.NET equivalent of this native code using crypto api

Question

Hi,

I have following native code (Powerbuilder) with uses the Crypto API to encrypt a string. I need C# code to decrypt the encrypted data. Can somebody give me a hint or sample?

Thanks, Jaap

private function blob of_encryptdecrypt (blob ablob_data, string as_password, boolean ab_encrypt)
// -----------------------------------------------------------------------------
// SCRIPT:     n_cryptoapi.of_EncryptDecrypt
//
// PURPOSE:    This function will encrypt/decrypt the blob passed to it. Both
//      encrypt/decrypt have the same api calls except one so they
//      are combined to save coding.
//
// ARGUMENTS:  ablob_data   - The blob to be decrypted
//      as_password - The secret password
//
// RETURN:   Blob containing the decrypted data.
//
// DATE        PROG/ID   DESCRIPTION OF CHANGE / REASON
// ----------  --------  -----------------------------------------------------
// 12/26/2006   RolandS  Initial Coding
// -----------------------------------------------------------------------------

ULong hCryptProv, hHash, hKey
ULong lul_datalen, lul_buflen, lul_error
Blob lblob_buffer, lblob_value
String ls_msgtext
string ls_password

// Get handle to CSP
If Not CryptAcquireContext(hCryptProv, KEY_CONTAINER, is_cryptoservice, PROV_RSA_FULL, 0) Then
    If Not CryptAcquireContext(hCryptProv, KEY_CONTAINER, is_cryptoservice, PROV_RSA_FULL, CRYPT_NEWKEYSET) Then
     of_GetLastError(lul_error, ls_msgtext)
     SignalError(lul_error, "CryptAcquireContext:~r~n~r~n" + ls_msgtext)
    End If
End If

// Create a hash object
If Not CryptCreateHash(hCryptProv, CALG_MD5, 0, 0, hHash) Then
    of_GetLastError(lul_error, ls_msgtext)
    SignalError(lul_error, "CryptCreateHash:~r~n~r~n" + ls_msgtext)
End If

// Hash the password
If Not CryptHashData(hHash,as_password, Len(as_password), 0) Then
    of_GetLastError(lul_error, ls_msgtext)
    SignalError(lul_error, "CryptHashData:~r~n~r~n" + ls_msgtext)
End If

// Derive a session key from the hash object
If Not CryptDeriveKey(hCryptProv, ENCRYPT_ALGORITHM, hHash, 0, hKey) Then
    of_GetLastError(lul_error, ls_msgtext)
    SignalError(lul_error, "CryptDeriveKey:~r~n~r~n" + ls_msgtext)
End If

// allocate buffer space
lul_datalen = Len(ablob_data)
lblob_buffer = ablob_data + Blob(Space(8))
lul_buflen = Len(lblob_buffer)

If ab_encrypt Then
    // Encrypt data
    If CryptEncrypt(hKey, 0, True, 0, lblob_buffer, lul_datalen, lul_buflen) Then
     lblob_value = BlobMid(lblob_buffer, 1, lul_datalen)
    Else
     of_GetLastError(lul_error, ls_msgtext)
     SignalError(lul_error, "CryptEncrypt:~r~n~r~n" + ls_msgtext)
    End If
Else
    // Decrypt data
    If CryptDecrypt(hKey, 0, True, 0, lblob_buffer, lul_datalen) Then
     lblob_value = BlobMid(lblob_buffer, 1, lul_datalen)
    Else
     of_GetLastError(lul_error, ls_msgtext)
     SignalError(lul_error, "CryptDecrypt:~r~n~r~n" + ls_msgtext)
    End If
End If

// Destroy session key
If hKey > 0 Then
    CryptDestroyKey(hKey)
End If

// Destroy hash object
If hHash > 0 Then
    CryptDestroyHash(hHash)
End If

// Release CSP handle
If hCryptProv > 0 Then
    CryptReleaseContext(hCryptProv, 0)
End If

Return lblob_value
end function

Answer 1

Without going over the code you provided - you can start at Walkthrough: Creating a Cryptographic Application - this will show you basic stuff on System.Security.Cryptography namespace.

Answer 2

You may check this site for PInvoke reference to the CryptoAPI functions: http://www.pinvoke.net

For example: [DllImport("advapi32.dll", SetLastError=true)] public static extern bool CryptHashData(IntPtr hHash, byte[] pbData, uint dataLen, uint flags);

Answer 3

You can do the same in C# using classes from System.Security.Cryptography.

You're effectively doing an MD5 hash of the password and then using the algorithm defined in a global constant ENCRYPT_ALGORITHM. You'd have to post the value of that variable to get a better answer. However, if you're using anything common, then there is likely to be a *CryptoServiceProvider wrapper for you. For example, AesCryptoServiceProvider.

By the way, using a straight (e.g. non-salted) MD5 hash of the password can be bad. See this article for more information.

Answer 4

Maybe you can get some hints on how to translate this from native cryptoapi to .Net using the VB example at:

How to encrypt a string in Visual Basic 6.0 and how to decrypt the string in Visual Basic .NET or in Visual Basic 2005