StaticcomputeCompute SRP handshake for authentication
User's email
User's password
Salt from auth/info (base64)
Modulus from auth/info (base64, PGP-signed)
Server ephemeral from auth/info (base64)
Auth version (default 4)
OptionalserverUsername: stringStaticverifyVerify server proof matches expected value
Proof from server
Expected proof calculated during handshake
SRP-6a implementation for Proton Drive authentication Adapted from ProtonMail WebClients