DER ⇄ 生形式の署名変換ツール
ASN.1 DER と生の r‖s（IEEE P1363）のどちらかの形式で ECDSA 署名を貼り付けると、両方の形式で結果を得られます。r と s の抽出、high-S の検出、正規形での再エンコードも行います。
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How it works
An ECDSA signature is just two big integers, r and s — but two ecosystems package them differently. TLS, X.509 certificates and OpenSSL wrap them in ASN.1 DER, a self-describing structure (SEQUENCE of two INTEGERs) whose length varies from signature to signature. JWS/JWT, WebCrypto and most blockchains instead use the raw IEEE P1363 form: r and s zero-padded to the curve size and simply concatenated, so a P-256 signature is always exactly 64 bytes. Feeding one format to a verifier that expects the other is one of the most common reasons a perfectly good signature "fails" to verify.
Two more subtleties trip people up. For every valid signature (r, s) the mirrored pair (r, n − s) is also valid, so strict systems — Bitcoin most famously — canonicalize by requiring the low-S half (s ≤ n/2); this tool flags high-S values and can normalize them. And DER itself has exactly one legal encoding per value: extra leading zeros, missing sign-padding or long-form lengths make a signature that lenient parsers accept but strict ones reject, so the converter warns about them and always emits the canonical bytes. Everything runs locally in your browser — nothing you paste is ever sent anywhere.