If you paste these characters into any online base64 decoder, you'll get the following:
<!DOCTYPE HTML>
<html>
<head id="htmlHead">
<meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0; user-scalable=no;" />
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta name="apple-mobile-web-app-capable" content="no" />
<title>Promote Groups or Pages</title>
<link rel="apple-touch-icon-precomposed" href="data:image/png;base64,
</head>
<body>
<a id="redirect" href="fb-groups://group?id=1503507809911018&s=s"></a>
</body>
</html>
<script type="text/javascript">
function jump() {
var e = document.getElementById('redirect');
var ev = document.createEvent('MouseEvents');
ev.initEvent('click', true, true, document.defaultView, 1, 0, 0, 0, 0, false, false, false, false, 0, null);
e.dispatchEvent(ev);
window.close();
}
if (("standalone" in window.navigator) && window.navigator.standalone) {
document.body.style.backgroundColor = '#000000';
jump();
} else {
var time = 1422371365;
var timeout = new Date().getTime() / 1000;
if (timeout > time + 4) {
document.write('</head><body bgcolor="#fff"><div align="center"></div>');
jump();
} else {
document.write(
####
)}
}
</script>
Instead of #### there is the following:
<style>
html,
body,
div,
span,
object,
iframe,
h1,
h2,
h3,
h4,
h5,
h6 {
margin: 0;
padding: 0;
border: 0;
font-size: 100%;
vertical-align: baseline;
font-family: HelveticaNeue-Light, sans-serif;
-webkit-touch-callout: none;
-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none
}
a {
color: inherit;
text-decoration: none
}
#headerBox {
width: 100%;
height: 48px;
border-bottom: .5px solid #979797;
background-color: #F6F6F6;
text-align: center
}
#popoverBox {
position: absolute;
bottom: 15px;
width: 290px;
height: 132px;
-webkit-border-radius: 11px;
border-radius: 11px;
background-color: #2891F7;
border: none;
margin-left: -145px;
left: 50%
}
#popoverBox:after {
top: 100%;
left: 50%;
border: solid transparent;
content: " ";
height: 0;
width: 0;
position: absolute;
pointer-events: none;
border-color: rgba(40, 145, 247, 0);
border-top-color: #2891F7;
border-width: 13px;
margin-left: -13px
}
.icon {
margin-bottom: 5px
}
#groupIconContainer {
margin-left: 40px;
width: 60px;
float: left;
height: 100%
}
#groupIcon {
height: 60px;
width: 60px;
-webkit-border-radius: 14px;
border-radius: 14px;
background-color: #FFF;
margin-top: 25px
}
#addToHomeIconContainer {
margin-right: 40px;
width: 60px;
float: right;
height: 100%
}
#addToHomeIcon {
margin-top: 25px
}
#arrow {
margin: 0 auto;
display: block;
margin-top: 49px
}
#infoText {
color: #141823;
font-size: 18px;
line-height: 28px;
text-align: center;
width: 280px;
height: 160px;
margin: auto;
position: absolute;
top: 0;
left: 0;
bottom: 0;
right: 0
}
.iconLabelContainer {
font-size: 12px;
color: #FFF;
line-height: 14px;
width: 100px;
height: 30px;
margin-left: -20px;
text-align: center
}
#groupCoverImage {
box-sizing: border-box;
-webkit-box-sizing: border-box;
background-size: cover;
background-position: center;
background-image: url("data:image/png;base64,
width: 52px;
height: 52px;
border-radius: 50%;
position: relative;
top: 4px;
left: 4px;
border: 0.5px solid rgba(0, 0, 0, 0.10);
}
</style>
<body>
<div id="headerBox"><span style="font-size:17px;color:#2891F7;line-height:20px;position:relative;top:13.5px"><a href="fb-groups://">Отмена</a></span>
</div>
<div id="infoText">Коснитесь кнопки
<svg style="vertical-align:text-bottom" width="22px" height="30px" viewbox="0 0 44 60" version="1.1">
<defs></defs>
<g id="Final" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
<g id="Add-Group-to-Home-Screen" transform="translate(-299.000000, -459.000000)">
<g id="Tap-+-below-+-Share-Icon" transform="translate(231.000000, 458.000000)">
<g id="share-icon" transform="translate(60.000000, 0.000000)">
<path d="M20.4,12 L30,2.4 L39.6,12" id="Shape" stroke="#3F75FF" stroke-width="2"></path>
<path d="M30,39.0000013 L30,2.7996" id="Shape" stroke="#3F75FF" stroke-width="2"></path>
<rect id="Rectangle-path" x="0" y="0" width="60" height="60"></rect>
<path d="M20.2682927,20 L9,20 L9,60 L51,60 L51,20 L39.7317073,20" id="Shape" stroke="#3F75FF" stroke-width="2"></path>
</g>
</g>
</g>
</g>
</svg> ниже,
<br>Затем коснитесь “Добавить на основной экран”</div>
<div id="popoverBox">
<div id="groupIconContainer">
<div id="groupIcon" class="icon">
<div id="groupCoverImage"></div>
</div>
<div class="iconLabelContainer">Promote Groups or Pages</div>
<!-- Replace with dynamic name -->
</div>
<div id="addToHomeIconContainer">
<svg id="addToHomeIcon" width="60px" height="60px" viewbox="0 0 120 120" version="1.1">
<defs></defs>
<g id="Add-Group-to-Home-Screen-Spec" transform="translate(-410.000000, -802.000000)">
<g id="Add-to-Home-Screen" transform="translate(367.000000, 802.000000)">
<g id="Add-to-Home-Screen-Icon" transform="translate(43.000000, 0.000000)">
<rect id="Rectangle-6" fill="#FFFFFF" x="0" y="0" width="120" height="120" rx="28"></rect>
<rect id="Rectangle-6" fill="#686870" x="25" y="25" width="70" height="70" rx="15"></rect>
<path d="M58,58 L58,46.991617 C58,45.8978404 58.8954305,45 60,45 C61.1122704,45 62,45.8916773 62,46.991617 L62,58 L73.008383,58 C74.1021596,58 75,58.8954305 75,60 C75,61.1122704 74.1083227,62 73.008383,62 L62,62 L62,73.008383 C62,74.1021596 61.1045695,75 60,75 C58.8877296,75 58,74.1083227 58,73.008383 L58,62 L46.991617,62 C45.8978404,62 45,61.1045695 45,60 C45,58.8877296 45.8916773,58 46.991617,58 L58,58 Z" id="Rectangle-8" fill="#FFFFFF"></path>
</g>
</g>
</g>
</svg>
<div class="iconLabelContainer">Добавить на
<br>основной экран</div>
</div>
<svg id="arrow" width="12px" height="20px" viewbox="0 0 24 40" version="1.1">
<defs></defs>
<g id="Explorations" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
<g id="Add-Group-to-Home-Screen" transform="translate(-307.000000, -847.000000)" stroke-linecap="round" stroke="#FFFFFF" stroke-width="5">
<g id="Group" sketch:type="MSLayerGroup" transform="translate(30.000000, 752.000000)">
<path d="M297,132 L280,115 L297,98" id="Path-35" transform="translate(288.500000, 115.000000) rotate(-180.000000) translate(-288.500000, -115.000000) "></path>
</g>
</g>
</g>
</svg>
</div>
<div id="popoverNub"></div>');
So I believe their implementation is as follows:
1) There is a web server embedded in the application (something like RoutingHTTPServer). It's running on some port like 5555 and configured to return a page with contents like:
<HTML><script>window.location.href=[data:text/html;charset=UTF-8;base64, tens of thousands of characters in an alpha-numeric string]</script></HTML>
The syntax is wrong, but the idea is to replace the current URL (http://localhost:5555) with that base64 encoded data.
2) When you tap add to home page in the app, it opens the link http://localhost:5555 in the Safari, the web server inside the app responds with the web page, containing a script that replaces the URL with base64 encoded data (this data contains the current time, when the script was generated)
3)From the contents of the base64 encoded data you can see that there are two checks:
a)if the app is running not in Safari, but standalone
b)if some time has passed since the script was generated
If any of these statements is true, you are redirected into the app using deep link.
If both of them are false, the page prompting user to add link to home screen is shown (that's what I replaced with ####).
When the link is added to home screen, all these scripts and web pages are embedded into that link as base64 encoded data.
