cpython/Doc/library/base64.rst

:mod:`base64` --- RFC 3548: Base16, Base32, Base64 Data Encodings
=================================================================

.. module:: base64
   :synopsis: RFC 3548: Base16, Base32, Base64 Data Encodings


.. index::
   pair: base64; encoding
   single: MIME; base64 encoding

This module provides data encoding and decoding as specified in :rfc:`3548`.
This standard defines the Base16, Base32, and Base64 algorithms for encoding and
decoding arbitrary binary strings into text strings that can be safely sent by
email, used as parts of URLs, or included as part of an HTTP POST request.  The
encoding algorithm is not the same as the :program:`uuencode` program.

There are two interfaces provided by this module.  The modern interface supports
encoding and decoding string objects using both base-64 alphabets defined
in :rfc:`3548` (normal, and URL- and filesystem-safe).  The legacy
interface provides for encoding and decoding to and from file-like objects as
well as strings, but only using the Base64 standard alphabet.

The modern interface, which was introduced in Python 2.4, provides:


.. function:: b64encode(s[, altchars])

   Encode a string using Base64.

   *s* is the string to encode.  Optional *altchars* must be a string of at least
   length 2 (additional characters are ignored) which specifies an alternative
   alphabet for the ``+`` and ``/`` characters.  This allows an application to e.g.
   generate URL or filesystem safe Base64 strings.  The default is ``None``, for
   which the standard Base64 alphabet is used.

   The encoded string is returned.


.. function:: b64decode(s[, altchars])

   Decode a Base64 encoded string.

   *s* is the string to decode.  Optional *altchars* must be a string of at least
   length 2 (additional characters are ignored) which specifies the alternative
   alphabet used instead of the ``+`` and ``/`` characters.

   The decoded string is returned.  A :exc:`TypeError` is raised if *s* is
   incorrectly padded.  Characters that are neither
   in the normal base-64 alphabet nor the alternative alphabet are
   discarded prior to the padding check.


.. function:: standard_b64encode(s)

   Encode string *s* using the standard Base64 alphabet.


.. function:: standard_b64decode(s)

   Decode string *s* using the standard Base64 alphabet.


.. function:: urlsafe_b64encode(s)

   Encode string *s* using the URL- and filesystem-safe
   alphabet, which substitutes ``-`` instead of
   ``+`` and ``_`` instead of ``/`` in the standard Base64 alphabet.  The result
   can still contain ``=``.


.. function:: urlsafe_b64decode(s)

   Decode string *s* using the URL- and filesystem-safe
   alphabet, which substitutes ``-`` instead of
   ``+`` and ``_`` instead of ``/`` in the standard Base64 alphabet.


.. function:: b32encode(s)

   Encode a string using Base32.  *s* is the string to encode.  The encoded string
   is returned.


.. function:: b32decode(s[, casefold[, map01]])

   Decode a Base32 encoded string.

   *s* is the string to decode.  Optional *casefold* is a flag specifying whether a
   lowercase alphabet is acceptable as input.  For security purposes, the default
   is ``False``.

   :rfc:`3548` allows for optional mapping of the digit 0 (zero) to the letter O
   (oh), and for optional mapping of the digit 1 (one) to either the letter I (eye)
   or letter L (el).  The optional argument *map01* when not ``None``, specifies
   which letter the digit 1 should be mapped to (when *map01* is not ``None``, the
   digit 0 is always mapped to the letter O).  For security purposes the default is
   ``None``, so that 0 and 1 are not allowed in the input.

   The decoded string is returned.  A :exc:`TypeError` is raised if *s* is
   incorrectly padded or if there are non-alphabet characters present in the
   string.


.. function:: b16encode(s)

   Encode a string using Base16.

   *s* is the string to encode.  The encoded string is returned.


.. function:: b16decode(s[, casefold])

   Decode a Base16 encoded string.

   *s* is the string to decode.  Optional *casefold* is a flag specifying whether a
   lowercase alphabet is acceptable as input.  For security purposes, the default
   is ``False``.

   The decoded string is returned.  A :exc:`TypeError` is raised if *s* were
   incorrectly padded or if there are non-alphabet characters present in the
   string.

The legacy interface:


.. function:: decode(input, output)

   Decode the contents of the *input* file and write the resulting binary data to
   the *output* file. *input* and *output* must either be file objects or objects
   that mimic the file object interface. *input* will be read until
   ``input.read()`` returns an empty string.


.. function:: decodestring(s)

   Decode the string *s*, which must contain one or more lines of base64 encoded
   data, and return a string containing the resulting binary data.


.. function:: encode(input, output)

   Encode the contents of the *input* file and write the resulting base64 encoded
   data to the *output* file. *input* and *output* must either be file objects or
   objects that mimic the file object interface. *input* will be read until
   ``input.read()`` returns an empty string.  :func:`encode` returns the encoded
   data plus a trailing newline character (``'\n'``).


.. function:: encodestring(s)

   Encode the string *s*, which can contain arbitrary binary data, and return a
   string containing one or more lines of base64-encoded data.
   :func:`encodestring` returns a string containing one or more lines of
   base64-encoded data always including an extra trailing newline (``'\n'``).

An example usage of the module:

   >>> import base64
   >>> encoded = base64.b64encode('data to be encoded')
   >>> encoded
   'ZGF0YSB0byBiZSBlbmNvZGVk'
   >>> data = base64.b64decode(encoded)
   >>> data
   'data to be encoded'


.. seealso::

   Module :mod:`binascii`
      Support module containing ASCII-to-binary and binary-to-ASCII conversions.

   :rfc:`1521` - MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies
      Section 5.2, "Base64 Content-Transfer-Encoding," provides the definition of the
      base64 encoding.