Url-safe Base64 to bitstring

README.md

Urlsafe Base64 to bit strings

I have been doing a lot of work with binary protocols (esp. for crytography) that encode ciphertext in base64.

To help with debugging these protocols and data, I wanted something that could show me the ones and zeros associated with a piece of data.

You may be able to use sed, base64, and hexdump to get the same functionality on a unix system, but I didn't mess around with that too much.

Example use

In an iPython shell

In [1]: %paste

In [2]: urlsafeb64_to_bitstring("NX6-")
Out[2]: '00110101 01111110 10111110'

In [3]: s = urlsafeb64_to_bitstring("NX6-")

In [4]: s
Out[4]: '00110101 01111110 10111110'

In [5]: bitstring_to_urlsafeb64(s)
Out[5]: 'NX6-'

bitstring.py

import base64

def urlsafeb64_to_bitstring(keyid):
  """Convert a base64 encoded string into a space-separated list of bits for debugging.
  """
  s = base64.urlsafe_b64decode(keyid)
  bitstring = ""
  bts = bytearray(s)
  # Iterate over bytes
  for bt in bts:
      # Iterate over bits, starting at left (high) bits so we build the string left to right
      for i in reversed(xrange(8)):
          v = bt >> i & 1 # check if bit is set
          bitstring += "%d" % (v)
      bitstring += " "
  return bitstring[:-1] # extra space

def bitstring_to_urlsafeb64(bitstring):
    """Convert a space-separated list of bits into a base64 encoded string.
    """
    byte_list = [] # will contain integer representations of each bit
    for bitstring_component in bitstring.split(" "):
        current_val = 0
        for bit in bitstring_component:
            # Shift up
            current_val = current_val << 1
            if bit == "1":
                # Set the lowest bit
                current_val = current_val | 1
        byte_list.append(current_val)
    return base64.urlsafe_b64encode(bytearray(byte_list))