Recently, we were working on a project where we needed to save an RSA public key into our SQL database. However, when attempting to convert the RSA public key to a string, we encountered an error stating that Bouncy Castle couldn't convert RsaKeyParameters to bytes.
To provide some context, we used BouncyCastle in C# to generate an RSA key pair. After generating the keys, we extracted the private and public keys into variables. Our goal was to store the public key for later verification within the application.So let's discuess that how we can do that.

To resolve this issue, we need to serialize the public key into a format that can be stored in the database and later deserialized for verification. One common approach is to encode the public key using a standard format such as PEM (Privacy-Enhanced Mail) or DER (Distinguished Encoding Rules). Bouncy Castle provides utility classes to perform these conversions.

For example, we can use the SubjectPublicKeyInfoFactory class to convert the public key into a byte array representation using the DER format:
SubjectPublicKeyInfo publicKeyInfo = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(rsaPublicKey);
byte[] publicKeyBytes = publicKeyInfo.ToAsn1Object().GetEncoded();
Then, we can store publicKeyBytes in the database as a binary blob or base64-encoded string. When retrieving the public key from the database, we can reverse process by decoding the byte array back into a SubjectPublicKeyInfo object and then reconstructing the RSA public key, so this approach ensures that the public key can be stored and retrieved correctly from the database without losing any information or encountering conversion errors.

You can try my solution is above one is suitable for you if you need to convert an RSA public key to a string using BouncyCastle in C#, follow these steps:

  1. We need to ensure that we have the BouncyCastle library installed in our C# project. We can do this using NuGet Package Manager or by manually adding the BouncyCastle DLL file to our project.
  2. Retrieve RSA Public Key: Obtain the RSA public key from our RSA key pair. This key pair can be generated using the BouncyCastle library or obtained from another source.
  3. Convert Public Key to SubjectPublicKeyInfo: BouncyCastle represents RSA public keys using the SubjectPublicKeyInfo structure. Convert our RSA public key to SubjectPublicKeyInfo using the GetEncoded() method.
  4. Encode SubjectPublicKeyInfo to Base64 String: Finally here, we are going encode the SubjectPublicKeyInfo structure to a Base64 string.

Here's  an example code showing how to convert an RSA public key to a string using BouncyCastle:

        using System;
        using Org.BouncyCastle.Crypto;
        using Org.BouncyCastle.OpenSsl;
        using Org.BouncyCastle.Security;
        class Program
            static void Main(string[] args)
                // Assuming rsaPublicKey is your RSA public key
                AsymmetricKeyParameter rsaPublicKey = GetRSAPublicKey();

                // Convert RSA public key to SubjectPublicKeyInfo
                SubjectPublicKeyInfo publicKeyInfo = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(rsaPublicKey);

                // Convert SubjectPublicKeyInfo to Base64 string
                string publicKeyString = Convert.ToBase64String(publicKeyInfo.GetEncoded());

                Console.WriteLine("RSA Public Key as String: " + publicKeyString);

            static AsymmetricKeyParameter GetRSAPublicKey()
                // Here you would retrieve or generate your RSA public key
                // For the sake of example, we'll just return a dummy public key
                return null;

Using above code you can effectively convert an RSA public key to a string representation in C#.