Decrypt C#-encrypted data in C++ using Windows AES crypto provider

I need to encrypt the data in C# using RijndaelManaged and decrypt in C++ code.

C# encryption code:

static string Encrypt(string plainText)
{
    byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);

    var keyBytes = new byte[] { /* ... 32 bytes of a key */};
    byte[] iv = new byte[] { /* ... 16 bytes of IV */ };


    var symmetricKey = new RijndaelManaged() 
    { 
        Mode = CipherMode.CBC, 
        Padding = PaddingMode.Zeros, 
        BlockSize = 128, // Must be 128 to be compatible with AES
        KeySize = 256 
    };

    var encryptor = symmetricKey.CreateEncryptor(keyBytes, iv);

    byte[] cipherTextBytes;
    using(var memoryStream = new MemoryStream())
    {
        using(var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
        {
            cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
            cryptoStream.FlushFinalBlock();
            cipherTextBytes = memoryStream.ToArray();
            cryptoStream.Close();
        }
        memoryStream.Close();
    }
    return Convert.ToBase64String(cipherTextBytes);
}

But when decrypting it in C++ code I always get NTE_BAD_DATA reply from CryptDecrypt. Here is C++ code (all checks are removed for clarity):

__declspec(dllexport) DWORD  Decrypt(char* stringBuffer)
{
string encryptedString(stringBuffer);

// Decode base64 string to byte array. Works ok, the binary array is the same as the one in C# code.
vector<BYTE> encryptionBuffer = Base64::decode(encryptedString);
DWORD bufferSize = encryptionBuffer.size();

struct CryptoBlob {
    BLOBHEADER header;
    DWORD cbKeySize;
    BYTE rgbKeyData[32];
} keyBlob;

keyBlob.header.bType = PLAINTEXTKEYBLOB;
keyBlob.header.bVersion = CUR_BLOB_VERSION;
keyBlob.header.reserved = 0;
keyBlob.header.aiKeyAlg = CALG_AES_256;
keyBlob.cbKeySize = 32;

BYTE keyData[32] = { /* 32 bytes of a key the same as in C# code */ };
BYTE ivData[16] = { /* 16 bytes of IV the same as in C# code */ };

memcpy(keyBlob.rgbKeyData, keyData, 32);

HCRYPTKEY hPubKey;
HCRYPTPROV hProv;

CryptAcquireContext(
    &hProv,
    NULL,
    NULL,
    PROV_RSA_AES,
    CRYPT_VERIFYCONTEXT);

CryptImportKey(hProv, (const LPBYTE)&keyBlob, sizeof(keyBlob), 0, 0, &hPubKey);
CryptSetKeyParam(hPubKey, KP_IV, ivData, 0);

// Here the error happens, the value returned is 0x80090005 (NTE_BAD_DATA)
DWORD err = CryptDecrypt(hPubKey, 0, TRUE, 0, encryptionBuffer.data(), &bufferSize);

// overwrite the input buffer with decrypted data
memset(stringBuffer, 0, encryptedString.length());
memcpy(stringBuffer, encryptionBuffer.data(), bufferSize);

return 0;
}

Any idea what could be wrong? Thanks!

When you pass TRUE as the 3rd parameter to CryptDecrypt it attempts to undo PKCS#7 padding. When it cannot undo that padding it emits NTE_BAD_DATA.

Since you've changed the padding mode for the encryption to a value other than Pkcs7, you need to pass FALSE and perform manual depadding.

Since PaddingMode.Zeros is not inherently undoable, there's no depadding to perform.