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Asked: 2026-05-11T06:59:32+00:00

Ok I’m trying to use the Win32 Crypto API in C++ to decrypt a

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Ok I’m trying to use the Win32 Crypto API in C++ to decrypt a string encrypted in C# (.NET 2) with the RijndaelManaged Class. But I’m having no luck at all i get jibberish or a bad data Win32 error code. All my keys, IV and salt match, I’ve looked in the watch for both test apps. I’ve spent all say looking at it and I’m officialy stuck.

Anyway here is the C#

            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(GetPassPhrase(), salt, 1000);         RijndaelManaged rijndael = new RijndaelManaged();         rijndael.BlockSize = 128;         rijndael.KeySize = 256;         rijndael.Mode = CipherMode.CBC;          rijndael.Key = pdb.GetBytes(m_KeySize);         rijndael.IV = GetIV(iv);          ICryptoTransform encryptor = rijndael.CreateEncryptor();          MemoryStream msEncrypt = new MemoryStream();         CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write);         Byte[] encryptedBytes = null;         Byte[] toBeEncrypted = UnicodeEncoding.Unicode.GetBytes(value);          csEncrypt.Write(toBeEncrypted, 0, toBeEncrypted.Length);         csEncrypt.FlushFinalBlock();         encryptedBytes = msEncrypt.ToArray();

The C++ to decrypt it is:

                                    keyBlob.hdr.bType = PLAINTEXTKEYBLOB;                 keyBlob.hdr.bVersion = CUR_BLOB_VERSION;                 keyBlob.hdr.reserved = 0;                 keyBlob.hdr.aiKeyAlg = CALG_AES_256;                 keyBlob.cbKeySize = KEY_SIZE;                 keyBlob.rgbKeyData = &byKey[0];                  if ( CryptImportKey( hProv, (const LPBYTE) &keyBlob, sizeof(BLOBHEADER) + sizeof(DWORD) + KEY_SIZE, 0, CRYPT_EXPORTABLE, &hKey ) )                 {                      if ( CryptSetKeyParam( hKey, KP_IV, (const BYTE *) &byIV, 0))                     {                         DWORD dwLen = iDestLen;                         if ( CryptDecrypt( hKey, 0, TRUE, 0, pbyData, &dwLen))                         {                              if ( dwLen < (DWORD) *plOutSize)                             {                                 memcpy_s(pbyOutput, *plOutSize, pbyData, dwLen);                                  *plOutSize = dwLen;                                  bRet = TRUE;                              }                         }                         else                         {                             // Log                             DWORD dwErr = ::GetLastError();                             int y =0;                         }                     }                 }

I’m calling CryptAcquireContext successfully and my C++ is executing fine. Can anyone spot the error in my ways. It’s starting to depress me know 🙁

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1 Answer

  1. Ok my fault, I didn’t include the Struct def for the keyblob in the C++ and it turns out you need a contigous block of data for the key with the header but I was using the MSDN example that had a pointer to the key data. Which is wrong!

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