Doc/library/base64.rst

: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), the :ref:`Base85 encoding
<base64-base-85>` specified in `PDF 2.0
<https://pdfa.org/resource/iso-32000-2/>`_, and non-standard variants
of Base85 used elsewhere.

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, *, padded=True, wrapcol=0)

   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.

   If *padded* is true (default), pad the encoded data with the '='
   character to a size multiple of 4.
   If *padded* is false, do not add the pad characters.

   If *wrapcol* is non-zero, insert a newline (``b'\n'``) character
   after at most every *wrapcol* characters.
   If *wrapcol* is zero (default), do not insert any newlines.

   .. versionchanged:: 3.15
      Added the *padded* and *wrapcol* parameters.


.. function:: b64decode(s, altchars=None, validate=False, *, padded=True, canonical=False)
              b64decode(s, altchars=None, validate=True, *, ignorechars, padded=True, canonical=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.

   If *padded* is true, the last group of 4 base 64 alphabet characters must
   be padded with the '=' character.
   If *padded* is false, padding is neither required nor recognized:
   the '=' character is not treated as padding but as a non-alphabet
   character, which means it is silently discarded when *validate* is false,
   or causes an :exc:`~binascii.Error` when *validate* is true unless
   b'=' is included in *ignorechars*.

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

   If *ignorechars* is specified, it should be a :term:`bytes-like object`
   containing characters to ignore from the input when *validate* is true.
   If *ignorechars* contains the pad character ``'='``,  the pad characters
   presented before the end of the encoded data and the excess pad characters
   will be ignored.
   The default value of *validate* is ``True`` if *ignorechars* is specified,
   ``False`` otherwise.

   If *validate* is false, characters that are neither
   in the normal base-64 alphabet nor (if *ignorechars* is not specified)
   the alternative alphabet are
   discarded prior to the padding check, but the ``+`` and ``/`` characters
   keep their meaning if they are not in *altchars* (they will be discarded
   in future Python versions).

   If *validate* is true, these non-alphabet characters in the input
   result in a :exc:`binascii.Error`.

   If *canonical* is true, non-zero padding bits are rejected.
   See :func:`binascii.a2b_base64` for details.

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

   .. versionchanged:: 3.15
      Added the *canonical*, *ignorechars*, and *padded* parameters.

   .. deprecated:: 3.15
      Accepting the ``+`` and ``/`` characters with an alternative alphabet
      is now deprecated.


.. 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, *, padded=True)

   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 ``=`` if *padded* is true (default).

   .. versionchanged:: 3.15
      Added the *padded* parameter.


.. function:: urlsafe_b64decode(s, *, padded=False)

   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`.

   .. versionchanged:: 3.15
      Added the *padded* parameter.
      Padding of input is no longer required by default.

   .. deprecated:: 3.15
      Accepting the ``+`` and ``/`` characters is now deprecated.


.. function:: b32encode(s, *, padded=True, wrapcol=0)

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

   If *padded* is true (default), pad the encoded data with the '='
   character to a size multiple of 8.
   If *padded* is false, do not add the pad characters.

   If *wrapcol* is non-zero, insert a newline (``b'\n'``) character
   after at most every *wrapcol* characters.
   If *wrapcol* is zero (default), do not add any newlines.

   .. versionchanged:: 3.15
      Added the *padded* and *wrapcol* parameters.


.. function:: b32decode(s, casefold=False, map01=None, *, padded=True, ignorechars=b'', canonical=False)

   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.

   If *padded* is true, the last group of 8 base 32 alphabet characters must
   be padded with the '=' character.
   If *padded* is false, padding is neither required nor recognized:
   the '=' character is not treated as padding but as a non-alphabet
   character, which means it raises an :exc:`~binascii.Error` unless
   b'=' is included in *ignorechars*.

   *ignorechars* should be a :term:`bytes-like object` containing characters
   to ignore from the input.

   If *canonical* is true, non-zero padding bits are rejected.
   See :func:`binascii.a2b_base32` for details.

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

   .. versionchanged:: 3.15
      Added the *canonical*, *ignorechars*, and *padded* parameters.


.. function:: b32hexencode(s, *, padded=True, wrapcol=0)

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

   .. versionadded:: 3.10

   .. versionchanged:: 3.15
      Added the *padded* and *wrapcol* parameters.


.. function:: b32hexdecode(s, casefold=False, *, padded=True, ignorechars=b'', canonical=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

   .. versionchanged:: 3.15
      Added the *canonical*, *ignorechars*, and *padded* parameters.


.. function:: b16encode(s, *, wrapcol=0)

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

   If *wrapcol* is non-zero, insert a newline (``b'\n'``) character
   after at most every *wrapcol* characters.
   If *wrapcol* is zero (default), do not add any newlines.

   .. versionchanged:: 3.15
      Added the *wrapcol* parameter.


.. function:: b16decode(s, casefold=False, *, ignorechars=b'')

   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``.

   *ignorechars* should be a :term:`bytes-like object` containing characters
   to ignore from the input.

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

   .. versionchanged:: 3.15
      Added the *ignorechars* parameter.


.. _base64-base-85:

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

Base85 encoding is a family of algorithms which represent four bytes
using five ASCII characters.  Originally implemented in the Unix
``btoa(1)`` utility, a version of it was later adopted by Adobe in the
PostScript language and is standardized in PDF 2.0 (ISO 32000-2).
This version, in both its ``btoa`` and PDF variants, is implemented by
:func:`a85encode`.

A separate version, using a different output character set, was
defined as an April Fool's joke in :rfc:`1924` but is now used by Git
and other software.  This version is implemented by :func:`b85encode`.

Finally, a third version, using yet another output character set
designed for safe inclusion in programming language strings, is
defined by ZeroMQ and implemented here by :func:`z85encode`.

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

* Whether to include and expect enclosing ``<~`` and ``~>`` markers.
* Whether to fold the input into multiple lines.
* The set of ASCII characters used for encoding.
* Compact encodings of sequences of spaces and null bytes.
* The encoding of zero-padding bytes applied to the input.

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 encoding used in PDF.

   If *wrapcol* is non-zero, insert a newline (``b'\n'``) character
   after at most every *wrapcol* characters.
   If *wrapcol* is zero (default), do not insert any newlines.

   *pad* controls whether zero-padding applied to the end of the input
   is fully retained in the output encoding, as done by ``btoa``,
   producing an exact multiple of 5 bytes of output. This is not part
   of the standard encoding used in PDF, as it does not preserve the
   length of the data.

   *adobe* controls whether the encoded byte sequence is framed with
   ``<~`` and ``~>``, as in a PostScript base-85 string literal.  Note
   that while ASCII85Decode streams in PDF documents *must* be
   terminated with ``~>``, they *must not* use a leading ``<~``.

   .. versionadded:: 3.4


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

   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 used in
   PDF and PostScript.

   *adobe* controls whether the ``<~`` and ``~>`` markers are
   present. While the leading ``<~`` is not required, the input must
   end with ``~>``, or a :exc:`ValueError` is raised.

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

   If *canonical* is true, non-canonical encodings are rejected.
   See :func:`binascii.a2b_ascii85` for details.

   .. versionadded:: 3.4

   .. versionchanged:: 3.15
      Added the *canonical* parameter.
      Single-character final groups are now always rejected as encoding
      violations.


.. function:: b85encode(b, pad=False, *, wrapcol=0)

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

   The input is padded with ``b'\0'`` so its length is a multiple of 4
   bytes before encoding.  If *pad* is true, all the resulting
   characters are retained in the output, which will always be a
   multiple of 5 bytes, and thus the length of the data may not be
   preserved on decoding.

   If *wrapcol* is non-zero, insert a newline (``b'\n'``) character
   after at most every *wrapcol* characters.
   If *wrapcol* is zero (default), do not add any newlines.

   .. versionadded:: 3.4

   .. versionchanged:: 3.15
      Added the *wrapcol* parameter.


.. function:: b85decode(b, *, ignorechars=b'', canonical=False)

   Decode the base85-encoded :term:`bytes-like object` or ASCII string *b* and
   return the decoded :class:`bytes`.

   *ignorechars* should be a :term:`bytes-like object` containing characters
   to ignore from the input.

   If *canonical* is true, non-canonical encodings are rejected.
   See :func:`binascii.a2b_base85` for details.

   .. versionadded:: 3.4

   .. versionchanged:: 3.15
      Added the *canonical* and *ignorechars* parameters.
      Single-character final groups are now always rejected as encoding
      violations.


.. function:: z85encode(s, pad=False, *, wrapcol=0)

   Encode the :term:`bytes-like object` *s* using Z85 (as used in ZeroMQ)
   and return the encoded :class:`bytes`.

   The input is padded with ``b'\0'`` so its length is a multiple of 4
   bytes before encoding.  If *pad* is true, all the resulting
   characters are retained in the output, which will always be a
   multiple of 5 bytes, as required by the ZeroMQ standard.

   If *wrapcol* is non-zero, insert a newline (``b'\n'``) character
   after at most every *wrapcol* characters.
   If *wrapcol* is zero (default), do not add any newlines.

   .. versionadded:: 3.13

   .. versionchanged:: 3.15
      The *pad* parameter was added.

   .. versionchanged:: 3.15
      Added the *wrapcol* parameter.


.. function:: z85decode(s, *, ignorechars=b'', canonical=False)

   Decode the Z85-encoded :term:`bytes-like object` or ASCII string *s* and
   return the decoded :class:`bytes`.

   *ignorechars* should be a :term:`bytes-like object` containing characters
   to ignore from the input.

   If *canonical* is true, non-canonical encodings are rejected.
   See :func:`binascii.a2b_base85` for details.

   .. versionadded:: 3.13

   .. versionchanged:: 3.15
      Added the *canonical* and *ignorechars* parameters.
      Single-character final groups are now always rejected as encoding
      violations.


.. _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.

   `ISO 32000-2 Portable document format - Part 2: PDF 2.0 <https://pdfa.org/resource/iso-32000-2/>`_
      Section 7.4.3, "ASCII85Decode Filter," provides the definition
      of the Ascii85 encoding used in PDF and PostScript, including
      the output character set and the details of data length preservation
      using zero-padding and partial output groups.

   `ZeroMQ RFC 32/Z85 <https://rfc.zeromq.org/spec/32/>`_
      The "Formal Specification" section provides the character set used in Z85.