[erlang-questions] How to use crypto:hmac_init() and friends?

[Frans Schneider]

Ok, thanks. It does work indeed.
But what do you mean with 'Ugly'"?

/Frans

On 11/19/2015 03:18 PM, Sverker Eriksson wrote:
> 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, 
>> Dhi_pub),
>>                    crypto:hmac_update(Mac_ctxt1, Dhir_pub);
>>                2 ->
>>                    Mac_ctxt_init
>>            end,
>> ...
>> ...
>> ...
>> LMac_ctxt = crypto:hmac_update(Mac_ctxt, <<Version/binary, 
>> Serialized/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 = 
>> <<9,138,157,154,221,37,75,104,28,207,127,232,193,207,68,39,22,88,62,219,15,114,22,236,68,226,141,39,69,212,143,123>>.
>> <<9,138,157,154,221,37,75,104,28,207,127,232,193,207,68,
>>   39,22,88,62,219,15,114,22,236,68,226,141,39,69,...>>
>> 2> crypto:hmac_init(sha256, K).
>> <<>>
>> 3>
>>
>> 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.
>
>
> /Sverker, Erlang/OTP
>
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