Convert string to byte in crypto++

Question

I am using the AES cipher with Crypto++. I have a key that recorded in the file during encryption. I extract key from file to string and try to use for decrypt.

Is there any way i can convert the string that contains the key string s1 to CryptoPP::byte? There fragment of code.

encoded.clear();
StringSource(key, sizeof(key), true,
    new HexEncoder(
        new StringSink(encoded)
));

ofstream fout1("key.txt"); 
fout1 << encoded; 
fout1.close(); 

string s1;                
ifstream TextFile1("key.txt");            

while (!TextFile1.eof())                       
{
    if (TextFile1.eof())
        break;
    TextFile1 >> s1;
}

To convert a `std::string` to a `CryptoPP::byte*` see [Convert Hex string to bytes in Crypto++](https://stackoverflow.com/q/17816312/608639), [Decoding Hex Encoded Value with Crypto++](https://stackoverflow.com/q/17306752/608639), [Decoding Hex Encoded Value with Crypto++](https://stackoverflow.com/q/17306752/608639) and [Get hexadecimal encrypted string in AES 256 Crypto++](https://stackoverflow.com/q/21896874/608639). — jww, Mar 28 '18 at 04:23
Also see [Storing the IV with the ciphertext Crypto++ CBC AES encryption](https://stackoverflow.com/q/45461770/608639) for a similar question. — jww, Mar 28 '18 at 04:30

jww · Answer 1 · 2018-03-28T16:59:48.267

I have a key that recorded in the file during encryption. I extract key from file to string and try to use for decrypt.

It sounds like you have a file structured similar to below:

[...key...][...iv...][...encrypted data...]

It is a bit unusual to store a key like that, but here is how it goes... Below I assume you are using AES-128 in CTR mode. That means [...key...] is 16 bytes, and [...iv...] is 16 bytes. The remainder is encrypted data.

I generate the sample file with:

$ head -c 128 < /dev/urandom > message.enc
$ hexdump -C message.enc
00000000  17 44 79 6b e6 96 ff d0  9e 3e 8c c4 fe 57 56 a2  |.Dyk.....>...WV.|
00000010  bb 59 9c a6 fb ab 73 de  a7 a9 4a 22 14 6e c4 af  |.Y....s...J".n..|
00000020  31 13 04 4d f2 79 f8 7c  7a 0b 16 2c bd be 6e 4c  |1..M.y.|z..,..nL|
00000030  b6 61 0a 6c 33 d3 f0 73  25 44 ec f5 cd f5 cd da  |.a.l3..s%D......|
00000040  3d 13 72 98 65 19 e1 c5  f8 49 1e 07 c7 dc ac b7  |=.r.e....I......|
00000050  ce 03 d1 90 94 08 aa 9d  a0 8b b0 cd ff 9c b9 67  |...............g|
00000060  8a 2c 6f d9 7e fa d2 07  0f a0 48 99 57 77 2b d1  |.,o.~.....H.Ww+.|
00000070  c7 28 2a bc 80 22 21 fb  4a ba cb b2 0e b6 2c ff  |.(*.."!.J.....,.|

The key is 17 44 ... 56 a2. The iv is bb 59 ... c4 af. The encrypted data starts at 31 13 04 4d....

And the program:

$ cat test.cxx
#include "filters.h"
#include "files.h"
#include "modes.h"
#include "aes.h"
#include "hex.h"

#include <iostream>

int main()
{
    using namespace CryptoPP;
    // Key and iv are stored at the head of the file
    SecByteBlock key(16), iv(16);
    FileSource fs("message.enc", false /* DO NOT Pump All */);

    // Attach new filter
    ArraySink ak(key, key.size());
    fs.Detach(new Redirector(ak));
    fs.Pump(16);  // Pump first 16 bytes

    // Attach new filter
    ArraySink av(iv, iv.size());
    fs.Detach(new Redirector(av));
    fs.Pump(16);  // Pump next 16 bytes

    CTR_Mode<AES>::Decryption decryptor;
    decryptor.SetKeyWithIV(key, key.size(), iv, iv.size());

    // Detach previously attached filter, attach new filter
    ByteQueue queue;
    fs.Detach(new StreamTransformationFilter(decryptor, new Redirector(queue)));
    fs.PumpAll();  // Pump remainder of bytes

    std::cout << "Key: ";
    StringSource(key, key.size(), true, new HexEncoder(new FileSink(std::cout)));
    std::cout << std::endl;

    std::cout << "IV: ";
    StringSource(iv, iv.size(), true, new HexEncoder(new FileSink(std::cout)));
    std::cout << std::endl;

    std::cout << "Message: ";
    HexEncoder hex(new FileSink(std::cout));
    queue.TransferTo(hex);
    std::cout << std::endl;

    return 0;
}

You can call either Attach or Detach to attach a new filter. Both attach a new filter. The difference is, Attach returns the old filter and you have to free it. Detach deletes the old filter for you.

Finally:

$ g++ -Wall -I . test.cxx ./libcryptopp.a -o test.exe
$ ./test.exe
Key: 1744796BE696FFD09E3E8CC4FE5756A2
IV: BB599CA6FBAB73DEA7A94A22146EC4AF
Message: 84F6DC079CA04BDFACB645CB11CC2F828573F1841B1B9267CB296B6A977BE19D68B05FA
AF41AB73498F45629EE050B132174A2798C12C29A7033ADD1999BECD00B101F2616112D7E6968EA0
A1BE159CD0EE43549BA6534C8D4AB8F5E7D9E3E44

Unrelated to Crypto++, you usually want to avoid this:

while (!TextFile1.eof())                       
{
    if (TextFile1.eof())
        break;
    TextFile1 >> s1;
}

I believe it is usually better to follow Why is iostream::eof inside a loop condition considered wrong? (I even think TextFile1 >> s1; should be in between the TextFile1.eof() checks. You don't reach eof() until you try to perform a read).

Convert string to byte in crypto++

1 Answers1