How can I find the known plain text P2 based on the given information? Assuming that the encryption is made using a common initialization vector and the mode used for encryption is OFB.

Plaintext  (P1): This is a known message!
Ciphertext (C1): a469b1c502c1cab966965e50425438e1bb1b5f9037a4c159
Plaintext  (P2): (unknown)
Ciphertext (C2): bf73bcd3509299d566c35b5d450337e1bb175f903fafc159
  • 1
    $\begingroup$ Welcome to Cryptography. Clearly this is homework!. What do you know about the OFB -mode? $\endgroup$
    – kelalaka
    Oct 11, 2019 at 14:17
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    $\begingroup$ Hint: How are P1 and C1 related? How are P2 and C2 related? Is there a cross-relation? $\endgroup$
    – SEJPM
    Oct 11, 2019 at 17:04
  • $\begingroup$ @kelalaka an OFB is an improved version of CFB. The basic difference among the two is that in OFB, the output of block cipher encryption is fed as a feedback to calculate the next block. The first block is calculated using an initialization vector (normally a string of random digits and alphabets). $\endgroup$
    – faizan
    Oct 12, 2019 at 18:41
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    $\begingroup$ I managed to solve it.First, I calculated the key by converting P1 into HEX and then XORed both P1 and C1 to get the Key. The key is then XORed with C2 which gave me a HEX of P2. This HEX is then converted finally back to ASCII and I got the answer. Thanks @SEJPM for the hint. $\endgroup$
    – faizan
    Oct 12, 2019 at 18:44
  • $\begingroup$ nice. Welcome to Cryptography. Note: only one can be notified with @ $\endgroup$
    – kelalaka
    Oct 12, 2019 at 18:48