4.11 The base64 Module

The base64 encoding scheme is used to convert arbitrary binary data to plain text. To do this, the encoder stores each group of three binary bytes as a group of four characters from the following set:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz
0123456789+/

In addition, the = character is used for padding at the end of the data stream.

Example 4-18 shows how the encode and decode functions work on file objects.

Example 4-18. Using the base64 Module to Encode Files

File: base64-example-1.py

import base64

MESSAGE = "life of brian"

file = open("out.txt", "w")
file.write(MESSAGE)
file.close()

base64.encode(open("out.txt"), open("out.b64", "w"))
base64.decode(open("out.b64"), open("out.txt", "w"))

print "original:", repr(MESSAGE)
print "encoded message:", repr(open("out.b64").read())
print "decoded message:", repr(open("out.txt").read())

original:  'life of brian'
encoded message: 'bGlmZSBvZiBicmlhbg==\012'
decoded message: 'life of brian'

Example 4-19 shows the encodestring and decodestring functions converting between strings. The functions are currently implemented as wrappers on top of encode and decode, using StringIO objects for input and output.

Example 4-19. Using the base64 Module to Encode Strings

File: base64-example-2.py

import base64

MESSAGE = "life of brian"

data = base64.encodestring(MESSAGE)

original_data = base64.decodestring(data)

print "original:", repr(MESSAGE)
print "encoded data:", repr(data)
print "decoded data:", repr(original_data)

original: 'life of brian'
encoded data: 'bGlmZSBvZiBicmlhbg==\012'
decoded data: 'life of brian'

Example 4-20 shows how to convert a username and a password to an HTTP basic authentication string. (Note that you don't really have to work for the NSA to be able to decode this format.)

Example 4-20. Using the base64 Module for Basic Authentication

File: base64-example-3.py

import base64

def getbasic(user, password):
    # basic authentication (according to HTTP)
    return base64.encodestring(user + ":" + password)    

print getbasic("Aladdin", "open sesame")

'QWxhZGRpbjpvcGVuIHNlc2FtZQ=='

Finally, Example 4-21 shows a small utility that converts a GIF image to a Python script, for use with the Tkinter library.

Example 4-21. Using the base64 Module to Wrap GIF Images for Tkinter

File: base64-example-4.py

import base64, sys

if not sys.argv[1:]:
    print "Usage: gif2tk.py giffile >pyfile"
    sys.exit(1)

data = open(sys.argv[1], "rb").read()

if data[:4] != "GIF8":
    print sys.argv[1], "is not a GIF file"
    sys.exit(1)

print '# generated from', sys.argv[1], 'by gif2tk.py'
print
print 'from Tkinter import PhotoImage' 
print
print 'image = PhotoImage(data="""'
print base64.encodestring(data),
print '""")'

# generated from samples/sample.gif by gif2tk.py

from Tkinter import PhotoImage

image = PhotoImage(data="""
R0lGODlhoAB4APcAAAAAAIAAAACAAICAAAAAgIAAgACAgICAgAQEBIwEBIyMBJRUlISE/LRUBAQE
...
AjmQBFmQBnmQCJmQCrmQDNmQDvmQEBmREnkRAQEAOw==
""")