/**
* Copyright 2011 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package de.schildbach.wallet.digitalcoin.util;
import java.math.BigInteger;
import java.util.Arrays;
import com.google.digitalcoin.core.Utils;
/**
* Base43, derived from Base58
*/
public class Base43
{
private static final String ALPHABET = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ$*+-./:";
private static final BigInteger BASE = BigInteger.valueOf(ALPHABET.length());
public static String encode(byte[] input)
{
// TODO: This could be a lot more efficient.
BigInteger bi = new BigInteger(1, input);
StringBuffer s = new StringBuffer();
while (bi.compareTo(BASE) >= 0)
{
BigInteger mod = bi.mod(BASE);
s.insert(0, ALPHABET.charAt(mod.intValue()));
bi = bi.subtract(mod).divide(BASE);
}
s.insert(0, ALPHABET.charAt(bi.intValue()));
// Convert leading zeros too.
for (byte anInput : input)
{
if (anInput == 0)
s.insert(0, ALPHABET.charAt(0));
else
break;
}
return s.toString();
}
public static byte[] decode(String input) throws IllegalArgumentException
{
byte[] bytes = decodeToBigInteger(input).toByteArray();
// We may have got one more byte than we wanted, if the high bit of the next-to-last byte was not zero. This
// is because BigIntegers are represented with twos-compliment notation, thus if the high bit of the last
// byte happens to be 1 another 8 zero bits will be added to ensure the number parses as positive. Detect
// that case here and chop it off.
boolean stripSignByte = bytes.length > 1 && bytes[0] == 0 && bytes[1] < 0;
// Count the leading zeros, if any.
int leadingZeros = 0;
for (int i = 0; input.charAt(i) == ALPHABET.charAt(0); i++)
{
leadingZeros++;
}
// Now cut/pad correctly. Java 6 has a convenience for this, but Android can't use it.
byte[] tmp = new byte[bytes.length - (stripSignByte ? 1 : 0) + leadingZeros];
System.arraycopy(bytes, stripSignByte ? 1 : 0, tmp, leadingZeros, tmp.length - leadingZeros);
return tmp;
}
public static BigInteger decodeToBigInteger(String input) throws IllegalArgumentException
{
BigInteger bi = BigInteger.valueOf(0);
// Work backwards through the string.
for (int i = input.length() - 1; i >= 0; i--)
{
int alphaIndex = ALPHABET.indexOf(input.charAt(i));
if (alphaIndex == -1)
{
throw new IllegalArgumentException("Illegal character " + input.charAt(i) + " at " + i);
}
bi = bi.add(BigInteger.valueOf(alphaIndex).multiply(BASE.pow(input.length() - 1 - i)));
}
return bi;
}
/**
* Uses the checksum in the last 4 bytes of the decoded data to verify the rest are correct. The checksum is removed
* from the returned data.
*
* @throws IllegalArgumentException
* if the input is not base 43 or the checksum does not validate.
*/
public static byte[] decodeChecked(String input) throws IllegalArgumentException
{
byte[] tmp = decode(input);
if (tmp.length < 4)
throw new IllegalArgumentException("Input too short");
byte[] checksum = new byte[4];
System.arraycopy(tmp, tmp.length - 4, checksum, 0, 4);
byte[] bytes = new byte[tmp.length - 4];
System.arraycopy(tmp, 0, bytes, 0, tmp.length - 4);
tmp = Utils.doubleDigest(bytes);
byte[] hash = new byte[4];
System.arraycopy(tmp, 0, hash, 0, 4);
if (!Arrays.equals(hash, checksum))
throw new IllegalArgumentException("Checksum does not validate");
return bytes;
}
}