[quick-dirty] html+js hack for cracking the substitution encryption.

freq.html

<h3>encrypted sentence</h3>
<script src="http://code.jquery.com/jquery-2.1.0.min.js"></script>

<input name="text" size="40" />
<button id="mk">make</button>

<hr />
<h3>decrypted sentence</h3>
<div id="box"></div>
<hr />

<h3>substitution chars</h3>
<table><tr><td>
<ul>
    <li>a = <input name="a" size="1" /></li>
    <li>b = <input name="b" size="1" /></li>
    <li>c = <input name="c" size="1" /></li>
    <li>d = <input name="d" size="1" /></li>
    <li>e = <input name="e" size="1" /></li>
    <li>f = <input name="f" size="1" /></li>
    <li>g = <input name="g" size="1" /></li>
    <li>h = <input name="h" size="1" /></li>
    <li>i = <input name="i" size="1" /></li>
    <li>j = <input name="j" size="1" /></li>
    <li>k = <input name="k" size="1" /></li>
    <li>l = <input name="l" size="1" /></li>
    <li>m = <input name="m" size="1" /></li>
</ul>
</td><td>
<ul>
    <li>n = <input name="n" size="1" /></li>
    <li>o = <input name="o" size="1" /></li>
    <li>p = <input name="p" size="1" /></li>
    <li>q = <input name="q" size="1" /></li>
    <li>r = <input name="r" size="1" /></li>
    <li>s = <input name="s" size="1" /></li>
    <li>t = <input name="t" size="1" /></li>
    <li>u = <input name="u" size="1" /></li>
    <li>v = <input name="v" size="1" /></li>
    <li>w = <input name="w" size="1" /></li>
    <li>x = <input name="x" size="1" /></li>
    <li>y = <input name="y" size="1" /></li>
    <li>z = <input name="z" size="1" /></li>
</ul>
</td></tr></table>
<button id="try">try</button>

<hr />

<h3>stats</h3>
<div id="stat"></div>

<script>
var sentence = 'aqjj szf. jqfk rxo\'z pqsc kbhi zq!';
$('button[id=mk]').click(function(){
    sentence = $('input[name=text]').val();
    $('div#box').html('<span style="color: red;">'+sentence+'</span>');
    var stats = {};
    for (var i=0; i<sentence.length; i++) {
        var ch = sentence[i];
        typeof stats[ch] == 'undefined' ?
            stats[ch] = 0 :
            stats[ch] += 1 ;
    }
    var sorted = [];
    for (var ch in stats) sorted.push([ch, stats[ch]]);
    sorted.sort(function(a, b){a[1]-b[1]});
    for (var i=0; i<sorted.length; i++) sorted[i] = $('<li>').html(sorted[i][0] + ' = ' + sorted[i][1]);
    $('div#stat').html($('<ul>').html(sorted));
});
$('button[id=try]').click(function(){
    var out = '';
    for (var i=0; i<sentence.length; i++) {
        var ch = sentence[i];
        if ('a' <= ch && ch <= 'z') {
            var sb = $('input[name='+ch+']').val();
            var rd = '<span style="color: red;">'+ch+'</span>'
            out += sb.length == 1 ? sb : rd ;
        } else {
            out += ch
        }
    }
    $('div#box').html(out);
});
</script>