PNG steganography in less than a kilo minified

stego.js

//PNG steganography utils
//ratio: about 2 pixels per 1 byte of text message
//Auto EOD byte as 255
//Use only non-transparent images as the algo has to modify alpha-channel of affected pixels
//to protect RGB values from in-browser rounding

+function(w) {
  var bitMapping = [0, 1, 2, 4, 5, 6, 8, 9], img = new Image(), cnv = document.createElement('canvas'), ctx=cnv.getContext('2d')
  w.Stego = {
    hide: function(imglink, msg, cb) { //image data URI (not only PNG) and message as input, resulting PNG image data URI as output (to callback)
      var data = btoa(unescape(encodeURIComponent(msg)))+'\xff', dlen = data.length
      img.onload = function() {
        cnv.width = this.width
        cnv.height = this.height
        ctx.drawImage(this, 0, 0)
        var idata=ctx.getImageData(0, 0, this.width, this.height)
        for(var p = -1, i = 0;i < (dlen<<4);i += 16) {
          for(var c = data.charCodeAt(++p), bmi = 0; bmi < 8; bmi++)
           idata.data[i+bitMapping[bmi]] = (idata.data[i+bitMapping[bmi]]&~1)|((c>>bmi)&1)
          idata.data[i+3] = idata.data[i+7] = idata.data[i+11] = 255
        }
        ctx.putImageData(idata, 0, 0)
        cb(cnv.toDataURL('image/png'))
      }
      img.src=imglink
    },
    reveal: function(imglink, cb) { //PNG image data URI as input, message as output (to callback)
      img.onload = function() {
        cnv.width = this.width
        cnv.height = this.height
        ctx.drawImage(this, 0, 0)
        var idata = ctx.getImageData(0, 0, this.width, this.height).data, data = [], i, c
        for(i = 0;i < idata.length;i += 16) {
          for(var c = 0, bmi = 0; bmi < 8; bmi++)
            c |= (idata[i+bitMapping[bmi]]&1)<<bmi
          if(c===255) break
          else if(c) data.push(String.fromCharCode(c))
        }
        cb(decodeURIComponent(escape(atob(data.join('')))))
      }
      img.src=imglink
    }
  }
}(window)