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

[Sverker Eriksson]

That was self-criticism.
I was referring to the empty binary that's not really empty.

/Sverker, Erlang/OTP

On 11/19/2015 03:34 PM, Frans Schneider wrote:
> 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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