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Doc/library/base64.rst
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[3.14] gh-134201: Expand explanation of Base85 encodings in base64 docs (GH-134288) (#134298)
20 май 2025, 03:05
20 май 2025, 03:05e20f05f
:mod:`!base64` --- Base16, Base32, Base64, Base85 Data Encodings
================================================================

.. module:: base64
   :synopsis: RFC 4648: Base16, Base32, Base64 Data Encodings;
              Base85 and Ascii85

**Source code:** :source:`Lib/base64.py`

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

--------------

This module provides functions for encoding binary data to printable
ASCII characters and decoding such encodings back to binary data.
This includes the :ref:`encodings specified in <base64-rfc-4648>`
:rfc:`4648` (Base64, Base32 and Base16)
and the non-standard :ref:`Base85 encodings <base64-base-85>`.

There are two interfaces provided by this module.  The modern interface
supports encoding :term:`bytes-like objects <bytes-like object>` to ASCII
:class:`bytes`, and decoding :term:`bytes-like objects <bytes-like object>` or
strings containing ASCII to :class:`bytes`.  Both base-64 alphabets
defined in :rfc:`4648` (normal, and URL- and filesystem-safe) are supported.

The :ref:`legacy interface <base64-legacy>` does not support decoding from strings, but it does
provide functions for encoding and decoding to and from :term:`file objects
<file object>`.  It only supports the Base64 standard alphabet, and it adds
newlines every 76 characters as per :rfc:`2045`.  Note that if you are looking
for :rfc:`2045` support you probably want to be looking at the :mod:`email`
package instead.


.. versionchanged:: 3.3
   ASCII-only Unicode strings are now accepted by the decoding functions of
   the modern interface.

.. versionchanged:: 3.4
   Any :term:`bytes-like objects <bytes-like object>` are now accepted by all
   encoding and decoding functions in this module.  Ascii85/Base85 support added.


.. _base64-rfc-4648:

RFC 4648 Encodings
------------------

The :rfc:`4648` encodings are suitable for encoding binary data so that it can be
safely sent by email, used as parts of URLs, or included as part of an HTTP
POST request.

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

   Encode the :term:`bytes-like object` *s* using Base64 and return the encoded
   :class:`bytes`.

   Optional *altchars* must be a :term:`bytes-like object` of length 2 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.

   May assert or raise a :exc:`ValueError` if the length of *altchars* is not 2.  Raises a
   :exc:`TypeError` if *altchars* is not a :term:`bytes-like object`.


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

   Decode the Base64 encoded :term:`bytes-like object` or ASCII string
   *s* and return the decoded :class:`bytes`.

   Optional *altchars* must be a :term:`bytes-like object` or ASCII string
   of length 2 which specifies the alternative alphabet used instead of the
   ``+`` and ``/`` characters.

   A :exc:`binascii.Error` exception is raised
   if *s* is incorrectly padded.

   If *validate* is ``False`` (the default), characters that are neither
   in the normal base-64 alphabet nor the alternative alphabet are
   discarded prior to the padding check.  If *validate* is ``True``,
   these non-alphabet characters in the input result in a
   :exc:`binascii.Error`.

   For more information about the strict base64 check, see :func:`binascii.a2b_base64`

   May assert or raise a :exc:`ValueError` if the length of *altchars* is not 2.

.. function:: standard_b64encode(s)

   Encode :term:`bytes-like object` *s* using the standard Base64 alphabet
   and return the encoded :class:`bytes`.


.. function:: standard_b64decode(s)

   Decode :term:`bytes-like object` or ASCII string *s* using the standard
   Base64 alphabet and return the decoded :class:`bytes`.


.. function:: urlsafe_b64encode(s)

   Encode :term:`bytes-like object` *s* using the
   URL- and filesystem-safe alphabet, which
   substitutes ``-`` instead of ``+`` and ``_`` instead of ``/`` in the
   standard Base64 alphabet, and return the encoded :class:`bytes`.  The result
   can still contain ``=``.


.. function:: urlsafe_b64decode(s)

   Decode :term:`bytes-like object` or ASCII string *s*
   using the URL- and filesystem-safe
   alphabet, which substitutes ``-`` instead of ``+`` and ``_`` instead of
   ``/`` in the standard Base64 alphabet, and return the decoded
   :class:`bytes`.


.. function:: b32encode(s)

   Encode the :term:`bytes-like object` *s* using Base32 and return the
   encoded :class:`bytes`.


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

   Decode the Base32 encoded :term:`bytes-like object` or ASCII string *s* and
   return the decoded :class:`bytes`.

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

   :rfc:`4648` 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.

   A :exc:`binascii.Error` is raised if *s* is
   incorrectly padded or if there are non-alphabet characters present in the
   input.


.. function:: b32hexencode(s)

   Similar to :func:`b32encode` but uses the Extended Hex Alphabet, as defined in
   :rfc:`4648`.

   .. versionadded:: 3.10


.. function:: b32hexdecode(s, casefold=False)

   Similar to :func:`b32decode` but uses the Extended Hex Alphabet, as defined in
   :rfc:`4648`.

   This version does not allow the digit 0 (zero) to the letter O (oh) and digit
   1 (one) to either the letter I (eye) or letter L (el) mappings, all these
   characters are included in the Extended Hex Alphabet and are not
   interchangeable.

   .. versionadded:: 3.10


.. function:: b16encode(s)

   Encode the :term:`bytes-like object` *s* using Base16 and return the
   encoded :class:`bytes`.


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

   Decode the Base16 encoded :term:`bytes-like object` or ASCII string *s* and
   return the decoded :class:`bytes`.

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

   A :exc:`binascii.Error` is raised if *s* is
   incorrectly padded or if there are non-alphabet characters present in the
   input.

.. _base64-base-85:

Base85 Encodings
-----------------

Base85 encoding is not formally specified but rather a de facto standard,
thus different systems perform the encoding differently.

The :func:`a85encode` and :func:`b85encode` functions in this module are two implementations of
the de facto standard. You should call the function with the Base85
implementation used by the software you intend to work with.

The two functions present in this module differ in how they handle the following:

* Whether to include enclosing ``<~`` and ``~>`` markers
* Whether to include newline characters
* The set of ASCII characters used for encoding
* Handling of null bytes

Refer to the documentation of the individual functions for more information.

.. function:: a85encode(b, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)

   Encode the :term:`bytes-like object` *b* using Ascii85 and return the
   encoded :class:`bytes`.

   *foldspaces* is an optional flag that uses the special short sequence 'y'
   instead of 4 consecutive spaces (ASCII 0x20) as supported by 'btoa'. This
   feature is not supported by the "standard" Ascii85 encoding.

   *wrapcol* controls whether the output should have newline (``b'\n'``)
   characters added to it. If this is non-zero, each output line will be
   at most this many characters long, excluding the trailing newline.

   *pad* controls whether the input is padded to a multiple of 4
   before encoding. Note that the ``btoa`` implementation always pads.

   *adobe* controls whether the encoded byte sequence is framed with ``<~``
   and ``~>``, which is used by the Adobe implementation.

   .. versionadded:: 3.4


.. function:: a85decode(b, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\v')

   Decode the Ascii85 encoded :term:`bytes-like object` or ASCII string *b* and
   return the decoded :class:`bytes`.

   *foldspaces* is a flag that specifies whether the 'y' short sequence
   should be accepted as shorthand for 4 consecutive spaces (ASCII 0x20).
   This feature is not supported by the "standard" Ascii85 encoding.

   *adobe* controls whether the input sequence is in Adobe Ascii85 format
   (i.e. is framed with <~ and ~>).

   *ignorechars* should be a :term:`bytes-like object` or ASCII string
   containing characters to ignore
   from the input. This should only contain whitespace characters, and by
   default contains all whitespace characters in ASCII.

   .. versionadded:: 3.4


.. function:: b85encode(b, pad=False)

   Encode the :term:`bytes-like object` *b* using base85 (as used in e.g.
   git-style binary diffs) and return the encoded :class:`bytes`.

   If *pad* is true, the input is padded with ``b'\0'`` so its length is a
   multiple of 4 bytes before encoding.

   .. versionadded:: 3.4


.. function:: b85decode(b)

   Decode the base85-encoded :term:`bytes-like object` or ASCII string *b* and
   return the decoded :class:`bytes`.  Padding is implicitly removed, if
   necessary.

   .. versionadded:: 3.4


.. function:: z85encode(s)

   Encode the :term:`bytes-like object` *s* using Z85 (as used in ZeroMQ)
   and return the encoded :class:`bytes`.  See `Z85  specification
   <https://rfc.zeromq.org/spec/32/>`_ for more information.

   .. versionadded:: 3.13


.. function:: z85decode(s)

   Decode the Z85-encoded :term:`bytes-like object` or ASCII string *s* and
   return the decoded :class:`bytes`.  See `Z85  specification
   <https://rfc.zeromq.org/spec/32/>`_ for more information.

   .. versionadded:: 3.13


.. _base64-legacy:

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.readline()`` returns an
   empty bytes object.


.. function:: decodebytes(s)

   Decode the :term:`bytes-like object` *s*, which must contain one or more
   lines of base64 encoded data, and return the decoded :class:`bytes`.

   .. versionadded:: 3.1


.. 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` inserts a newline character (``b'\n'``)
   after every 76 bytes of the output, as well as ensuring that the output
   always ends with a newline, as per :rfc:`2045` (MIME).


.. function:: encodebytes(s)

   Encode the :term:`bytes-like object` *s*, which can contain arbitrary binary
   data, and return :class:`bytes` containing the base64-encoded data, with newlines
   (``b'\n'``) inserted after every 76 bytes of output, and ensuring that
   there is a trailing newline, as per :rfc:`2045` (MIME).

   .. versionadded:: 3.1


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'

.. _base64-security:

Security Considerations
-----------------------

A new security considerations section was added to :rfc:`4648` (section 12); it's
recommended to review the security section for any code deployed to production.

.. 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.