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

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 **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. Additionally, the decoding functions of the
modern interface also accept Unicode strings containing only ASCII
characters. 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.

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

The modern interface provides:

base64.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.

base64.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 ``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 ``binascii.Error``.

base64.standard_b64encode(s)

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

base64.standard_b64decode(s)

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

base64.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 ``=``.

base64.urlsafe_b64decode(s)

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

base64.b32encode(s)

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

base64.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 ``TypeError`` is raised if
   *s* were incorrectly padded or if there are non-alphabet characters
   present in the string.

base64.b16encode(s)

   Encode a byte string using Base16.

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

base64.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 ``TypeError`` is raised if
   *s* were incorrectly padded or if there are non-alphabet characters
   present in the string.

The legacy interface:

base64.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 *file objects*. *input* will be read until ``input.read()``
   returns an empty bytes object.

base64.decodebytes(s)
base64.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.

   New in version 3.1.

base64.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 *file objects*. *input* will be read until
   ``input.read()`` returns an empty bytes object. ``encode()``
   returns the encoded data plus a trailing newline character
   (``b'\n'``).

base64.encodebytes(s)
base64.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. ``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'

See also:

   Module ``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.
