# `base64` —— Base16、Base32、Base64、Base85 資料編碼

Source code: Lib/base64.py

這個模組提供將二進位資料編碼成可顯示 ASCII 字元以及解碼回原始資料的功能，包括了 RFC 3548中的 Base16、Base32、Base64 等編碼方式，以及標準 Ascii85、Base85 編碼等。

RFC 3548標準編碼可以使電子郵件、URL，或是 HTTP POST 內容等傳輸管道安全地傳遞資料。這些編碼演算法與uuencode 並不相同。

There are two interfaces provided by this module. The modern interface supports encoding bytes-like objects to ASCII `bytes`, and decoding bytes-like objects or strings containing ASCII to `bytes`. Both base-64 alphabets defined in RFC 3548 (normal, and URL- and filesystem-safe) are supported.

The legacy interface does not support decoding from strings, but it does provide functions for encoding and decoding to and from file objects. 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 `email` package instead.

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

3.4 版更變: Any bytes-like objects are now accepted by all encoding and decoding functions in this module. Ascii85/Base85 support added.

The modern interface provides:

`base64.``b64encode`(s, altchars=None)

  Encode the bytes-like object s using Base64 and return the encoded `bytes`.

  Optional altchars must be a bytes-like object 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.

`base64.``b64decode`(s, altchars=None, validate=False)

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

  Optional altchars must be a bytes-like object or ASCII string of at least length 2 (additional characters are ignored) which specifies the alternative alphabet used instead of the `+` and `/` characters.

  A `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 `binascii.Error`.

`base64.``standard_b64encode`(s)

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

`base64.``standard_b64decode`(s)

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

`base64.``urlsafe_b64encode`(s)

  Encode 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 `bytes`. The result can still contain `=`.

`base64.``urlsafe_b64decode`(s)

  Decode 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 `bytes`.

`base64.``b32encode`(s)

  Encode the bytes-like object s using Base32 and return the encoded `bytes`.

`base64.``b32decode`(s, casefold=False, map01=None)

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

  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.

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

`base64.``b16encode`(s)

  Encode the bytes-like object s using Base16 and return the encoded `bytes`.

`base64.``b16decode`(s, casefold=False)

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

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

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

`base64.``a85encode`(b, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)

  Encode the bytes-like object b using Ascii85 and return the encoded `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.

  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.

  3.4 版新加入.

`base64.``a85decode`(b, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\v')

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

  3.4 版新加入.

`base64.``b85encode`(b, pad=False)

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

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

  3.4 版新加入.

`base64.``b85decode`(b)

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

  3.4 版新加入.

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.readline()` returns an empty bytes object.

`base64.``decodebytes`(s)

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

  3.1 版新加入.

`base64.``decodestring`(s)

  Deprecated alias of `decodebytes()`.

  3.1 版後已棄用.

`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()` 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).

`base64.``encodebytes`(s)

  Encode the bytes-like object s, which can contain arbitrary binary data, and return `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).

  3.1 版新加入.

`base64.``encodestring`(s)

  Deprecated alias of `encodebytes()`.

  3.1 版後已棄用.

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'

也參考

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.