Implement Hashcash algorithm to fix 402 PaymentRequired errors on login

[tr4wzified]

Took inspiration from qgustavor/mega#239 (comment) and implemented the hashcash in this library which should fix #248

[gpailler]

Thank you very much for your contribution. I’ve added a minor comment regarding some duplicated code.

Once this PR is merged, I’ll publish a version on MyGet. If everything works as expected, I’ll release an official version on NuGet.

Thanks again!

[Bl4Cc4t]

Suggested change

	hashPrefix = ReverseBytes(hashPrefix);
	hashPrefix = BinaryPrimitives.ReverseEndianness(hashPrefix);

this part was failing for me, so I tried replaced it with the builtin function for reversing the endianess and it seems to work

[tr4wzified]

Have not implemented this yet because it requires installing System.Memory, but let me know if you prefer this @gpailler

[gpailler]

I prefer to not introduce a new dependency here.
@Bl4Cc4t Did you have a chance to investigate why the original code was failing?

@tr4wzified Maybe you can use the following instead

if (BitConverter.IsLittleEndian)
{
  Array.Reverse(hash);
}
      
var hashPrefix = BitConverter.ToUInt32(hash, 0);

[gpailler]

@Bl4Cc4t @tr4wzified ? I'm just waiting for your feedback in order to merge this PR and deploy a first version on MyGet.

[tr4wzified]

I'll check this out later today

[tr4wzified]

Changed it, looks like everything's working here

[andrescirulo]

Hi, just to know, is there an estimation of when will this merge be done? Is it stall?
Thanks a lot

[tr4wzified]

Hey, sorry, I kind of forgot about this. I will have a look in a sec to adjust the PR based on the review and comments here.

[tr4wzified]

I replaced the functions with the existing ones found in the extensions, I had missed those. I haven't used BinaryPrimitives because it required installing another NuGet package, but let me know if that is the preferred way of handling it.