(Visual Basic 6.0) RSA Encrypting Symmetric Secret Key

The RSA encryption algorithm is computationally expensive. It is not the best choice for encrypting large amounts of data. Symmetric encryption algorithms such as AES (i.e. Rijndael) or Blowfish are much more efficient. A typical application scenario is that you want to send encrypted messages to a partner, but you don't want to send the symmetric key unprotected. A solution is to generate a public/private RSA key pair and provide your partner with the public key (in advance). You may then encrypt the symmetric algorithm's key using the RSA private key. Next, encrypt the message using the symmetric algorithm, and send your partner both the encrypted key and encrypted message. Your partner decrypts by first RSA decrypting the key, and then uses the decrypted symmetric key to decrypt the message content.

Note: This example requires Chilkat v11.0.0 or greater.

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

Dim success As Long
success = 0

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

Dim rsa As New ChilkatRsa

' Load a public/private key pair from a .snk key file.
' Assume you have already provided your partner with 
' the public key part of the key pair.
Dim xmlKey As String
xmlKey = rsa.SnkToXml("qa_data/rsaKeys/test.snk")
If (rsa.LastMethodSuccess = 0) Then
    Debug.Print rsa.LastErrorText
    Exit Sub
End If

Dim pubKey As New PublicKey
success = pubKey.LoadFromString(xmlKey)
If (success = 0) Then
    Debug.Print pubKey.LastErrorText
    Exit Sub
End If

success = rsa.UsePublicKey(pubKey)

' Our message data will be encrypted using 128-bit AES
' encryption, using CBC (cipher-block chaining).
Dim crypt As New ChilkatCrypt2
crypt.CryptAlgorithm = "aes"
crypt.CipherMode = "cbc"
crypt.KeyLength = 128

' Generate 128-bit (16 bytes) secret key and return as a hex string.
crypt.EncodingMode = "hex"
Dim secretKey As String
secretKey = crypt.GenRandomBytesENC(16)
Debug.Print "Unencrypted Key: " & secretKey

' Use the key we generated:
crypt.SetEncodedKey secretKey,"hex"

' RSA encrypt the secret key and return as a hex string:
rsa.EncodingMode = "hex"
Dim bUsePrivateKey As Long
bUsePrivateKey = 0
Dim encryptedKey As String
encryptedKey = rsa.EncryptStringENC(secretKey,bUsePrivateKey)

' Symmetric encrypt a message.  For this example the message
' is very short, but typically this is where a large amount
' of data may be encrypted.
crypt.EncodingMode = "base64"
Dim encryptedText As String
encryptedText = crypt.EncryptStringENC("Hello World!")

' Show our encrypted key and encrypted text:
Debug.Print "Encrypted Key: " & encryptedKey
Debug.Print "Encrypted Text: " & encryptedText

' Assume we sent these strings to our partner...

' Here's what we do at the partner end:
Dim privKey As New PrivateKey
success = privKey.LoadXml(xmlKey)
If (success = 0) Then
    Debug.Print privKey.LastErrorText
    Exit Sub
End If

success = rsa.UsePrivateKey(privKey)

' First, decrypt the encryptedKey:
bUsePrivateKey = 1
Dim decryptedKey As String
decryptedKey = rsa.DecryptStringENC(encryptedKey,bUsePrivateKey)
Debug.Print "Decrypted Key: " & decryptedKey

' Set our crypt object's properties and secret key:
Dim crypt2 As New ChilkatCrypt2
crypt2.CryptAlgorithm = "aes"
crypt2.CipherMode = "cbc"
crypt2.KeyLength = 128
crypt2.EncodingMode = "base64"
crypt2.SetEncodedKey decryptedKey,"hex"

' Decrypt the message:
Dim decryptedText As String
decryptedText = crypt2.DecryptStringENC(encryptedText)
Debug.Print "Decrypted Text: " & decryptedText