The key-scheduling part of the algorithm basically creates an array of 256 bytes from the secret key, which is just another, bigger version of the key. This array will be the key that is used to encrypt and decrypt the data afterwards. This part consists of the following two parts:
- It creates an array with values from 0 to 256 sequentially:
for i from 0 to 255
S[i] := i
endfor
- It swaps bytes based on the key—this generates an index number, j, based on the secret key:
for i from 0 to 255
j := (j + S[i] + key[i mod keylength]) mod 256
swap values of S[i] and S[j]
endfor
Once this initiation part for the key is done, the decryption algorithm starts. In most cases, the KSA part is written in a separate function that takes only the secret key as an argument, without the data that needs to be encrypted or decrypted.
