/**
* A Base64 encoder/decoder.
*
* <p>
* This class is used to encode and decode data in Base64 format as described in RFC 1521.
*
* <p>
* Project home page: <a href="http://www.source-code.biz/base64coder/java/">www.source-code.biz/base64coder/java</a><br>
* Author: Christian d'Heureuse, Inventec Informatik AG, Zurich, Switzerland<br>
* Multi-licensed: EPL / LGPL / GPL / AL / BSD.
*/
public class Base64Coder {

// The line separator string of the operating system.
private static final String systemLineSeparator = System.getProperty("line.separator");

// Mapping table from 6-bit nibbles to Base64 characters.
private static char[]    map1 = new char[64];
   static {
      int i=0;
      for (char c='A'; c<='Z'; c++) map1[i++] = c;
      for (char c='a'; c<='z'; c++) map1[i++] = c;
      for (char c='0'; c<='9'; c++) map1[i++] = c;
      map1[i++] = '+'; map1[i++] = '/'; }

// Mapping table from Base64 characters to 6-bit nibbles.
private static byte[]    map2 = new byte[128];
   static {
      for (int i=0; i<map2.length; i++) map2[i] = -1;
      for (int i=0; i<64; i++) map2[map1[i]] = (byte)i; }

/**
* Encodes a string into Base64 format.
* No blanks or line breaks are inserted.
* @param s  A String to be encoded.
* @return   A String containing the Base64 encoded data.
*/
public static String encodeString (String s) {
   return new String(encode(s.getBytes())); }

/**
* Encodes a byte array into Base 64 format and breaks the output into lines of 76 characters.
* This method is compatible with <code>sun.misc.BASE64Encoder.encodeBuffer(byte[])</code>.
* @param in  An array containing the data bytes to be encoded.
* @return    A String containing the Base64 encoded data, broken into lines.
*/
public static String encodeLines (byte[] in) {
   return encodeLines(in, 0, in.length, 76, systemLineSeparator); }

/**
* Encodes a byte array into Base 64 format and breaks the output into lines.
* @param in            An array containing the data bytes to be encoded.
* @param iOff          Offset of the first byte in <code>in</code> to be processed.
* @param iLen          Number of bytes to be processed in <code>in</code>, starting at <code>iOff</code>.
* @param lineLen       Line length for the output data. Should be a multiple of 4.
* @param lineSeparator The line separator to be used to separate the output lines.
* @return              A String containing the Base64 encoded data, broken into lines.
*/
public static String encodeLines (byte[] in, int iOff, int iLen, int lineLen, String lineSeparator) {
   int blockLen = (lineLen*3) / 4;
   if (blockLen <= 0) throw new IllegalArgumentException();
   int lines = (iLen+blockLen-1) / blockLen;
   int bufLen = ((iLen+2)/3)*4 + lines*lineSeparator.length();
   StringBuilder buf = new StringBuilder(bufLen);
   int ip = 0;
   while (ip < iLen) {
      int l = Math.min(iLen-ip, blockLen);
      buf.append (encode(in, iOff+ip, l));
      buf.append (lineSeparator);
      ip += l; }
   return buf.toString(); }

/**
* Encodes a byte array into Base64 format.
* No blanks or line breaks are inserted in the output.
* @param in  An array containing the data bytes to be encoded.
* @return    A character array containing the Base64 encoded data.
*/
public static char[] encode (byte[] in) {
   return encode(in, 0, in.length); }

/**
* Encodes a byte array into Base64 format.
* No blanks or line breaks are inserted in the output.
* @param in    An array containing the data bytes to be encoded.
* @param iLen  Number of bytes to process in <code>in</code>.
* @return      A character array containing the Base64 encoded data.
*/
public static char[] encode (byte[] in, int iLen) {
   return encode(in, 0, iLen); }

/**
* Encodes a byte array into Base64 format.
* No blanks or line breaks are inserted in the output.
* @param in    An array containing the data bytes to be encoded.
* @param iOff  Offset of the first byte in <code>in</code> to be processed.
* @param iLen  Number of bytes to process in <code>in</code>, starting at <code>iOff</code>.
* @return      A character array containing the Base64 encoded data.
*/
public static char[] encode (byte[] in, int iOff, int iLen) {
   int oDataLen = (iLen*4+2)/3;       // output length without padding
   int oLen = ((iLen+2)/3)*4;         // output length including padding
   char[] out = new char[oLen];
   int ip = iOff;
   int iEnd = iOff + iLen;
   int op = 0;
   while (ip < iEnd) {
      int i0 = in[ip++] & 0xff;
      int i1 = ip < iEnd ? in[ip++] & 0xff : 0;
      int i2 = ip < iEnd ? in[ip++] & 0xff : 0;
      int o0 = i0 >>> 2;
      int o1 = ((i0 &   3) << 4) | (i1 >>> 4);
      int o2 = ((i1 & 0xf) << 2) | (i2 >>> 6);
      int o3 = i2 & 0x3F;
      out[op++] = map1[o0];
      out[op++] = map1[o1];
      out[op] = op < oDataLen ? map1[o2] : '='; op++;
      out[op] = op < oDataLen ? map1[o3] : '='; op++; }
   return out; }

/**
* Decodes a string from Base64 format.
* No blanks or line breaks are allowed within the Base64 encoded input data.
* @param s  A Base64 String to be decoded.
* @return   A String containing the decoded data.
* @throws   IllegalArgumentException If the input is not valid Base64 encoded data.
*/
public static String decodeString (String s) {
   return new String(decode(s)); }

/**
* Decodes a byte array from Base64 format and ignores line separators, tabs and blanks.
* CR, LF, Tab and Space characters are ignored in the input data.
* This method is compatible with <code>sun.misc.BASE64Decoder.decodeBuffer(String)</code>.
* @param s  A Base64 String to be decoded.
* @return   An array containing the decoded data bytes.
* @throws   IllegalArgumentException If the input is not valid Base64 encoded data.
*/
public static byte[] decodeLines (String s) {
   char[] buf = new char[s.length()];
   int p = 0;
   for (int ip = 0; ip < s.length(); ip++) {
      char c = s.charAt(ip);
      if (c != ' ' && c != '\r' && c != '\n' && c != '\t')
         buf[p++] = c; }
   return decode(buf, 0, p); }

/**
* Decodes a byte array from Base64 format.
* No blanks or line breaks are allowed within the Base64 encoded input data.
* @param s  A Base64 String to be decoded.
* @return   An array containing the decoded data bytes.
* @throws   IllegalArgumentException If the input is not valid Base64 encoded data.
*/
public static byte[] decode (String s) {
   return decode(s.toCharArray()); }

/**
* Decodes a byte array from Base64 format.
* No blanks or line breaks are allowed within the Base64 encoded input data.
* @param in  A character array containing the Base64 encoded data.
* @return    An array containing the decoded data bytes.
* @throws    IllegalArgumentException If the input is not valid Base64 encoded data.
*/
public static byte[] decode (char[] in) {
   return decode(in, 0, in.length); }

/**
* Decodes a byte array from Base64 format.
* No blanks or line breaks are allowed within the Base64 encoded input data.
* @param in    A character array containing the Base64 encoded data.
* @param iOff  Offset of the first character in <code>in</code> to be processed.
* @param iLen  Number of characters to process in <code>in</code>, starting at <code>iOff</code>.
* @return      An array containing the decoded data bytes.
* @throws      IllegalArgumentException If the input is not valid Base64 encoded data.
*/
public static byte[] decode (char[] in, int iOff, int iLen) {
   if (iLen%4 != 0) throw new IllegalArgumentException ("Length of Base64 encoded input string is not a multiple of 4.");
   while (iLen > 0 && in[iOff+iLen-1] == '=') iLen--;
   int oLen = (iLen*3) / 4;
   byte[] out = new byte[oLen];
   int ip = iOff;
   int iEnd = iOff + iLen;
   int op = 0;
   while (ip < iEnd) {
      int i0 = in[ip++];
      int i1 = in[ip++];
      int i2 = ip < iEnd ? in[ip++] : 'A';
      int i3 = ip < iEnd ? in[ip++] : 'A';
      if (i0 > 127 || i1 > 127 || i2 > 127 || i3 > 127)
         throw new IllegalArgumentException ("Illegal character in Base64 encoded data.");
      int b0 = map2[i0];
      int b1 = map2[i1];
      int b2 = map2[i2];
      int b3 = map2[i3];
      if (b0 < 0 || b1 < 0 || b2 < 0 || b3 < 0)
         throw new IllegalArgumentException ("Illegal character in Base64 encoded data.");
      int o0 = ( b0       <<2) | (b1>>>4);
      int o1 = ((b1 & 0xf)<<4) | (b2>>>2);
      int o2 = ((b2 &   3)<<6) |  b3;
      out[op++] = (byte)o0;
      if (op<oLen) out[op++] = (byte)o1;
      if (op<oLen) out[op++] = (byte)o2; }
   return out; }

// Dummy constructor.
private Base64Coder() {}

} // end class Base64Coder



// How to export the private key from keystore?
// Does keytool not have an option to do so?
// This example use the "testkeys" file that comes with JSSE 1.0.3
// Alexey Zilber: Ported to work with Base64Coder: http://www.source-code.biz/snippets/java/2.htm

import java.security.cert.Certificate;
import java.security.*;
import java.io.File;
import java.io.FileInputStream;

class ExportPriv {
	public static void main(String args[]) throws Exception {
		if (args.length != 4) {
			// Yes I know this sucks (the password is visible to other users via
			// ps
			// but this was a quick-n-dirty fix to export from a keystore to
			// pkcs12
			// someday I may fix, but for now it'll have to do.
			System.err
					.println("Usage: java ExportPriv <keystore> <alias> <store-password> <key-password>");
			System.exit(1);
		}
		ExportPriv myep = new ExportPriv();
		myep.doit(args[0], args[1], args[2], args[3]);
	}

	public void doit(String fileName, String aliasName, String storepassword,
			String keyPassword) throws Exception {

		KeyStore ks = KeyStore.getInstance("JKS");

		char[] storePwdPhrase = storepassword.toCharArray();
		char[] keyPasswordPhrase = keyPassword.toCharArray();
		// BASE64Encoder myB64 = new BASE64Encoder();

		File certificateFile = new File(fileName);
		ks.load(new FileInputStream(certificateFile), storePwdPhrase);

		KeyPair kp = getPrivateKey(ks, aliasName, keyPasswordPhrase);

		PrivateKey privKey = kp.getPrivate();

		char[] b64 = Base64Coder.encode(privKey.getEncoded());

		System.out.println("-----BEGIN PRIVATE KEY-----");
		System.out.println(b64);
		System.out.println("-----END PRIVATE KEY-----");

	}

	// From http://javaalmanac.com/egs/java.security/GetKeyFromKs.html

	public KeyPair getPrivateKey(KeyStore keystore, String alias,
			char[] password) {
		try {
			// Get private key
			Key key = keystore.getKey(alias, password);
			if (key instanceof PrivateKey) {
				// Get certificate of public key
				Certificate cert = keystore.getCertificate(alias);

				// Get public key
				PublicKey publicKey = cert.getPublicKey();

				// Return a key pair
				return new KeyPair(publicKey, (PrivateKey) key);
			}
		} catch (UnrecoverableKeyException e) {
		} catch (NoSuchAlgorithmException e) {
		} catch (KeyStoreException e) {
		}
		return null;
	}

}


ExportPriv <keystore> <alias> <store-password> <key-password>

http://conshell.net/wiki/index.php/Keytool_to_OpenSSL_Conversion_tips

Export Private Key from a keystore

Java