Document IV truncation for .NET Framework but not .NET Core for TripleDESCryptoServiceProvider

[vcsjones]

Issue description

👋 Hi docs folks! On Corefx an issue was filed because of a compatibility difference between .NET Framework and .NET Core for the TripleDESCryptoServiceProvider class that isn't documented anywhere except in the comments of the .NET Framework reference source.

The gist is the initialization vector on TripleDESCryptoServiceProvider (either the IV property or the rbgIV parameter on CreateEncryptor and CreateDecryptor methods) accepts a byte array. In .NET Core, that byte array for IV must be equal to a valid block size of the algorithm. For 3DES, that's 64-bits (8 bytes).

In .NET Framework, it would silently just use the first 8 bytes, even if you gave it 9, or 20.

When migrating from the .NET Framework to .NET Core, users that were erroneously passing in more than 8 bytes started getting exceptions. The fix for this is to just change your code during the migration to pass in just the first 8 bytes.

I'm happy to try to submit a pull request for this, but I'm not sure the best way or place to communicate this. In the remarks section for the class?

Target framework

Check the .NET target framework(s) being used, and include the version number(s).

• .NET Core (2.0, 2.1)
• .NET Framework (all of them)
• .NET Standard

[rpetrusha]

Thanks for opening this issue, @vcsjones. The Remarks section is the appropriate place for this information. We'd be happy to review a PR if you'd like to submit one. If you have any questions, please don't hesitate to ask.