Android : Implementation of OAuth request in android

I am trying to implement OAuth request to webservice using HmacSHA1 signature. I need to send two fields: data and method. Here is my code:

String HMAC_SHA1_ALGORITHM = "HmacSHA1";
String response = "";
        try {
            HttpClient client = new DefaultHttpClient();
            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
            HttpPost request = new HttpPost(url);
            CommonsHttpOAuthConsumer co = new CommonsHttpOAuthConsumer(key,
                    secret);
            SecretKeySpec signingKey = new SecretKeySpec(key.getBytes(),
                    HMAC_SHA1_ALGORITHM);
            // get an hmac_sha1 Mac instance and initialize with the signing
            // key
            Mac mac = Mac.getInstance(HMAC_SHA1_ALGORITHM);
            mac.init(signingKey);
            // compute the hmac on input data bytes
            byte[] rawHmac = mac.doFinal(data.getBytes());

            // base64-encode the hmac
            String result = new String(rawHmac, "UTF-8");// Encoding.EncodeBase64(rawHmac);
            nameValuePairs.add(new BasicNameValuePair("data", result));
            nameValuePairs.add(new BasicNameValuePair("method",
                    "UpdateFilms"));
            co.sign(request);
            request.setEntity(new UrlEncodedFormEntity(nameValuePairs));
            HttpResponse httpResponse = client.execute(request);
            HttpEntity httpEntity = httpResponse.getEntity();
            InputStream is = httpEntity.getContent();

            BufferedReader reader = new BufferedReader(
                    new InputStreamReader(is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            is.close();
            response = sb.toString();
            try {
            } catch (Exception e) {
                e.printStackTrace();
            }

            Log.v("response", "" + response);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return response;

But the server is returning error "Invalid Signature". Is my implementation correct?