Despite all comments, I still lack understanding of your intentions. Therefore, the sample code below may not provide what you exactly want, but at least should give an idea how to employ cryptographic functions. Particularly, the most notable difference from your approach is that the encryption key and initialization vector are computed once and for all messages, rather than reevaluated on each occasion, because the latter is prone to synchronization errors — such as when you reuse single crypto object to communicate with multiple parties, or when some messages get lost in transmission.
Public Shared Sub Test()
' Note: You should not actually hard-code any sensitive information in your source files, ever!
Dim sKeyPreimage As String = "MySuperPassword"
Dim oMyCrypto As New MyCrypto(sKeyPreimage)
Dim sPlaintext As String = "My super secret data"
Dim sEncrypted As String = oMyCrypto.EncryptText(sPlaintext)
Dim sDecrypted As String = oMyCrypto.DecryptText(sEncrypted)
Console.Out.WriteLine("Plaintext: {0}", sPlaintext) ' "My super secret data"
Console.Out.WriteLine("Encrypted: {0}", sEncrypted) ' "72062997872DC4B4D1BCBF48D5D30DF0D498B20630CAFA28D584CCC3030FC5F1"
Console.Out.WriteLine("Decrypted: {0}", sDecrypted) ' "My super secret data"
End Sub
Public Class MyCrypto
Private Shared TextEncoding As Text.Encoding = Text.Encoding.UTF8
Private CipherEngine As System.Security.Cryptography.SymmetricAlgorithm
' Note: Unlike in the question, same key and IV are reused for all messages.
Private CipherKey() As Byte
Private CipherIV() As Byte
Public Sub New(ByVal sKeyPreimage As String)
Dim abKeyPreimage() As Byte = TextEncoding.GetBytes(sKeyPreimage)
Dim abKeySalt() As Byte = TextEncoding.GetBytes("Ivan Medvedev")
Const KeyDerivationRounds As Integer = 1 << 12
Dim oKeyDerivationEngine As New System.Security.Cryptography.Rfc2898DeriveBytes(abKeyPreimage, abKeySalt, KeyDerivationRounds)
Me.CipherEngine = System.Security.Cryptography.Aes.Create()
Me.CipherEngine.Padding = Security.Cryptography.PaddingMode.PKCS7
Me.CipherKey = oKeyDerivationEngine.GetBytes(Me.CipherEngine.KeySize >> 3)
Me.CipherIV = oKeyDerivationEngine.GetBytes(Me.CipherEngine.BlockSize >> 3)
End Sub
Public Function Encrypt(ByVal abPlaintext() As Byte) As Byte()
Dim abCiphertext() As Byte
Using hStreamSource As New System.IO.MemoryStream(abPlaintext),
hStreamCipher As New System.Security.Cryptography.CryptoStream(
hStreamSource,
Me.CipherEngine.CreateEncryptor(Me.CipherKey, Me.CipherIV),
Security.Cryptography.CryptoStreamMode.Read),
hStreamTarget As New System.IO.MemoryStream
hStreamCipher.CopyTo(hStreamTarget)
abCiphertext = hStreamTarget.ToArray()
End Using
Return abCiphertext
End Function
Public Function Decrypt(ByVal abCiphertext() As Byte) As Byte()
Dim abPlaintext() As Byte
Using hStreamSource As New System.IO.MemoryStream(abCiphertext),
hStreamCipher As New System.Security.Cryptography.CryptoStream(
hStreamSource,
Me.CipherEngine.CreateDecryptor(Me.CipherKey, Me.CipherIV),
Security.Cryptography.CryptoStreamMode.Read),
hStreamTarget As New System.IO.MemoryStream
hStreamCipher.CopyTo(hStreamTarget)
abPlaintext = hStreamTarget.ToArray()
End Using
Return abPlaintext
End Function
Public Function EncryptText(ByVal sPlaintext As String) As String
Dim abPlaintext() As Byte = TextEncoding.GetBytes(sPlaintext)
Dim abCiphertext() As Byte = Me.Encrypt(abPlaintext)
Dim sCiphertext As String = Hex.Format(abCiphertext)
Return sCiphertext
End Function
Public Function DecryptText(ByVal sCiphertext As String) As String
Dim abCiphertext() As Byte = Hex.Parse(sCiphertext)
Dim abPlaintext() As Byte = Me.Decrypt(abCiphertext)
Dim sPlaintext As String = TextEncoding.GetChars(abPlaintext)
Return sPlaintext
End Function
End Class
Public Class Hex
Public Shared Function Format(ByVal abValue() As Byte) As String
Dim asChars(0 To abValue.Length * 2 - 1) As Char
Dim ndxChar As Integer = 0
For ndxByte As Integer = 0 To abValue.Length - 1
Dim bNibbleHi As Byte = abValue(ndxByte) >> 4, bNibbleLo As Byte = CByte(abValue(ndxByte) And &HFUS)
asChars(ndxChar) = Convert.ToChar(If(bNibbleHi <= 9, &H30US + bNibbleHi, &H37US + bNibbleHi)) : ndxChar += 1
asChars(ndxChar) = Convert.ToChar(If(bNibbleLo <= 9, &H30US + bNibbleLo, &H37US + bNibbleLo)) : ndxChar += 1
Next
Return New String(asChars)
End Function
Public Shared Function Parse(ByVal sValue As String) As Byte()
If String.IsNullOrEmpty(sValue) Then Return New Byte() {}
If (sValue.Length Mod 2) > 0 Then Return Nothing
Dim ndxText As Integer = 0
Dim ndxByteMax As Integer = (sValue.Length \ 2) - 1
Dim abValue(0 To ndxByteMax) As Byte
Try
For ndxByte As Integer = 0 To ndxByteMax
abValue(ndxByte) = Convert.ToByte(sValue.Substring(ndxText, 2), 16)
ndxText += 2
Next
Catch ex As Exception
Return Nothing
End Try
Return abValue
End Function
End Class
Again, please, note that this is just an example. I am not endorsing any kind of protection techniques shown here, especially because your task remains unknown. The code above simply illustrates the syntax and semantics — not how to do it right.
