How to store a long hex value message in C++

Question

I'm learning a crypto class and one of the assignment asked us to xor a bunch of hex ciphertext and try to find the encrypted message.

I know that you can do '0x' in front of int or long to hold a hex value in a variable, but what if my message is this long:

271946f9bbb2aeadec111841a81abc300ecaa01bd8069d5cc91005e9fe4aad6e04d513e96d99de2569bc5e50eeeca709b50a8a987f4264edb6896fb537d0a716132ddc938fb0f836480e06ed0fcd6e9759f40462f9cf57f4564186a2c1778f1543efa270bda5e933421cbe88a4a52222190f471e9bd15f652b653b7071aec59a2705081ffe72651d08f822c9ed6d76e48b63ab15d0208573a7eef027

I would get an overflow. Is there a way to put the whole message into one variable? I could split the message into subparts, but I prefer it to be in variable instead of many (if that is possible). I tried to use string to hold the massage, but how can I use the operator, '^', for xor?

Or is there a more simple technique that I do not know of?

Thanks

Answer 1

For something like this, you'd typically use a string or a vector<char> to hold the data. You can't use the entire string/vector as an operand to ^ , but you can apply it one byte at a time.

If you want to simplify the rest of the code, you could create a class that overloaded operator^ to do a byte-wise XOR, so your code would look something like result = key ^ message; .

Answer 2

You could use an array of, well, any size integer, and apply your operators to it an element at a time (which will probably be a bit more efficient than an array of characters). @JerryCoffin's idea of wrapping it inside a class w/ an overloaded operator is a good one, regardless of the actual representation you use.

Answer 3

Put it in a separate text file

read the file into a buffer

convert ascii chars to hex values

Answer 4

Jerry & Scott have sound suggestions. Another option is to use an existing library: for example, the GNU GMP arbitrary-precision maths library at http://gmplib.org , which supports XOR (see http://gmplib.org/manual/Integer-Logic-and-Bit-Fiddling.html#Integer-Logic-and-Bit-Fiddling ) and a "scanf" style function to read in hex (see http://gmplib.org/manual/Formatted-Input-Strings.html#Formatted-Input-Strings ), and explicitly aims to provide excellent support for cryptography.