Why does my implementation of hex to Base58check produce different results than the example?

Question

Step 8 of this wiki page gives this hex value: 00010966776006953D5567439E5E39F86A0D273BEED61967F6

Step 9 converts it to this base58 string:16UwLL9Risc3QfPqBUvKofHmBQ7wMtjvM

I'm trying to implement the conversion function using the pseudocode from this wiki page. This is my implementation (in Java):

    String input = "00010966776006953D5567439E5E39F86A0D273BEED61967F6"
    BigInteger bigInteger = new BigInteger(input , 16);
    String code_string = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
    StringBuilder output = new StringBuilder();

    while(bigInteger.compareTo(BigInteger.ZERO) == 1){
        BigInteger[] divAndRemainder = bigInteger.divideAndRemainder(BigInteger.valueOf(58));
        output.append(code_string.charAt(divAndRemainder[1].intValue()));
        bigInteger = divAndRemainder[0];
    }

    int i=0;
    while(concat.charAt(i) == '0'){
        i++;
        output.append(code_string.charAt(0));
    }
    System.out.println(output.reverse());

This prints out 1116UwLL9Risc3QfPqBUvKofHmBQ7wMtjvM. This is close to what the wiki page produces, but not quite, there are 2 extra leading 1s. This is from the 2 leading 0s in the input string. Why doesn't the wiki example get my result?

the leading zeros are not covered correctly. Haven't looked into the code, but here is a very good discussion thread, including java references: https://bitcointalk.org/index.php?topic=1026.0 - also there is this java implementation here: http://rosettacode.org/wiki/Bitcoin/address_validation#Java — pebwindkraft, Apr 04 '18 at 03:55

score 1 · Answer 1 · answered Apr 06 '18 at 07:33

Each hexadecimal character has four bits of information. Two hex characters contains eight bits of information, so they form a byte.

For each byte in front of the address, a 1 should be put. Since two characters form a byte:

int i=0;
while(concat.charAt(i) == '0' && concat.charAt(i + 1) == '0'){
    i += 2;
    output.append(code_string.charAt(0));
}

BTW, it would look better if it was a for loop:

for (int i = 0; concat.charAt(i++) == '0' && concat.charAt(i++) == '0';) {
    output.append(code_string.charAt(0));
}

Also, probably you've made a mistake while copying the code: You haven't declared a variable named concat. It's input.

Last note: If you're going to use this encoding for things other than Bitcoin addresses (which have checksum), make sure that i will always be smaller than concat.length()

Why does my implementation of hex to Base58check produce different results than the example?

1 Answers1