Crypto++ CTR mode manual implement - crypto++

Question

I am trying to make CTR manually on top of ECB mode (but still) using Crypto++.
The idea is:
For single block: Just use ECB For multiple block, use CTR algorithm
(AFAIK):
//We have n block of plain data -> M
PlainData M[n];
key;
iv;
char *CTR;
cipher ="";
for(i = 0; i<n; i++ ){
if(i ==0){
CTR = iv;
}
ei = encryptECB(CTR + i)
cipherI = xor(ei, M[i])
cipher += cipherI;
}
//My xor() to XOR two char array
void xor(char *s1, char* s2, char *& result, int len){
try{
int i;
for (i = 0; i < len; i++){
int u = s1[i] ^ s2[i];
result[i] = u;
}
result[i] = '\0';
}
catch (...){
cout << "Errp";
}
}
Test 1: 100% Crypto++ CTR
string auto_ctr(char * s1, long size){
CTR_Mode< AES >::Encryption e;
e.SetKeyWithIV(key, sizeof(key), iv, sizeof(iv));
string cipherZ;
StringSource s(s1, true,
new StreamTransformationFilter(e,
new StringSink(cipherZ), BlockPaddingSchemeDef::BlockPaddingScheme::NO_PADDING
)
);
return cipherZ;
}
Test 2: Manual CTR based on ECB
string encrypt(char* s1, int size){
ECB_Mode< AES >::Encryption e;
e.SetKey(key, size);
string cipher;
string s(s1, size);
StringSource ss1(s, true,
new StreamTransformationFilter(e,
new StringSink(cipher), BlockPaddingSchemeDef::BlockPaddingScheme::NO_PADDING
) // StreamTransformationFilter
); // StringSource
return cipher;
}
static string manual_ctr(char *plain, long &size){
int nBlocks = size / BLOCK_SIZE;
char* encryptBefore = new char[BLOCK_SIZE];
char *ci = new char[BLOCK_SIZE] ;
string cipher;
for (int i = 0; i < nBlocks; i++){
//If the first loop, CTR = IV
if (i == 0){
memcpy(encryptBefore, iv, BLOCK_SIZE);
}
encryptBefore[BLOCK_SIZE] = '\0';
memcpy(encryptBefore, encryptBefore + i, BLOCK_SIZE);
char *buffer = new char[BLOCK_SIZE];
memcpy(buffer, &plain[i], BLOCK_SIZE);
buffer[BLOCK_SIZE] = '\0';
//Encrypt the CTR
string e1 = encrypt(encryptBefore, BLOCK_SIZE);
//Xor it with m[i] => c[i]
xor((char*)e1.c_str(), buffer, ci, BLOCK_SIZE);
//Append to the summary cipher
/*for (int j = 0; j < BLOCK_SIZE/2; j++){
SetChar(cipher, ci[j], i*BLOCK_SIZE + j);
}*/
cipher += ci;
//Set the cipher back to iv
//memcpy(encryptBefore, ci, BLOCK_SIZE);
}
return cipher;
}
And this is Main for testing:
void main(){
long size = 0;
char * plain = FileUtil::readAllByte("some1.txt", size);
string auto_result = auto_ctr(plain, size);
string manual_result = manual_ctr(plain, size);
getchar();
}
The auto_result is:
"Yž+eÞsÂÙ\bü´\x1a¨Ü_ÙR•L¸Ð€¦å«ÎÍÊ[w®Ÿg\fT½\ý7!p\r^ÍdžúP\bîT\x3\x1cZï.s%\x1ei{ÚMˆØ…Pä¾õ\x46\r5\tâýï‚ú\x16ç’Qiæ²\x15š€á^ªê]W
ÊNqdŒ¥ ˆ†¾j%8.Ìù\x6Þ›ÔÏ’[c\x19"
The manual_result is:
"Yž+eÞsÂÙ\bü´\x1a¨Ü_Ù·\x18ýuù\n\nl\x11Á\x19À†Žaðƒºñ®GäþŽá•\x11ÇYœf+^Q\x1a\x13B³‘QQµºëÑÌåM\"\x12\x115â\x10¿Ô„›s°‰=\x18*\x1c:²IF'n#ŠŠ¾mGÂzõžÀ\x1eÏ\SëYU¼í‘"
>
What is the problem with my implement?

Answer 1

Since your first block seems to be working fine, I've only searched for problems in the management of the counter itself and here is what seems me wrong :
memcpy(encryptBefore, encryptBefore + i, BLOCK_SIZE);
Here you are trying to increment your IV i times, I presume, but this is not what happens, what you do is trying to copy into your encryptBefore pointer the content of the encryptBefore+i pointer spanning over BLOCK_SIZE bytes. This is not at all incrementing the IV, but it works for the first block because then i=0.
What you want to do is actually creating a big integer using CryptoPP::Integer to use as an IV and increment that integer and then convert it into a byte array using the Encode(byte *output, size_t outputLen, Signedness sign=UNSIGNED) const function from the CryptoPP Integer class when you need to use bytes instead of integers.
Ps: when performing i/o operations, I recommend you to use hexadecimal strings, take a look at the CryptoPP::HexEncoder and HexDecoder classes, they both are well documented on CryptoPP wiki.