CFB mode: similar texts yield similar results?

[Roland Bock]

Hi,

I want to use AES encryption in CFB mode. For experiments, I am using 
the attached program. It generates a random key and an initialization 
vector. It then encrypts all command line arguments and displays the HEX 
encoded encrypted strings. Here is what it looks like:

$ testEncryptor 1111 1112 1113 1114
1111: E2B2334F
1112: E2B2334C
1113: E2B2334D
1114: E2B2334A

The encrypted results are almost identical.

I am aware that I am using the same IV for all arguments, but still I 
would have expected similar input would NOT result in similar output. I 
meam, the four input values differ in just one Byte and same is true for 
their encrypted version? Somehow it seems that encryption is done Byte 
by Byte. Can I change that?


Thanks and regards,

Roland
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http://www.cryptopp.com.// System includes
#include <iostream>

// Library includes
#include <cryptopp/aes.h>
#include <cryptopp/modes.h>
#include <cryptopp/filters.h>
#include <cryptopp/osrng.h>
#include <cryptopp/hex.h>

using namespace std;
using namespace CryptoPP;

int main(int argc, char* argv[])
{
   if (argc < 2)
   {
      cerr << "Usage: " << argv[0] << " {<strings to be encrypted>}" << endl;
      return 1;
   }


   AutoSeededRandomPool rnd;

   // Generate a random key
   byte key[AES::DEFAULT_KEYLENGTH];
   rnd.GenerateBlock(key, AES::DEFAULT_KEYLENGTH);

   // Generate a random IV
   byte iv[AES::BLOCKSIZE];
   rnd.GenerateBlock(iv, AES::BLOCKSIZE);


   for (int i = 1; i < argc; ++i)
   {
      string cipherText;
      CFB_Mode<AES>::Encryption encryptor( key, sizeof(key), iv);
      StringSource encryptionFilter(argv[i], true, new StreamTransformationFilter(encryptor, new HexEncoder(new StringSink(cipherText))));
      encryptionFilter.PumpAll();

      cout << argv[i] << ": " << cipherText << endl;
   }

   return 0;
}