Storing the IV with the ciphertext Crypto++ CBC AES encryption

Question

I\'m trying to encrypt(and decrypt after) a file using AES in CBC mode and Crypto++ library

Here\'s what I already did:

using namespace CryptoPP;
Aut         


        

Answer 1

Storing the IV with the ciphertext Crypto++ CBC AES encryption

Some of the code you are using is a bit unusual to me. I'll pick a couple things out and show you some of the Crypto++ ways.

Before you begin, take a look at Pipelines and Pumping Data on the Crypto++ wiki. Remember that data flows from sources to sinks. In between the data encounters filters which transform the data.

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std::ifstream fin(file_path, std::ios::binary);
if (!fin)
{
    std::cout << "error";
}
std::ostringstream ostrm;
ostrm << fin.rdbuf();
std::string plaintext(ostrm.str());
fin.close();

A Crypto++ FileSource has a constructor that takes a std::istream. You could do something like the following. Also see FileSource on the Crypto++ wiki.

std::ifstream fin(file_path, std::ios::binary);
FileSource source(fin, true /*pump all*/, NULLPTR);
...

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AES::Encryption aesEncryption(key, CryptoPP::AES::MAX_KEYLENGTH);
CBC_Mode_ExternalCipher::Encryption cbcEncryption(aesEncryption, iv);

ExternalCipher are for the FIPS DLL. You can use them without the DLL, but they exist for the DLL. Usually you use:

 CBC_Mode::Encryption encryptor;

Also, you usually want to avoid a confidentiality-only mode. Typically you want to use an Authenticated Encryption mode of operation. It provides confidentiality and authenticity.

Crypto++ provides CCM, EAX and GCM authenticated encryption modes of operation. OCB and EAX are very good choices. EAX mode is documented at EAX Mode on the Crypto++ wiki. OCB is not available at the moment. We are getting ready to check-in OCB mode.

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Now,i want to write the encrypted string to a file,and store the IV with it,since the iv doesn't need to be kept secret,ideally at the beginning or the end of the ciphertext

Use something like the following. I did not compile it, so you will need to fix the typos.

AutoSeededRandomPool prng;
SecByteBlock key(AES::MAXIMUM_KEYLENGTH), iv(AES::BLOCKSIZE);

RandomNumberSource rs1(prng, AES::MAXIMUM_KEYLENGTH, new ArraySink(key, key.size()));
RandomNumberSource rs2(prng, AES::BLOCKSIZE, new ArraySink(iv, iv.size()));

HexEncoder encoder(new FileSink(std::cout));

std::cout << "Key: ";
encoder.Put(key, key.size());
encoder.MessageEnd();
std::cout << std::endl;

std::cout << "IV: ";
encoder.Put(iv, iv.size());
encoder.MessageEnd();
std::cout << std::endl;

EAX::Encryption encryptor;
encryptor.SetKeyWithIV(key, key.size(), iv, iv.size());

// Plaintext message
std::string message;

// Output file
FileSink file("message.enc");

// Source wrappers
ArraySource as(iv, iv.size(), true,
    new Redirector(file));

// Source wrapper
StringSource ss(message, true,
    new StreamTransformationFilter(encryptor,
       new Redirector(file)));

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When i will try to decrypt this file,how can i extract the iv and ciphertext separately ?

Use something like the following.

// Key is from previous example. It cannot change
SecByteBlock key(AES::MAXIMUM_KEYLENGTH), iv(AES::BLOCKSIZE);    
FileSource fs("message.enc", false /* DO NOT Pump All */);

// Attach new filter
ArraySink as(iv, iv.size());
fs.Attach(new Redirector(as));
fs.Pump(AES::BLOCKSIZE);  // Pump first 16 bytes

EAX::Decryption decryptor;
decryptor.SetKeyWithIV(key, key.size(), iv, iv.size());

// Detach previously attached filter, attach new filter
ByteQueue queue;
fs.Detach(new StreamTransformationFilter(decryptor, new Redirector(queue)));
fs.PumpAll();  // Pump remainder of bytes

The encrypted data will be in a ByteQueue. It does not provide C++ iterator-like functionality, like a pointer and a size. To get the data out of the ByteQueue you transfer it or copy it to another filter or a sink:

SecByteBlock block(queue.MaxRetrievable());
ArraySink sink(block, block.size());
queue.TransferTo(sink);

You can get the data out of the ByteQueue and put it in astd::string with:

std::string recovered;
StringSink sink(recovered);
queue.TransferTo(sink);

And you can print the IV recovered from the file with:

HexEncoder encoder(new FileSink(std::cout));

std::cout << "IV: ";
encoder.Put(iv, iv.size());
encoder.MessageEnd();
std::cout << std::endl;