Key derivation in c++ and VB.NET

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Date: Fri, 11 Feb 2005 19:37:02 -0800

I am trying to encrypt data in c++ and decrypt in VB.NET. I find that the
System.Cryptography classes don't align exactly with the CryptoAPI
functionality that is easily accessed from c++.

I believe that the problem I'm having is deriving the same key on the .NET
side as on the c++ side.

The c++ code (which encrypts and decrypts fine) looks like:

// Get handle to user default provider.
if (CryptAcquireContext(&hProv, NULL, NULL, PROV_RSA_FULL, 0))
{
  // Create hash object.
  if (CryptCreateHash(hProv, CALG_SHA1, 0, 0, &hHash))
  {
    // Hash password string.
    if (CryptHashData(hHash, pbKey, cbKey, 0))
    {
      // Create block cipher session key based on hash of the password.
      if (CryptDeriveKey(hProv, CALG_3DES, hHash, CRYPT_EXPORTABLE, &hKey))
      {
        // Allocate memory.
        pbBuffer = (LPBYTE)malloc(*pcbCipherText);
        if (pbBuffer != NULL)
        {
          memcpy(pbBuffer, pbPlainText, cbPlainText);
          cbBuffer = *pcbCipherText;
          *pcbCipherText = cbPlainText;

          // Encrypt data
          if (CryptEncrypt(hKey, 0, TRUE, 0, pbBuffer, pcbCipherText,
cbBuffer))
          {

The VP.NET code (where the problem is):

Dim ivNull() As Byte = {0, 0, 0, 0, 0, 0, 0, 0}
Dim csp As New TripleDESCryptoServiceProvider ' CSP context
Dim bHash As PasswordDeriveBytes
Dim outData As New MemoryStream
Dim Cipher As CryptoStream
Dim transform As ICryptoTransform

bHash = New PasswordDeriveBytes(ToString(Key), ivNull)
csp.Key = bHash.CryptDeriveKey("TripleDES", "SHA1", 0, ivNull)
csp.IV = ivNull

Cipher = New CryptoStream(outData, _
                                          csp.CreateDecryptor(), _
                                          CryptoStreamMode.Write)
Cipher.Write(inData, 0, inData.Length)
Cipher.Close()

The VB code throws an exception on the last (close) statement. I believe
that the reason is that the derived key for decryption doesn't match the one
that was used for encryption.

Any suggestions?

Thanks,

</hjp>

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