csrf_token.nx

csrf_token.nx -- Cross-Site Request Forgery token generation. Standard defence against forged cross-origin form submits: server issues a token tied to the user's session, embeds it in forms, rejects submissions whose token doesn't HMAC-match the session. Since cross-origin JS can't read the session cookie or the issued token, attacker can't forge a valid request. OWASP recommended pattern: \"Signed Double-Submit Cookie\" -- token = random + HMAC(session_id, random). Verification splits, re-derives HMAC, constant-time compares. Composes rand.nx + hmac.nx + base64.nx + ct.nx. Token format (URL-safe, no padding): base64url(random_32_bytes) . base64url(hmac[0..16]) 32 bytes random = 256 bits unpredictability; 128-bit truncated HMAC = collision-resistant auth tag for this use. Total URI- safe length ~65 chars. Invariants: CT1 Token split at '.'; left = random, right = MAC. CT2 Verification re-derives MAC from (session_id, random) and ct_memcmp against provided MAC. CT3 Session ID opaque to this module -- any byte-stable session identifier works.