How to validate bitcoin address

Question

I have found the following code that validates bitcoin address using python:

from hashlib import sha256

digits58 = '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz'

def decode_base58(bc, length):

    n = 0

    for char in bc:

        n = n * 58 + digits58.index(char)

    return n.to_bytes(length, 'big')

def check_bc(bc):

    bcbytes = decode_base58(bc, 25)

    return bcbytes[-4:] == sha256(sha256(bcbytes[:-4]).digest()).digest()[:4]

if __name__ == '__main__':

    bc = '1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i'

    assert check_bc(bc)

    assert not check_bc( bc.replace('N', 'P', 1) )

    assert check_bc('1111111111111111111114oLvT2')

    assert check_bc("17NdbrSGoUotzeGCcMMCqnFkEvLymoou9j")

Now I want to validate bitcoin address using Java, javascript or php. Any idea how to do it?

slayer · Answer 1 · Aug 17, 2018

I found these 2 scripts online:

Javascript:

<html>

<head>

<script type="text/javascript" src="/u/28441300/BigInt.js"></script>

<script type="text/javascript" src="u/28441300/sha256.js"></script>

</head>

<body>


<div id="text">

</div>


<script type="text/javascript">

var address = "1Eym7pyJcaambv8FG4ZoU8A4xsiL9us2zz";

if (check(address)) {

    document.getElementById('text').innerHTML += "valid";

} else {

    document.getElementById('text').innerHTML += "invalid";

}



function check(address) {

  var decoded = base58_decode(address);    

  if (decoded.length != 25) return false;


  var cksum = decoded.substr(decoded.length - 4);

  var rest = decoded.substr(0, decoded.length - 4); 


  var good_cksum = hex2a(sha256_digest(hex2a(sha256_digest(rest)))).substr(0, 4);


  if (cksum != good_cksum) return false;

  return true;

}


function base58_decode(string) {

  var table = '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz';

  var table_rev = new Array();


  var i;

  for (i = 0; i < 58; i++) {

    table_rev[table[i]] = int2bigInt(i, 8, 0);

  }


  var l = string.length;

  var long_value = int2bigInt(0, 1, 0); 


  var num_58 = int2bigInt(58, 8, 0);


  var c;

  for(i = 0; i < l; i++) {

    c = string[l - i - 1];

    long_value = add(long_value, mult(table_rev[c], pow(num_58, i)));

  }


  var hex = bigInt2str(long_value, 16); 


  var str = hex2a(hex); 


  var nPad;

  for (nPad = 0; string[nPad] == table[0]; nPad++); 


  var output = str;

  if (nPad > 0) output = repeat("\0", nPad) + str;


  return output;

}


function hex2a(hex) {

    var str = '';

    for (var i = 0; i < hex.length; i += 2)

        str += String.fromCharCode(parseInt(hex.substr(i, 2), 16));

    return str;

}


function a2hex(str) {

    var aHex = "0123456789abcdef";

    var l = str.length;

    var nBuf;

    var strBuf;

    var strOut = "";

    for (var i = 0; i < l; i++) {

      nBuf = str.charCodeAt(i);

      strBuf = aHex[Math.floor(nBuf/16)];

      strBuf += aHex[nBuf % 16];

      strOut += strBuf;

    }

    return strOut;

}


function pow(big, exp) {

    if (exp == 0) return int2bigInt(1, 1, 0);

    var i;

    var newbig = big;

    for (i = 1; i < exp; i++) {

        newbig = mult(newbig, big);

    }


    return newbig;

}


function repeat(s, n){

    var a = [];

    while(a.length < n){

        a.push(s);

    }

    return a.join('');

}

</script>

</body>

</html>

PHP:

<?php


function checkAddress($address)

{

    $origbase58 = $address;

    $dec = "0";


    for ($i = 0; $i < strlen($address); $i++)

    {

        $dec = bcadd(bcmul($dec,"58",0),strpos("123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz",substr($address,$i,1)),0);

    }


    $address = "";


    while (bccomp($dec,0) == 1)

    {

        $dv = bcdiv($dec,"16",0);

        $rem = (integer)bcmod($dec,"16");

        $dec = $dv;

        $address = $address.substr("0123456789ABCDEF",$rem,1);

    }


    $address = strrev($address);


    for ($i = 0; $i < strlen($origbase58) && substr($origbase58,$i,1) == "1"; $i++)

    {

        $address = "00".$address;

    }


    if (strlen($address)%2 != 0)

    {

        $address = "0".$address;

    }


    if (strlen($address) != 50)

    {

        return false;

    }


    if (hexdec(substr($address,0,2)) > 0)

    {

        return false;

    }


    return substr(strtoupper(hash("sha256",hash("sha256",pack("H*",substr($address,0,strlen($address)-8)),true))),0,8) == substr($address,strlen($address)-8);

}


?>

Omkar · Answer 2 · Aug 21, 2018

Is there any better solution for validating bitcoin address ?
Can you explain me the javascript code?

answered Aug 21, 2018 by anonymous

An easier solution for validating bitcoin address would be to use an online Bitcoin address validator.

commented Aug 22, 2018 by Omkar
• 69,180 points

You could also do it w CSS, like this:

.container {
  padding: 10px;
}

.msg {
  margin: 10px 0px;
  color: #000;
  &.fail {
    color: red;
  }
  &.pass {
    color: green;
  }
}

input,
button {
  font-size: 18px;
  user-select: none;
}

.link-action {
  color: green;
  text-decoration: underline;
  cursor: pointer;
  display: inline-block;
  margin: 10px;
}

commented Aug 22, 2018 by Christine
• 15,750 points

Before explaning the Javascript to validate the Bitcoin address, let us look at some key factors of a valid Bitcoin address:

1. A Bitcoin address is between 25 and 34 characters long.
2. The address always starts with a 1 (for P2PKH address format) or 3 (for P2SH address format).
3. An address can contain all alphanumeric characters, with the exceptions of 0(zero), O(Uppercase letter 'O'), I(Uppercase i)
and l(Lowercase L) to avoid visual ambiguity.

Explanation of Javascript:

The programming checks if the given address is a valid Bitcoin address or not. If it is valid, it prints Valid and if it is
not valid then it prints invalid

function check(address)
This is the function from where the procedure to check whether the given address is valid or not starts.

base58_decode(address)
This is the function that is used to convert the address from its encoded form to normal form.
Base58 is a group of binary-to-text encoding schemes used to represent large integers as alphanumeric text, introduced by Satoshi Nakamoto for use with Bitcoin.

var cksum = decoded.substr(decoded.length - 4);
The checksum of an address is the string of the address excluding the first 4 characters.

var rest = decoded.substr(0, decoded.length - 4);
This is the rest of the address (from character 5 to the last character in the address)

function hex2a(hex)
Converts a hex string into an alpha-numerical string

function a2hex(str)
Converts an alpha-numerical into hex string

Conversion from alpha-numerical to hex and the reverse is done during the encoding and decoding so these functions are used
to convert the strings back to their original form for validation.

All these functions are used to validate the Bitcoin address in the following way:
First the address is decoded from base58. Then the checksum and the rest of the address is set to the variables "cksum" and
"rest". The code checks whether the checksum of the address is a good checksum by comparing it to the standard checksum.
If all these conditions are satisfied then the given address is valid.