C# – Securely Handling Private Keys from Windows Key Store

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I'm working on an application that encrypts configuration file settings for an ASP.Net web server. The encryption method I'm implementing is enveloping data using AES and RSA. I'm generating an AES Key, using that AES key to encrypt my data, then encrypting the AES key with the public key from a certificate in my Windows Certificate Store. I then save the data, along with the encrypted AES key, and the certificate serial number used to encrypt it, back to the configuration file.

When I want to access this data, I read the serial number of the certificate used to encrypt the AES key, pull out the private key from that certificate, and decrypt the AES key, then decrypt the data using that key. Here is where I am unsure of my level of security.

In order to access the private key associated with the certificate, I must mark the private key as exportable, and give the user my application runs under access to the private key. Since this is an ASP.Net server, that user is "Network Services". I'm sure you can see why this would be a security issue. Basically if anyone were to break into my ASP.Net server over the web, perhaps through some sort of injection, they would have access to that private key, would they not?

Does anyone have any suggestions as to how I could make my model more secure?

Decrypt Function:

//need an rsa crytpo provider to decrypt the aes key with the private key associated with the certificate
using (RSACryptoServiceProvider rsaCSP = (RSACryptoServiceProvider) decryptionCertificate.PrivateKey)
{
    //decrypt the aes key with the cert's private key
    byte[] aesKey = rsaCSP.Decrypt(encryptedAESKey, false);

    //decrypt data using the decrypted aes key and the IV passed in
    decryptedData = decryptWithAes(encryptedData, aesKey, aesIV);
}

Thanks.

Best Answer

Just by writing

RSACryptoServiceProvider rsaCSP = (RSACryptoServiceProvider)decryptionCertificate.PrivateKey

it doesn't mean that you are assigning the actual "key". You can make the private key Non-Exportable and still would be able to perform decryption.

Related Solutions

C# – Simple insecure two-way data “obfuscation”

Other answers here work fine, but AES is a more secure and up-to-date encryption algorithm. This is a class that I obtained a few years ago to perform AES encryption that I have modified over time to be more friendly for web applications (e,g. I've built Encrypt/Decrypt methods that work with URL-friendly string). It also has the methods that work with byte arrays.

NOTE: you should use different values in the Key (32 bytes) and Vector (16 bytes) arrays! You wouldn't want someone to figure out your keys by just assuming that you used this code as-is! All you have to do is change some of the numbers (must be <= 255) in the Key and Vector arrays (I left one invalid value in the Vector array to make sure you do this...). You can use https://www.random.org/bytes/ to generate a new set easily:

Using it is easy: just instantiate the class and then call (usually) EncryptToString(string StringToEncrypt) and DecryptString(string StringToDecrypt) as methods. It couldn't be any easier (or more secure) once you have this class in place.

using System;
using System.Data;
using System.Security.Cryptography;
using System.IO;


public class SimpleAES
{
    // Change these keys
    private byte[] Key = __Replace_Me__({ 123, 217, 19, 11, 24, 26, 85, 45, 114, 184, 27, 162, 37, 112, 222, 209, 241, 24, 175, 144, 173, 53, 196, 29, 24, 26, 17, 218, 131, 236, 53, 209 });

    // a hardcoded IV should not be used for production AES-CBC code
    // IVs should be unpredictable per ciphertext
    private byte[] Vector = __Replace_Me__({ 146, 64, 191, 111, 23, 3, 113, 119, 231, 121, 2521, 112, 79, 32, 114, 156 });


    private ICryptoTransform EncryptorTransform, DecryptorTransform;
    private System.Text.UTF8Encoding UTFEncoder;

    public SimpleAES()
    {
        //This is our encryption method
        RijndaelManaged rm = new RijndaelManaged();

        //Create an encryptor and a decryptor using our encryption method, key, and vector.
        EncryptorTransform = rm.CreateEncryptor(this.Key, this.Vector);
        DecryptorTransform = rm.CreateDecryptor(this.Key, this.Vector);

        //Used to translate bytes to text and vice versa
        UTFEncoder = new System.Text.UTF8Encoding();
    }

    /// -------------- Two Utility Methods (not used but may be useful) -----------
    /// Generates an encryption key.
    static public byte[] GenerateEncryptionKey()
    {
        //Generate a Key.
        RijndaelManaged rm = new RijndaelManaged();
        rm.GenerateKey();
        return rm.Key;
    }

    /// Generates a unique encryption vector
    static public byte[] GenerateEncryptionVector()
    {
        //Generate a Vector
        RijndaelManaged rm = new RijndaelManaged();
        rm.GenerateIV();
        return rm.IV;
    }


    /// ----------- The commonly used methods ------------------------------    
    /// Encrypt some text and return a string suitable for passing in a URL.
    public string EncryptToString(string TextValue)
    {
        return ByteArrToString(Encrypt(TextValue));
    }

    /// Encrypt some text and return an encrypted byte array.
    public byte[] Encrypt(string TextValue)
    {
        //Translates our text value into a byte array.
        Byte[] bytes = UTFEncoder.GetBytes(TextValue);

        //Used to stream the data in and out of the CryptoStream.
        MemoryStream memoryStream = new MemoryStream();

        /*
         * We will have to write the unencrypted bytes to the stream,
         * then read the encrypted result back from the stream.
         */
        #region Write the decrypted value to the encryption stream
        CryptoStream cs = new CryptoStream(memoryStream, EncryptorTransform, CryptoStreamMode.Write);
        cs.Write(bytes, 0, bytes.Length);
        cs.FlushFinalBlock();
        #endregion

        #region Read encrypted value back out of the stream
        memoryStream.Position = 0;
        byte[] encrypted = new byte[memoryStream.Length];
        memoryStream.Read(encrypted, 0, encrypted.Length);
        #endregion

        //Clean up.
        cs.Close();
        memoryStream.Close();

        return encrypted;
    }

    /// The other side: Decryption methods
    public string DecryptString(string EncryptedString)
    {
        return Decrypt(StrToByteArray(EncryptedString));
    }

    /// Decryption when working with byte arrays.    
    public string Decrypt(byte[] EncryptedValue)
    {
        #region Write the encrypted value to the decryption stream
        MemoryStream encryptedStream = new MemoryStream();
        CryptoStream decryptStream = new CryptoStream(encryptedStream, DecryptorTransform, CryptoStreamMode.Write);
        decryptStream.Write(EncryptedValue, 0, EncryptedValue.Length);
        decryptStream.FlushFinalBlock();
        #endregion

        #region Read the decrypted value from the stream.
        encryptedStream.Position = 0;
        Byte[] decryptedBytes = new Byte[encryptedStream.Length];
        encryptedStream.Read(decryptedBytes, 0, decryptedBytes.Length);
        encryptedStream.Close();
        #endregion
        return UTFEncoder.GetString(decryptedBytes);
    }

    /// Convert a string to a byte array.  NOTE: Normally we'd create a Byte Array from a string using an ASCII encoding (like so).
    //      System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
    //      return encoding.GetBytes(str);
    // However, this results in character values that cannot be passed in a URL.  So, instead, I just
    // lay out all of the byte values in a long string of numbers (three per - must pad numbers less than 100).
    public byte[] StrToByteArray(string str)
    {
        if (str.Length == 0)
            throw new Exception("Invalid string value in StrToByteArray");

        byte val;
        byte[] byteArr = new byte[str.Length / 3];
        int i = 0;
        int j = 0;
        do
        {
            val = byte.Parse(str.Substring(i, 3));
            byteArr[j++] = val;
            i += 3;
        }
        while (i < str.Length);
        return byteArr;
    }

    // Same comment as above.  Normally the conversion would use an ASCII encoding in the other direction:
    //      System.Text.ASCIIEncoding enc = new System.Text.ASCIIEncoding();
    //      return enc.GetString(byteArr);    
    public string ByteArrToString(byte[] byteArr)
    {
        byte val;
        string tempStr = "";
        for (int i = 0; i <= byteArr.GetUpperBound(0); i++)
        {
            val = byteArr[i];
            if (val < (byte)10)
                tempStr += "00" + val.ToString();
            else if (val < (byte)100)
                tempStr += "0" + val.ToString();
            else
                tempStr += val.ToString();
        }
        return tempStr;
    }
}

.net – Is it possible to encrypt with private key using .net RSACryptoServiceProvider

Just to clear things up a bit:

RSA can be used either for encryption (ensuring that Eve cannot read messages that Alice sends to Bob) or for signing (ensuring that if Alice sends a message to Bob, Bob knows that it was actually Alice that sent the message, and not Eve pretending to be Alice)

RSA generates a pair of keys - a public key and a private key. RSA is designed so that if you apply the public key and then apply the private key, or vice versa, you will get the same message back. And the public key can be derived from the private key, but the opposite is impossible.

To use RSA for encryption, Alice encrypts the message using Bob's public key. The only way to read this message is with Bob's private key, which only he has. Thus Eve can't read the message because he does not have this key. On the other hand, this provides no authentication of the source of the message. Eve can also get Bob's public key (since it's public) and send messages to Bob, pretending to be Alice.

To use RSA for signing, Alice takes a hash of the message, encrypts the hash using her own private key, and appends the result (this is the signature) to the message. Eve can of course still decrypt this using Alice's public key. However, Bob can decrypt the signature using Alice's public key and see if it matches. If it does, it must have been encrypted using Alice's private key, which only she has, so it must have come from Alice.

Now, I'm not familiar with the .NET cryptography API, so I'm not sure if it works exactly as described here. But this explanation might help you understand some of the answers you are getting.

Best Answer

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C# – Simple insecure two-way data “obfuscation”

.net – Is it possible to encrypt with private key using .net RSACryptoServiceProvider

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