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The documentation claims that these classes are present, in version 126.96.36.199 of the System.Core.dll assembly. (See http://androidapi.xamarin.com/?link=T%3aSystem.Security.Cryptography.SHA256CryptoServiceProvider ).
The version of System.Core.dll that ships with Xamarin is 188.8.131.52, and 184.108.40.206 is nowhere to be found.
These CryptoServiceProviders have been part of .NET since 3.5 ( http://msdn.microsoft.com/en-us/library/system.security.cryptography.sha256cryptoserviceprovider(v=vs.110).aspx ) , and implementations of these classes have been part of Mono for 6 years ( https://github.com/mono/mono/tree/master/mcs/class/System.Core/System.Security.Cryptography ).
Other CryptoServiceProviders are already included (MD5 and SHA1).
We in general do not surface those APIs because there is a better option available and because it would increase the metadata for the code that we ship.
In general people should use the factory class, e.g. `SHA256.Create()`, to get the best implementation available
For example, on Xamarin.Mac and XAmarin.iOS those implementations use the hardware accelerated CommonCrypto implementations.
There may be a "better" option available if a developer only needs to target Xamarin.Android or Xamarin.iOS with their own code, but what about existing libraries from third parties? Getting other developers to care about the Xamarin platforms is more difficult when the code they already have requires significant rework just to compile.
There are so many useful NuGet packages in the wild that would be incredibly useful on mobile, but are unavailable because they rely on simple, fully managed classes that are available in desktop Mono but are excluded from Xamarin.
It's extra frustrating because no new code needs to be written. The code already exists in Mono - all that needs to be changed are some #if blocks.
How onerous is the metadata cost? These three classes have small API surface area. Is the additional metadata measured in bytes, or are we talking kilobytes? If the metadata is a concern, why was SHA1CryptoServiceProvider included but not SHA256CryptoServiceProvider?
If SHA256.Create() returns a hardware accelerated implementation on some platforms, why not use that to implement the SHA256CryptoServiceProvider?