Data size after encryption (AES encryption)
Posted on 2011-03-15
We have a problem in hand where we distribute an xml file to our clients. The data has to be visible to the clients on a particular date of the year (Embargo date). Since the data size is going to be large, we allow them to download the xml prior to the Embargo date, but want the data to be visible only on/after this date.
The data has two parts i. Share details which can be visible anytime ii. share portfolio details (group of data elements) which needs to remain secret till the Embargo date.
So we are thinking of encrypting the secret data elements only within the xml and publish the xml file much before the embargo date. On the morning of embargo date, we will reveal the Decryption key and the clients can use that to see the full data.
The clients on their end will download the xml prior to the embargo date and import into their system to prove that the file works fine. The portfolio details will be kept in staging area which show up as junk characters. Once the Decryption key is obtained, they can decrypt the data and see the actual characters.
We have decided to use AES encryption algorithm. The dev platform is .Net. Our objective is the keep the data file as small as possible to facilitate easy download. As I said, we don't want the whole data to be encrypted, only few data elements need to be encrypted.
The questions we are trying to answer is, how much the data size increases after encryption. This will help us take decision on whether
i. we will encrypt each secret data element individually or
ii. concatenate the secret data elements and encrypt it as one field
iii. We have another option, to separate non-secret and secret data into two files
I have seen that if I use 256 bit key, a 1 byte to 15 byte data becomes 24 bytes after incryption, 16 byte becomes 44, 100 byte becomes 152, 1000 bytes become 1344 etc.
Can anyone help me estimating the data size after applying AES encryption. This can be either using 128 bit or 256 bit key size.