[erlang-questions] hash_state() and Segmentation Fault

[Valentin Micic]

Hmmm… I may need to restate the question:

Does anyone know where can one find a description of the hash_state() structure, as used by crypto:hash_xxx functions?

Thanks in advance.

V/

On 10 Apr 2019, at 1:08 PM, Valentin Micic wrote:

> Hi, 
> 
> I've been investigating a feasibility of saving of hash_state() -- used as a part of erlang:md5_init/md5_update/md5_final and/or their functional equivalents in crypto library; so it could be used later to implicitly reinitialize hash calculations.
> Well, (duh!) of course it is feasible; however, my concern was that the structure of this opaque value may change between different versions of Erlang run-time, and I was interested to see how these functions would behave when fake values for hash_state()  are given.
> 
> The results are interesting.
> 
> A call to,  say, erlang:md5_final( <<0:88/unsigned_integer-unit:8>> ) (*), or 
>  erlang:md5_final( <<1212312312:88/unsigned_integer-unit:8>> ), or, indeed
> crypto:hash_final( {md5, <<0:92/unsigned-integer-unit:8>>} )
> 
> will all produce:
> 
> <<176,230,65,201,152,204,62,174,111,162,248,114,109,152,205,221>>
> 
> Presumably this may be due to some default value that has been used for all invalid values for hash_sate().
> 
> However, a call using a fake (yet "non-zero") value in  crypto:hash_final( {md5, <<1:92/unsigned-integer-unit:8>>} )  results in run-time crashing and reporting segmentation fault (and this cannot be a good thing, right?).
> 
> As it appears that some internal tests are performed in order to verify the hash_state() value, would  it possible to extend these test to cover other values without imposing unnecessary performance penalty?
> 
> Or, alternatively, is there any way that this test could be performed externally (e.g. when in doubt and before calling a function that may crash the run-time)… in other words, is it possible to publish descriptions (e.g. structure) of various hash_state() values?
> 
> Kind regards
> 
> V/
> 
> (*) 88 corresponds to a size (in octets) of the erlang:md5_xxx hash_state() value, and conversely, 92 is a number of octets in md5 hash_state() equivalent used by crypto library.
> 
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