File download logging

Rails
1

Is there any way of determining whether a file has been successfully
downloaded by a user or to put it another way, whether a dowload was
interrupted?

2

On 16/08/06, Adam G. [email protected] wrote:

Is there any way of determining whether a file has been successfully
downloaded by a user or to put it another way, whether a dowload was
interrupted?

Use javascript to verify the data’s hash matches what you have for the
file.
If the hash doesn’t match, it can be assumed the download wasn’t
completed
properly. A Javascript implementation of SHA-1 is below:
function sha1Hash(msg)
{
// constants [4.2.1]
var K = [0x5a827999, 0x6ed9eba1, 0x8f1bbcdc, 0xca62c1d6];

// PREPROCESSING

msg += String.fromCharCode(0x80); // add trailing '1' bit to string

[
5.1.1]

// convert string msg into 512-bit/16-integer blocks arrays of ints

[
5.2.1]
var l = Math.ceil(msg.length/4) + 2; // long enough to contain msg
plus
2-word length
var N = Math.ceil(l/16); // in N 16-int blocks
var M = new Array(N);
for (var i=0; i<N; i++) {
M[i] = new Array(16);
for (var j=0; j<16; j++) { // encode 4 chars per integer,
big-endian encoding
M[i][j] = (msg.charCodeAt(i64+j4)<<24) |
(msg.charCodeAt(i64+j4+1)<<16)
|
(msg.charCodeAt(i64+j4+2)<<8) | (msg.charCodeAt
(i64+j4+3));
}
}
// add length (in bits) into final pair of 32-bit integers
(big-endian)
[5.1.1]
M[N-1][14] = ((msg.length-1) >>> 30) * 8;
M[N-1][15] = ((msg.length-1)*8) & 0xffffffff;

// set initial hash value [5.3.1]
var H0 = 0x67452301;
var H1 = 0xefcdab89;
var H2 = 0x98badcfe;
var H3 = 0x10325476;
var H4 = 0xc3d2e1f0;

// HASH COMPUTATION [6.1.2]

var W = new Array(80); var a, b, c, d, e;
for (var i=0; i<N; i++) {

    // 1 - prepare message schedule 'W'
    for (var t=0;  t<16; t++) W[t] = M[i][t];
    for (var t=16; t<80; t++) W[t] = ROTL(W[t-3] ^ W[t-8] ^ W[t-14]

^
W[t-16], 1);

    // 2 - initialise five working variables a, b, c, d, e with

previous
hash value
a = H0; b = H1; c = H2; d = H3; e = H4;

    // 3 - main loop
    for (var t=0; t<80; t++) {
        var s = Math.floor(t/20); // seq for blocks of 'f' functions

and
‘K’ constants
var T = (ROTL(a,5) + f(s,b,c,d) + e + K[s] + W[t]) &
0xffffffff;
e = d;
d = c;
c = ROTL(b, 30);
b = a;
a = T;
}

    // 4 - compute the new intermediate hash value
    H0 = (H0+a) & 0xffffffff;  // note 'addition modulo 2^32'
    H1 = (H1+b) & 0xffffffff;
    H2 = (H2+c) & 0xffffffff;
    H3 = (H3+d) & 0xffffffff;
    H4 = (H4+e) & 0xffffffff;
}

return H0.toHexStr() + H1.toHexStr() + H2.toHexStr() + H3.toHexStr()

H4.toHexStr();
}

//
// function ‘f’ [4.1.1]
//
function f(s, x, y, z)
{
switch (s) {
case 0: return (x & y) ^ (~x & z);
case 1: return x ^ y ^ z;
case 2: return (x & y) ^ (x & z) ^ (y & z);
case 3: return x ^ y ^ z;
}
}

//
// rotate left (circular left shift) value x by n positions [3.2.5]
//
function ROTL(x, n)
{
return (x<<n) | (x>>>(32-n));
}

//
// extend Number class with a tailored hex-string method
// (note toString(16) is implementation-dependant, and
// in IE returns signed numbers when used on full words)
//
Number.prototype.toHexStr = function()
{
var s=“”, v;
for (var i=7; i>=0; i–) { v = (this>>>(i*4)) & 0xf; s +=
v.toString(16);
}
return s;
}

3

Thanks for the reply Hasan,

I’m a bit of a newbie I’m afraid and so I don’t understand why the hash
is necessary.
Correct me if I’m wrong but would the process be somewhat like this? :

User clicks on link
action generates http get for file and created field in db table
‘downloads’ with filename and hash.

Download commences

Success -> javacript sends its hash to application which is checked
against tabe downloads.

Failure -> nothing happens.

How is the javascript triggered?

Regards

Adam