X.509 証明書デコーダー
PEM 形式の証明書、チェーン全体、または base64/16 進数の DER を貼り付けてください。身元情報、有効期間、鍵と署名のアルゴリズム、拡張、フィンガープリントをすべてローカルで解析し、アルゴリズムの健全性についてわかりやすく解説します。
100% ローカル：すべてブラウザー内で処理され、どのサーバーにも何も送信されません
How it works
Your certificate never leaves this page. The input is split into PEM blocks (or decoded from base64/hex DER), and each block is parsed with an ASN.1 parser compiled into the page itself — subject, validity, key parameters and extensions are read directly from the DER structure. The SHA-256 fingerprint is computed in JavaScript in your browser as well, so no bytes are uploaded anywhere. Assessment notes are rule-based: signature hashes are checked against what modern clients accept, key algorithms against the NIST IR 8547 post-quantum timeline, and lifetimes against the CA/Browser Forum schedule.