将C ++指针代码转换为python

Question

I have a code snippet in C++ which needs to converted to Python:

static void DecryptBuff (const unit8_t key, char* buf, const size_t n) {
   for (auto ptr = buf; ptr < buf +n; ++ptr)
      *ptr = *ptr ^ key;
}

The value being sent to the "DecryptBuff" is address of some variable. The data type of the variable can be anything and so the "for" is not easy to use.

I know that python does not have pointers, is there any other way I can approach this problem?

Answer 1

You are using your pointer basically as an array. You can use array.array , which works in a similar way to C arrays.

Answer 2

You could write this using a Python list for the C char *buf , mutating the list so that it contains the result, but a more Pythonic way is to create a new buffer containing the xor-encoded bytes and return it.

With older versions of Python you'd create a list of the ord() values of each char in the buf string, xor the elements of that list with your key, convert the ints back to chars using chr() , and then join() the individual chars back into a single string. But with more modern versions of Python you can get bytearray to do most of the dirty work.

#! /usr/bin/env python

def xor_crypt(key, buf):
    return str(bytearray([i ^ key for i in bytearray(buf)]))

plaintxt = "Hello, world!"
key = 42

print 'original ', `plaintxt`

encrypted = xor_crypt(key, plaintxt)
print 'encrypted', `encrypted`

decrypted = xor_crypt(key, encrypted)
print 'decrypted', `decrypted`

output

original  'Hello, world!'
encrypted 'bOFFE\x06\n]EXFN\x0b'
decrypted 'Hello, world!'

But if you really want to imitate the C code more closely and mutate buf , you can easily do that, like this:

#! /usr/bin/env python

def xor_crypt(key, buf):
    buf[:] = bytearray([i ^ key for i in buf])

plaintxt = "Hello, world!"
key = 42

buf = bytearray(plaintxt)
print 'original ', `buf`, str(buf)

xor_crypt(key, buf)
print 'encrypted', `buf`

xor_crypt(key, buf)
print 'decrypted', `buf`

output

original  bytearray(b'Hello, world!') Hello, world!
encrypted bytearray(b'bOFFE\x06\n]EXFN\x0b')
decrypted bytearray(b'Hello, world!')