Question

i try to read file of bytes written in hex for example: (909090) which is nop nop nop in assembly, i want to xor each two digit as a byte with the 0x91.

how do i print the value in hex? now it prints just numbers with no meaning.

in general i want to build a packer to my assembly code.

def add_prefix(a):
    a = str(a)
    a = '0x' + a
    a = int(a, 16)
    return a

fr = open('C:\code.txt', 'r')
word = ''
b = ''
for line in fr:
    b += line

count = 0

for char in b:
    char = str(char)
    word += char
    if count%2 == 0:
        word = int(str(add_prefix(word)^91), 16)
        print word
        word = ""

Answer 1

The algorithm needed is quite simple:

1. read two characters at a time
2. convert these to an integer
3. Xor the integer with your mask (0x91)
4. Convert the integer back to a hex string (with leading zero)

---

with open(file_path, 'r') as fp:
    result = []
    while True:
        # 1.
        s = fp.read(2)
        if not s:
            break

        # 2. 
        number = int(s, 16) # No need for '0x' prefix!

        # 3.
        number = number ^ 0x91  # '91' needs '0x' prefix!

        # 4.
        result.append(format(number, '02x'))

print ''.join(result)