whois: show negotiated TLS version/group/cipher in 671; sslgroup crate reads the KEX group

sslgroup/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "sslgroup"
+version = "0.1.0"
+edition = "2021"
+description = "Read the negotiated TLS key-exchange group name via SSL_get0_group_name (not exposed by the safe openssl crate)."
+license = "MIT"
+
+[dependencies]
+# Same versions echoircd resolves, so the SslRef / SSL pointer types match.
+openssl = "0.10"
+openssl-sys = "0.9"
+foreign-types = "0.3"

sslgroup/src/lib.rs

@@ -0,0 +1,23 @@
+//! Reads the negotiated TLS key-exchange group name (e.g. `X25519MLKEM768`) via
+//! OpenSSL's `SSL_get0_group_name`, which the safe `openssl` crate does not expose.
+//! Isolated in its own crate so the daemon can stay `#![forbid(unsafe_code)]` — the
+//! single `unsafe` FFI call lives here, exactly like the FFI inside `openssl`/`ring`.
+
+use foreign_types::ForeignTypeRef;
+use openssl::ssl::SslRef;
+use std::ffi::CStr;
+
+/// The TLS key-exchange group name for an accepted session (OpenSSL 3.2+), or
+/// `None` if unavailable (before the handshake, or an OpenSSL without the API).
+pub fn group_name(ssl: &SslRef) -> Option<String> {
+    // SAFETY: `ssl` is a live, accepted `SSL`. `SSL_get0_group_name` returns a
+    // NUL-terminated string OpenSSL owns for the session's lifetime; we copy it out
+    // here, so the borrow does not escape.
+    unsafe {
+        let name = openssl_sys::SSL_get0_group_name(ssl.as_ptr());
+        if name.is_null() {
+            return None;
+        }
+        CStr::from_ptr(name).to_str().ok().map(String::from)
+    }
+}