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Your problem is due to the
fact that your "automatic" stream decryptor is reading the file from
the very beginning. And due to the error in configuring encryptor, the
first block can't be decrypted properly. See the attached source code for correction. I've tested the corrected code under Cygwin with Crypto++ v5.5.1 and it worked properly. Rash wrote: Ok here is what I have been trying, Now the problem is I'm unable to retrieve the first 5 bytes. If the input is "The library is an powerful and elegant tool for performing complex cryptography" I get the output as "}ý1/4 tÛ=rary is an powerful and elegant tool for performing complex cryptography" --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the "Crypto++ Users" Google Group. To unsubscribe, send an email to [EMAIL PROTECTED] More information about Crypto++ and this group is available at http://www.cryptopp.com. -~----------~----~----~----~------~----~------~--~--- |
#include <iostream>
#include <cryptopp/aes.h>
#include <cryptopp/modes.h>
#include <cryptopp/sha.h>
#include <cryptopp/osrng.h>
#include <cryptopp/files.h>
#include <cryptopp/filters.h>
USING_NAMESPACE(CryptoPP);
USING_NAMESPACE(std);
/**************************************************************/
/* Code to illustrate usage of cryptographic primitives */
/* provided by Crypto++ library v5.5.1 of Wei Dai. */
/* */
/* In particular, the program provides two functions: */
/* */
/* 1. Encrypt a file (key based on password) with random IV;*/
/* */
/* 2. Decrypt previously encrypted file. */
/* */
/* Issue with the original code: */
/* - set IV in the encryptor but also used it as random */
/* prefix, so garbage was the 1st encrypted block; */
/* - did not "fast-forward" the encrypted file past the */
/* first block (which was encrypted random garbage). */
/* */
/* Corrections performed: */
/* - fixed encryptor by leaving random block prefixed, */
/* but removing redundancy (having it in encryptor); */
/* - fixed decryptor (see the source code). */
/* */
/*==========================================================*/
/* Written by: */
/* Rash <[EMAIL PROTECTED]> */
/* */
/* Modified by: */
/* Mouse <[EMAIL PROTECTED]> */
/* Sun Sep 2, 12:25:00 2007 */
/* */
/************************************************************/
/* */
/* Copyright (C) 2007 Rash <[EMAIL PROTECTED]> */
/* Copyright (C) 2007 Mouse <[EMAIL PROTECTED]> */
/* */
/************************************************************/
void encryptFile(const char* password, const char* inputFileName,
const char* outputFileName)
{
byte pass[AES::BLOCKSIZE]; // digest of password
byte iv[16]; // Initial Vector (IV), misused
// by original author
byte true_iv[16]; // real IV used - set to zero
AutoSeededRandomPool rng;
// digest password
StringSource(password, true,
new HashFilter(*(new SHA256),
new ArraySink(pass, AES::BLOCKSIZE)));
// random Initial Vector
rng.GenerateBlock(iv, 16);
memset(true_iv, 0, 16);
// create object for encrypting
AES::Encryption aesEncryption(pass,
CryptoPP::AES::DEFAULT_KEYLENGTH);
CBC_Mode_ExternalCipher::Encryption cbcEncryption(aesEncryption,
true_iv);
StreamTransformationFilter *encryptor;
encryptor = new StreamTransformationFilter(cbcEncryption,
new FileSink(outputFileName) );
encryptor->Put(iv, 16); // this prefixes the file with random block (not IV)
// Cryptographically it is equivalent to IV, so just as good
// "bind" a file and encrypt one
FileSource(inputFileName, true, encryptor);
}
void decryptFile(const char* password, const char*inputFileName,
const char* outputFileName)
{
byte pass[AES::BLOCKSIZE];
byte iv[16]; // here's 1st problem: AES IV is 16 bytes
byte head_file[16]; // so must skip 16 bytes, not 8.
memset(iv, 0, 16); // very correct - in fact the encryptor prefixes file
// with a random block, so no need to pass the IV explicitly.
try {
StringSource(password, true, new HashFilter(*(new SHA256), new
ArraySink(pass,AES::BLOCKSIZE)));
CryptoPP::AES::Decryption aesDecryption(pass,
CryptoPP::AES::DEFAULT_KEYLENGTH);
CryptoPP::CBC_Mode_ExternalCipher::Decryption cbcDecryption(aesDecryption,
iv);
StreamTransformationFilter *decryptor;
decryptor = new
StreamTransformationFilter(cbcDecryption, new
FileSink(outputFileName));
// decryptor->Get(head_file, 16); // does not do anything useful, wrong here
// We must somehow decrypt 1st block of the input file, without sending the
// result into the output file.
char garbage[16], iv_garbage[16]; // place for IV stuff
ifstream inf;
inf.open(inputFileName); inf.read(iv_garbage, 16); // absorb random prefix
// Decrypt random prefix (with zero IV) to some dummy buffer to get
// (a) decryptor state adjusted to IV, and
// (b) file position pointer advanced to the past-IV spot.
cbcDecryption.ProcessData((byte *)garbage, (const byte *)iv_garbage, 16);
// NOW can run the decryption engine in "automatic" mode
FileSource(inf, true, decryptor);
inf.close(); // to be nice
}
catch(CryptoPP::Exception &e)
{
cerr << "Caught exception during decryption!" << endl;
return;
}
}
int main(int argc, char *argv[])
{
const char *pwd = "not-so-complicated parole";
const char *pfn = "plain.txt";
const char *cfn = "cipher.dat";
const char *ofn = "plain2.txt";
// Create test plaintext file "plain.txt"
ofstream plain;
plain.open(pfn);
plain << "An excellent test of file encryption techniques.\n";
plain << "This test will not demonstrate many tricks though.\n";
plain.close();
// Invoke encryptor to encrypt into "cipher.dat"
encryptFile(pwd, pfn, cfn);
// Invoke decryptor to decrypt into "plain2.txt"
decryptFile(pwd, cfn, ofn);
exit(0);
}
