diff --git a/run.py b/run.py
index c3298d3..4beca5a 100644
--- a/run.py
+++ b/run.py
@@ -1,25 +1,65 @@
-"""Dev/Prod entry point. Runs the Flask app, over TLS when certs exist."""
+"""Entrypoint. Runs the Flask app over HTTPS (self-signed cert in dev).
+
+TLS is mandatory in this design: the passphrase and all plaintext stay
+in the browser, and the API only makes sense over an encrypted channel
+so browsers refuse to send crypto JS/code over an untrusted channel (
+mixed content / HSTS-chaining attacks).
+
+Two modes:
+  - certs/cert.pem + certs/key.pem present -> serve https://127.0.0.1:8443
+  - otherwise -> generate a self-signed certificate (only for local dev)
+    and start on https://127.0.0.1:8443
+
+If openssl is unavailable, falls back to plain HTTP on 5000 with a loud
+warning — dev convenience only, NOT suitable for production.
+"""
 
 import os
 import ssl
+import subprocess
+import sys
 
 from backend.app import app
 from backend import storage
 
+CERT = os.path.join("certs", "cert.pem")
+KEY = os.path.join("certs", "key.pem")
+
+
+def _ensure_self_signed_cert():
+    if os.path.exists(CERT) and os.path.exists(KEY):
+        return
+    os.makedirs("certs", exist_ok=True)
+    subprocess.run(
+        [
+            "openssl", "req", "-x509", "-newkey", "rsa:2048",
+            "-keyout", KEY, "-out", CERT,
+            "-days", "365", "-nodes",
+            "-subj", "/CN=localhost",
+            "-addext", "subjectAltName=DNS:localhost,IP:127.0.0.1",
+        ],
+        check=True,
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+    )
+    print("Self-signed dev certificate created in certs/.")
+
 
 def main():
     storage.init_db()
 
-    cert = os.path.join("certs", "cert.pem")
-    key = os.path.join("certs", "key.pem")
+    port = int(os.environ.get("PORT", "8443"))
+    host = os.environ.get("HOST", "127.0.0.1")
 
-    if os.path.exists(cert) and os.path.exists(key):
+    try:
+        _ensure_self_signed_cert()
         context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
-        context.load_cert_chain(cert, key)
-        app.run(host="127.0.0.1", port=8443, ssl_context=context)
-    else:
-        print("WARNING: no certs/ found, running WITHOUT TLS (dev only!).")
-        app.run(host="127.0.0.1", port=5000)
+        context.load_cert_chain(CERT, KEY)
+        print(f"Serving on https://{host}:{port} (self-signed dev TLS).")
+        app.run(host=host, port=port, ssl_context=context)
+    except (FileNotFoundError, subprocess.CalledProcessError) as e:
+        print(f"WARNING: no TLS available ({e}); falling back to HTTP.", file=sys.stderr)
+        app.run(host=host, port=5000)
 
 
 if __name__ == "__main__":