Encrypting files with simontool and the Simon Cipher

Outside of the authors of the simon cipher, I probably know more about the cipher than anyone else. I've written the tools for hardware implementation (one day, the IEEE will approve the paper). I've spent a lot of time encrypting multiple blocks and looking at the strobe outputs. The simontool program can encrypt a file as EBC. I do not support CBC yet because I cannot make assumptions about the hardware chaining.

Let's use simontool to encrypt a file with the simon cipher. It helps to start with the manual file, but here's the summary. I do not give you a way to generate a key from text at this point, so you will need a key. Let's assume SIMON128/128 and pick the test key from the 0f0e0d0c0b0a09080706050403020100. This key is the encryption key, so we first need to also generate a decryption key.

simontool -e -b 128 -k 128 -s 0f0e0d0c0b0a09080706050403020100 -t 0 -y

This will print out the complete key expansion, and the key we want is the "final key" entry. The -t flag is generally used to encrypt a single block and the -y flag dumps the key expansion to stdout.

key[65]: e4eaaf3ba3196adfd49c399343c9c09a 
key[66]: 29b0397872648490e4eaaf3ba3196adf 
key[67]: 1cb8547cd58c4df929b0397872648490 
final key: 6413494fda72360d1cb8547cd58c4df9 
data: 13914e4e9aec8f25bb849374e01139aa 

Encrypt the file, testfile.txt:

simontool -e -b 128 -k 128 -s 0f0e0d0c0b0a09080706050403020100 -i testfile.txt -o testfile.simon

Decrypt the file testfile.simon, use the expanded key:

simontool.elf -d -b 128 -k 128 -s 6413494fda72360d1cb8547cd58c4df9 -i testfile.simon -o testfile.txt

Now, what if you forget you key? I cannot help you, but if you forget the format, the header of the encrypted file has some information:

SIMON                 11 128 128

The format from the manual:

 header is 32-bytes in total:    
          8 for the word SIMON
          8 for nothing
          4 for nothing
          4 for offset padding (most likely a value < 8)
          4 for key size information 
          4 for block size information