比特币钱包地址 Base58 编码简介

Base64 是很多人熟知的一种编码，它是一种基于可打印字符来表示二进制数据的方法，类似的还有 Base32、Base16 等 编码，区别就是选取的基不一样。

本文要讨论的是 Base58, 它更多的被用在比特币的钱包地址编码上，其 基 是58个字母和数字组成，列表如下：

123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz

注意，少了容易混淆的数字0，大写字母I，大写字母O，小写字母l，一共是： 10+26×2-4=58 个。

这个编码到底有什么好处呢？

• 首先，便于阅读，没有容易混淆的字母和数字
• 第二，没有标点符号 + 和 /，显的更美观
• 第三，整个成词，双击更容易选中，也不会随意折行
• 第四，这实际是一个混淆编码，看上去像Base64,但是其实不是，可以当作简单加密

附一份代码

import java.util.Arrays;

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 RuntimeException if the given string is not a valid base58 string
     */
    public static byte[] decode(String input) throws RuntimeException {
        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 RuntimeException("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);
    }

    /**
     * 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;
    }

}