JWT: Trust Nothing in the Header

Many JWT breaches come from trusting the token's own alg header (none, or an RS256→HS256 swap). The first line is T; each next line is a token header.payload.signature (base64url, no padding). For each token print:

• REJECT malformed if it doesn't have exactly 3 parts, or the header or payload isn't base64url-encoded JSON objects;
• REJECT alg if alg isn't exactly one of HS256, RS256, ES256 (case-sensitive — none, None, HS512 are rejected);
• REJECT signature if the signature part is empty;
• otherwise OK <alg> <sub>, where <sub> is the payload's sub string, or - if missing.

You only pre-screen here — real code must still verify the signature with a key chosen by the server.