Decrypt an encoded string in ASP.Net

i want to decrypt the follwing string

AQAqyBYDDzTEV9lvWvWfR7S-9my4tamudiNkRLJnWoUO3prPBmVUsn4qGTxI75T32u71H8i8y3OcpXVirm5uuKZNg8nUGPlZZzI3avsYj47dUKlaOmW4SPRz_xIP5d2UILKZpIWoisPLcmi675jvBorjzui2MmG73i9iU0QQcRtFfIK2Tpc_2D-j020wQoulquKAi_lFeEeXvR4035CotE4-

for that i use following code

  private string Decryptdata(string encryptpwd) { string decryptpwd = string.Empty; UTF8Encoding encodepwd = new UTF8Encoding(); Decoder Decode = encodepwd.GetDecoder(); byte[] todecode_byte = Convert.FromBase64String(encryptpwd); int charCount = Decode.GetCharCount(todecode_byte, 0, todecode_byte.Length); char[] decoded_char = new char[charCount]; Decode.GetChars(todecode_byte, 0, todecode_byte.Length, decoded_char, 0); decryptpwd = new String(decoded_char); return decryptpwd; }

but i get error like Invalid character in a Base-64 string

kindly help me