[ACCEPTED]-How does MD5Sum algorithm work?-md5

The algorithm Message Digest 5 or MD5, it 24 is a one-way cryptographic hash function. MD5 23 performs many binary operations on the "message" (binary 22 data, for example of an iso image) to compute 21 a 128-bit "hash".

It is useful to check the 20 integrity of a downloaded package such as 19 ubuntu because generating the MD5 hash will 18 be identical if the package is exactly the 17 same as the authenticated source. If there 16 are errors in the download (from network 15 interference, etc.) or if you downloaded 14 a malicious package that was made to look 13 like ubuntu but contained other code/whatever, then 12 the MD5 would not match and you would know 11 something was different and you should download 10 again from a different source.

UPDATE

If the above 9 doesn't explain "how it works" quite literally 8 enough for you, here is my own javascript 7 implementation of the MD5 algorithm:

/**
 * JavaScript Class Implementation of RSA Data Security, Inc.
 * Message Digest 5 (MD5) Algorithm, as defined in RFC 1321.
 * 
 * @author    Dustin C. Fineout <dustin.fineout@gmail.com>
 * @created   10/10/2009 11:23:29 AM
 * @modified  10/10/2009 12:34:56 PM
 * @version   0.1 WORKING
**/

/**
 * Usage example:
 * var md5 = new MD5();
 * alert(md5.hash('Dustin Fineout')); // 8844be37f4e8b3973b48b95b0c69f0b1
**/
function MD5()
{
  this.F = function(x,y,z) { return (x & y) | ((~x) & z); };
  this.G = function(x,y,z) { return (x & z) | (y & (~z)); };
  this.H = function(x,y,z) { return (x ^ y ^ z); };
  this.I = function(x,y,z) { return (y ^ (x | (~z))); };
  this.C = function(q,a,b,x,s,ac) { return this.addu(this.rol(this.addu(this.addu(a,q),this.addu(x,ac)),s),b); };
  this.FF = function(a,b,c,d,x,s,ac) { return this.C((b & c) | ((~b) & d),a,b,x,s,ac); };
  this.GG = function(a,b,c,d,x,s,ac) { return this.C((b & d) | (c & (~d)),a,b,x,s,ac); };
  this.HH = function(a,b,c,d,x,s,ac) { return this.C(b ^ c ^ d,a,b,x,s,ac); };
  this.II = function(a,b,c,d,x,s,ac) { return this.C(c ^ (b | (~d)),a,b,x,s,ac); };

  this.hash = function(message)
  {
    var xl,x=[],k,aa,bb,cc,dd,a=0x67452301,b=0xEFCDAB89,c=0x98BADCFE,d=0x10325476;
    x = this.convertToWordArray(this.utf8Encode(message));
    xl = x.length;
    for (var j = 0; j < xl; j += 16) {
      aa=a; bb=b; cc=c; dd=d;
      a=this.FF(a,b,c,d,x[j+0],7,0xD76AA478);
      d=this.FF(d,a,b,c,x[j+1],12,0xE8C7B756);
      c=this.FF(c,d,a,b,x[j+2],17,0x242070DB);
      b=this.FF(b,c,d,a,x[j+3],22,0xC1BDCEEE);
      a=this.FF(a,b,c,d,x[j+4],7,0xF57C0FAF);
      d=this.FF(d,a,b,c,x[j+5],12,0x4787C62A);
      c=this.FF(c,d,a,b,x[j+6],17,0xA8304613);
      b=this.FF(b,c,d,a,x[j+7],22,0xFD469501);
      a=this.FF(a,b,c,d,x[j+8],7,0x698098D8);
      d=this.FF(d,a,b,c,x[j+9],12,0x8B44F7AF);
      c=this.FF(c,d,a,b,x[j+10],17,0xFFFF5BB1);
      b=this.FF(b,c,d,a,x[j+11],22,0x895CD7BE);
      a=this.FF(a,b,c,d,x[j+12],7,0x6B901122);
      d=this.FF(d,a,b,c,x[j+13],12,0xFD987193);
      c=this.FF(c,d,a,b,x[j+14],17,0xA679438E);
      b=this.FF(b,c,d,a,x[j+15],22,0x49B40821);
      a=this.GG(a,b,c,d,x[j+1],5,0xF61E2562);
      d=this.GG(d,a,b,c,x[j+6],9,0xC040B340);
      c=this.GG(c,d,a,b,x[j+11],14,0x265E5A51);
      b=this.GG(b,c,d,a,x[j+0],20,0xE9B6C7AA);
      a=this.GG(a,b,c,d,x[j+5],5,0xD62F105D);
      d=this.GG(d,a,b,c,x[j+10],9,0x2441453);
      c=this.GG(c,d,a,b,x[j+15],14,0xD8A1E681);
      b=this.GG(b,c,d,a,x[j+4],20,0xE7D3FBC8);
      a=this.GG(a,b,c,d,x[j+9],5,0x21E1CDE6);
      d=this.GG(d,a,b,c,x[j+14],9,0xC33707D6);
      c=this.GG(c,d,a,b,x[j+3],14,0xF4D50D87);
      b=this.GG(b,c,d,a,x[j+8],20,0x455A14ED);
      a=this.GG(a,b,c,d,x[j+13],5,0xA9E3E905);
      d=this.GG(d,a,b,c,x[j+2],9,0xFCEFA3F8);
      c=this.GG(c,d,a,b,x[j+7],14,0x676F02D9);
      b=this.GG(b,c,d,a,x[j+12],20,0x8D2A4C8A);
      a=this.HH(a,b,c,d,x[j+5],4,0xFFFA3942);
      d=this.HH(d,a,b,c,x[j+8],11,0x8771F681);
      c=this.HH(c,d,a,b,x[j+11],16,0x6D9D6122);
      b=this.HH(b,c,d,a,x[j+14],23,0xFDE5380C);
      a=this.HH(a,b,c,d,x[j+1],4,0xA4BEEA44);
      d=this.HH(d,a,b,c,x[j+4],11,0x4BDECFA9);
      c=this.HH(c,d,a,b,x[j+7],16,0xF6BB4B60);
      b=this.HH(b,c,d,a,x[j+10],23,0xBEBFBC70);
      a=this.HH(a,b,c,d,x[j+13],4,0x289B7EC6);
      d=this.HH(d,a,b,c,x[j+0],11,0xEAA127FA);
      c=this.HH(c,d,a,b,x[j+3],16,0xD4EF3085);
      b=this.HH(b,c,d,a,x[j+6],23,0x4881D05);
      a=this.HH(a,b,c,d,x[j+9],4,0xD9D4D039);
      d=this.HH(d,a,b,c,x[j+12],11,0xE6DB99E5);
      c=this.HH(c,d,a,b,x[j+15],16,0x1FA27CF8);
      b=this.HH(b,c,d,a,x[j+2],23,0xC4AC5665);
      a=this.II(a,b,c,d,x[j+0],6,0xF4292244);
      d=this.II(d,a,b,c,x[j+7],10,0x432AFF97);
      c=this.II(c,d,a,b,x[j+14],15,0xAB9423A7);
      b=this.II(b,c,d,a,x[j+5],21,0xFC93A039);
      a=this.II(a,b,c,d,x[j+12],6,0x655B59C3);
      d=this.II(d,a,b,c,x[j+3],10,0x8F0CCC92);
      c=this.II(c,d,a,b,x[j+10],15,0xFFEFF47D);
      b=this.II(b,c,d,a,x[j+1],21,0x85845DD1);
      a=this.II(a,b,c,d,x[j+8],6,0x6FA87E4F);
      d=this.II(d,a,b,c,x[j+15],10,0xFE2CE6E0);
      c=this.II(c,d,a,b,x[j+6],15,0xA3014314);
      b=this.II(b,c,d,a,x[j+13],21,0x4E0811A1);
      a=this.II(a,b,c,d,x[j+4],6,0xF7537E82);
      d=this.II(d,a,b,c,x[j+11],10,0xBD3AF235);
      c=this.II(c,d,a,b,x[j+2],15,0x2AD7D2BB);
      b=this.II(b,c,d,a,x[j+9],21,0xEB86D391);
      a=this.addu(a,aa); b=this.addu(b,bb); c=this.addu(c,cc); d=this.addu(d,dd);
    }
    return (this.wordToHex(a)+this.wordToHex(b)+this.wordToHex(c)+this.wordToHex(d)).toLowerCase();
  };

  this.test = function()
  {
    if (this.hash('Dustin Fineout') == '8844be37f4e8b3973b48b95b0c69f0b1') {
      return true;
    }
    return false;
  };

  this.addu = function(x, y)
  {
    var ls = (x & 0xFFFF) + (y & 0xFFFF);
    return (((x >> 16) + (y >> 16) + (ls >> 16)) << 16) | (ls & 0xFFFF);
  };

  this.rol = function(v, s)
  {
    return (v << s) | (v >>> (32 - s));
  };

  this.utf8Encode = function(str)
  {
    return unescape(encodeURIComponent(str));
  };

  this.convertToWordArray = function(str)
  {
    var lWordCount;
    var lMessageLength = str.length;
    var lNumberOfWords_temp1=lMessageLength + 8;
    var lNumberOfWords_temp2=(lNumberOfWords_temp1-(lNumberOfWords_temp1 % 64))/64;
    var lNumberOfWords = (lNumberOfWords_temp2+1)*16;
    var lWordArray=new Array(lNumberOfWords-1);
    var lBytePosition = 0;
    var lByteCount = 0;
    while ( lByteCount < lMessageLength ) {
      lWordCount = (lByteCount-(lByteCount % 4))/4;
      lBytePosition = (lByteCount % 4)*8;
      lWordArray[lWordCount] = (lWordArray[lWordCount] | (str.charCodeAt(lByteCount)<<lBytePosition));
      lByteCount++;
    }
    lWordCount = (lByteCount-(lByteCount % 4))/4;
    lBytePosition = (lByteCount % 4)*8;
    lWordArray[lWordCount] = lWordArray[lWordCount] | (0x80<<lBytePosition);
    lWordArray[lNumberOfWords-2] = lMessageLength<<3;
    lWordArray[lNumberOfWords-1] = lMessageLength>>>29;
    return lWordArray;
  };

  this.wordToHex = function(lValue)
  {
    var wordToHexValue="",wordToHexValue_temp="",lByte,lCount;
    for (lCount = 0;lCount<=3;lCount++) {
      lByte = (lValue>>>(lCount*8)) & 255;
      wordToHexValue_temp = "0" + lByte.toString(16);
      wordToHexValue = wordToHexValue + wordToHexValue_temp.substr(wordToHexValue_temp.length-2,2);
    }
    return wordToHexValue;
  };
}

I'll 6 be happy to annotate or expand as desired 5 :) Also, please note this isn't the singleton 4 pattern I still use for JavaScript namespaces 3 - it was actually my first attempt at such 2 a pattern :p There are many delicious singleton 1 patterns available from Douglas Crockford.

MD5Sum is a file checksum generating tool using 12 MD5 as the hashing algorithm. It provides the 11 user with a reasonable assurance that the 10 file was untampered with.

In order to do 9 this, the user will need to (via shell integration 8 or manually) process their own MD5 hash 7 and compare it to the hash provided by the 6 uploader.

An MD5Sum hash won't assure you 5 that the uploader who provided the checksum 4 didn't fiddle with the file, to do that 3 you should trust the source of the MD5 checksum 2 (ie. get the checksum from Ubuntu instead 1 of the download site providing the image).

md5sum is a tool used to verify the integrity 12 of files, as virtually any change to a file 11 will cause its MD5 hash to change.

Most commonly, md5sum is 10 used to verify that a file has not changed 9 as a result of a faulty file transfer, a 8 disk error or non-malicious meddling. The 7 md5sum program is included in most Unix-like operating 6 systems or compatibility layers such as 5 Cygwin.

You will need to run md5sum on the ISO 4 image and it will output something as below:

Open 3 terminal and execute the following:

md5sum myisofile.iso > out.txt

cat out.txt (this will output as below)

myisofile.iso   f6de8b885f7a1917e5368fcb947d1738

This 2 MD5 hash need to be compared with the source 1 myisofile.iso. If both MD5 hash is the same, means the ISO is untampered.