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`Base64` divides the sequence of bytes to be encoded into groups of three bytes (with the last group possibly being partially filled). Each group of three bytes is then divided into four six-bit fields and each six bit field is encoded as a character from the table below.
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The `Content-Transfer-Encoding:` header line is used to specify how the message has been encoded. When MIME was designed, some email servers were only able to process messages containing characters encoded using the 7 bits ASCII character set. MIME allows the utilization of other character encodings.
 
Inside the email header, the `Content-Type:` header line indicates how the MIME email message is structured. :rfc:`2046` defines the utilization of this header line. The two most common structures for MIME messages are :
 
`Content-Type: multipart/mixed`. This header line indicates that the MIME message contains several independent parts. For example, such a message may contain a part in plain text and a binary file.
 
`Content-Type: multipart/alternative`. This header line indicates that the MIME message contains several representations of the same information. For example, a `multipart/alternative` message may contain both a plain text and an HTML version of the same text.
 
To support these two types of MIME messages, the recipient of a message must be able to extract the different parts from the message. In :rfc:`822`, an empty line was used to separate the header lines from the body. Using an empty line to separate the different parts of an email body would be difficult as the body of email messages often contains one or more empty lines. Another possible option would be to define a special line, e.g. `*-LAST_LINE-*` to mark the boundary between two parts of a MIME message. Unfortunately, this is not possible as some emails may contain this string in their body (e.g. emails sent to students to explain the format of MIME messages). To solve this problem, the `Content-Type:` header line contains a second parameter that specifies the string that has been used by the sender of the MIME message to delineate the different parts. In practice, this string is often chosen randomly by the mail client.
 
The email message below, copied from :rfc:`2046` shows a MIME message containing two parts that are both in plain text and encoded using the ASCII character set. The string `simple boundary` is defined in the `Content-Type:` header as the marker for the boundary between two successive parts. Another example of MIME messages may be found in :rfc:`2046`.
 
The `Content-Type:` header can also be used inside a MIME part. In this case, it indicates the type of data placed in this part. Each data type is specified as a type followed by a subtype. A detailed description may be found in :rfc:`2046`. Some of the most popular `Content-Type:` header lines are :
 
`text`. The message part contains information in textual format. There are several subtypes : `text/plain` for regular ASCII text, `text/html` defined in :rfc:`2854` for documents in :term:`HTML` format or the `text/enriched` format defined in :rfc:`1896`. The `Content-Type:` header line may contain a second parameter that specifies the character set used to encode the text. `charset=us-ascii` is the standard ASCII character table. Other frequent character sets include `charset=UTF8` or `charset=iso-8859-1`. The `list of standard character sets <http://www.iana.org/assignments/character-sets>`_ is maintained by :term:`IANA`.
 
`image`. The message part contains a binary representation of an image. The subtype indicates the format of the image such as `gif <http://en.wikipedia.org/wiki/Graphics_Interchange_Format>`_, `jpg <http://en.wikipedia.org/wiki/Jpeg>`_ or `png <http://en.wikipedia.org/wiki/Portable_Network_Graphics>`_.
 
`audio`. The message part contains an audio clip. The subtype indicates the format of the audio clip like `wav <http://en.wikipedia.org/wiki/Wav>`_ or `mp3 <http://en.wikipedia.org/wiki/Mp3>`_.
 
`video`. The message part contains a video clip. The subtype indicates the format of the video clip like `avi <http://en.wikipedia.org/wiki/Audio_Video_Interleave>`_ or `mp4 <http://en.wikipedia.org/wiki/Mp4>`_.
 
`application`. The message part contains binary information that was produced by the particular application listed as the subtype. Email clients use the subtype to launch the application that is able to decode the received binary information.
 
From ASCII to Unicode
 
The first computers used different techniques to represent characters in memory and on disk. During the 1960s, computers began to exchange information via tape or telephone lines. Unfortunately, each vendor had its own proprietary character set and exchanging data between computers from different vendors was often difficult. The 7 bits ASCII character table :rfc:`20` was adopted by several vendors and by many Internet protocols. However, ASCII became a problem with the internationalization of the Internet and the desire of more and more users to use character sets that support their own written language. A first attempt at solving this problem was the definition of the ISO-8859_ character sets by :term:`ISO`. This family of standards specified various character sets that allowed the representation of many European written languages by using 8 bits characters. Unfortunately, an 8-bits character set is not sufficient to support some widely used languages, such as those used in Asian countries. Fortunately, at the end of the 1980s, several computer scientists proposed to develop a standard that supports all written languages used on Earth today. The Unicode standard [Unicode]_ has now been adopted by most computer and software vendors. For example, Java always uses Unicode to manipulate characters, Python can handle both ASCII and Unicode characters. Internet applications are slowly moving towards complete support for the Unicode character sets, but moving from ASCII to Unicode is an important change that can have a huge impact on current deployed implementations. See, for example, the work to completely internationalize email :rfc:`4952` and domain names :rfc:`5890`.
 
The last MIME header line is `Content-Transfer-Encoding:`. This header line is used after the `Content-Type:` header line, within a message part, and specifies how the message part has been encoded. The default encoding is to use 7 bits ASCII. The most frequent encodings are `quoted-printable` and `Base64`. Both support encoding a sequence of bytes into a set of ASCII lines that can be safely transmitted by email servers. `quoted-printable` is defined in :rfc:`2045`. We briefly describe `base64` which is defined in :rfc:`2045` and :rfc:`4648`.
 
`Base64` divides the sequence of bytes to be encoded into groups of three bytes (with the last group possibly being partially filled). Each group of three bytes is then divided into four six-bit fields and each six bit field is encoded as a character from the table below.
 
Value
 
Encoding
 
0
 
A
 
17
 
R
 
34
 
i
 
51
 
z
 
1
 
B
 
18
 
S
 
35
 

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Source string added
`Base64` divides the sequence of bytes to be encoded into groups of three bytes (with the last group possibly being partially filled). Each group of three bytes is then divided into four six-bit fields and each six bit field is encoded as a character from the table below.
4 years ago
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30966 String information

Source string location
../../protocols/email.rst:227
String age
4 years ago
Last updated
2 months ago
Source string age
4 years ago
Translation file
locale/fr/LC_MESSAGES/protocols/email.po, string 48