问腾讯云通信生成签名时异常，ECDSA KeyFactory not available？

/**
	 * @brief 生成 tls 票据，精简参数列表
	 * @param skdAppid 应用的 sdkappid
	 * @param identifier 用户 id
	 * @param privStr 私钥文件内容
	 * @param expire 有效期，以秒为单位，推荐时长一个月
	 * @return
	 * @throws IOException
	 */
	public static GenTLSSignatureResult GenTLSSignatureEx(
			long skdAppid,
			String identifier,
			String privStr,
			long expire) throws IOException {

		GenTLSSignatureResult result = new GenTLSSignatureResult();
		
        Security.addProvider(new BouncyCastleProvider());
        Reader reader = new CharArrayReader(privStr.toCharArray());
        JcaPEMKeyConverter converter = new JcaPEMKeyConverter();
        PEMParser parser = new PEMParser(reader);
        Object obj = parser.readObject();
        parser.close();
        
        //发生异常的代码  *******************
    	PrivateKey privKeyStruct = converter.getPrivateKey((PrivateKeyInfo) obj);
    		//   ****************************************
    
		String jsonString = "{" 
		+ "\"TLS.account_type\":\"" + 0 +"\","
		+"\"TLS.identifier\":\"" + identifier +"\","
		+"\"TLS.appid_at_3rd\":\"" + 0 +"\","
	    +"\"TLS.sdk_appid\":\"" + skdAppid +"\","
		+"\"TLS.expire_after\":\"" + expire +"\","
        +"\"TLS.version\": \"201512300000\""
		+"}";
		
		String time = String.valueOf(System.currentTimeMillis()/1000);
		String SerialString = 
			"TLS.appid_at_3rd:" + 0 + "\n" +
			"TLS.account_type:" + 0 + "\n" +
			"TLS.identifier:" + identifier + "\n" + 
			"TLS.sdk_appid:" + skdAppid + "\n" + 
			"TLS.time:" + time + "\n" +
			"TLS.expire_after:" + expire +"\n";
		
		try {
			//Create Signature by SerialString
			Signature signature = Signature.getInstance("SHA256withECDSA", "BC");
			signature.initSign(privKeyStruct);
			signature.update(SerialString.getBytes(Charset.forName("UTF-8")));
			byte[] signatureBytes = signature.sign();
			
			String sigTLS = Base64.encodeBase64String(signatureBytes);
			
			//Add TlsSig to jsonString
		    JSONObject jsonObject= new JSONObject(jsonString);
		    jsonObject.put("TLS.sig", (Object)sigTLS);
		    jsonObject.put("TLS.time", (Object)time);
		    jsonString = jsonObject.toString();
		    
		    //compression
		    Deflater compresser = new Deflater();
		    compresser.setInput(jsonString.getBytes(Charset.forName("UTF-8")));

		    compresser.finish();
		    byte [] compressBytes = new byte [512];
		    int compressBytesLength = compresser.deflate(compressBytes);
		    compresser.end();
		    String userSig = new String(base64_url.base64EncodeUrl(Arrays.copyOfRange(compressBytes,0,compressBytesLength)));
    
		    result.urlSig = userSig;
		}
		catch(Exception e)
		{
			e.printStackTrace();
			result.errMessage = "generate usersig failed";
		}
		
		return result;
	}