[erlang-questions] hash_state() and Segmentation Fault

[Sverker Eriksson]

I amend:
OTP 19 (or greater) + OpenSSL 1.0 (or greater) will give you type safe
crypto:hash_* functions.
On mån, 2019-04-15 at 15:45 +0200, Sverker Eriksson wrote:
> The crypto:hash_* functions have been type safe since OTP 19 (2016), you can
> no longer crash the Erlang VM  by passing a fake hash_state argument.
> 
> They are also pure functional. You can save a hash_state and use it as many
> times you want to fork off different hash caclulations. The only limitation is
> a hash_state only work in the VM instance that created it.
> 
> /Sverker, Erlang/OTP
> 
> On mån, 2019-04-15 at 12:03 +0200, Valentin Micic wrote:
> > 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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