these are the facts I know of the block cipher.

  • the encryption function E
  • the decryption function D
  • x,y so that D(key,x) = D(key,y)+1

However, I don't know the key.

But I have two datas (x,y) which are the encrypted values from n respectively its successor n+1.

Assuming I knew the key, I could decrypt:
  • D(key,x) = n+1
  • D(key,y) = n

That is equivalent:
  • E(key,n+1) = x
  • E(key,n) = y

Remember I have x, y.

If you have any idea, let me know.
Thank you.