How to solve NTE_BAD_DATA using CryptDecrypt at final step - c++

Question

I am working on a encryption algo with c++ using WINAPI. My encryption works flawless and my decryption too untill the last chunk to be decrypted with final = TRUE. I got the NTE_BAD_DATA error.
PS : I manualy check the buffer and the decryption works fine untill the last CryptDecrypt.
If someone have an idea, kindly help me :)
Here is my code :
PVOID test(PVOID buffer, DWORD* length, PCHAR key_str2,bool isdecrypt) {
CHAR default_key[] = "3igcZhRdWq96m3GUmTAiv9";
CHAR* key_str = default_key;
size_t len = lstrlenA(key_str);
DWORD dwStatus = 0;
BOOL bResult = FALSE;
wchar_t info[] = L"Microsoft Enhanced RSA and AES Cryptographic Provider";
HCRYPTPROV hProv;
if (!CryptAcquireContextW(&hProv, NULL, info, PROV_RSA_AES, CRYPT_VERIFYCONTEXT)) {
dwStatus = GetLastError();
CryptReleaseContext(hProv, 0);
return 0;
}
HCRYPTHASH hHash;
if (!CryptCreateHash(hProv, CALG_SHA_256, 0, 0, &hHash)) {
dwStatus = GetLastError();
CryptReleaseContext(hProv, 0);
return 0;
}
if (!CryptHashData(hHash, (BYTE*)key_str, len, 0)) {
DWORD err = GetLastError();
return 0;
}
HCRYPTKEY hKey;
if (!CryptDeriveKey(hProv, CALG_AES_128, hHash, 0, &hKey)) {
dwStatus = GetLastError();
CryptReleaseContext(hProv, 0);
return 0;
}
const size_t chunk_size = CHUNK_SIZE;
BYTE chunk[chunk_size] = { 0 };
DWORD out_len = 0;
BOOL isFinal = FALSE;
DWORD readTotalSize = 0;
DWORD inputSize = *length;
PVOID newBuff = VirtualAlloc(0, inputSize, MEM_COMMIT, PAGE_READWRITE);
while (true)
{
if (readTotalSize + chunk_size >= inputSize)
{
isFinal = TRUE;
memcpy(chunk, PVOID((DWORD)buffer + readTotalSize), inputSize - readTotalSize);
out_len = inputSize - readTotalSize;
}
else {
memcpy(chunk, PVOID((DWORD)buffer + readTotalSize), chunk_size);
out_len = chunk_size;
}
if (isdecrypt) {
if (!CryptDecrypt(hKey, NULL, isFinal, 0, chunk, &out_len)) {
int a = GetLastError();
break;
}
}
else {
if (!CryptEncrypt(hKey, NULL, isFinal, 0, chunk, &out_len, chunk_size)) {
break;
}
}
if (readTotalSize + chunk_size >= inputSize) {
memcpy(PVOID((DWORD)newBuff + readTotalSize), chunk, inputSize - readTotalSize);
readTotalSize += inputSize - readTotalSize;
}
else {
memcpy(PVOID((DWORD)newBuff + readTotalSize), chunk, chunk_size);
readTotalSize += chunk_size;
}
if (isFinal)
break;
memset(chunk, 0, chunk_size);
}
CryptReleaseContext(hProv, 0);
CryptDestroyKey(hKey);
CryptDestroyHash(hHash);
return newBuff;
}

Answer 1

You are not storing off the encryption the final block correctly.
The amount of data you pass into the encryption is in out_len, and the amount of data that is encrypted is put back into out_len. For a block cypher that doesn't matter for whole blocks, but your last block which is less than a whole block being passed into encryption probably creates a whole block of encrypted data.
This means that your encrypted data is probably a little larger than your plain text in the final block, but you make no effort to handle this so on when you decrypt you have an issue as you've dropped some of the encrypted data on the floor.
Use the value put into out_len by the encryption function to store off your data (as you overwrite your data stream you'll need to make sure there's space though)