 I'm learning about Feistel Ciphers at the moment, but having issues with the formulae, so I can't move on from it.

Encryption:

1. $$𝐿𝐸_{𝑖+1} = 𝑅𝐸_𝑖$$
2. $$𝑅𝐸_{𝑖+1} = 𝐿𝐸_𝑖 \oplus 𝐹( 𝑅𝐸_𝑖 , 𝐾_𝑖)$$

For the round function $$𝐹( 𝑅𝐸_𝑖 , 𝐾_𝑖)$$, could someone advise on what exactly I need to do? Let's say my key is $$1197$$, what needs to happen in between the brackets, $$(𝑅𝐸_𝑖 , 𝐾_𝑖)$$ , full diagram screenshot attached, thanks. • Welcome to Cryptography. I've edited your question, please check. The round function which is usually called the $F$ function performs confusion and diffusion. What is your actual question? Normally, one must define it as in DES – kelalaka Mar 2 '19 at 19:49
• Thanks for getting back to me. So If I have a plaintext and divide it into 2 parts, left and right, I need to put the right side through the round function. This should get me my value to XOR with the left side, but i'm not sure what exactly I have to do with the right side and round function in the 1st place. – terodee Mar 2 '19 at 19:54
• Yes, this is what the 1. and 2. equations already says. What is the problem? Did you see this question? – kelalaka Mar 2 '19 at 19:57
• Yes I checked he link you supplied but I can't really make sense of it. As I said Im only learning about it now, so still a bit raw with how it works. I did find something on a video from another site, it seems to imply that I must XOR the right side with Key. From here I would shift this value by 1 it and XOR it with the left side, does that sound correct? I have added a screenshot of this above. – terodee Mar 2 '19 at 21:22
• It is necessary to distinguish between on one hand Feistel cipher that is a structure or a tool that is used to construct block ciphers, on the other hand concrete applications of this tool. So $F$ is a function that takes two inputs, it could be any function but requirements are that it provides confusion and diffusion. The output of $F$ is then xored with the other part of the input. It might be worth mentioning that the keys $K_i$ are generated by some other procedures form input key.Now if you want to see a concrete implementation of $F$ then look at $DES$ for example. – Marc Ilunga Mar 2 '19 at 22:02

I guess it makes more sense if you write the two equations like this (referring to the nice diagram in Wikipedia): $$L_{i+1}=R_i,\; R_{i+1}=L_i\oplus F_{K_i}(R_i).$$ In each round, the round function $$F$$ takes as input the corresponding round key $$K_i$$ and the previous round's right output, then computes the output value, which is then XORed with the previous round's left output. $$F$$ is usually modeled as a pseudorandom function and has many instantiations (see here for a list of Feistel ciphers, of which each specifies its own $$F$$ instantiation and key schedule). This post discusses the requirements/goals for good $$F$$ instantiations.
Function $$F(x)$$ should be function with pseudo-random output (if you change 1 bit in intput there should be a chance to change half of output bits). There is no need to use a bijection, because in schema we do not need $$F^{-1}(x)$$.