how to decrypt an encrypted text using RSACryptoServiceProvider?

Question

I have encrypted a text using RSACryptoServiceProvider. I exported the public and private key. Obviously I just want to expose the public key inside the decoder application, so I have written a code as follows :

private const string PublicKey = "<RSAKeyValue><Modulus>sIzQmj4vqK0QPd7RXKigD7Oi4GKPwvIPoiUyiKJMGP0qcbUkRPioe2psE/d3c1a2NY9oj4Da2y1qetjvKKFad2QAhXuql/gPIb1WmI+f6q555GClvHWEjrJrD/ho7SLoHbWd6oY6fY609N28lWJUYO97RLVaeg2jfNAUSu5bGC8=</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>";

private string Decrypt()
        {
            byte[] encryptedKeyAsBytes = Convert.FromBase64String(_encryptedKey);
            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            rsa.FromXmlString(PublicKey);
            // read ciphertext, decrypt it to plaintext
            byte[] plainBytes = rsa.Decrypt(encryptedKeyAsBytes, false);
            string plainText = System.Text.Encoding.ASCII.GetString(plainBytes);

            return plainText;
        }

But an exception is thrown at line "byte[] plainBytes = rsa.Decrypt(encryptedKeyAsBytes, false);" and says "Key does not exist." However if I expose the whole private and public key then it runns happily. So how can I decrypt the data using only the public key information?

Answer 1

The short answer is: you can't. To decrypt messages you need the private key, that's the major principle of asymmetric cryptography.

You encrypt messages using someone's public key so that only the person in possession of the corresponding private key is able to decrypt them.

That's why the public key is called public - you may safely distribute it to the public so that they can encrypt messages to be read by you who is the sole owner of the corresponding private key.