I want to use a key derivation function to derive a key to another one.
I think HKDF is the easiest solution in my case.
The following picture gives a clear definition of it : http://webee.technion.ac.il/~hugo/kdf/hkdf-fig.jpg
In the case where the number of key bits request from the KDF is equal to the hash output length (i.e.
L = k in the definition) do I have to compute 2 HMAC :
PRK = HMAC(XTS, SKM) and
K(1) = HMAC(PRK, CTXinfo)
or can I directly compute :
K(1) = HMAC(SKM, CTXinfo) ?
Thank you in advance.