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// rot13-Algorithmus (für Entschlüsselung in JavaScript)
var map = new Array();
var s = "abcdefghijklmnopqrstuvwxyz";
  
for (var i = 0; i < s.length; i++)
	map[s.charAt(i)] = s.charAt((i + 13) % 26);

for (i = 0; i < s.length; i++)
	map[s.charAt(i).toUpperCase()] = s.charAt((i + 13) % 26).toUpperCase();

function rot13 (str)
{
	var rwert = 'mailto:';
	for (i = 0; i < str.length; i++)
	{
		var b = str.charAt(i);
		if (b == '+')
			rwert += '@';
		else
			rwert += ('A' <= b && b <= 'Z' || 'a' <= b && b <= 'z' ? map[b] : b);
	}
	return rwert;
}

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<a href="" onmouseover="this.href=rot13('znk+zhfgreznaa.qr');">Max Mustermann</a>

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function mailrot13 ($eml)
{
    return str_replace("@", "+", str_rot13($eml));
}