转换ASN。1个签名到IEEEP1363签名

我确实使用下面的代码生成了一个签名，但是，我想要一个 IEEE P1336（长度 80）格式的签名

           guard let signData = SecKeyCreateSignature(
               key,
               SecKeyAlgorithm.ecdsaSignatureMessageX962SHA256,
               signatureString.data(using: .utf8)! as CFData, &error) else {
               let e = error!.takeRetainedValue() as Error

               print("Signing Error (  e.localizedDescription)")
               return nil
           }
           let signedData = signData as Data
           let signedString = signedData.base64EncodedString(options: [])