Question

The octets from “Hello world” are: E8 32 9B FD 06 DD DF 72 36 19

And if apply padding it will become : D0 65 36 FB 0D BA BF E5 6C 32

(copy from wiki)

Can I know what’s the formula to encoding with 1-bit padding?

Here is the formula of convert string to PDU that I edited :(thanks to source provider)
*The whole complete formula is kinda long so I just post a part of it here…

    maxkeys = 160;

    function binToInt(x)//sp
   {
var total = 0;  
var power = parseInt(x.length)-0;   

for(var i=0;i<x.length;i++)
{
    if(x.charAt(i) == '1')
    {
     total = total +Math.pow(2,power);
    }
    power --;
}
return total;
  }

  function intToHex(i) //sp
    {
     var sHex = "0123456789ABCDEF"; 
     h = ""; 
     i = parseInt(i);   
    for(j = 0; j <= 3; j++)
     {
      h += sHex.charAt((i >> (j * 8 + 4)) & 0x0F) +
        sHex.charAt((i >> (j * 8)) & 0x0F);
      }
    return h.substring(0,2);
   }


 function stringToPDU(inpString,size) 
     {

var bitSize = size[0].value * size[0].checked | size[1].value * size[1].checked | size[2].value * size[2].checked;

var octetFirst = "";
var octetSecond = ""; 
var output = "";

var DATA_ENCODING = "00"; 

var VALID_PERIOD = "AA";

var userDataSize;
if (bitSize == 7)
{

    userDataSize = intToHex(inpString.length);

    for(var i=0;i<=inpString.length;i++)
    {
        if(i==inpString.length)
        {
            if (octetSecond != "") // AJA Fix overshoot
            {
                output = output + "" + (intToHex(binToInt(octetSecond)));
            }
            break;
        }
        var current = intToBin(getSevenBit(inpString.charAt(i)),7);

        var currentOctet;
        if(i!=0 && i%8!=0)
        {
            octetFirst = current.substring(7-(i)%8);
            currentOctet = octetFirst + octetSecond;    

            output = output + "" + (intToHex(binToInt(currentOctet)));
            octetSecond = current.substring(0,7-(i)%8); 
        }
        else
        {
            octetSecond = current.substring(0,7-(i)%8);
        }   
    }
}

//var data =  DATA_ENCODING + VALID_PERIOD + userDataSize;

var PDU =   output;

return  PDU;
      }

    <input onclick="pduOut.value= stringToPDU(document.stringToPduForm.smsText.value.substring (0, maxkeys),document.stringToPduForm.size);" type="button" size="11" value="  Convert  " name="stringButton">

I need to know what changes needed so that the formula can encode message with 1-bit padding..but I not really understand the concept,can someone teach me?
Thanks.

Answer 1

Write out those octets in binary (with the lowest bit first in each octet) and you’ll see it:

E8 32 9B FD 06 DD DF 72 36 19 = 00010111 01001100 11011001 10111111 01100000 10111011 11111011 01001110 01101100 10011000
D0 65 36 FB 0D BA BF E5 6C 32 = 00001011 10100110 01101100 11011111 10110000 01011101 11111101 10100111 00110110 01001100