Manual CBC encryption handing with Crypto++ - crypto++

Question

I am trying to play around with a manual encryption in CBC mode but still use Crypto++, just to know can I do it manually.
The CBC algorithm is (AFAIK):
Presume we have n block K[1]....k[n]
0. cipher = empty;
1. xor(IV, K1) -> t1
2. encrypt(t1) -> r1
3. cipher += r1
4. xor (r1, K2) -> t2
5. encrypt(t2) -> r2
6. cipher += r2
7. xor(r2, K3)->t3
8. ...
So I tried to implement it with Crypto++. I have a text file with alphanumeric characters only. Test 1 is read file chunk by chunk (16 byte) and encrypt them using CBC mode manually, then sum up the cipher. Test 2 is use Crypto++ built-in CBC mode.
Test 1
char* key;
char* iv;
//Iterate in K[n] array of n blocks
BSIZE = 16;
std::string vectorToString(vector<char> v){
string s ="";
for (int i = 0; i < v.size(); i++){
s[i] = v[i];
}
return s;
}
vector<char> xor( vector<char> s1, vector<char> s2, int len){
vector<char> r;
for (int i = 0; i < len; i++){
int u = s1[i] ^ s2[i];
r.push_back(u);
}
return r;
}
vector<char> byteToVector(byte *b, int len){
vector<char> v;
for (int i = 0; i < len; i++){
v.push_back( b[i]);
}
return v;
}
string cbc_manual(byte [n]){
int i = 0;
//Open a file and read from it, buffer size = 16
// , equal to DEFAULT_BLOCK_SIZE
std::ifstream fin(fileName, std::ios::binary | std::ios::in);
const int BSIZE = 16;
vector<char> encryptBefore;
//This function will return cpc
string cpc ="";
while (!fin.eof()){
char buffer[BSIZE];
//Read a chunk of file
fin.read(buffer, BSIZE);
int sb = sizeof(buffer);
if (i == 0){
encryptBefore = byteToVector( iv, BSIZE);
}
//If i == 0, xor IV with current buffer
//else, xor encryptBefore with current buffer
vector<char> t1 = xor(encryptBefore, byteToVector((byte*) buffer, BSIZE), BSIZE);
//After xored, encrypt the xor result, it will be current step cipher
string r1= encrypt(t1, BSIZE).c_str();
cpc += r1;
const char* end = r1.c_str() ;
encryptBefore = stringToVector( r1);
i++;
}
return cpc;
}
This is my encrypt() function, because we have only one block so I use ECB (?) mode
string encrypt(string s, int size){
ECB_Mode< AES >::Encryption e;
e.SetKey(key, size);
string cipher;
StringSource ss1(s, true,
new StreamTransformationFilter(e,
new StringSink(cipher)
) // StreamTransformationFilter
); // StringSource
return cipher;
}
And this is 100% Crypto++ made solution:
Test 2
encryptCBC(char * plain){
CBC_Mode < AES >::Encryption encryption(key, sizeof(key), iv);
StreamTransformationFilter encryptor(encryption, NULL);
for (size_t j = 0; j < plain.size(); j++)
encryptor.Put((byte)plain[j]);
encryptor.MessageEnd();
size_t ready = encryptor.MaxRetrievable();
string cipher(ready, 0x00);
encryptor.Get((byte*)&cipher[0], cipher.size());
}
Result of Test 1 and Test 2 are different. In the fact, ciphered text from Test 1 is contain the result of Test 2. Example:
Test 1's result aaa[....]bbb[....]ccc[...]...
Test 2 (Crypto++ built-in CBC)'s result: aaabbbccc...
I know the xor() function may cause a problem relate to "sameChar ^ sameChar = 0", but is there any problem relate to algorithm in my code?
This is my Test 2.1 after the 1st solution of jww.
static string auto_cbc2(string plain, long size){
CBC_Mode< AES >::Encryption e;
e.SetKeyWithIV(key, sizeof(key), iv, sizeof(iv));
string cipherText;
CryptoPP::StringSource ss(plain, true,
new CryptoPP::StreamTransformationFilter(e,
new CryptoPP::StringSink(cipherText)
, BlockPaddingSchemeDef::NO_PADDING
) // StreamTransformationFilter
); // StringSource
return cipherText;
}
It throw an error:
Unhandled exception at 0x7407A6F2 in AES-CRPP.exe: Microsoft C++
exception: CryptoPP::InvalidDataFormat at memory location 0x00EFEA74
I only got this error when use BlockPaddingSchemeDef::NO_PADDING, tried to remove BlockPaddingSchemeDef or using BlockPaddingSchemeDef::DEFAULT_PADDING, I got no error . :?

Answer 1

StringSource ss1(s, true,
new StreamTransformationFilter(e,
new StringSink(cipher)));
This uses PKCS padding by default. It takes a 16-byte input and produces a 32-byte output due to padding. You should do one of two things.
First, you can use BlockPaddingScheme::NO_PADDING. Something like:
StringSource ss1(s, true,
new StreamTransformationFilter(e,
new StringSink(cipher)
BlockPaddingScheme::NO_PADDING));
Second, you can process blocks manually, 16 bytes at a time. Something like:
AES::Encryption encryptor(key, keySize);
byte ibuff[<some size>] = ...;
byte obuff[<some size>];
ASSERT(<some size> % AES::BLOCKSIZE == 0);
unsigned int BLOCKS = <some size>/AES::BLOCKSIZE;
for (unsigned int i=0; i<BLOCKS; i==)
{
encryptor.ProcessBlock(&ibuff[i*16], &obuff[i*16]);
// Do the CBC XOR thing...
}
You may be able to call ProcessAndXorBlock from the BlockCipher base class and do it in one shot.