A cryptographic system is said to have Perfect Forward Secrecy (PFS) if it’s not possible for an attacker who obtains a user’s private key in the present to use this key to decrypt messages the user has sent or received in the past.
This property is achieved by using separate keys for individual sessions or short periods of time instead of always using one and the same key.
Threema supports Perfect Forward Secrecy both on the transport layer and on the end-to-end layer. Technical information can be found in the Cryptography Whitepaper.
