(C# UWP/WinRT) RSA Encrypt/Decrypt AES Key

Demonstrates how to use RSA to protect a key for AES encryption. It can be used in this scenario: You will provide your RSA public key to any number of counterparts. Your counterpart will generate an AES key, encrypt data (or a file) using it, then encrypt the AES key using your RSA public key. Your counterpart sends you both the encrypted data and the encrypted key. Since you are the only one with access to the RSA private key, only you can decrypt the AES key. You decrypt the key, then decrypt the data using the AES key.

This example will show the entire process. (1) Generate an RSA key and save both private and public parts to PEM files. (2) Encrypt a file using a randomly generated AES encryption key. (3) RSA encrypt the AES key. (4) RSA decrypt the AES key. (5) Use it to AES decrypt the file or data.

Chilkat Universal Windows Platform (UWP) / WinRT Downloads Chilkat for the Universal Windows Platform (UWP)

// This example assumes the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// ------------------
// Step 1.  Generate an RSA key and save to PEM files.

Chilkat.Rsa rsa = new Chilkat.Rsa();

// Generate a 1024-bit key. 
bool success = rsa.GenerateKey(1024);
if (success != true) {
    Debug.WriteLine(rsa.LastErrorText);
    return;
}

// Keys are exported in XML format:
string pubKeyXml = rsa.ExportPublicKey();

Chilkat.PublicKey pubKey = new Chilkat.PublicKey();
success = pubKey.LoadXml(pubKeyXml);
// For brevity, we are not checking the return value...
success = pubKey.SaveOpenSslPemFile("qa_temp/pubKey.pem");

string privKeyXml = rsa.ExportPrivateKey();

Chilkat.PrivateKey privKey = new Chilkat.PrivateKey();
success = privKey.LoadXml(privKeyXml);
success = privKey.SaveRsaPemFile("qa_temp/privKey.pem");

// Other methods exist for saving the private key in PKCS8 format,
// both encrypted and un-encrypted..

// ------------------
// Step 2.   This is the code your counterpart will run to 
// AES encrypt a file.  It generates a random AES key
// to use for the encryption.

Chilkat.Crypt2 crypt = new Chilkat.Crypt2();

crypt.CryptAlgorithm = "aes";
crypt.CipherMode = "cbc";
crypt.KeyLength = 256;

// Generate random bytes and return the random bytes
// in a hex string:
crypt.EncodingMode = "hex";
string randomKey = crypt.GenRandomBytesENC(32);
Debug.WriteLine("AES key = " + randomKey);

// Set the key.
crypt.SetEncodedKey(randomKey,"hex");

// Set the IV to a known value that will be used on both sides.
// (If desired, you could generate a random IV and protect it in the same
// way as the key...)
// The length of the IV for AES is always 16 bytes, regardless of the key size.
crypt.SetEncodedIV("000102030405060708090A0B0C0D0E0F","hex");

// AES Encrypt the file (the file may be any size because it will
// stream the file in/out.
success = await crypt.CkEncryptFileAsync("qa_data/hamlet.xml","qa_temp/aesEncrypted.dat");
if (success != true) {
    Debug.WriteLine(crypt.LastErrorText);
    return;
}

// ------------------
// Step 3.
// ------------------
// RSA encrypt the AES key.
// We'll pretend  your counter part has the public-key PEM file and needs
// to load it.  Therefore, we'll start with a new RSA object and load
// the public key from the PEM..
Chilkat.Rsa rsa2 = new Chilkat.Rsa();
Chilkat.PublicKey pubKey2 = new Chilkat.PublicKey();

// For brevity, don't check the success..  
success = pubKey2.LoadOpenSslPemFile("qa_temp/pubKey.pem");
pubKeyXml = pubKey2.GetXml();
success = rsa2.ImportPublicKey(pubKeyXml);

// RSA encrypt the AES key and return it as a base64 encoded string.
rsa2.EncodingMode = "base64";
bool bUsePrivateKey = false;
string encryptedAesKey = rsa2.EncryptStringENC(randomKey,bUsePrivateKey);

// At this point, your counterpart sends you the encryptedAesKey,
// and the encrypted file.

// ------------------
// Step 4 - RSA decrypt the AES key.
// ------------------

// Start with a new RSA object and load the private key from the PEM.
Chilkat.Rsa rsa3 = new Chilkat.Rsa();
Chilkat.PrivateKey privKey2 = new Chilkat.PrivateKey();

success = privKey2.LoadPemFile("qa_temp/privKey.pem");
privKeyXml = privKey2.GetXml();
success = rsa3.ImportPrivateKey(privKeyXml);

// The encrypted AES key is encoded using base64, so set
// our EncodingMode to "base64".
rsa3.EncodingMode = "base64";

bUsePrivateKey = true;
string decryptedAesKey = rsa3.DecryptStringENC(encryptedAesKey,bUsePrivateKey);
Debug.WriteLine("decryptedAesKey = " + decryptedAesKey);

// ------------------
// Step 5 - AES decrypt the file.
// ------------------

Chilkat.Crypt2 crypt2 = new Chilkat.Crypt2();

crypt2.CryptAlgorithm = "aes";
crypt2.CipherMode = "cbc";
crypt2.KeyLength = 256;

// Set the key.
crypt2.SetEncodedKey(decryptedAesKey,"hex");

// Set the IV 
crypt2.SetEncodedIV("000102030405060708090A0B0C0D0E0F","hex");

// AES Decrypt the file (the file may be any size because it will
// stream the file in/out.
success = await crypt2.CkDecryptFileAsync("qa_temp/aesEncrypted.dat","qa_temp/decrypted.xml");
if (success != true) {
    Debug.WriteLine(crypt2.LastErrorText);
    return;
}