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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