gistfile1.txt

var secret_key = 'abcdefghijklmnopqrstuvwxyz';
var hash = crypto.createHash('sha1');
hash.update(secret_key);
console.log(unsign_value('8a8d431c4bee603699e3fd3e9593a3264af0f57c', 'value1'));

function unsign_value(signature, value)
{
    var hmac = crypto.createHmac('sha1', hash.digest());
    hmac.update(value);
    console.log(signature);
    var new_signature = hmac.digest('hex');
    console.log(new_signature);
    return signature === new_signature;
}