SQL AES_256 Encryption and Decryption
Try defining the length of the varbinary in your convert statement as varbinary(max). Per https://technet.microsoft.com/en-us/library/ms188362(v=sql.110).aspx it would default to 30.

You can replicate that with the following script:

DECLARE @STRING VARCHAR(100);
SELECT  @STRING = REPLICATE('X', 100);

SELECT  LEN(@STRING) AS String_Length,
        LEN(CONVERT(VARBINARY, @STRING)) AS Varbinary_Length ,
        LEN(CONVERT(VARBINARY(MAX), @STRING)) AS VarbinaryMax_Length;