[erlang-questions] How to use crypto:hmac_init() and friends?
Thu Nov 19 15:18:42 CET 2015
On 11/19/2015 02:37 PM, Frans Schneider wrote:
> Dear list,
> I have to create the HMAC of a message by including data at several
> points in my code as shown below:
> Mac_ctxt_init = crypto:hmac_init(sha256, Mac_key),
> Mac_ctxt = case Version of
> 3 ->
> Mac_ctxt1 = crypto:hmac_update(Mac_ctxt_init,
> crypto:hmac_update(Mac_ctxt1, Dhir_pub);
> 2 ->
> LMac_ctxt = crypto:hmac_update(Mac_ctxt, <<Version/binary,
> LMac = crypto:hmac_final_n(LMac_ctxt, 8),
> This will always return <<>>! It already starts with the hmac_init()
> which returns <<>>.
> Erlang/OTP 18 [erts-7.1] [source] [64-bit] [smp:2:2]
> [async-threads:10] [hipe] [kernel-poll:false]
> Eshell V7.1 (abort with ^G)
> 1> K =
> 2> crypto:hmac_init(sha256, K).
> Using crypto:hmac(sha256, K, Data, 8) works fine, but is not usable.
> Any idea what goes wrong or what I am doing wrong?
Nothing is wrong (ugly maybe, but not wrong).
That empty binary is not really empty. It's a magic binary!
It's referring some internal emulator state (the hmac context in this case).
Just pass it along to hmac_update and hmac_final and you will get a nice
regular binary in the end.
The magic binary looks and behaves like an empty binary. It will even
lose its magic powers
and turn into a regular binary if serialized with term_to_binary.
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