How to Read a PEM RSA Private Key in .NET using Bouncy Castle?

Reading a PEM RSA Private Key in .NET

Developers often encounter the need to read a PEM RSA private key within a .NET environment. To address this, let's explore a solution utilizing both the .NET Framework and a popular open-source library called Bouncy Castle.

Introducing Bouncy Castle

Bouncy Castle is a comprehensive cryptography library that provides extensive support for various algorithms and formats. It includes robust functionality for handling RSA keys in PEM format.

Implementing the Solution

To read a PEM RSA private key from .NET using Bouncy Castle, follow these steps:

1. Install the Bouncy Castle library in your project.
2. Obtain the PEM-encoded RSA private key and store it in a variable called privateKey.

3. Create an instance of the AsymmetricCipherKeyPair class:

   AsymmetricCipherKeyPair keyPair;

4. Read the PEM file containing the private key into a TextReader object:

   using (var reader = File.OpenText(@"c:\myprivatekey.pem"))
   {
    // Read the private key from the file using a PEM reader
    keyPair = (AsymmetricCipherKeyPair) new PemReader(reader).ReadObject();
   }

5. Create an instance of the Pkcs1Encoding class with an RSA engine:

   var decryptEngine = new Pkcs1Encoding(new RsaEngine());

6. Initialize the Pkcs1Encoding engine for decryption using the private key:

   decryptEngine.Init(false, keyPair.Private);

7. Decode the base64-encoded bytes to be decrypted (previously saved in bytesToDecrypt) using the Pkcs1Encoding engine:

   var decrypted = Encoding.UTF8.GetString(decryptEngine.ProcessBlock(bytesToDecrypt, 0, bytesToDecrypt.Length));

Conclusion

This detailed walkthrough demonstrates how to effectively read and use a PEM RSA private key in .NET. By leveraging Bouncy Castle's capabilities, developers can seamlessly integrate RSA decryption into their applications.

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