gistfile1.txt

public static String signSha256WithRsa(String toSign, String privateKeyString) {
        if (toSign == null || privateKeyString == null) {
            return null;
        }

        try {
            KeyFactory kf = KeyFactory.getInstance("RSA");
            KeySpec keySpec = new PKCS8EncodedKeySpec(org.apache.commons.codec.binary
                    .Base64.decodeBase64(privateKeyString.getBytes("utf-8")));

            PrivateKey prvKey = kf.generatePrivate(keySpec);
            Signature sig = Signature.getInstance("SHA256withRSA");
            sig.initSign(prvKey);
            sig.update(toSign.getBytes());
            byte[] sigBytes = sig.sign();

            return new String(org.apache.commons.codec.binary.Base64.encodeBase64(sigBytes));
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        } catch (InvalidKeySpecException e) {
            e.printStackTrace();
        } catch (InvalidKeyException e) {
            e.printStackTrace();
        } catch (SignatureException e) {
            e.printStackTrace();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }

        return null;
    }