HTML5 Canvas指紋及反追蹤介紹

1 Canvas指紋的簡介

很多網站通過Canvas指紋來跟蹤用戶。browserleaks[1]是一個在線檢測canvas指紋的網站。一般的指紋實現原理即通過canvas畫布繪製一些圖形，填寫一些文字，隨後獲取圖形的base64編碼，再經過hash後得到最終的指紋。

下面是一個簡單的例子，最終調用toDataUrl會生成一段編碼。

  var getCanvasFp = function (options) {
    var result = []
    // Very simple now, need to make it more complex (geo shapes etc)
    var canvas = document.createElement('canvas')
    canvas.width = 2000
    canvas.height = 200
    canvas.style.display = 'inline'
    var ctx = canvas.getContext('2d')
    // detect browser support of canvas winding
    // http://blogs.adobe.com/webplatform/2013/01/30/winding-rules-in-canvas/
    // https://github.com/Modernizr/Modernizr/blob/master/feature-detects/canvas/winding.js
    ctx.rect(0, 0, 10, 10)
    ctx.rect(2, 2, 6, 6)
    result.push('canvas winding:' + ((ctx.isPointInPath(5, 5, 'evenodd') === false) ? 'yes' : 'no'))

    ctx.textBaseline = 'alphabetic'
    ctx.fillStyle = '#f60'
    ctx.fillRect(125, 1, 62, 20)
    ctx.fillStyle = '#069'
    // https://github.com/Valve/fingerprintjs2/issues/66
    if (options.dontUseFakeFontInCanvas) {
      ctx.font = '11pt Arial'
    } else {
      ctx.font = '11pt no-real-font-123'
    }
    ctx.fillText('Cwm fjordbank glyphs vext quiz, \ud83d\ude03', 2, 15)
    ctx.fillStyle = 'rgba(102, 204, 0, 0.2)'
    ctx.font = '18pt Arial'
    ctx.fillText('Cwm fjordbank glyphs vext quiz, \ud83d\ude03', 4, 45)

    // canvas blending
    // http://blogs.adobe.com/webplatform/2013/01/28/blending-features-in-canvas/
    // http://jsfiddle.net/NDYV8/16/
    ctx.globalCompositeOperation = 'multiply'
    ctx.fillStyle = 'rgb(255,0,255)'
    ctx.beginPath()
    ctx.arc(50, 50, 50, 0, Math.PI * 2, true)
    ctx.closePath()
    ctx.fill()
    ctx.fillStyle = 'rgb(0,255,255)'
    ctx.beginPath()
    ctx.arc(100, 50, 50, 0, Math.PI * 2, true)
    ctx.closePath()
    ctx.fill()
    ctx.fillStyle = 'rgb(255,255,0)'
    ctx.beginPath()
    ctx.arc(75, 100, 50, 0, Math.PI * 2, true)
    ctx.closePath()
    ctx.fill()
    ctx.fillStyle = 'rgb(255,0,255)'
    // canvas winding
    // http://blogs.adobe.com/webplatform/2013/01/30/winding-rules-in-canvas/
    // http://jsfiddle.net/NDYV8/19/
    ctx.arc(75, 75, 75, 0, Math.PI * 2, true)
    ctx.arc(75, 75, 25, 0, Math.PI * 2, true)
    ctx.fill('evenodd')

    if (canvas.toDataURL) { result.push('canvas fp:' + canvas.toDataURL()) }
    return result
  }

2 防止被追蹤

通過1的介紹，爲了防止被追蹤，需要對生成進行調整，這樣避免暴露自己的數據。變化的形式有很多，例如變換顏色，透明度及字體。此外canvas提供了getImageData[2]得到一個圖形的像素數據，對這裏的數據進行變化也能對結果進行變化。這裏以anticanvas[3]爲例，其它的處理方法原理類似。 下面的樣例代碼原理很簡單，就是通過對原始方法進行攔截來對當前結果改變。在編碼中通過閉包[4]保存原始方法。最終實現代理的效果，原始方法調用前對參數進行修改。

(function(){
    CanvasRenderingContext2D.prototype.getImageData = function(a) {
      return function() {
        console.log("Context");
        spoofFromContext(this,a);
        return a.apply(this, arguments);
      };
    }(CanvasRenderingContext2D.prototype.getImageData);
    HTMLCanvasElement.prototype.toDataURL=(function(){
        var original = HTMLCanvasElement.prototype.toDataURL;
        return function() {
            spoof(this);
            return original.apply(this,arguments);
        };
    })();
    function spoof(canvas){
        	var ctx = canvas.getContext("2d");
            spoofFromContext(ctx);    
    }
    function spoofFromContext(ctx,a){        
        	if(!a) a = ctx.getImageData;
 			var data = a.call(ctx,0, 0, ctx.canvas.width, ctx.canvas.height);
            for(var c=0; c<data.data.length; c=c+4){
                var r = data.data[c];
                var g = data.data[c+1];
                var b = data.data[c+2];
                var a = data.data[c+3];
                if(a!=0){
                    data.data[c]=r-Math.random();
                    data.data[c+1]=g-Math.random();
                    data.data[c+2]=b-Math.random();
                    data.data[c+3]=a-Math.random();
                }                               
         	}   
            ctx.putImageData(data, 0, 0);   
            console.log("Spoofed");     
    } 
})();

3參考

[1].canvas指紋,https://browserleaks.com/canvas#how-does-it-work
[2].getImageData使用簡介,https://blog.csdn.net/FE_dev/article/details/82586698
[3].anticanvas,https://github.com/WolfspiritM/AntiCF/blob/master/anticanvas.js
[4].JS閉包http://www.ruanyifeng.com/blog/2009/08/learning_javascript_closures.html