Spaces when converting byte array to hex std::string c++

Question

I am using the following function to convert a byte array (Crypto++ key) to a Hex String

std::string Hexa::byte_to_hex_encoder(unsigned char *array, int len){
    std::stringstream ss;
    for(int i=0;i<len;++i)
        ss << std::hex << std::uppercase <<std::setw(2) <<(int)array[i];
    return ss.str();
}

The byte array is of size 16 and when I don't use setw(2) I get a hex string with lesser characters like 30 or sometimes 31. When I am using setw(2) I get random spaces in the hex string like

5CA0 138C5487D2C6D929EC36B694890

How can I convert a byte array to hex string and vice versa without spaces in the hex string?

Answer 1

You also need setfill('0') so that the numbers are properly padded.

Without the setw, a number like 7 comes out as just 7, making your string short as you have seen. With the setw but no setfill, it's padded to the right length, but with a space.

Adding the setfill ensures it gets padded with zeroes.

For your code that would be:

ss << std::hex
   << std::uppercase
   << std::setw(2)
   << std::setfill('0')
   << (int)array[i];

Answer 2

Since you have the Crypto++ tag, here's are two ways to do it in Crypto++. From the HexEncoder wiki page.

First, using Crypto++ pipelines:

byte decoded[] = { 0xFF, 0xEE, 0xDD, 0xCC, 0xBB, 0xAA, 0x99, 0x88,
                   0x77, 0x66, 0x55, 0x44, 0x33, 0x22, 0x11, 0x00 };
string encoded;

StringSource ss(decoded, sizeof(decoded), true,
    new HexEncoder(
        new StringSink(encoded)
    ) // HexEncoder
); // StringSource

cout << encoded << endl;

As with the previous example, a run produces the following output.

$ ./cryptopp-test.exe
FFEEDDCCBBAA99887766554433221100

---

And second, using functions:

byte decoded[] = { 0xFF, 0xEE, 0xDD, 0xCC, 0xBB, 0xAA, 0x99, 0x88,
                   0x77, 0x66, 0x55, 0x44, 0x33, 0x22, 0x11, 0x00 };
string encoded;

HexEncoder encoder;
encoder.Put(decoded, sizeof(decoded));
encoder.MessageEnd();

word64 size = encoder.MaxRetrievable();
if(size)
{
    encoded.resize(size);       
    encoder.Get((byte*)encoded.data(), encoded.size());
}

cout << encoded << endl;

A run of the above program produces the following output.

$ ./cryptopp-test.exe
FFEEDDCCBBAA99887766554433221100

---

There's a HexDecoder too so you can decode the encoded strings. It works nearly the same way.