The algorithm Message Digest 5 or MD5, it is a one-way cryptographic hash function. MD5 performs many binary operations on the "message" (binary data, for example of an iso image) to compute a 128-bit "hash".
It is useful to check the integrity of a downloaded package such as ubuntu because generating the MD5 hash will be identical if the package is exactly the same as the authenticated source. If there are errors in the download (from network interference, etc.) or if you downloaded a malicious package that was made to look like ubuntu but contained other code/whatever, then the MD5 would not match and you would know something was different and you should download again from a different source.
UPDATE
If the above doesn't explain "how it works" quite literally enough for you, here is my own javascript 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
* @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)<>>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 be happy to annotate or expand as desired :) Also, please note this isn't the singleton pattern I still use for JavaScript namespaces - it was actually my first attempt at such a pattern :p There are many delicious singleton patterns available from Douglas Crockford.
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