How do to handle the case, when the string to be encrypted using AES is less than 16 bytes?

AES code requires the string to be 16 bytes. How do I handle a case, when the string to be converted is less than 16 bytes both during encryption and decryption? Appreciate, if you could paste the code snippet?

You may refer to AES code at http://buchholz.hs-bremen.de/aes/aes.htm

Thanks a lot!

Answer 1

[Geoff Hayes]

If the string (to encrypt) is less than 16 bytes, then why not just pad with zeros? Looking at the documentation referenced from your above link ( http://buchholz.hs-bremen.de/aes/AES.pdf) the plaintext input to the cipher function is just an array of 16 doubles (presumably, the software only considers the first 8 bits from each double) converted from the plaintext string via hex2dec. So you could do something like the following with your plaintext hexadecimal string:

ptLen = length(plaintext);   % get the length of the plaintext string
if ptLen>0 && ptLen<16       
  plaintext = [plaintext repmat({'00'},1,16-ptLen)];  % pad the plaintext 
end                                                   % string with zeros

Now your plaintext string has length of 16, and the conversion from hexadecimal to decimal, and the encryption can proceed (as per the documentation).

The decryption shouldn't matter as the cipher text will be 16-bytes (I'm assuming this given my brief read of the document).

[Sunil]

Thank Jeff for providing me the right direction. Replacing the 3rd line of your code with the below code, helped to solve my problem.

plaintext = [plaintext zeros(1,16-ptLen)];

since I got the below error, with your code.

_??? Error using ==> cipher at 21 Plaintext has to be a vector (not a cell array) with 16 elements._

[Geoff Hayes]

Cool. In the documentation, the author seemed to be using cell arrays but maybe in the code, something different was being used. Great that you got it working!

[Anis Zaitunah]

If the string (to encrypt) is more than 16 bytes, how to encryp?

[Geoff Hayes]

Anis - I think that would would encrypt every block of 16 bytes and then, when you have a block that is less than 16 bytes, pad it with zeros. Perhaps.