Sample code used to create signed URLs. Gist created to support troubleshooting question at http://stackoverflow.com/questions/19890062/put-files-to-google-cloud-storage-gcs-via-signed-urls.

GcsSigner.java

import org.apache.commons.codec.binary.Base64;

import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;
import java.security.KeyStore;
import java.security.PrivateKey;
import java.security.Signature;
import java.security.UnrecoverableKeyException;

// http://commons.apache.org/codec/

public class GcsSigner {
    public static void main(String[] args) throws Exception {
        // hardcode the values for testing purpose
        String keyFile = "/mypath/privatekey.p12";
        String keyPassword = "notasecret";
        String stringToSign = "PUT\n" +
                "\n" +
                "text/plain\n" +
                "1384084959392\n" +
                "x-goog-api-version:2\n" +
                "x-goog-project-id:1234\n" +
                "/test-bucket/bob.txt";

        PrivateKey key = loadKeyFromPkcs12(keyFile, keyPassword.toCharArray());
        String signature = signData(key, stringToSign);

        // URL encode signature
        System.out.println(URLEncoder.encode(signature, "UTF-8"));
    }

    private static PrivateKey loadKeyFromPkcs12(String filename, char[] password) throws Exception {
        FileInputStream fis = new FileInputStream(filename);
        KeyStore ks = KeyStore.getInstance("PKCS12");
        try {
            ks.load(fis, password);
        } catch (IOException e) {
            if (e.getCause() != null && e.getCause() instanceof UnrecoverableKeyException) {
                System.err.println("Incorrect password");
            }
            throw e;
        }
        return (PrivateKey)ks.getKey("privatekey", password);
    }


    private static String signData(PrivateKey key, String data) throws Exception {
        Signature signer = Signature.getInstance("SHA256withRSA");
        signer.initSign(key);
        signer.update(data.getBytes("UTF-8"));
        byte[] rawSignature = signer.sign();
        String encodedSignature = new String(Base64.encodeBase64(rawSignature, false), "UTF-8");
        return encodedSignature;
    }
}