crash in 4.2 when decrypting "damaged" excrypted strings

Ewan Speirs Fri, 11 Feb 2005 05:22:50 -0800

Crypto++ Ver:   4.2
Dev Tool:         msvc++ 6

This is a real entry-level question so bear with me if I explain it badly. I�m starting to get in to cryptography so I�m looking at the test code supplied with crypto v4.2, this is the sample usages of the crypto library found in the test.cpp file.

I�m looking at simple string encryption/decryption with a key, so from that example I�m using the routines:

string EncryptString(const char *instr, const char *passPhrase);
string DecryptString(const char *instr, const char *passPhrase);

which in turn use DefaultEncryptorWithMAC, and HexDecoder.

The worked example works fine, however when I try to decrypt an input string that is not perfect (assume its lost a few characters, or its been corrupted or a someone has tried to fake/guess what some input should be) the test application crashes in both console on windows.

It seems there is no tolerance for �bad� encrypted string in the decrypt routine? Its not unreasonable to think that at some point an input will be wrong in some way so how can I check this rather than just having the application crash?

Code is passed bellow.

Thanks for any guidance
Ewan


===============

#include "pch.h"

#include "md5.h"
#include "sha.h"
#include "ripemd.h"
#include "files.h"
#include "rng.h"
#include "hex.h"
#include "gzip.h"
#include "default.h"
#include "rsa.h"
#include "randpool.h"
#include "ida.h"
#include "socketft.h"

#include "validate.h"
#include "bench.h"

#include <iostream>

#if (_MSC_VER >= 1000)
#include <crtdbg.h>               // for the debug heap
#endif

#if defined(__MWERKS__) && defined(macintosh)
#include <console.h>
#endif

USING_NAMESPACE(CryptoPP)
USING_NAMESPACE(std)

const int MAX_PHRASE_LENGTH=250;

string EncryptString(const char *plaintext, const char *passPhrase);
string DecryptString(const char *ciphertext, const char *passPhrase);

int main(int argc, char *argv[]) { // try with these values - works fine //pass: 1234 //cyphertext: C833582F5D00AB48E8B59ED5F1C6D7122F86689E8E86C042CC03A6DF4326ACE1AFC0DD1BABD13E78C20642F8F54C06A2CF28610FBE73B40A //plain: try to decrypt this

// try with these values - crash every time //pass: 1234 //cyphertext: SOMETHINGAHACKERMIGHTTRYORAENCRYPTEDTEXTWITHAMISSINGCHARACTER

   char passPhrase[MAX_PHRASE_LENGTH], ciphertext[1024];

   cout << "Passphrase: ";

   cin.getline(passPhrase, MAX_PHRASE_LENGTH);

   cout << "\nCiphertext: ";
   cin.getline(ciphertext, 1024);

   string decrypted = DecryptString(ciphertext, passPhrase);
   cout << "\nDecrypted: " << decrypted << endl;
}

// statics exactly the same as those supplied in v4.2 text.cpp file
string EncryptString(const char *instr, const char *passPhrase)
{
        string outstr;

DefaultEncryptorWithMAC encryptor(passPhrase, new HexEncoder(new StringSink(outstr))); encryptor.Put((byte *)instr, strlen(instr)); encryptor.MessageEnd();

        return outstr;
}

string DecryptString(const char *instr, const char *passPhrase)
{
        string outstr;

HexDecoder decryptor(new DefaultDecryptorWithMAC(passPhrase, new StringSink(outstr))); decryptor.Put((byte *)instr, strlen(instr)); decryptor.MessageEnd();

        return outstr;
}

crash in 4.2 when decrypting "damaged" excrypted strings

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