[erlang-questions] hash_state() and Segmentation Fault

[Valentin Micic]

On 15 Apr 2019, at 4:09 PM, Sverker Eriksson wrote:

> I amend:
> 
> OTP 19 (or greater) + OpenSSL 1.0 (or greater) will give you type safe crypto:hash_* functions.
> 

Thanks Sverker,

I like what you're saying, however that does not correspond to my experience. 
I've been performing my tests using Erlang/OTP 21.1, but when I called

>>>> crypto:hash_final( {md5, <<1:92/unsigned-integer-unit:8>>} )

therefore, using a fake hash_state() value which tend to cause VM  to bail out reporting a segmentation fault.

Could you please run the line above on one of your run-times (VM) and confirm your assertion.

BTW: Thanks for the info about limiting usage of a hash_state() variable to the VM that created it -- that answered my (somewhat) silly question.

Kind regards

V/



> 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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