Python - counting up in hex

Question

What is a good way to count up in hex and append the result to the end of a larger hex string? I am trying to format a string to be used in a padding oracle attack. The strings will be concatenated together to form an HTTP request.

I have a two 32 character hex strings. 'g' is a guess, pad is the padding oracle. Basically what I need to do is have the last byte of g count up in hex from 0x00 to 0xff.The code I have so far is:

split = [value[x:x+32] for x in range (0, len(value), 32)]  #Split the CT into 16 byte chunks
IV = unhexlify(split[0])
c0 = unhexlify(split[1])
c1 = unhexlify(split[2])
c2 = unhexlify(split[3])


g = unhexlify("00000000000000000000000000000000")  
pad = unhexlify("00000000000000000000000000000001")  

pad_xor_guess = xorb(g, pad)
c1_prime = xorb(pad_xor_guess, c1)

attack = str(hexlify(c1_prime + c2).decode())

'attack' will be passed into the query method that will append the attack string to the web address. Now the part I am stuck on is that I have to basically send up to 256 HTTP requests to guess one byte of the plaintext. How can I use a for loop to "count up" from 00 to ff, appending the result to g in such a way that it can be xor'd with the pad and the chosen ciphertext block? SO far I have been going down this path, but I am stuck on how to make this work with the hex strings.

for i in range(0, 20):
    #g = bytes([i])
    print(bytes([i]),end=' ')
    #print(g, end=' ')

Answer 1

For given int value, hex function will give you the hex string preceded with 0x, so hex(i)[2:] gives you the hex number itself, zfill will make sure you get two digits for the single digits numbers

for i in range(256):
    print(hex(i)[2:].zfill(2))

You might also want to consider making it all caps, since some parsers rely on hex being written in capital letters, so the example will be:

for i in range(256):
    print(hex(i)[2:].zfill(2).upper())

And if you just need the full string, you don't need to append them one by one, you can create the string in one go:

hex_str = "".join([hex(i)[2:].zfill(2).upper() for i in range(256)])

Answer 2

I guess you mean something like:

>>> for i in range(256):
    print "{:02x}".format(i)  # or X for uppercase


00
01
02
...
fd
fe
ff