Key Scheduling of International Data Encryption Algorithm (IDEA)

Question

How to perform the key scheduling in International Data Encryption Algorithm (IDEA), I took a research but I can't understand how to perform it,

"further groups of eight keys are created by rotating the main key left 25 bits between each group of eight.", this is the part where I cannot understand the key scheduling of IDEA

Answer 1

Have a look at the corresponding paper.

The key is a 128-bit string $K$. The round keys for round $r=1,...,8$ are derived as follows:

• take the version of $K$ cyclically rotated to the left $25\cdot r$ times;
• split the string obtained into eight substrings and call them $Z_1^{(r)}$, ..., $Z_8^{(r)}$.

The keys $Z_7^{(r)}$ and $Z_8^{(r)}$ are never used. The subkeys $Z_1^{(9)}$, ..., $Z_4^{(9)}$ are used to mask the output.