Encrypt and Decrypt text file conatins using AES::CBC mode

[Alok Sethi]

Hi,

I want to encrypt and decrypt file contains using AES::CBC mode. I tried the following for encryption and decryption.
It works fine but I get the below two exceptions.

1. Run-Time Check Failure #2 - Stack around the variable 'iVector' was corrupted.
2. Run-Time Check Failure #2 - Stack around the variable 'key' was corrupted.

I don't know exactly where I am doing wrong. Please help me to fix this problem.

To encrypt the file here is my code :-
==================================
void Encrypt(string inFileName, string outFileName)
{

    byte key[CryptoPP::AES::DEFAULT_KEYLENGTH];
    key[sizeof(key)] = 0;

    byte iVector[CryptoPP::AES::BLOCKSIZE];
    iVector[sizeof(iVector)] = 0;
   
    ifstream in(inFileName.c_str(), ios::binary);

    const string orgStrObj((istreambuf_iterator<char>(in)), istreambuf_iterator<char>());
    in.close();
   
    CryptoPP::AES::Encryption cipher(key, sizeof(key));
   
    CryptoPP::CBC_Mode_ExternalCipher::Encryption encryption(cipher, iVector);
    CryptoPP::StreamTransformationFilter strFilter(encryption, new CryptoPP::StringSink(encryptStr));
    strFilter.Put(reinterpret_cast<const byte*>(orgStrObj.c_str()), orgStrObj.size());
    strFilter.MessageEnd();
   
     ofstream out(outFileName.c_str(), ios::binary);
    out.write(encryptStr.c_str(), encryptStr.size());
    out.close();
}


To decrypt the file here is my code :-
================================
void Crypto::Decrypt(string inFileName, string outFileName)
{
    byte key[CryptoPP::AES::DEFAULT_KEYLENGTH];
    key[sizeof(key)] = 0;

    byte iVector[CryptoPP::AES::BLOCKSIZE];
    iVector[sizeof(iVector)] = 0;
   
    ifstream in(inFileName.c_str(), ios::binary);

    const string encryptStrObj((istreambuf_iterator<char>(in)), istreambuf_iterator<char>());
    in.close();
 
    CryptoPP::AES::Decryption cipher(key, sizeof(key));

    CryptoPP::CBC_Mode_ExternalCipher::Decryption decryption(cipher, iVector);
    CryptoPP::StreamTransformationFilter strFilter(decryption, new CryptoPP::StringSink(decryptStr));
    strFilter.Put(reinterpret_cast<const byte*>(encryptStrObj.c_str()), encryptStrObj.size());
    strFilter.MessageEnd();
   
    ofstream out(outFileName.c_str(), ios::binary);
    out.write(decryptStr.c_str(), decryptStr.size());
    out.close();
}

[Fraser Hutchison]

Hi there,

The cause of this error is where you have key[sizeof(key)] = 0; and iVector[sizeof(iVector)] = 0; in each of your functions.  You're trying to access an element one past the end of each array.

I guess you're trying to null-terminate these arrays, but there's no need here.  In fact, it's valid for either the key or IV to contain '0' char(s) anywhere within.

Cheers,
Fraser.

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[Alok Sethi]

Hi Fraser,

Thanks for your quick reply. As per your suggestion I tried by removing these two lines. But getting the same error message.
Also I tried to set the memory to zero as below.

memset( key, 0x01, CryptoPP::AES::DEFAULT_KEYLENGTH );
memset( iVector, 0x01, CryptoPP::AES::BLOCKSIZE );

It would be great if you could suggest any other alternative solutions.


Thanks,
Alok.

[David Irvine,,,]

Alok Sethi writes:

> « HTML content follows »

>
> Hi Fraser,
>
> Thanks for your quick reply. As per your suggestion I tried by removing these
> two lines. But getting the same error message.
> Also I tried to set the memory to zero as below.
>
> memset( key, 0x01, CryptoPP::AES::DEFAULT_KEYLENGTH );
> memset( iVector, 0x01, CryptoPP::AES::BLOCKSIZE );
>
> It would be great if you could suggest any other alternative solutions.

Apart from these stack errors, what are you trying to do, it seems you are
setting key and iv to zero, is this intentional. Should you not take a key
and iv in through your parameters and set these to the size of the constants
you have to create the key ivector. If you did want to create a key and iv
of zero then set each array member to zero, but it would be quite strange.

Apart from that you are setting one past the end of the byte array, the end
of an array size 64 is element 63.

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[Da Co]

I was not able to encrypt with these parameters set to zero. You need 16 meaningful bytes in key and IV.

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[David Irvine,,,]

Da Co writes:

> « HTML content follows »

>
> I was not able to encrypt with these parameters set to zero. You need 16
> meaningful bytes in key and IV.

Excellent, I hope you are on the way now. I think oyu were makig use of
undefined behaviour, so setting al bytes will fix that.

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