# Rivest Cipher 6 (RC6) Key Scheduler

I am trying to understand the Key Scheduler used in RC6 and I have a total of 3 questions. The RC6 Wikipedia page says that the only difference between the RC5 and RC6 key scheduler is that more words are produced from the key in RC6. In my application, I am using w=32bits, r=20rounds and b=16bytes.

My first question is are these are two separate for loops or is the second for loop that iterates from s-1 to v inside the first for loop that iterates from 1 to 2r+3? My second question is why are we setting A=B=i=j=0 and is this command and the v=3xmax{c,2r+4} inside the first for loop?

The RC6 paper also supplies this pseudo code:

Input: User-supplied b byte key preloaded into the c-word array L[0... c - 1]

Number r of rounds

Output: w-bit round keys S[0... 2r + 3]

Procedure: S[0] = Pw

for i = 1 to 2r + 3 do:
S[i] = S[i - 1] + Qw
A = B = i = j = 0
v = 3 x max{c,2r+4}

for s = 1 to v do:
{
A = S[i] = (S[i] + A + B)<<<3
B = L[j] = (L[j] + A + B)<<<(A + B)
i = (i + 1)mod(2r + 4)
j = (j + 1)modc
}


My third question is on how to preload the word array L with my 16byte key. The RC5 paper says:

The first algorithmic step of key expansion is to copy the secret key K[0...b-1] into an array L[0...c-1] of c=b/u words where u= w/8 is the number of bytes/word. This operation is done in a natural manner using u consecutive key bytes of K to fill up each successive word in L, low-order byte to high-order byte. Any unfilled byte positions of L are zeroed.

And the pseudo code for this operation is:

for i=b-1 downto 0 do:
L[i/u] = (L[i/u]<<<8)+K[i];


To elaborate on my third question, if i is going from 15 down to 0, and my u = 32/8=4, how can I use a decimal to index L? For example, when i is 15 we have L[15/4]=(L[15/4]<<<8 + K[i]);