Why is this error by using the algorithm Rijndael Cryptografia and decrypt?

Hello, everyone shall be based on post this site on the link below
 
http://forums.asp.net/p/25511/2758314.aspx#2758314
 

Adapted in a class of Cryptografia and then to my project in a routine that recovers the value of a Field ID (INT-type field, field key, autoincrement a basis MySql) and that passes the datalist of my code as you can see below
 
 protected static string RetornaPreço(object obj)

{

DataRowView dv = obj as DataRowView;

if (Convert.ToBoolean(dv["autorizaranuncioac"]) != false)

{

//string codigo = dv["IDAnuncio"].ToString();

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