Why does a self-signed certificate throw a warning?

Why does a self-signed certificate throw a warning?

If encryption is identical, why does a self-signed certificate produce a full-page browser interstitial while a Let's Encrypt one does not? Because the browser's trust decision has nothing to do with the cipher and everything to do with the signature chain.

A self-signed certificate is signed by its own private key — it vouches for itself. The verification logic in every TLS (Transport Layer Security) client walks the chain from the leaf certificate up to a root in the local trust store. A self-signed leaf chains to nothing the browser already trusts, so path validation (RFC 5280, section 6) fails with an "unknown issuer" error.

A common misconception worth correcting: self-signed certificates are not "weaker encryption." The same cipher suites, the same AES-256-GCM, the same key sizes are available. What is missing is the third-party attestation of identity. You get confidentiality against a passive eavesdropper but zero protection against an active man-in-the-middle, because anyone can mint a self-signed cert claiming to be your domain.

Legitimate uses exist: internal services, local development, mutual-TLS between machines you control where you pin the cert explicitly.

Further reading: RFC 5280 section 6 (Certification Path Validation).

Bottom line: self-signed = same crypto, no trusted vouching. Fine inside your own perimeter, never for the public web.