package javasecurity;
import java.io.*;
import java.security.*;
import java.security.spec.*;
class VerSig
{
	public static void main(String[] args)
	{
		if (args.length != 3)
		{
			System.out.println
				("Usage: VerSig publickeyfile signaturefile datafile");
		}
		else
		{
			try
			{
				/* 導入經編碼后的公共密鑰 */
				FileInputStream keyfis = new FileInputStream(args[0]);
				byte[] encKey = new byte[keyfis.available()];
				keyfis.read(encKey);
				keyfis.close();
				X509EncodedKeySpec pubKeySpec = 
					new X509EncodedKeySpec(encKey);
				KeyFactory keyFactory = 
					KeyFactory.getInstance("DSA", "SUN");
				PublicKey pubKey =
					keyFactory.generatePublic(pubKeySpec);
				/* 輸入證書數據 */
				FileInputStream sigfis = new FileInputStream(args[1]);
				byte[] sigToVerify = new byte[sigfis.available()];
				sigfis.read(sigToVerify);
				sigfis.close();
				/* 建立一個簽名并用公共密鑰初始化 */
				Signature sig = 
					Signature.getInstance("SHA1withDSA", "SUN");
				sig.initVerify(pubKey);
				/* 更新并校驗數據 */
				FileInputStream datafis = new FileInputStream(args[2]);
				BufferedInputStream bufin = 
					new BufferedInputStream(datafis);
				byte[] buffer = new byte[1024];
				int len;
				while (bufin.available() != 0)
				{
					len = bufin.read(buffer);
					sig.update(buffer, 0, len);
				};
				bufin.close();
				boolean verifies = sig.verify(sigToVerify);
				System.out.println("signature verifies: " + verifies);
			}
			catch (Exception e)
			{
				System.err.println("Caught exception " + e.toString());
			};
		}
	}
}