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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.

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HKDF stage 1 is useful for when the SKM value is derived using some deterministic method, such as a key exchange, or from a source that may not be fully trusted. SKM may be substantially larger than the keys you want, but may have the entropy not evenly distributed. Stage 1 compacts and distributes the entropy into a key that is correctly sized for the hash function in stage 2.

If you have a source key from a true random number generator, or that has already gone through a strong KDF, you can substitute PRK with SKM, and skip the first HMAC operation.

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