Question

Is it ‘safe’ to store cipher parameters in the (unencrypted) header of an encrypted file? Is there anything (other than the key of course!) that shouldn’t be stored/transmitted in the clear?

Answer 1

You are using a symmetric encryption, where storing the blocksize, blockmode and keysize would be safe, since you don’t (mustn’t) make keys available as you stated.

But all such params are in general useful to attackers. If the file cannot easily be associated with a cipher and used params (or the software respectively), an attacker would have considerably more work to do and that’s what encryption basically is for. A cipher is secure, while (and because) everyone can see how it works. Additionally trying to hide some information can also add some security.

• AES has a fixed block size of 128bits, which itself is not a critical information, knowing of AES itself already. So this one is not needed inside the file header.

• The keysize is given by the key itself, so it can be left out too.

• The blockmode is the remaining parameter. Just never use ECB. Permanently use a single blockmode like OCB and you don’t need to store it in the file aswell.

Predefining all params at both sides is a solution, if you don’t intend to change them per file.
Error checking can be done using checksums, which are also critical information, so you may encrypt them together with the data or provide them together with the key.

Perhaps, following approaches can help if you have to transmit the params anyway:

• Transmit params in the key file, if you’re up to define the format yourself and the keys were distributed on a per file basis.

• You could also define different settings by mapping them to some randomly defined enumerators, which don’t provide valuable information without knowing the software.