How to convert char string of Hex characters to unsigned char string of binary data

I am trying to convert a hexadecimal character array to a binary string (unsigned char) that I can use in crypto functions using Cryptlib or OpenSSL. Basically I have a Hex representation of a key like (02726d40f378e716981c4321d60ba3a3) which as a character string is 32 characters but as a binary string should be 16 bytes.

The character string is being read in from a file, how can I convert it to its binary representation as an unsigned char array?

Thanks.

If it's being read in as characters, read in two at a time and write one byte.

How?

The string is being read from a file with fgets() and being stored in a char*

Then go through the stored string in your char* two at a time and write out one byte at a time.

I don't think that will work. I'm reading in characters from a file and I need to convert them to binary. I need one character in an unsigned char array to represent one byte (two characters) from a char array.

Quote:

---

Originally Posted by maxhavoc

I don't think that will work. I'm reading in characters from a file and I need to convert them to binary. I need one character in an unsigned char array to represent one byte (two characters) from a char array.

---

Of course it's possible; in fact, it's trivial. If you need it spelled out, well, let's spell it out:

1. Decide the numerical value of the first character in a byte pair (0-15).
2. Shift this value over four bits to the left.
3. Add the numerical value of the second character in a byte pair (0-15).
4. Lather, rinse, repeat.

If you can come up with a way to only give strtoul two characters at a time, it becomes even more trivial.

Or do repeated

Code:

---

sscanf(&input_string[i], "%02x", &number);
byte_string[j] = (unsigned char)number

---

every 2 bytes.

Ok, sorry guys, I'm a bit slow. Here's the function I've put together based on nonoob's answer:

Code:

---

void stringToHex(unsigned char* hexString, char* string, int stringLength)
{
        unsigned char uChar = 0;
       
        for (int x = 0; x < stringLength; x++)
        {
                sscanf_s(&string[x], "%02x", &uChar);
                hexString[x] = uChar;
        }
}

---

I'm getting a runtime error about stack corruption on uChar. What am I doing wrong?

You need to put the result into an int first. Then cast it and assign to char like in my example. Your x should go up by 2 indexing string[]. But where your destination is into hexString the index should go up by 1s.

sscanf is really the best option here, of course - this is just an example of doing it 'by hand':

Code:

---

int
hex_char_to_binary( char ch, char* ret )
{
      ch = tolower( ch );     
      if( isdigit( ch ) )
            *ret = ch - '0';           
      else if( ch >= 'a' && ch <= 'f' )
            *ret = ( ch - 'a' ) + 10; 
      else
            return *ret = 0;
      return 1;     
}
 
char*
hex_to_binary_copy( const char* str, char* ret )
{
      char
            * cpy = ret,
            low,
            high;
      if( strlen( str ) &#37; 2 )
      {
            *ret = 0;
            return 0;
      }
      for( const char* ptr = str; *ptr; ptr += 2 )
      {
            if
            (
                  !hex_char_to_binary( *ptr, &high )
                  ||
                  !hex_char_to_binary( *( ptr + 1 ), &low )
            )
            {
                  *ret = 0;
                  return 0;
            }
            *cpy++ = low | ( high << 4 ); 
      }
      *cpy = 0;
      return ret;                             
}

---

[edit]
Or perhaps better yet:
[/edit]

Code:

---

int
hex_to_binary( const char* str, void* buf, int* siz )
{
      char
            * cpy = ( char* )buf,
            low,
            high;
      const char
            * ptr = str;
      int
            unused,
            len = strlen( str ),
            half = len / 2,
            odd = len % 2;
      if( siz == 0 )
            siz = &unused;
      if( odd )
      {
      /*
            a bit of a hack...
      */
            --ptr;
            ++half;
            high = 0;
            goto skip;
      }
      for( ; *ptr; ptr += 2 )
      {
            if( !hex_char_to_binary( *ptr, &high ) )
                  return *siz = 0;
            skip:                 
            if( !hex_char_to_binary( *( ptr + 1 ), &low ) )
                  return *siz = 0;
            *cpy++ = low | ( high << 4 ); 
      }
      return *siz = half;                             
}

---

I know this post was a while ago but thanks for your answers! Specially to nonoob...
Saved me time and effort...

Click the "like" link then, instead of digging up dead threads, as mentioned in the rules as something NOT to do.