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 ASCII-only byte 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 ASCII byte string objects using all three
alphabets.  The legacy interface provides for encoding and decoding to and from
file-like objects as well as byte strings, but only using the Base64 standard
alphabet.

The modern interface provides:

.. function:: b64encode(s, altchars=None)

   Encode a byte 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 byte string is returned.


.. function:: b64decode(s, altchars=None, validate=False)

   Decode a Base64 encoded byte string.

   *s* is the byte 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:`binascii.Error` exception is raised
   if *s* is incorrectly padded.

   If *validate* is ``False`` (the default), non-base64-alphabet characters are
   discarded prior to the padding check.  If *validate* is ``True``,
   non-base64-alphabet characters in the input result in a
   :exc:`binascii.Error`.


.. function:: standard_b64encode(s)

   Encode byte string *s* using the standard Base64 alphabet.


.. function:: standard_b64decode(s)

   Decode byte string *s* using the standard Base64 alphabet.


.. function:: urlsafe_b64encode(s)

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


.. function:: urlsafe_b64decode(s)

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


.. function:: b32encode(s)

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


.. function:: b32decode(s, casefold=False, map01=None)

   Decode a Base32 encoded byte string.

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


.. function:: b16encode(s)

   Encode a byte string using Base16.

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


.. function:: b16decode(s, casefold=False)

   Decode a Base16 encoded byte 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 byte 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 binary *input* file and write the resulting binary
   data to the *output* file. *input* and *output* must be :term:`file objects
   <file object>`. *input* will be read until ``input.read()`` returns an empty
   bytes object.


.. function:: decodebytes(s)
              decodestring(s)

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


.. function:: encode(input, output)

   Encode the contents of the binary *input* file and write the resulting base64
   encoded data to the *output* file. *input* and *output* must be :term:`file
   objects <file object>`. *input* will be read until ``input.read()`` returns
   an empty bytes object. :func:`encode` returns the encoded data plus a trailing
   newline character (``b'\n'``).


.. function:: encodebytes(s)
              encodestring(s)

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


An example usage of the module:

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