<?php  
if(!isset($_GET['txt']))  
{  
exit();  
}  
  
header ("Content-type: image/png");  
$string = $_GET['txt'];  
$font = 4;  
$width = ImageFontWidth($font) * strlen($string);  
$height = ImageFontHeight($font);  
  
$im = @imagecreate ($width,$height);  
$background_color = imagecolorallocate ($im, 255, 255, 255); //white background  
$text_color = imagecolorallocate ($im, 0, 0,0);//black text  
imagestring ($im, $font, 0, 0, $string, $text_color);  
imagepng ($im);  
?>

Public Function Decrypt(ByVal encryptedBytes As String, ByVal secretKey As String) As String
        Dim plainText As String = Nothing
        Using inputStream As MemoryStream = New MemoryStream(Convert.FromBase64String(encryptedBytes))
            Dim algorithm As RijndaelManaged = getAlgorithm(secretKey)
            Using cryptoStream As CryptoStream = New CryptoStream(inputStream, algorithm.CreateDecryptor(), CryptoStreamMode.Read)
                Dim outputBuffer(0 To CType(inputStream.Length - 1, Integer)) As Byte
                Dim readBytes As Integer = cryptoStream.Read(outputBuffer, 0, CType(inputStream.Length, Integer))
                plainText = Encoding.Unicode.GetString(outputBuffer, 0, readBytes)
            End Using
        End Using
        Return plainText
    End Function

    Private Function getAlgorithm(ByVal secretKey As String) As RijndaelManaged
        Const salt As String = "aqui el salt"
        Const keySize As Integer = 256

        Dim keyBuilder As Rfc2898DeriveBytes = New Rfc2898DeriveBytes(secretKey, Encoding.Unicode.GetBytes(salt))
        Dim algorithm As RijndaelManaged = New RijndaelManaged()
        algorithm.KeySize = keySize
        algorithm.IV = keyBuilder.GetBytes(CType(algorithm.BlockSize / 8, Integer))
        algorithm.Key = keyBuilder.GetBytes(CType(algorithm.KeySize / 8, Integer))
        algorithm.Padding = PaddingMode.PKCS7
        Return algorithm
    End Function