RC2 key schedule

Question

Can someone explain how the RC2 key schedule works (particularly the very beginning of it)? i know it uses little endian, but my implementation is not working for any key except "0000 0000 0000 0000"

Test Vector
Key = 88bc a90e 9087 5a
Plaintext = 0000 0000 0000 0000
Ciphertext = 6ccf 4308 974c 267f

im assuming that the first thing to do with the key would be to change it into

bc88 0ea9 8790 5a

and yes i know RC2 is not even used anymore, but i would still like to know

Answer 1

The RFC says:

The key expansion algorithm begins by placing the supplied T-byte key into bytes L[0], ..., L[T-1] of the key buffer.

So if your key is 88bc a90e 9087 5a you get L[0]=0x88, L[1]=0xbc, ... L[6]=0x5a.

No need to consider any endianess here.

If you want to treat the key-buffer as 16-bit words you get:

K[i] = L[2*i] + 256*L[2*i+1]

I.e. K[0] = 0xbc88, K[1] = 0xa90e, K[2] = 0x8790. L[7] is only assigned later in the key-expansion step, so strictly speaking K[3] is undefined at this point. Feel free to choose any value you want however, since it makes no difference to the algorithm. If you select 0, you get K[3] = 0x005a.