As an extension to [this quesiton][1], or [this one][2] how would you go about attacking a re used pad if the plaintext was randomized before being encrypted?

      0 0 0 1...         Plaintext

      0 1 1 1            seed(truly random number)
      0 1 0 1            Key1
      0 0 1 0            seed + Key1 mod 2 (=Cipher1)

      0 1 1 0...         Plaintext + PRNG(seed) mod 2 (=CipherR)
      1 0 1 0...         Key2(repeating 2 bit key "1 0") 
      1 1 0 0...         CipherR + Key2 mod 2 (=Cipher2) 

      0 0 1 0 1 1 0 0... Cipher1 & Cipher2 → ciphertext

This is using a OTP cipher to transmit the seed of a PRNG, then using a Vernam cipher with a repeating key to transmit the plaintext XORed with the PRN. Increasing the message length a little, hopefully to avoid cribbing.

How do you decode the Vernam cipher after the plain text has been randomized?


  [1]: https://crypto.stackexchange.com/q/2249/6417
  [2]: https://crypto.stackexchange.com/a/108/6417