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Hi I am new to cryptography and am confused as to how HKDF generates two separate keys from one single key/password. Is it as follows?

Key 1: HKDF-Extract(salt, IKM) -> PRK

Key 2: HKDF-Expand(PRK, info, L) -> OKM

OR is it that

Key 1: HKDF-Extract(salt, IKM) -> PRK

Key 2: SHA-512(PRK)

Where Key 2 would be HMAC-key?

Could it be that one key is derived using pb-kdf and hmac-key is derived using hkdf?

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The extract and expand steps of HKDF are just two phases of one algorithm run. The PRK created my the extract step is not meant to be used as a key (even though it would not be a problem securitywise).

So if you need to derive two keys from one IKM value, you run the whole algorithm (that is: extract & expand) once with an appropriate info string (like "key1", for instance) and then you run the whole thing a second time with the same salt and IKM but a different info string (like "key2").

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OK I think I found my answer. It depends on how you use it. So in my case i am using codeIgnitor library which generates two keys as follows:

Key 1: hkdf(key, 'sha512', NULL, key.length, 'encryption') Key 2: hkdf(key, 'sha512', NULL, NULL, 'authentication')

where Parameters are:

$key (string) – Input key material

$digest (string) – A SHA-2 family digest algorithm

$salt (string) – Optional salt

$length (int) – Optional output length

$info (string) – Optional context/application-specific info

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