# How round constants are chosen in a block cipher?

Block ciphers that are NOT LIGHTWEIGHT use relatively complex key scheduling algorithms to enhance their security and thwart some of the known cryptographic attacks. In such ciphers round constants are also added for removing symmetries in the structure (if these exist) but the selection criteria for the round constants is never mentioned.

How should these constants be selected for any block cipher?

• It is a matter of removing the attack surface of slide attacks. However, one need to see the action on their schedule. Jun 3 at 7:34

Usually, if there is no explanation provided, the constants are typically generated from some sort of well-known constant or pattern. For example, MD5 uses $$sin(x)$$, SHA-2 uses square and cube roots, and numerous algorithms use $$\pi$$, $$e$$, or $$\phi$$, and the reader is expected to recognize those constants. (Neither MD5 nor SHA-2 are block ciphers, but the same principle applies.) For example, SEED uses $$\phi$$.