Question

I’m trying to convert this Python code to C. But for the life of me, I can’t figure out what this line here does. The rest of the program seems simple.

self.payload = "\x02\x00%s%s" % (
    pack(">b", length),
    "".join(random.choice(string.printable) for i in range(length)))

If anybody could give me a rough idea of what this is doing, it’d be much appreciated!

Answer 1

First line:

• The correct translation of length = random.randint(18, 20) is int length = rand() % 3 + 18.

Now let’s dissect the dense second line piece by piece.

• "\x02\x00%s%s" % (x, y) means to substitute the format string with the given arguments (like sprintf()). In this case it means concatenating the bytes 0x02, 0x00 with two strings x and y to follow.

• x = pack(">b", length) uses struct.pack(). In this case, it means to convert the integer value length into one byte representing its value. It’s almost equivalent to using chr().

• y = "".join(z) means to take each element in the list z (which must be a string) and concatenate them with “” (nothing) between them. (For example, "@".join(["a","b","c"]) --> "a@b@c".)

• z = (random.choice(string.printable) for i in range(length)) returns a generator object. You can think of it as a list whose elements are computed on demand. In this case, it generates length elements where each element is one character randomly chosen from the string string.printable.

All in all, the second line yields a string that starts wxth 0x02 0x00, followed by (char)length, followed by length random characters each uniformly chosen from the set of chars string.printable.