AFAIK (there's too many profile in PCL 2.0) System.Security.dll
is not part of any portable library profile.
- .NET: part of the framework
- Mono: part of the framework
- MonoTouch : not part of the profile
- Mono for Android : not part of the profile
- WinRT : not sure
- Silverlight : not part of the profile
- MonoGame : not a profile - it will depend on which profile it is used
The good news is that you could build your own using Mono's source code. That should be pretty easy for MonoTouch and Mono for Android.
The only problem I can foresee is Silverlight since it did not (last time I checked) support RSA and Mono's BigInteger implementation requires unsafe code (which was also not allowed under most circumstances). OTOH, and with a bit more work, you could re-implement RSA using SL's System.Numerics.dll
.