing long-term keypairs in place of the Signal protocol massively simplifies 1-1 messaging.

For those unaware, PFS protects your data/messages from future exploits and breaches. With PFS, each message’s encryption is isolated, preventing compromise of current and past interactions [2].

A simple example to illustrate why PFS is beneficial. Lets assume any 3 letter agency is collecting all Signal/Session messages - on top of the tons of data they’re already capturing. The great thing is that your messages are encrypted, they can’t see anything - YAY - but they’re storing them basically forever.

Two ways they may be able to compromise your privacy and view ALL your messages:

1. A flaw is discovered that allows them to crack/brute force the encryption in weeks instead of years/decades/eternity. If you were using Sessions, because you use the same key for every message, they now have access to everything you’ve ever said. If you were using Signal, they have access to that one message and need to spend considerable resources trying to crack every other message.

2. Your phone is compromised and they take your encryption keys. If you were using Sessions, this again gives them access to your entire message history. If you were using Signal, because the keys are always rotating (known as ephemeral) they can only use them to unlock the most recent received messages.

It’s important to state that both cases above only really matter if you delete your messages after a certain time. Otherwise, yes, all they have to do is take your phone and get access to your entire message history - which is why ephemeral messaging (i.e. auto deleting messages after a certain time) is crucial if you suspect you may be targeted.

[0] https://getsession.org/blog/session-protocol-explained

[1] https://getsession.org/blog/session-protocol-technical-information

[2] https://www.signal.org/blog/advanced-ratcheting/