nx_x509_validate.nx

nx_x509_validate.nx -- X.509 certificate validation (Gap I.2.A). Phase 0b §I.2 of the Nishi TLS 1.3 stack per docs/NISHI_TLS13_GAP_AUDIT.md. Composes the shipped ed25519_verify_full (T11) with the X509Cert parse offsets (nx_x509, extended in T6) into the cert-validation primitives that the TLS client's accept-any stub will eventually call. What it does today: - x509_verify_signature_ed25519(buf, cert, signer_pubkey32): read cert.tbs + cert.sig from buf, verify the signature under signer_pubkey32 using shipped ed25519_verify_full. Returns NX_X509_VAL_OK (1) on valid, NX_X509_VAL_BAD_SIG (0) on invalid. Negative on malformed (e.g. sig wrong length). - Sealed verdict + validity gate. What it doesn't do yet (sub-pieces I.2.B + I.2.C, ~1 session each): - validity period check (UTCTime / GeneralizedTime parse + compare against caller-supplied now_unix_secs). Needs: * fn x509_validity_get(cert, buf, out_notBefore, out_notAfter) * ASN.1 time-string parser (Year Month Day Hour Min Sec Z) * compose against nx_time / unix-epoch math - SAN extension walk (parse cert.extensions_off/len for OID 2.5.29.17 SubjectAltName, walk GeneralName SEQUENCE, extract dNSName entries, match against SNI hostname). - ECDSA-P256 signature verify (for real-world server certs; covers ~80% of Web PKI). - RSA-PSS signature verify (for the remaining ~15%). - Full RFC 5280 chain walk + trust-anchor matching (Gap I.3). KAT verified: - Self-signed Ed25519 cert (RFC 8032 §7.1 TEST 1 keypair; fake tbs blob; sign with ed25519_sign_full at test time; wrap into a synthetic X509Cert offset structure) validates. - Same cert with one tbs byte flipped: validation FAILS. - Same cert with a different signer_pubkey: validation FAILS. - Same cert with sig byte flipped: validation FAILS. Composes with: