/**
  * 獲取私鑰。
  *
  * @param filename 私鑰文件路徑  (required)
  * @return 私鑰對象
  */
public static PrivateKey getPrivateKey(String filename) throws IOException {

  String content = new String(Files.readAllBytes(Paths.get(filename)), "utf-8");
  try {
    String privateKey = content.replace("-----BEGIN PRIVATE KEY-----", "")
        .replace("-----END PRIVATE KEY-----", "")
        .replaceAll("\\s+", "");

    KeyFactory kf = KeyFactory.getInstance("RSA");
    return kf.generatePrivate(
        new PKCS8EncodedKeySpec(Base64.getDecoder().decode(privateKey)));
  } catch (NoSuchAlgorithmException e) {
    throw new RuntimeException("當前Java環(huán)境不支持RSA", e);
  } catch (InvalidKeySpecException e) {
    throw new RuntimeException("無效的密鑰格式");
  }
}

/**
* Read private key from file
*
* @param string    $filepath     PEM encoded private key file path
* 
* @return resource|bool     Private key resource identifier on success, or FALSE on error
*/
public static function getPrivateKey($filepath) {
    return openssl_get_privatekey(file_get_contents($filepath));
}

protected string sign(string message)
{
    // 需去除私鑰文件中的-----BEGIN/END PRIVATE KEY-----
    string privateKey = "MIIEvgIBADANBgkqhkiG...30HBe+GD1tntZgf6I1Y0ZpHZ";
    byte[] keyData = Convert.FromBase64String(privateKey);
    using (CngKey cngKey = CngKey.Import(keyData, CngKeyBlobFormat.Pkcs8PrivateBlob))
    using (RSACng rsa = new RSACng(cngKey))
    {
        byte[] data = System.Text.Encoding.UTF8.GetBytes(message);
        return Convert.ToBase64String(rsa.SignData(data, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
    }
}

package main
import (
  "crypto/x509"
  "encoding/pem"
  "fmt"
  "log"
)

func main() {
  var pemData = []byte(`
-----BEGIN PRIVATE KEY-----
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
-----END PRIVATE KEY-----`)

  block, rest := pem.Decode(pemData)
  if block == nil || block.Type != "PRIVATE KEY" {
    log.Fatal("failed to decode PEM block containing public key")
  }

  pri, err := x509.ParsePKCS8PrivateKey(block.Bytes)
  if err != nil {
    log.Fatal(err)
  }

  fmt.Printf("Got a %T, with remaining data: %q", pri, rest)
}