1:mod:`base64` --- Base16, Base32, Base64, Base85 Data Encodings
2===============================================================
3
4.. module:: base64
5   :synopsis: RFC 3548: Base16, Base32, Base64 Data Encodings;
6              Base85 and Ascii85
7
8**Source code:** :source:`Lib/base64.py`
9
10.. index::
11   pair: base64; encoding
12   single: MIME; base64 encoding
13
14--------------
15
16This module provides functions for encoding binary data to printable
17ASCII characters and decoding such encodings back to binary data.
18It provides encoding and decoding functions for the encodings specified in
19:rfc:`3548`, which defines the Base16, Base32, and Base64 algorithms,
20and for the de-facto standard Ascii85 and Base85 encodings.
21
22The :rfc:`3548` encodings are suitable for encoding binary data so that it can
23safely sent by email, used as parts of URLs, or included as part of an HTTP
24POST request.  The encoding algorithm is not the same as the
25:program:`uuencode` program.
26
27There are two interfaces provided by this module.  The modern interface
28supports encoding :term:`bytes-like objects <bytes-like object>` to ASCII
29:class:`bytes`, and decoding :term:`bytes-like objects <bytes-like object>` or
30strings containing ASCII to :class:`bytes`.  Both base-64 alphabets
31defined in :rfc:`3548` (normal, and URL- and filesystem-safe) are supported.
32
33The legacy interface does not support decoding from strings, but it does
34provide functions for encoding and decoding to and from :term:`file objects
35<file object>`.  It only supports the Base64 standard alphabet, and it adds
36newlines every 76 characters as per :rfc:`2045`.  Note that if you are looking
37for :rfc:`2045` support you probably want to be looking at the :mod:`email`
38package instead.
39
40
41.. versionchanged:: 3.3
42   ASCII-only Unicode strings are now accepted by the decoding functions of
43   the modern interface.
44
45.. versionchanged:: 3.4
46   Any :term:`bytes-like objects <bytes-like object>` are now accepted by all
47   encoding and decoding functions in this module.  Ascii85/Base85 support added.
48
49The modern interface provides:
50
51.. function:: b64encode(s, altchars=None)
52
53   Encode the :term:`bytes-like object` *s* using Base64 and return the encoded
54   :class:`bytes`.
55
56   Optional *altchars* must be a :term:`bytes-like object` of at least
57   length 2 (additional characters are ignored) which specifies an alternative
58   alphabet for the ``+`` and ``/`` characters.  This allows an application to e.g.
59   generate URL or filesystem safe Base64 strings.  The default is ``None``, for
60   which the standard Base64 alphabet is used.
61
62
63.. function:: b64decode(s, altchars=None, validate=False)
64
65   Decode the Base64 encoded :term:`bytes-like object` or ASCII string
66   *s* and return the decoded :class:`bytes`.
67
68   Optional *altchars* must be a :term:`bytes-like object` or ASCII string of
69   at least length 2 (additional characters are ignored) which specifies the
70   alternative alphabet used instead of the ``+`` and ``/`` characters.
71
72   A :exc:`binascii.Error` exception is raised
73   if *s* is incorrectly padded.
74
75   If *validate* is ``False`` (the default), characters that are neither
76   in the normal base-64 alphabet nor the alternative alphabet are
77   discarded prior to the padding check.  If *validate* is ``True``,
78   these non-alphabet characters in the input result in a
79   :exc:`binascii.Error`.
80
81
82.. function:: standard_b64encode(s)
83
84   Encode :term:`bytes-like object` *s* using the standard Base64 alphabet
85   and return the encoded :class:`bytes`.
86
87
88.. function:: standard_b64decode(s)
89
90   Decode :term:`bytes-like object` or ASCII string *s* using the standard
91   Base64 alphabet and return the decoded :class:`bytes`.
92
93
94.. function:: urlsafe_b64encode(s)
95
96   Encode :term:`bytes-like object` *s* using the
97   URL- and filesystem-safe alphabet, which
98   substitutes ``-`` instead of ``+`` and ``_`` instead of ``/`` in the
99   standard Base64 alphabet, and return the encoded :class:`bytes`.  The result
100   can still contain ``=``.
101
102
103.. function:: urlsafe_b64decode(s)
104
105   Decode :term:`bytes-like object` or ASCII string *s*
106   using the URL- and filesystem-safe
107   alphabet, which substitutes ``-`` instead of ``+`` and ``_`` instead of
108   ``/`` in the standard Base64 alphabet, and return the decoded
109   :class:`bytes`.
110
111
112.. function:: b32encode(s)
113
114   Encode the :term:`bytes-like object` *s* using Base32 and return the
115   encoded :class:`bytes`.
116
117
118.. function:: b32decode(s, casefold=False, map01=None)
119
120   Decode the Base32 encoded :term:`bytes-like object` or ASCII string *s* and
121   return the decoded :class:`bytes`.
122
123   Optional *casefold* is a flag specifying
124   whether a lowercase alphabet is acceptable as input.  For security purposes,
125   the default is ``False``.
126
127   :rfc:`3548` allows for optional mapping of the digit 0 (zero) to the letter O
128   (oh), and for optional mapping of the digit 1 (one) to either the letter I (eye)
129   or letter L (el).  The optional argument *map01* when not ``None``, specifies
130   which letter the digit 1 should be mapped to (when *map01* is not ``None``, the
131   digit 0 is always mapped to the letter O).  For security purposes the default is
132   ``None``, so that 0 and 1 are not allowed in the input.
133
134   A :exc:`binascii.Error` is raised if *s* is
135   incorrectly padded or if there are non-alphabet characters present in the
136   input.
137
138
139.. function:: b16encode(s)
140
141   Encode the :term:`bytes-like object` *s* using Base16 and return the
142   encoded :class:`bytes`.
143
144
145.. function:: b16decode(s, casefold=False)
146
147   Decode the Base16 encoded :term:`bytes-like object` or ASCII string *s* and
148   return the decoded :class:`bytes`.
149
150   Optional *casefold* is a flag specifying whether a
151   lowercase alphabet is acceptable as input.  For security purposes, the default
152   is ``False``.
153
154   A :exc:`binascii.Error` is raised if *s* is
155   incorrectly padded or if there are non-alphabet characters present in the
156   input.
157
158
159.. function:: a85encode(b, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)
160
161   Encode the :term:`bytes-like object` *b* using Ascii85 and return the
162   encoded :class:`bytes`.
163
164   *foldspaces* is an optional flag that uses the special short sequence 'y'
165   instead of 4 consecutive spaces (ASCII 0x20) as supported by 'btoa'. This
166   feature is not supported by the "standard" Ascii85 encoding.
167
168   *wrapcol* controls whether the output should have newline (``b'\n'``)
169   characters added to it. If this is non-zero, each output line will be
170   at most this many characters long.
171
172   *pad* controls whether the input is padded to a multiple of 4
173   before encoding. Note that the ``btoa`` implementation always pads.
174
175   *adobe* controls whether the encoded byte sequence is framed with ``<~``
176   and ``~>``, which is used by the Adobe implementation.
177
178   .. versionadded:: 3.4
179
180
181.. function:: a85decode(b, *, foldspaces=False, adobe=False, ignorechars=b' \\t\\n\\r\\v')
182
183   Decode the Ascii85 encoded :term:`bytes-like object` or ASCII string *b* and
184   return the decoded :class:`bytes`.
185
186   *foldspaces* is a flag that specifies whether the 'y' short sequence
187   should be accepted as shorthand for 4 consecutive spaces (ASCII 0x20).
188   This feature is not supported by the "standard" Ascii85 encoding.
189
190   *adobe* controls whether the input sequence is in Adobe Ascii85 format
191   (i.e. is framed with <~ and ~>).
192
193   *ignorechars* should be a :term:`bytes-like object` or ASCII string
194   containing characters to ignore
195   from the input. This should only contain whitespace characters, and by
196   default contains all whitespace characters in ASCII.
197
198   .. versionadded:: 3.4
199
200
201.. function:: b85encode(b, pad=False)
202
203   Encode the :term:`bytes-like object` *b* using base85 (as used in e.g.
204   git-style binary diffs) and return the encoded :class:`bytes`.
205
206   If *pad* is true, the input is padded with ``b'\0'`` so its length is a
207   multiple of 4 bytes before encoding.
208
209   .. versionadded:: 3.4
210
211
212.. function:: b85decode(b)
213
214   Decode the base85-encoded :term:`bytes-like object` or ASCII string *b* and
215   return the decoded :class:`bytes`.  Padding is implicitly removed, if
216   necessary.
217
218   .. versionadded:: 3.4
219
220
221The legacy interface:
222
223.. function:: decode(input, output)
224
225   Decode the contents of the binary *input* file and write the resulting binary
226   data to the *output* file. *input* and *output* must be :term:`file objects
227   <file object>`. *input* will be read until ``input.readline()`` returns an
228   empty bytes object.
229
230
231.. function:: decodebytes(s)
232
233   Decode the :term:`bytes-like object` *s*, which must contain one or more
234   lines of base64 encoded data, and return the decoded :class:`bytes`.
235
236   .. versionadded:: 3.1
237
238.. function:: decodestring(s)
239
240   Deprecated alias of :func:`decodebytes`.
241
242   .. deprecated:: 3.1
243
244
245.. function:: encode(input, output)
246
247   Encode the contents of the binary *input* file and write the resulting base64
248   encoded data to the *output* file. *input* and *output* must be :term:`file
249   objects <file object>`. *input* will be read until ``input.read()`` returns
250   an empty bytes object. :func:`encode` inserts a newline character (``b'\n'``)
251   after every 76 bytes of the output, as well as ensuring that the output
252   always ends with a newline, as per :rfc:`2045` (MIME).
253
254
255.. function:: encodebytes(s)
256
257   Encode the :term:`bytes-like object` *s*, which can contain arbitrary binary
258   data, and return :class:`bytes` containing the base64-encoded data, with newlines
259   (``b'\n'``) inserted after every 76 bytes of output, and ensuring that
260   there is a trailing newline, as per :rfc:`2045` (MIME).
261
262   .. versionadded:: 3.1
263
264.. function:: encodestring(s)
265
266   Deprecated alias of :func:`encodebytes`.
267
268   .. deprecated:: 3.1
269
270
271An example usage of the module:
272
273   >>> import base64
274   >>> encoded = base64.b64encode(b'data to be encoded')
275   >>> encoded
276   b'ZGF0YSB0byBiZSBlbmNvZGVk'
277   >>> data = base64.b64decode(encoded)
278   >>> data
279   b'data to be encoded'
280
281
282.. seealso::
283
284   Module :mod:`binascii`
285      Support module containing ASCII-to-binary and binary-to-ASCII conversions.
286
287   :rfc:`1521` - MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies
288      Section 5.2, "Base64 Content-Transfer-Encoding," provides the definition of the
289      base64 encoding.
290
291