Always getting same encrypted string with CryptoAPI - encryption

Question

Is there a way to get a non-deterministic output from the CryptoAPI? In other words, a different string output when encrypting a string.
For example, using CALG_AES_256 when deriving a crypt key with password of 'password' and string to encrypt of 'a', it always returns "SnÆwÞ¢L\x1e?6FÏLþw"
I'm somewhat of a n00b in using CryptoAPI, so any assistance is appreciated.
Edited:
Here is the cryptography code from Microsoft's example code decrypte and encrypt This is the same code, just shortened/compacted. This code was compiled in VS 2017 as a Win32 Console app. pszSource and pszDest are two files in the C:\temp folder. source.txt has the letter we're trying to encrypt in it.
The problem I'm having is that this crypt/decrypt code from the CryptoAPI does not allow certain strings to be encrypted and then decrypted (i.e. n, t, L, p, aa, ab, ac, ad, ae, etc). If someone can tell me why, that would be very helpful.
#include <windows.h>
#include <tchar.h>
#include <wincrypt.h>
#define KEYLENGTH 0x00800000
#define ENCRYPT_BLOCK_SIZE 8
bool MyDecryptFile(LPTSTR szSource,LPTSTR szDestination,LPTSTR szPassword);
bool MyEncryptFile(LPTSTR szSource,LPTSTR szDestination,LPTSTR szPassword);
int _tmain(int argc, _TCHAR* argv[])
{
LPTSTR pszSource = L"c:\\temp\\source.txt";
LPTSTR pszDestination = L"c:\\temp\\dest.txt";
LPTSTR pszPassword = L"t";
if (MyEncryptFile(pszSource, pszDestination, pszPassword))
{
_tprintf(TEXT("Encryption of the file %s was successful. \n"),pszSource);
_tprintf(TEXT("The encrypted data is in file %s.\n"),pszDestination);
}
if (MyDecryptFile(pszSource, pszDestination, pszPassword))
{
_tprintf(TEXT("Encryption of the file %s was successful. \n"),pszSource);
_tprintf(TEXT("The encrypted data is in file %s.\n"),pszDestination);
}
return 0;
}
bool MyEncryptFile(LPTSTR pszSourceFile,LPTSTR pszDestinationFile,LPTSTR pszPassword)
{
bool fReturn = false;
HANDLE hSourceFile = INVALID_HANDLE_VALUE, hDestinationFile = INVALID_HANDLE_VALUE;
HCRYPTPROV hCryptProv = NULL;
HCRYPTKEY hKey = NULL, hXchgKey = NULL;
HCRYPTHASH hHash = NULL;
PBYTE pbBuffer = NULL;
DWORD dwBlockLen, dwBufferLen, dwCount;
hSourceFile = CreateFile(pszSourceFile,FILE_READ_DATA,FILE_SHARE_READ,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,NULL);
if (INVALID_HANDLE_VALUE == hSourceFile)
goto Exit_MyEncryptFile;
hDestinationFile = CreateFile(pszDestinationFile,FILE_WRITE_DATA,FILE_SHARE_READ,NULL,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL,NULL);
if (INVALID_HANDLE_VALUE == hDestinationFile)
goto Exit_MyEncryptFile;
CryptAcquireContext(&hCryptProv,NULL,MS_ENH_RSA_AES_PROV,PROV_RSA_AES,0);
CryptCreateHash(hCryptProv,CALG_SHA_256,0,0,&hHash);
CryptHashData(hHash,(BYTE *)pszPassword,lstrlen(pszPassword),0);
CryptDeriveKey(hCryptProv,CALG_AES_256,hHash,CRYPT_EXPORTABLE,&hKey);
dwBlockLen = 1000 - 1000 % ENCRYPT_BLOCK_SIZE;
if (ENCRYPT_BLOCK_SIZE > 1)
dwBufferLen = dwBlockLen + ENCRYPT_BLOCK_SIZE;
else
dwBufferLen = dwBlockLen;
pbBuffer = (BYTE *)malloc(dwBufferLen);
bool fEOF = FALSE;
do
{
if (ReadFile(hSourceFile,pbBuffer,dwBlockLen,&dwCount,NULL))
{
if (dwCount < dwBlockLen)
fEOF = TRUE;
if (CryptEncrypt(hKey,NULL,fEOF,0,pbBuffer,&dwCount,dwBufferLen))
WriteFile(hDestinationFile,pbBuffer,dwCount,&dwCount,NULL);
}
}
while (!fEOF);
fReturn = true;
Exit_MyEncryptFile:
if (hSourceFile) CloseHandle(hSourceFile);
if (hDestinationFile) CloseHandle(hDestinationFile);
if (pbBuffer) free(pbBuffer);
if (hHash) {CryptDestroyHash(hHash);hHash = NULL;}
if (hKey) CryptDestroyKey(hKey);
if (hCryptProv) CryptReleaseContext(hCryptProv, 0);
return fReturn;
}
bool MyDecryptFile(LPTSTR pszSourceFile,LPTSTR pszDestinationFile,LPTSTR pszPassword)
{
bool fReturn = false;
HANDLE hSourceFile = INVALID_HANDLE_VALUE, hDestinationFile = INVALID_HANDLE_VALUE;
HCRYPTKEY hKey = NULL;
HCRYPTHASH hHash = NULL;
HCRYPTPROV hCryptProv = NULL;
PBYTE pbBuffer = NULL;
DWORD dwCount, dwBlockLen, dwBufferLen;
hSourceFile = CreateFile(pszDestinationFile,FILE_READ_DATA,FILE_SHARE_READ,NULL,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL,NULL);
if (INVALID_HANDLE_VALUE == hSourceFile)
goto Exit_MyDecryptFile;
hDestinationFile = CreateFile(pszSourceFile,FILE_WRITE_DATA,FILE_SHARE_READ,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,NULL);
if (INVALID_HANDLE_VALUE == hDestinationFile)
goto Exit_MyDecryptFile;
CryptAcquireContext(&hCryptProv,NULL,MS_ENH_RSA_AES_PROV,PROV_RSA_AES,0);
CryptCreateHash(hCryptProv,CALG_SHA_256,0,0,&hHash);
CryptHashData(hHash,(BYTE *)pszPassword,lstrlen(pszPassword),0);
CryptDeriveKey(hCryptProv,CALG_AES_256,hHash,CRYPT_EXPORTABLE,&hKey);
dwBlockLen = 1000 - 1000 % ENCRYPT_BLOCK_SIZE;
dwBufferLen = dwBlockLen;
pbBuffer = (PBYTE)malloc(dwBufferLen);
bool fEOF = false;
do
{
if (!ReadFile(hSourceFile,pbBuffer,dwBlockLen,&dwCount,NULL))
goto Exit_MyDecryptFile;
if (dwCount <= dwBlockLen)
fEOF = TRUE;
LONG rv = CryptDecrypt(hKey,0,fEOF,0,pbBuffer,&dwCount);
if (rv==0)
{
DWORD dwErr = GetLastError(); // <--- fails if password and string are n, t, L, p, aa, ab, ac, ad , ae
goto Exit_MyDecryptFile;
}
if (!WriteFile(hDestinationFile,pbBuffer,dwCount,&dwCount,NULL))
goto Exit_MyDecryptFile;
}
while (!fEOF);
fReturn = true;
Exit_MyDecryptFile:
if (pbBuffer) free(pbBuffer);
if (hSourceFile) CloseHandle(hSourceFile);
if (hDestinationFile) CloseHandle(hDestinationFile);
if (hHash) {CryptDestroyHash(hHash);hHash = NULL;}
if (hKey) CryptDestroyKey(hKey);
if (hCryptProv) CryptReleaseContext(hCryptProv, 0);
return fReturn;
}
What about using this to get the KP_IV option?
BOOL bRV;
bRV = CryptAcquireContextW(&hCryptProv, NULL, MS_ENH_RSA_AES_PROV, PROV_RSA_AES, 0);
bRV = CryptGenKey(hCryptProv, CALG_AES_256,0,&hKey);
DWORD dwMode = CRYPT_MODE_CBC;
bRV = CryptSetKeyParam(hKey,KP_MODE,(BYTE*)&dwMode,0);
BYTE pbData[16];
memcpy(pbData,"n",sizeof("n")); // <--- Hard coded password
bRV = CryptSetKeyParam(hKey,KP_IV,pbData,0);
enter code here

Answer 1

If you want to obtain different cypertext when encrypting the same plaintext with the same key, you have to use the CBC mode of operation:
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
In order to correctly encrypt with CBC, you need to generate a different random Initialization Vector (IV) every time.
In order to decrypt, you need to know the IV used during encryption.
So, the IV must be associated (in clear) to the cyphertext.
In reference to your example, when calling the CryptDeriveKey function, the CBC is the default mode but it uses an IV set to zero and this invalidates the utility of the CBC operating mode:
https://msdn.microsoft.com/en-us/library/windows/desktop/aa379916(v=vs.85).aspx
In order to set the random IV you need to call the CryptSetKeyParam function, which accept the KP_IV param:
https://msdn.microsoft.com/en-us/library/windows/desktop/aa380272(v=vs.85).aspx
Bye
Giovanni