HTML5 Canvas指纹及反追踪介绍

1 Canvas指纹的简介

很多网站通过Canvas指纹来跟踪用户。browserleaks[1]是一个在线检测canvas指纹的网站。一般的指纹实现原理即通过canvas画布绘制一些图形，填写一些文字，随后获取图形的base64编码，再经过hash后得到最终的指纹。

下面是一个简单的例子，最终调用toDataUrl会生成一段编码。

  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 防止被追踪

通过1的介绍，为了防止被追踪，需要对生成进行调整，这样避免暴露自己的数据。变化的形式有很多，例如变换颜色，透明度及字体。此外canvas提供了getImageData[2]得到一个图形的像素数据，对这里的数据进行变化也能对结果进行变化。这里以anticanvas[3]为例，其它的处理方法原理类似。 下面的样例代码原理很简单，就是通过对原始方法进行拦截来对当前结果改变。在编码中通过闭包[4]保存原始方法。最终实现代理的效果，原始方法调用前对参数进行修改。

(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");     
    } 
})();

3参考

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