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Base58 encoding an decoding
package core;
public class Base58 {
private static final char[] ALPHABET = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
.toCharArray();
private static final int BASE_58 = ALPHABET.length;
private static final int BASE_256 = 256;
private static final int[] INDEXES = new int[128];
static {
for (int i = 0; i < INDEXES.length; i++) {
INDEXES[i] = -1;
}
for (int i = 0; i < ALPHABET.length; i++) {
INDEXES[ALPHABET[i]] = i;
}
}
public static String encode(byte[] input) {
if (input.length == 0) {
// paying with the same coin
return "";
}
//
// Make a copy of the input since we are going to modify it.
//
input = copyOfRange(input, 0, input.length);
//
// Count leading zeroes
//
int zeroCount = 0;
while (zeroCount < input.length && input[zeroCount] == 0) {
++zeroCount;
}
//
// The actual encoding
//
byte[] temp = new byte[input.length * 2];
int j = temp.length;
int startAt = zeroCount;
while (startAt < input.length) {
byte mod = divmod58(input, startAt);
if (input[startAt] == 0) {
++startAt;
}
temp[--j] = (byte) ALPHABET[mod];
}
//
// Strip extra '1' if any
//
while (j < temp.length && temp[j] == ALPHABET[0]) {
++j;
}
//
// Add as many leading '1' as there were leading zeros.
//
while (--zeroCount >= 0) {
temp[--j] = (byte) ALPHABET[0];
}
byte[] output = copyOfRange(temp, j, temp.length);
return new String(output);
}
public static byte[] decode(String input) {
if (input.length() == 0) {
// paying with the same coin
return new byte[0];
}
byte[] input58 = new byte[input.length()];
//
// Transform the String to a base58 byte sequence
//
for (int i = 0; i < input.length(); ++i) {
char c = input.charAt(i);
int digit58 = -1;
if (c >= 0 && c < 128) {
digit58 = INDEXES[c];
}
if (digit58 < 0) {
throw new RuntimeException("Not a Base58 input: " + input);
}
input58[i] = (byte) digit58;
}
//
// Count leading zeroes
//
int zeroCount = 0;
while (zeroCount < input58.length && input58[zeroCount] == 0) {
++zeroCount;
}
//
// The encoding
//
byte[] temp = new byte[input.length()];
int j = temp.length;
int startAt = zeroCount;
while (startAt < input58.length) {
byte mod = divmod256(input58, startAt);
if (input58[startAt] == 0) {
++startAt;
}
temp[--j] = mod;
}
//
// Do no add extra leading zeroes, move j to first non null byte.
//
while (j < temp.length && temp[j] == 0) {
++j;
}
return copyOfRange(temp, j - zeroCount, temp.length);
}
private static byte divmod58(byte[] number, int startAt) {
int remainder = 0;
for (int i = startAt; i < number.length; i++) {
int digit256 = (int) number[i] & 0xFF;
int temp = remainder * BASE_256 + digit256;
number[i] = (byte) (temp / BASE_58);
remainder = temp % BASE_58;
}
return (byte) remainder;
}
private static byte divmod256(byte[] number58, int startAt) {
int remainder = 0;
for (int i = startAt; i < number58.length; i++) {
int digit58 = (int) number58[i] & 0xFF;
int temp = remainder * BASE_58 + digit58;
number58[i] = (byte) (temp / BASE_256);
remainder = temp % BASE_256;
}
return (byte) remainder;
}
private static byte[] copyOfRange(byte[] source, int from, int to) {
byte[] range = new byte[to - from];
System.arraycopy(source, from, range, 0, range.length);
return range;
}
}
@DeflateAwning
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DeflateAwning commented Jan 28, 2021

Sample usage to use this class to decode Base 58 into a string:

import java.nio.charset.StandardCharsets;
public class Main {
	public static void main(String[] args) {
		Base58 b58 = new Base58();

		String testInput = "6Ewm5RQaA8KxLE6AqH6UvppH37UQFni9wmxL978p";
		String testOutput;

		// Convert b58.decode byte array to String (Source: https://mkyong.com/java/how-do-convert-byte-array-to-string-in-java/)
		testOutput = new String(b58.decode(testInput), StandardCharsets.UTF_8);

		System.out.println(testOutput);
	}
}

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