Which step am I missing - DES-OFB Key
#10
You have mis-understood the implementation of DES-OFB somewhat. Each round of DES-OFB gives you 64 bits of keystream, that is XORed with your plaintext to give you 64-bits of ciphertext. I'm not sure how or why you changed it to only use the first 8 bits of each DES round...

Example:-

Round 1 - You an 64-bit IV (which is just the first input round) and encrypt this with the DES key to give 64 bits of output from the DES cipher. This is XORed with 64 bits of plaintext to give you 64 bits of ciphertext.

Round 2 - you take the output of the previous DES register and encrypt it again with your key. This yields a further 64 bits of keystream that is XORed with the next 64 bits of plaintext.. and so on...

To break DES-OFB, you only need to know 128 consecutive bits of keystream, as long as they align with the 64-bit DES input and output blocks. Yes - it does work.

(02-09-2017, 01:18 AM)MaxPower Wrote:
(02-09-2017, 12:46 AM)ccprime Wrote: XOR is not supported by hashcat, i think

Sorry if I made that hard to understand, the XOR is performed at the end of the encryption to obtain the cipher that is transmitted. I manually XOR the encryption output with the plain text to show that it produces the same cipher text as a straight DES-OFB encryption.

To obtain the encrypted text necessary for the Hashcat function you need to XOR the cipher text with the Known plain text. But that will only give the encrypted text for the first byte of each cycle.

This diagram may help to explain the encryption process
[Image: 601px-OFB_encryption.svg.png]

I cant seem to work out how to get a complete 8 bytes of encrypted text to be able to run the Hashcat DES option. As the final cipher text has one byte of output from each DES encryption cycle.

As I said earlier, I only need to find the key for any stage of the cycle as the same key is used everytime. I know the input text in full for all stages and the first byte of encrypted text for every stage.

Is it even possible to find the key in DES-OFB even if you know the plain text?


Messages In This Thread
RE: Which step am I missing - DES-OFB Key - by MattSR - 06-23-2017, 03:26 AM