org.bitcoinj.core.Base58 Maven / Gradle / Ivy
/*
* 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 org.bitcoinj.core;
import java.math.BigInteger;
import java.util.Arrays;
/**
* Base58 is a way to encode Bitcoin addresses (or arbitrary data) as alphanumeric strings.
*
* Note that this is not the same base58 as used by Flickr, which you may find referenced around the Internet.
*
* You may want to consider working with {@link VersionedChecksummedBytes} instead, which
* adds support for testing the prefix and suffix bytes commonly found in addresses.
*
* Satoshi explains: why base-58 instead of standard base-64 encoding?
*
* - Don't want 0OIl characters that look the same in some fonts and
* could be used to create visually identical looking account numbers.
* - A string with non-alphanumeric characters is not as easily accepted as an account number.
* - E-mail usually won't line-break if there's no punctuation to break at.
* - Doubleclicking selects the whole number as one word if it's all alphanumeric.
*
*
* However, note that the encoding/decoding runs in O(n²) time, so it is not useful for large data.
*
* The basic idea of the encoding is to treat the data bytes as a large number represented using
* base-256 digits, convert the number to be represented using base-58 digits, preserve the exact
* number of leading zeros (which are otherwise lost during the mathematical operations on the
* numbers), and finally represent the resulting base-58 digits as alphanumeric ASCII characters.
*/
public class Base58 {
public static final char[] ALPHABET = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz".toCharArray();
private static final char ENCODED_ZERO = ALPHABET[0];
private static final int[] INDEXES = new int[128];
static {
Arrays.fill(INDEXES, -1);
for (int i = 0; i < ALPHABET.length; i++) {
INDEXES[ALPHABET[i]] = i;
}
}
/**
* Encodes the given bytes as a base58 string (no checksum is appended).
*
* @param input the bytes to encode
* @return the base58-encoded string
*/
public static String encode(byte[] input) {
if (input.length == 0) {
return "";
}
// Count leading zeros.
int zeros = 0;
while (zeros < input.length && input[zeros] == 0) {
++zeros;
}
// Convert base-256 digits to base-58 digits (plus conversion to ASCII characters)
input = Arrays.copyOf(input, input.length); // since we modify it in-place
char[] encoded = new char[input.length * 2]; // upper bound
int outputStart = encoded.length;
for (int inputStart = zeros; inputStart < input.length; ) {
encoded[--outputStart] = ALPHABET[divmod(input, inputStart, 256, 58)];
if (input[inputStart] == 0) {
++inputStart; // optimization - skip leading zeros
}
}
// Preserve exactly as many leading encoded zeros in output as there were leading zeros in input.
while (outputStart < encoded.length && encoded[outputStart] == ENCODED_ZERO) {
++outputStart;
}
while (--zeros >= 0) {
encoded[--outputStart] = ENCODED_ZERO;
}
// Return encoded string (including encoded leading zeros).
return new String(encoded, outputStart, encoded.length - outputStart);
}
/**
* Decodes the given base58 string into the original data bytes.
*
* @param input the base58-encoded string to decode
* @return the decoded data bytes
* @throws AddressFormatException if the given string is not a valid base58 string
*/
public static byte[] decode(String input) throws AddressFormatException {
if (input.length() == 0) {
return new byte[0];
}
// Convert the base58-encoded ASCII chars to a base58 byte sequence (base58 digits).
byte[] input58 = new byte[input.length()];
for (int i = 0; i < input.length(); ++i) {
char c = input.charAt(i);
int digit = c < 128 ? INDEXES[c] : -1;
if (digit < 0) {
throw new AddressFormatException("Illegal character " + c + " at position " + i);
}
input58[i] = (byte) digit;
}
// Count leading zeros.
int zeros = 0;
while (zeros < input58.length && input58[zeros] == 0) {
++zeros;
}
// Convert base-58 digits to base-256 digits.
byte[] decoded = new byte[input.length()];
int outputStart = decoded.length;
for (int inputStart = zeros; inputStart < input58.length; ) {
decoded[--outputStart] = divmod(input58, inputStart, 58, 256);
if (input58[inputStart] == 0) {
++inputStart; // optimization - skip leading zeros
}
}
// Ignore extra leading zeroes that were added during the calculation.
while (outputStart < decoded.length && decoded[outputStart] == 0) {
++outputStart;
}
// Return decoded data (including original number of leading zeros).
return Arrays.copyOfRange(decoded, outputStart - zeros, decoded.length);
}
public static BigInteger decodeToBigInteger(String input) throws AddressFormatException {
return new BigInteger(1, decode(input));
}
/**
* Decodes the given base58 string into the original data bytes, using the checksum in the
* last 4 bytes of the decoded data to verify that the rest are correct. The checksum is
* removed from the returned data.
*
* @param input the base58-encoded string to decode (which should include the checksum)
* @throws AddressFormatException if the input is not base 58 or the checksum does not validate.
*/
public static byte[] decodeChecked(String input) throws AddressFormatException {
byte[] decoded = decode(input);
if (decoded.length < 4)
throw new AddressFormatException("Input too short");
byte[] data = Arrays.copyOfRange(decoded, 0, decoded.length - 4);
byte[] checksum = Arrays.copyOfRange(decoded, decoded.length - 4, decoded.length);
byte[] actualChecksum = Arrays.copyOfRange(Sha256Hash.hashTwice(data), 0, 4);
if (!Arrays.equals(checksum, actualChecksum))
throw new AddressFormatException("Checksum does not validate");
return data;
}
/**
* Divides a number, represented as an array of bytes each containing a single digit
* in the specified base, by the given divisor. The given number is modified in-place
* to contain the quotient, and the return value is the remainder.
*
* @param number the number to divide
* @param firstDigit the index within the array of the first non-zero digit
* (this is used for optimization by skipping the leading zeros)
* @param base the base in which the number's digits are represented (up to 256)
* @param divisor the number to divide by (up to 256)
* @return the remainder of the division operation
*/
private static byte divmod(byte[] number, int firstDigit, int base, int divisor) {
// this is just long division which accounts for the base of the input digits
int remainder = 0;
for (int i = firstDigit; i < number.length; i++) {
int digit = (int) number[i] & 0xFF;
int temp = remainder * base + digit;
number[i] = (byte) (temp / divisor);
remainder = temp % divisor;
}
return (byte) remainder;
}
}