Designing an E2E-Encrypted Terminal Chat in C++17: SRP-6a, HKDF, and a Relay-Blind Server

There is a class of security properties that most hobby chat implementations simply skip: the server should not be able to read your messages, and authentication should not require trusting the server with a password hash. cmd_chat is a deliberately minimal C++17 implementation that takes both of these seriously — using SRP-6a , HKDF-SHA256 , and a Fernet-compatible AEAD scheme — without hiding the mechanics behind a TLS library. This post is about the design decisions, the trade-offs, and the p