From the site:

System.Security.Cryptography namespace of the .NET Framework.
The following classes and interfaces are part of the System.Security.Cryptography namespace of the .NET Framework Base Class Library.
SymmetricAlgorithm – is the top level abstraction for all symmetric cryptography algorithms

AsymmetricAlgorithm – is the top level abstraction for all asymmetric cryptography algorithms

RC2 – the RC2 abstraction, subclassing from SymmetricAlgorithm

DES – the DES algorithm abstraction, subclassing from SymmetricAlgorithm

TripleDES – the Triple DES algorithm abstraction, deriving from SymmetricAlgorithm

Rijndael – the Rijdeal abstraction, deriving from SymmetricAlgorithm

RSA – the RSA public key cipher agbstraction, deriving from AsymmetricAlgorithm

DSA – the DSA public key cipher abstraction, deriving from AsymmetricAlgorithm

ICryptoTransform – the interface definition for encryption and decryption transforms. Objects of this type are generated by methods on SymmetricAlgorithm

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So it isn't like they are releasing full source. More like bits and pieces, from what it sounds like.