Round Function
A round function is the transformation a block cipher applies to the data block in each round, the core component of an iterative cipher.
Purpose
Iterating it over many rounds accumulates confusion and diffusion until the cipher reaches its security strength.
Mechanism
A round function composes nonlinear substitution (S-boxes), linear diffusion (permutation/mixing), and key addition. A Feistel round function F(R,K) modifies half a block; an SPN round function applies S-P transformations to the full block.
Use cases
- AES/DES round functions
- Feistel networks
- SPN structures