Thanks for your answer,

Your statement about security makes sense, the thing is that this code was 
written 20 years ago by some developer and I do not know his 
intention/philosophy behind this encryption/decryption mechanism.
With that being said, fixing the current construction, which is considered 
dangerous according to your story, is sufficient. At least for now, since 
my only 'task' is to have our project running on various platforms 
(Android/iOS).

One final request; could you help me write the code for the HMAC? I barely 
know the cryptopp api and I know even less about cryptography.
I figured I need something like the following,

CryptoPP::HMAC<CryptoPP::SHA256> hmac(pKey, dwKeySize);
CryptoPP::StringSource ss(m_bufSeed, true,
  new CryptoPP::HashFilter(hmac,
  new CryptoPP::StringSink(hmac)
      )
);



Op woensdag 21 september 2016 15:00:04 UTC+2 schreef jean-pierre.muench:
>
> The issue you seem to have is that the RNG is, well, actually producing 
> random results.
>
> If you want deterministic behavior, you should avoid RNGs if possible.
>
> To clarify, I'm trying to draw a picture of the old IV generation:
>
> Key -> SHA256 -> RandomPool -> SHA256 -> XOR "compression" -> IV
>   |                             |
>   +---------------------+
>
> From what I can see, your implementation always tries to produce a static 
> IV, which is only dependent on the key. This is only fine if you never 
> re-use the key, e.g. you never instantiate (and use) a second instance with 
> the same key (for encryption). Otherwise this is an IV re-use which is 
> horribly broken and allows for really easy attacks.
>
> Another issue that I see in this code is the complete absence of 
> authentication. If an attacker manipulates the cipher text you've no chance 
> of (cryptographically reliably) detecting it. The fix would be to use a 
> mode like GCM or EAX.
>
> To fix your current construction (and get the same dangerous behavior 
> back), you can replace RandomPool with HMAC<SHA256> and feed the key in as 
> the key and the hashed key as the message.
>
> To actually fix the problems, you should use
>
>    1. Authenticated encryption 
>    2. Explicitly or implicitly set a new IV / nonce for each message 
>    (either by using a deterministic counter or by generating the IV / nonce 
> at 
>    random) 
>
> Here are the related wiki pages for EAX and GCM
>
>    - https://cryptopp.com/wiki/EAX_Mode 
>    - https://cryptopp.com/wiki/GCM_Mode
>    
> BR
>
> JPM
>
> Am 21.09.2016 um 11:06 schrieb edwin schriek:
>
> Hi, 
>
> We are updating a project which uses the ancient cryptopp 4.2, to the 
> latest version in order to have better cross platform support (4.2 suffers 
> from undefined behaviour under platforms like Android/iOS).
> In this project, we use AES to decrypt/encrypt some data, nothing to fancy.
>
> Now, the problem when moving to the newer version lies in generation the 
> IV.
> The code used to generate our IV is as follows:
>
> BOOL CCryptorAES101::Init(BYTE* pKey, DWORD dwKeySize)
> {
>  ASSERT(pKey);
>
>
>  m_bInit = true;
>
>
>  try
>  {
>  CryptoPP::SHA256         hash;
>  CryptoPP::RandomPool     rng;
>  CryptoPP::MySecByteBlock bufSeedIV(IV_SEED_SIZE);
>
>
>  m_bufSeed.Resize(hash.DigestSize());
>  m_bufIV  .Resize(IV_SIZE);
>
>
>  hash.CalculateDigest(m_bufSeed,pKey,dwKeySize);
>  rng.Put(pKey,dwKeySize);
>  rng.GenerateBlock(bufSeedIV,bufSeedIV.Size());
>
>
>  CryptoPP::MySecByteBlock bufHash(hash.DigestSize());
>
>
>  hash.CalculateDigest(bufHash,bufSeedIV.Begin(),bufSeedIV.Size());
>
>
>  ASSERT(2 * m_bufIV.Size() == bufHash.Size());
>
>
>  for (int i = 0; i < m_bufIV.Size(); i++)
>  m_bufIV[i] = bufHash[i] ^ bufHash[i + 16];
>
>
>  }
>  catch (CryptoPP::Exception const& e)
>  {
>  UNUSED_ALWAYS(e);
>
>
>  m_bInit = false;
>  
>  return false;
>  }
>
>
>  return true;
> }
>
>
> The following code does the actual decryption:
>
> BOOL CCryptorAES101::Decrypt(BYTE* pData, DWORD dwSize)
> {
>  if (!m_bInit)
>  return FALSE;
>
>
>  try
>  {
> #if CRYPTO_VER == 42
>  CryptoPP::AESEncryption aes(m_bufSeed,m_bufSeed.Size());
>  CryptoPP::CFBDecryption decryptor(aes,m_bufIV);
> #else
>  CryptoPP::AES::Encryption                     aes(m_bufSeed,m_bufSeed.
> size());
>  CryptoPP::CFB_Mode_ExternalCipher::Decryption decryptor(aes,m_bufIV);
> #endif
>  decryptor.ProcessString(pData,dwSize);
>  }
>  catch (CryptoPP::Exception const& e)
>  {
>  UNUSED_ALWAYS(e);
>
>
>  return FALSE;
>  }
>
>
>  return TRUE;
> }
>
>
>
> Under cryptopp 4.2, bufSeedIV is always the same, resulting in a bufHash 
> and m_bufIV which are always the same. 
> Under cryptopp 5.6.4 however, bufSeedIV is always different, this is our 
> problem.
>
> As far as I can figure, the RandomPool implementation changed over time, 
> but I could be wrong.
>
> Any guidance would be appreciated!
>
> Kind regards,
>
> Edwin
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