AES IV into file with FileSource and FileSink - c++

Question

I have the need to crypt big files (multi GB) with crypto++. I managed to find an example on the documentation that helped me create the 2 followings functions :
bool AESEncryptFile(const std::string& clearfile, const std::string& encfile, const std::string& key) {
try {
byte iv[CryptoPP::AES::BLOCKSIZE] = {};
CryptoPP::CBC_Mode<CryptoPP::AES>::Encryption encryptor;
encryptor.SetKeyWithIV((unsigned char*)key.c_str(), CryptoPP::AES::DEFAULT_KEYLENGTH, iv);
CryptoPP::StreamTransformationFilter filter(encryptor);
CryptoPP::FileSource source(clearfile.c_str(), false);
CryptoPP::FileSink sink(encfile.c_str());
source.Attach(new CryptoPP::Redirector(filter));
filter.Attach(new CryptoPP::Redirector(sink));
const CryptoPP::word64 BLOCK_SIZE = 4096;
CryptoPP::word64 processed = 0;
while (!EndOfFile(source) && !source.SourceExhausted()) {
source.Pump(BLOCK_SIZE);
filter.Flush(false);
processed += BLOCK_SIZE;
}
filter.MessageEnd();
return true;
} catch (const CryptoPP::Exception& ex) {
return false;
}
}
bool AESDecryptFile(const std::string& encfile, const std::string& clearfile, const std::string& key) {
try {
byte iv[CryptoPP::AES::BLOCKSIZE] = {};
CryptoPP::CBC_Mode<CryptoPP::AES>::Decryption decryptor;
decryptor.SetKeyWithIV((unsigned char*)key.c_str(), CryptoPP::AES::DEFAULT_KEYLENGTH, iv);
CryptoPP::StreamTransformationFilter filter(decryptor);
CryptoPP::FileSource source(encfile.c_str(), false);
CryptoPP::FileSink sink(clearfile.c_str());
source.Attach(new CryptoPP::Redirector(filter));
filter.Attach(new CryptoPP::Redirector(sink));
const CryptoPP::word64 BLOCK_SIZE = 4096;
CryptoPP::word64 processed = 0;
while (!EndOfFile(source) && !source.SourceExhausted()) {
source.Pump(BLOCK_SIZE);
filter.Flush(false);
processed += BLOCK_SIZE;
}
.
filter.MessageEnd();
return true;
} catch (const CryptoPP::Exception& ex) {
return false;
}
}
This is working great. On 8 GB files i'm using very little memory.
But as you can see the IV is (empty for now) hardcoded and i would like to :
While encrypting , put it a the end of the file.
While decrypting : get the IV from the file to init the decryptor.
Is there a way to do that with crypto++ or should i handle it manually after/before the enc/decryption process ?

Answer 1

Thanks to all the differents comments here is what i managed to do. As suggested by #Sam Mason i put the iv at the beginning of the file :
So before starting to encrypt i 'm putting the iv at the beginning of the file:
CryptoPP::ArraySource(iv, sizeof(iv), true,
new CryptoPP::Redirector(sink)
);
// Encrypt
And then when decrypting i'm getting the IV back like this :
unsigned char iv[CryptoPP::AES::BLOCKSIZE];
CryptoPP::ArraySink ivSink(iv, sizeof(iv));
source.Attach(new CryptoPP::Redirector(ivSink));
source.Pump(CryptoPP::AES::BLOCKSIZE);
// Decrypt
Note for future reader : Don't use an empty IV like show in my OP , instead generate one randomly , for example :
CryptoPP::AutoSeededRandomPool prng;
unsigned char iv[CryptoPP::AES::BLOCKSIZE];
prng.GenerateBlock(iv, sizeof(iv));