LemonEmail looks like a very interesting implementation of 100% decentralized messaging system with end-to-end encryption. From what I understand from the information available, it uses the Ethereum blockchain to transport the ‘metadata’ of messages sent through the system, and it uses IPFS to transport the encrypted messages.
But, does LemonEmail support Perfect Forward Secrecy? In other words, if a user's private key is compromised, can an attacker use it to go back and decrypt ALL of the messages that were EVER sent to that user, that were encrypted using the corresponding public key (being that the metadata and the encrypted messages could easily be accessed by the attacker, as they are presumably stored indefinitely in the Ethereum blockchain and IPFS)?
Reading LemonEmail's blog post, 'Behind-the-scenes look at Lemon Email', after 'What happens behind the scenes when you click send', it reads, 'The email is encrypted locally. This ensures we can never read your data. If the recipient is a Lemon Email user, the email is encrypted using their public key'. However, there is no mention here of any sort of ephemeral key exchange, as necessary for Perfect Forward Secrecy.
Does anyone know of anything to the contrary, that LemonEmail does in fact support Perfect Forward Secrecy?