Perfect Forward Secrecy
The largest change in the V2 protocol will be the addition of perfect forward secrecy (PFS). As previously discussed, Session Protocol V1 does not achieve forward secrecy currently, as all messages are encrypted under the same LTK; this key does not rotate unless the user creates a new Session account.
Session initially launched with PFS, inherited from the Signal Protocol, but the Signal Protocol was later deprecated in Session due to the significant issues it created for users with multiple linked devices. During the time it was enabled, Session users frequently raised issues with:
Sent messages not properly syncing between linked devices and remaining in a "failed to decrypt" state.
Messages received from other users generally failing to decrypt.
Accounts recovered via the user's Recovery Password not properly recovering message history.
These issues stem from the fact that the Signal Protocol was not designed for use in a decentralized network, and has very limited support for linked devices. To this day, Signal only supports up to five linked devices (one primary mobile device and up to four other linked desktop devices). The Signal Protocol is designed to depend on a centralized server to help clients publish pre-key bundles and keep ratcheting keys in sync between linked devices.
Based on this, it is clear that a new protocol is needed—one that takes cues from the Signal Protocol (still a strong choice in terms of encryption and security) but is designed for a decentralized network (like Session’s) and resolves the challenge of supporting many linked mobile and desktop devices.
Source:
https://getsession.org/blog/session-protocol-v2