Paypal mobile express checkout with java - paypal

Question

I have developed a mobile application which required to integrate with paypal, my back end server is RESTfull java server. So in order to integrate with paypal, i obtain "deviceReferenceTokenWithAppId" from client end calls a servlet at back end server along with other details (cartId..etc), When the request comes to servlet i do some back end operations and calls paypal to get the specific "SetExpressCheckout" token required for mobile payment.
StringBuilder postData = new StringBuilder();
postData.append(USER).append("=").append(Constants.PAYPAL_MERCHANT_USER);
postData.append("&").append(PASSWORD).append("=").append(Constants.PAYPAL_MERCHANT_PASSWORD);
postData.append("&").append(SIGNATURE).append("=").append(Constants.PAYPAL_MERCHANT_SIGNATURE);
postData.append("&").append(METHOD).append("=").append("SetExpressCheckout");
postData.append("&").append(VERSION).append("=").append("88");
postData.append("&").append(AMOUNT).append("=").append(amount.toString());
postData.append("&").append(CANCEL_URL).append("=").append(Constants.PAYPAL_MERCHANT_CANCEL_URL);
postData.append("&").append(RETURN_URL).append("=").append(Constants.PAYPAL_MERCHANT_RETURN_URL);
byte[] postDataArr = postData.toString().getBytes(UTF-8);
// Hit the the URL.
URL url = new URL("https://api-3t.sandbox.paypal.com/nvp");
HttpsURLConnection.setDefaultHostnameVerifier(new CustomizedHostnameVerifier());
HttpsURLConnection conn = (HttpsURLConnection) url.openConnection();
conn.setDoOutput(true);
conn.setDoInput(true);
conn.setUseCaches(false);
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded;charset=UTF-8");
conn.setRequestProperty("Content-Length", Integer.toString(postDataArr.length));
DataOutputStream output = new DataOutputStream( conn.getOutputStream());
output.write( postDataArr );
output.flush();
output.close ();
rc = conn.getResponseCode();
My problem is if i call the paypal nvp, i dont get a valid token as the response, instead i get following error,
Key - TIMESTAMP - value : 2012-06-11T18:16:02Z
Key - CORRELATIONID - value : 207108cab758a
Key - ACK - value : Failure
Key - VERSION - value : 88
Key - BUILD - value : 2975009
Key - L_ERRORCODE0 - value : 10002
Key - L_SHORTMESSAGE0 - value : Security error
Key - L_LONGMESSAGE0 - value : Security header is not valid
Key - L_SEVERITYCODE0 - value : Error
Can somebody tell me why do i get this error? have i missed anything here?
If i can successfully get the TOKEN, next i have to redirect my servlet response to following Url to get mobile checkout view to mobile application.
https://www.sandbox.paypal.com/webscr?cmd=_express-checkout-mobile&drt="+<deviceReferenceTokenWithAppId>+"&token="+<TOKEN from payal nvp>
Appreciate if someone can guide me to a correct path if i have done a mistake here..

Answer 1

It means your API credentials are incorrect for the endpoint you're accessing.
Try replacing "PASSWORD" by "PWD"; authentication is done with a 'USER', 'PWD' and 'SIGNATURE' with PayPal.