RSA asymmetric encryption typically involves public key encryption and private key decryption. However, some scenarios require the reverse approach: private key encryption in Java and public key decryption in C#.

Certificate formats differ between platforms: .pfx certificates contain both public and private keys, while .cer certificates only include public keys. C#'s built-in RSA implementation doesn't support public key decryption, requiring the use of third-party libraries like BouncyCastle.

public static RsaKeyParameters ConvertDotNetPublicKeyToJava(string publicKeyXml)
{
    XmlDocument xmlDoc = new XmlDocument();
    xmlDoc.LoadXml(publicKeyXml);
    
    string modulusBase64 = xmlDoc.SelectSingleNode("//Modulus").InnerText;
    string exponentBase64 = xmlDoc.SelectSingleNode("//Exponent").InnerText;
    
    BigInteger modulus = new BigInteger(1, Convert.FromBase64String(modulusBase64));
    BigInteger exponent = new BigInteger(1, Convert.FromBase64String(exponentBase64));
    
    return new RsaKeyParameters(false, modulus, exponent);
}

public string ExtractPublicKeyFromCertificate(string certificatePath)
{
    X509Certificate2 certificate = new X509Certificate2(certificatePath);
    RSACryptoServiceProvider rsaProvider = (RSACryptoServiceProvider)certificate.PublicKey.Key;
    return rsaProvider.ToXmlString(false);
}

public string DecryptWithPublicKey(string publicKeyXml, string encryptedBase64)
{
    RsaKeyParameters javaPublicKey = ConvertDotNetPublicKeyToJava(publicKeyXml);
    
    IBufferedCipher cipher = CipherUtilities.GetCipher("RSA/ECB/PKCS1Padding");
    cipher.Init(false, javaPublicKey);
    
    byte[] encryptedData = Convert.FromBase64String(encryptedBase64);
    byte[] decryptedBytes = cipher.DoFinal(encryptedData);
    
    return Encoding.UTF8.GetString(decryptedBytes);
}

Common issues include incorrect Base64 encoding of ciphertext, which results in "Unknown block type" errors during decrypiton. Ensure the encrypted data from Java is properly encoded and matches the expected format.