Using the Flickr Authentication API - part I - generating the api_sig

All Flickr API calls using an authentication token must be signed. In addition, calls to the flickr.auth.* methods and redirections to the auth page on flickr must also be signed.

The api_sig is to be appended to the URL for all such API calls. The following code can be used to generate the api_sig (of course, you can use a higher-level library such as flickrj if you enjoy eating microwave ready to eat meals over growing your own tomatoes ;-).

package com.jasondchambers.flickr;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;

/**
* Generates an api_sig from a list of parameter key value pairs
*
* The signing process goes like this. The parameters are sorted
* e.g. foo=1, bar=2, baz=3 sorts to bar=2, baz=3, foo=1
* The secret and the parameters are concatenated together as follows to provide
* the raw signature
* e.g. SECRETbar2baz3foo1
* An MD5 hash is created, converted to hex and returned - this is to be used as
* the api_sig parameter
*
* See section 8 of [1]
*
* [1] http://www.flickr.com/services/api/auth.spec.html
*
* @author Jason Chambers
*
*/
public class ApiSigGenerator {

 private String secret;

 public ApiSigGenerator(String secret) {
   this.secret = secret;
 }

 public String sign(String... paramKeyValuePairs) {
   try {
     // Sort the parameters first
     SortedMap<String, String> sortedParameterMap = sort(paramKeyValuePairs);
     // Generate the raw signature
     String rawApiSig = generateRawApiSig(sortedParameterMap);
     // Hash the raw signature and return
     return generateMd5(rawApiSig.toString());
   }
   catch (Exception e) {
     throw new FlickrClientException(e);
   }
 }

 private SortedMap<String, String> sort(String[] paramKeyValuePairs) {
   SortedMap<String, String> sortedParameterMap = new TreeMap<String, String>();
   final int KEY = 0;
   final int VALUE = 1;
   int i = KEY;
   String key = null;
   for (String o : paramKeyValuePairs) {
     if (i == KEY) {
       key = o;
     } else {
       sortedParameterMap.put(key, o);
     }
     i = ~i;
   }
   return sortedParameterMap;
 }

 private String generateRawApiSig(SortedMap<String, String> sortedParameterMap) {
   StringBuffer rawApiSig = new StringBuffer();
   rawApiSig.append(secret);
   Set<String> keySet = sortedParameterMap.keySet();
   for (String k1 : keySet) {
     rawApiSig.append(k1);
     rawApiSig.append(sortedParameterMap.get(k1));
   }
   return rawApiSig.toString();
 }

 private static String generateMd5(String input) throws NoSuchAlgorithmException {
   StringBuffer output = new StringBuffer();
   MessageDigest md;
     md = MessageDigest.getInstance("MD5");
   byte[] md5 = md.digest(input.getBytes());
   for (int i = 0; i < md5.length; i++) {
     String tmpStr = "0" + Integer.toHexString((0xff & md5[i]));
     output.append(tmpStr.substring(tmpStr.length() - 2));
   }
   return output.toString();
 }
}