KMime

Table of Contents

• Introduction
• Structure of this document
• Structure of MIME messages
  • A brief history of the MIME standard
  • MIME by examples
    • A simple message
    • Encodings and charsets
      • Charset Encoding
      • Content Transfer Encoding
      • Conclusion
    • Encoding in Header Fields
    • Messages with attachments
    • HTML Messages
    • HTML Messages with Inline Images
    • Encapsulated messages
    • Signed and Encrypted Messages
      • Signed MIME parts
      • Encrypted MIME parts
      • Inline Crypto Formats
    • Miscellaneous Points about Messages
      • Line Breaks
      • Header Folding and CFWS
• The two in-memory representations of messages # {#string-broken-down }
• Overview of KMime classes
• RFCs
• Further Reading

Introduction

KMime is a library for handling mail messages and newsgroup articles. Both mail messages and newsgroup articles are based on the same standard called MIME, which stands for Multipurpose Internet Mail Extensions. In this document, the term message is used to refer to both mail messages and newsgroup articles.

KMime deals solely with the in-memory representation of messages. Topics such as transport or storage of messages are handled by other libraries, for example by the mailtransport library or by the KIMAP library. Similarly, this library does not deal with displaying messages or advanced composing, for those there are the messageviewer and messagecomposer components in the KDE PIM messagelib module.

KMime's main function is to parse, modify and assemble messages in-memory. In a later section, parsing and assembling are actually explained. KMime provides high-level classes that make these tasks easy.

MIME is defined by various RFCs, see the RFC section for a list of them.

Structure of this document

This document will first give an introduction to the MIME specification, as it is essential to understand the basics of the structure of MIME messages for using this library. The introduction here is aimed at users of the library. It gives a broad overview with examples and omits some details. Developers who wish to modify KMime should read the corresponding RFCs as well, but this is not necessary for library users.

After the introduction to the MIME format, the two ways of representing a message in memory are discussed, the string representation and the broken down representation.

This is followed by a section giving an overview of the most important KMime classes.

The last sections give a list of relevant RFCs and provide links for further reading.

Structure of MIME messages

A brief history of the MIME standard

The MIME standard is quite new (1993), email and usenet existed way before the MIME standard came into existence. Because of this, the MIME standard has to keep backwards compatibility. The email standard before MIME lacked many capabilities, like encodings other than ASCII, or attachments. These and other things were later added by MIME. The standard for messages before MIME is defined in RFC 5233. In RFC 2045 to RFC 2049, several backward-compatible extensions to the basic message format are defined, adding support for attachments, different encodings and many others.

Actually, there is an even older standard, defined in RFC 733 (Standard for the format of ARPA network text messages, introduced in 1977). This standard is now obsoleted by RFC 5322, but backwards compatibility is in some cases supported, as there are still messages in this format around.

Since pre-MIME messages had no way to handle attachments, attachments were sometimes added to the message text in an uuencoded form. Although this is also obsolete, reading uuencoded attachments is still supported by KMime.

After MIME was introduced, people realized that there was no way to have the filename of attachments encoded in anything other than ASCII. Thus, RFC 2231 was introduced to allow arbitrary encodings for parameter values, such as the attachment filename.

MIME by examples

In the following sections, MIME message examples are shown, examined and explained, starting with a simple message and proceeding to more interesting examples. You can get additional examples by simply viewing the source of your own messages in your mail client, or by having a look at the examples in the various RFCs.

A simple message

Subject: First Mail
From: John Doe <john.doe@domain.com>
Date: Sun, 21 Feb 2010 19:16:11 +0100
MIME-Version: 1.0

Hello World!

The above example features a very simple message. The two main parts of this message are the header and the body, which are separated by an empty line. The body contains the actual message content, and the header contains metadata about the message itself. The header consists of several header fields, each of them in their own line. Header fields are made up from the header field name, followed by a colon, followed by the header field body.

The MIME-Version header field is mandatory for MIME messages. Subject, From and Date are important header fields; they are usually displayed in the message list of a mail client. The Subject header field can be anything, it does not have a special structure. It is a so-called unstructured header field. In contrast, the From and the Date header fields have to follow a special structure, they must be formed in a way that machines can parse. They are structured header fields. For example, a mail client needs to understand the Date header field so that it can sort the messages by date in the message list. The exact details of how the header field bodies of structured header fields should be formed are specified in an RFC.

In this example, the From header contains a single email address. More precisely, a single email address is called a mailbox, which is made up of the display name (John Doe) and the address specification (john..nosp@m.doe@.nosp@m.domai.nosp@m.n.co.nosp@m.m), which is enclosed in angle brackets. The addr-spec consists of the user name, the local part, and the domain name.

Many header fields can contain multiple email addresses, for example the To field for messages with multiple recipients can have a comma-separated list of mailboxes. A list of mailboxes, together with a display name for the list, forms a group, and multiple groups can form an address list. This is however rarely used, you'll most often see a simple list of plain mailboxes.

There are many more possible header fields than shown in this example, and the header can even contain arbitrary header fields, which usually are prefixed with X-, like X-Face.

Encodings and charsets

From: John Doe <john.doe@domain.com>
Date: Mon, 22 Feb 2010 00:42:45 +0100
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

Gr=FCezi Welt!

The above shows a message that is using a different charset than the standard US-ASCII charset. The message body contains the string "Grüezi Welt!", which is encoded in a special way.

The content-type of this message is text/plain, which means that the message is simple text. Later, other content types will be introduced, such as text/html. If there is no Content-Type header field, it is assumed that the content-type is text/plain.

Before MIME was introduced, all messages were limited to the US-ASCII charset. Only the lower 127 values of the bytes were allowed to be used, the so-called 7-bit range. Writing a message in another charset or using letters from the upper 127 byte values was not allowed.

Charset Encoding

When talking about charsets, it is important to understand how strings of text are converted to byte arrays, and the other way around. A message is nothing else than a big array of bytes. The bytes that form the body of the message somehow need to be interpreted as a text string. Interpreting a byte array as a text string is called decoding the text. Converting a text string to a byte array is called encoding the text. A codec (**co**der-**dec**oder) is a utility that can encode and decode text. In Qt, the class for text strings is QString, and the class for byte arrays is QByteArray.

With the US-ASCII charset, encoding and decoding text is easy, one just has to look at an ASCII table to be able to convert text strings to byte arrays and byte arrays to text strings. For example, the letter 'A' is represented by a single byte with the value of 65. When encountering a byte with the value 84, we can look that up in the table and see that it represents the letter 'T'. With the US-ASCII charset, each letter is represented by exactly one byte, which is very convenient. Even better, all letters commonly used in English text have byte values below 127, so the 7-bit limit of messages is no problem for text encoded with the US-ASCII charset. Another example: The string "Hello World!" is represented by the following byte array:

48 65 6C 6C 6F 20 57 6F 72 6C 64

Note that the byte values are written in hexadecimal form here, not in decimal as earlier.

Now, what if we want to write a message that contains German umlauts or Chinese letters? Those are not in the ASCII table, therefore a different charset has to be used. There is a wealth of charsets to choose from. Not all charsets can handle all letters, for example the ISO-8859-1 charset can handle German umlauts, but cannot handle Chinese or Arabic letters. The Unicode standard is an attempt to introduce charsets that can handle all known letters in the world, in all languages. Unicode actually has several charsets, for example UTF-8 and UTF-16. In an ideal world, everyone would be using Unicode charsets, but for historic and legacy reasons, other charsets are still much in use.

Charsets other than US-ASCII don't generally have as nice properties: A single letter can be represented by multiple bytes, and generally the byte values are not in the 7-bit range. Pay attention to the UTF-8 charset: At first glance, it looks exactly like the US-ASCII charset, common latin letters like A - Z are encoded with the same byte values as with US-ASCII. However, letters other than A - Z are suddenly encoded with two or even more bytes. In general, one letter can be encoded in an abitrary number of bytes, depending on the charset. One can not rely on the 1 letter == 1 byte assumption.

Now, what should be done when the text string "Grüezi Welt!" should be sent in the body of a message? The first step is to choose a charset that can represent all of its letters. This already excludes US-ASCII. Once a charset is chosen, the text string is encoded into a byte array. "Grüezi Welt!" encoded with the ISO-8859-1 charset produces the following byte array:

47 72 FC 65 7A 69 20 57 65 6C 74 21

The letter 'ü' here is encoded using a single byte with the value FC. The same string encoded with UTF-8 looks slightly different:

47 72 C3 BC 65 7A 69 20 57 65 6C 74 21

Here, the letter 'ü' is encoded with two bytes, C3 BC. Still, one can see the similarity between the two charsets for the other letters.

You can try this out yourself: Open your favorite text editor and enter some text with non-latin letters. Then save the file and view it in a hex editor to see how the text was converted to a byte array. Make sure to try out setting different charsets in your text editor.

At this point, the text string is successfully converted to a byte array, using e.g. the ISO-8859-1 charset. To indicate which charset was used, a Content-Type header field has to be added, with the correct charset parameter. In our example above, that was done. If the charset parameter of the Content-Type, or even the complete Content-Type header field is left out, the receiver can not know how to interpret the byte array! In these cases, the byte array is usually decoded incorrectly, and the text strings contain wrong letters or lots of question marks. There is even a special term for such wrongly decoded text, Mojibake. It is important to always know what charset your byte array is encoded with, otherwise an attempt at decoding the byte array into a text string will fail and produce Mojibake. There is no such thing as plain text! If there is no Content-Type header field in a message, the message body should be interpreted as US-ASCII.

To learn more about charsets and encodings, read The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) and A tutorial on character code issues. Especially the first article should really be read, as the name indicates.

Content Transfer Encoding

Now, we can't use the byte array that was just created in a message. The string encoded with ISO-8859-1 has the byte value FC for the letter 'ü', which is decimal value 252. However, as said earlier, messages are only valid when all bytes are in the 7-bit range, i.e. have byte value below 127. So what should we do for byte values that are greater than 127, how can they be added to messages? The solution for this is to use a content transfer encoding (CTE). A content transfer encoding takes a byte array as input and transforms it. The output is another byte array, but one which only uses byte values in the 7-bit range. One such content transfer encoding is quoted-printable (QTP), which is used in the above example. Quoted-printable is easy to understand: When encountering a byte that has a value greater than 127, it is simply replaced by a '=', followed by the hexadecimal code of the byte value, represented as letters and digits encoded with ASCII. This means that a byte with the value 252 is replaced with the ASCII string =FC, since FC is the hexadecimal value of 252. The ASCII string =FC itself is now three bytes big, 3D 46 43. Therefore, the quoted-printable encoding replaces each byte outside of the 7-bit range with 3 new bytes. Decoding quoted-printable encoding is also easy: Each time a byte with the value 3D, which is the letter '=' in ASCII, is encountered, the next two following bytes are interpreted as the hex value of the resulting byte. The quoted-printable encoding was invented to make reading the byte array easy for humans.

The quoted-printable encoding is not a good choice when the input byte array contains lots of bytes outside the 7-bit range, as the resulting byte array will be three times as big in the worst case, which is a waste of space. Therefore another content transfer encoding was introduced, Base64. The details of the base64 encoding are too much to write about here; refer to the Wikipedia article or the RFC for details. As an example, the ISO-8859-1 encoded text string "Grüezi Welt!" is, after encoding it with base64, represented by the following ASCII string: R3L8ZXppIFdlbHQh. To express the same in byte arrays: The byte array 47 72 FC 65 7A 69 20 57 65 6C 74 21 is, after encoding it with base64, represented by the byte array 52 33 4C 38 5A 58 70 70 49 46 64 6C 62 48 51 68.

There are two other content transfer encodings besides quoted printable and base64: 7-bit and 8-bit. 7-bit is just a marker to indicate that no content transfer encoding is used. This is the case when the byte array is already completely in the 7-bit range, for example when writing English text using the US-ASCII charset. 8-bit is also a marker to indicate that no content transfer encoding was used. This time, not because it was not necessary, but because of a special exception, byte values outside of the 7-bit range are allowed. For example, some SMTP servers support the 8BITMIME extension, which indicates that they accept bytes outside of the 7-bit range. In this case, one can simply use the byte arrays as-is, without using any content transfer encoding. Creating messages with 8-bit content transfer encoding is currently not supported by KMime. The advantage of 8-bit is that there is no overhead in size, unlike with base64 or even quoted-printable.

When using one of the 4 content transfer encodings, i.e. quoted-printable, base64, 7-bit or 8-bit, this has to be indicated in the header field Content-Transfer-Encoding. If the header field is left out, it is assumed that the content transfer encoding is 7-bit. The example above uses quoted-printable.

From: John Doe <john.doe@domain.com>
Date: Mon, 22 Feb 2010 00:42:45 +0100
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: base64

R3L8ZXppIFdlbHQh

The same example, this time encoded with the base64 content transfer encoding.

From: John Doe <john.doe@domain.com>
Date: Mon, 22 Feb 2010 00:42:45 +0100
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="utf-8"
Content-Transfer-Encoding: base64

R3LDvGV6aSBXZWx0IQ==

Again the same example, this time using UTF-8 as the charset.

From: John Doe <john.doe@domain.com>
Date: Mon, 22 Feb 2010 00:42:45 +0100
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="utf-8"
Content-Transfer-Encoding: quoted-printable

Gr=C3=BCezi Welt!

The example with a combination of UTF-8 and quoted-printable CTE. As said somewhere above, with the UTF-8 encoding, the letter 'ü' is represented by the two bytes C3 BC.

From: John Doe <john.doe@domain.com>
Date: Mon, 22 Feb 2010 00:42:45 +0100
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="utf-8"
Content-Transfer-Encoding: 7-bit

Hello World

A different example, showing 7-bit content transfer encoding. Although the UTF-8 charset has lots of letters that are represented by bytes outside of the 7-bit range, the string "Hello World" can be fully represented in the 7-bit range here, even with UTF-8.

In the further reading section, you will find links to web applications that demonstrate encodings and charsets.

Conclusion

When adding a text string to the body of a message, it needs to be encoded twice: First, the encoding of the charset needs to be applied, which transforms the text string into a byte array. Afterwards, the content transfer encoding has to be applied, which transforms the byte array from the first step into a byte array that only has bytes in the 7-bit range.

When decoding, the same has to be done, in reverse: One first has decode the byte array with the content transfer encoding, to get a byte array that has all 256 possible byte values. Afterwards, the resulting byte array needs to be decoded with the correct charset, to transform it into a text string. For those two decoding steps, one has to look at the Content-Type and the Content-Transfer-Encoding header fields to find the correct charset and CTE for decoding.

It is important to always keep the charset and the content transfer encoding in mind. Byte arrays and strings are not to be confused. Byte arrays that are encoded with a CTE are not to be confused with byte arrays that are not encoded with a CTE.

This section showed how to use different charsets in the body of a message. The next section will show what to do when another charset is needed in one of the header field bodies.

Encoding in Header Fields

In the last section, we discussed how to use different charsets in the body of a message. But what if a different charset needs to be added to one of the header fields? For example one might want to write a mail to a mailbox with the display name "András Manţia" and with the subject "Grüezi!".

The header fields are limited to characters in the 7-bit range, and are interpreted as US-ASCII. That means the header field names, such as "From: ", are all encoded in US-ASCII. The header field bodies, such as the "1.0" of MIME-Version, are also encoded with US-ASCII. This is mandated by the RFC.

The Content-Type and the Content-Transfer-Encoding header fields only apply to the message body, they have no meaning for other header fields.

This means that any letter in a different charset has to be encoded in some way to satisfy the RFC. Letters with a different charset are only allowed in some of the header field bodies; the header field names always have to be in US-ASCII.

From: Thomas McGuire <thomas@domain.com>
Subject: =?iso-8859-1?q?Gr=FCezi!?=
Date: Mon, 22 Feb 2010 14:34:01 +0100
MIME-Version: 1.0
To: =?utf-8?q?Andr=C3=A1s?= =?utf-8?q?_Man=C5=A3ia?= <andras@domain.com>
Content-Type: Text/Plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

bla bla bla

The above example shows how text that is encoded with a different charset than US-ASCII is handled in the message header. This can be seen in the bodies of the Subject header field and the To header field. In this example, the body of the message is unimportant, it is just "bla bla bla" in US-ASCII. The way the header field bodies are encoded is sometimes referred to as a RFC2047 string or as an encoded word, which has its origin in the RFC where this encoding scheme is defined. RFC2047 strings are only allowed in some of the header fields, like Subject, and in the display name of mailboxes in header fields like From and To. In other header fields, such as Date and MIME-Version, they are not allowed, but they wouldn't make much sense there anyway, since those are structured header fields with a clearly defined structure.

RFC2047 strings start with "=?" and end with "?=". Between those markers, they consist of three parts:

• The charset, such as "iso-8859-1"
• The encoding, which is "q" or "b"
• The encoded text

These three parts are separated with a '?'. Encoding the third part, the text, is very similar to how text strings in the message body are encoded: First, the text string is encoded to a byte array using the charset encoding. Afterwards, the second encoding is used on the result, to ensure that all resulting bytes are within the 7-bit range.

The second encoding here is almost identical to the content transfer encoding. There are two possible encodings, b and q. The b encoding is the same as the base64 encoding of the content transfer encoding. The q encoding is very similar to the quoted-printable encoding of the content transfer encoding, but with some little differences that are described in the RFC.

Let's examine the subject of the message, =?iso-8859-1?q?Gr=FCezi!?=, in detail:

The first part of the RFC2027 string is the charset, so it is ISO-8859-1 in this case. The second part is the encoding, which is the q encoding here. The last part is the encoded text, which is Gr=FCezi!. As with the quoted-printable encoding, "=FC" is the encoding for the byte with the value FC, which in the ISO-8859-1 charset is the letter 'ü'. The complete decoded text is therefore "Grüezi!".

Each RFC2047 string in the header can use a different charset: In this example, the Subject uses ISO-8859-1, To uses UTF-8 and the message body uses US-ASCII.

In the To header field, two RFC2047 strings are used. A single, bigger, RFC2047 string for the whole display name could also have been used. In this case, the second RFC2047 string starts with an underscore, which is decoded as a space in the q encoding. The space between the two RFC2047 strings is ignored, it is just used to separate the two encoded words.

There are some restriction on RFC2047 strings: They are not allowed to be longer than 75 characters, which means two or more encoded words have to be used for long text strings. Also, there are some restrictions on where RFC2047 strings are allowed; most importantly, the address specification must not be encoded, to be backwards compatible. For further details, refer to the RFC.

Messages with attachments

Until now, we only looked at messages that had a single text part as the message body. In this section, we'll examine messages with attachments.

From: frank@domain.com
To: greg@domain.com
Subject: Nice Photo
Date: Sun, 28 Feb 2010 19:57:00 +0100
MIME-Version: 1.0
Content-Type: Multipart/Mixed;
  boundary="Boundary-00=_8xriL5W6LSj00Ly"

--Boundary-00=_8xriL5W6LSj00Ly
Content-Type: Text/Plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Hi Greg,

attached you'll find a nice photo.

--Boundary-00=_8xriL5W6LSj00Ly
Content-Type: image/jpeg;
  name="test.jpeg"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
  filename="test.jpeg"

/9j/4AAQSkZJRgABAQAAAQABAAD/4Q3XRXhpZgAASUkqAAgAAAAHAAsAAgAPAAAAYgAAAAABBAAB
[SNIP 800 lines]
ze5CdSH2Z8yTatHSV2veW0rKzeq30//Z

--Boundary-00=_8xriL5W6LSj00Ly--

Note: Since the image in this message would be really big, most of it is omitted / snipped here.

The above example consists of two parts: A normal text part and an image attachment. Messages that consist of multiple parts are called multipart messages. The top-level content-type therefore is multipart/mixed. Mixed simply means that the following parts have no relation to each other, it is just a random mixture of parts. Later, we will look at other types, such as multipart/alternative or multipart/related. A part is sometimes also called node, content or MIME part.

Each MIME part of the message is separated by a boundary, and that boundary is specified in the top-level content-type header as a parameter. In the message body, the boundary is prefixed with "--", and the last boundary is suffixed with "--", so that the end of the message can be detected. When creating a message, care must be taken that the boundary appears nowhere else in the message, for example in the text part, as the parser would get confused by this.

A MIME part begins right after the boundary. It consists of a MIME header and a MIME body, which are separated by an empty line. The MIME header should not be confused with the message header: The message header contains metadata about the whole message, like subject and date. The MIME header only contains metadata about the specific MIME part, like the content type of the MIME part. MIME header field names always start with "Content-". The example above shows the three most important MIME header fields. Usually those are the only ones used. The top-level header of a message actually mixes the message metadata and the MIME metadata into one header: In this example, the header contains the Date header field, which is an ordinary header field, and it contains the Content-Type header field, which is a MIME header field.

MIME parts can be nested, and therefore form a tree. The above example has the following tree:

multipart/mixed
|- text/plain
\- image/jpeg

The text/plain node is therefore a child of the multipart/mixed node. The multipart/mixed node is a parent of the other two nodes. The image/jpeg node is a sibling of the text/plain node. Multipart nodes are the only nodes that have children, other nodes are leaf nodes. The body of a multipart node consists of all complete child nodes (MIME header and MIME body), separated by the boundary.

Each MIME part can have a different content transfer encoding. In the above example, the text part has a 7bit CTE, while the image part has a base64 CTE. The multipart/mixed node does not specify a CTE, multipart nodes always have 7bit as the CTE. This is because the body of multipart nodes can only consist of bytes in the 7 bit range: The boundary is 7 bit, the MIME headers are 7 bit, and the MIME bodies are already encoded with the CTE of the child MIME part, and are therefore also 7 bit. This means no CTE for multipart nodes is necessary.

The MIME part for the image does not specify a charset parameter in the content type header field. This is because the body of that MIME part will not be interpreted as a text string, therefore the byte array does not need to be decoded to a string. Instead, the byte array is interpreted as an image, by an image renderer. The message viewer application passes the MIME part body as a byte array to the image renderer. The content type consists of a media type and a subtype. For example, the content type "text/html" has the media type "text" and the subtype "html". Only nodes that have the media type "text" need to specify a charset, as those nodes are the only nodes of which the body is interpreted as a text string.

The only header field not yet encountered in previous sections is the Content-Disposition header field, which is defined in a separate RFC. It describes how the message viewer application should display the MIME part. In the case of the image part, is should be presented as an attachment. The filename parameter tells the message viewer application which filename should be used by default when the user saves the attachment to disk.

The content type header field for the image MIME part has a name parameter, which is similar to the filename parameter of the Content-Disposition header field. The difference is that name refers to the name of the complete MIME part, whereas filename refers to the name of the attachment. The name parameter of the Content-Type header field in this case is superfluous and only exists for backwards compatibility, and can be ignored; the filename parameter of the Content-Disposition header field should be preferred when it is present.

From: Thomas McGuire <thomas@domain.com>
To: sebastian@domain.com
Subject: Help with SPARQL
Date: Sun, 28 Feb 2010 21:57:51 +0100
MIME-Version: 1.0
Content-Type: Multipart/Mixed;
  boundary="Boundary-00=_PjtiLU2PvHpvp/R"

--Boundary-00=_PjtiLU2PvHpvp/R
Content-Type: Text/Plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Hi Sebastian,

I have a problem with a SPARQL query, can you help me debug this? Attached is
the query and a screenshot showing the result.

--Boundary-00=_PjtiLU2PvHpvp/R
Content-Type: text/plain;
  charset="UTF-8";
  name="query.txt"
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment;
  filename="query.txt"

prefix nco:<http://www.semanticdesktop.org/ontologies/2007/03/22/nco#>

SELECT ?person
WHERE {
 ?person a nco:PersonContact .
 ?person nco:birthDate ?birthDate .
}"
--Boundary-00=_PjtiLU2PvHpvp/R
Content-Type: image/png;
  name="screenshot.png"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
  filename="screenshot.png"

AAAAyAAAAAEBBAABAAAAyAAAAA0BAgATAAAAcQAAABIBAwABAAAAAQAAADEBAgAPAAAAhAAAAGmH
[SNIP]
YXJlLmpwZWcAZGlnaUthbS0w

--Boundary-00=_PjtiLU2PvHpvp/R--

The above example message consists of three MIME parts: The main text part and two attachments. One attachment has the media type text, therefore a charset parameter is necessary to correctly display it. The MIME tree looks like this:

multipart/mixed
|- text/plain
|- text/plain
\- image/jpeg

HTML Messages

From: Thomas McGuire <thomas@domain.com>
Subject: HTML test
Date: Thu, 4 Mar 2010 13:59:18 +0100
MIME-Version: 1.0
Content-Type: multipart/alternative;
  boundary="Boundary-01=_m66jLd2/vZrH5oe"
Content-Transfer-Encoding: 7bit

--Boundary-01=_m66jLd2/vZrH5oe
Content-Type: text/plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Hello World

--Boundary-01=_m66jLd2/vZrH5oe
Content-Type: text/html;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">
<html>
  <head></head>
  <body>
    Hello <b>World</b>
  </body>
</html>
--Boundary-01=_m66jLd2/vZrH5oe--

The above example is a simple HTML message. It consists of a plain text and a HTML part, which are in a multipart/alternative container. The message has the following structure:

multipart/alternative
|- text/plain
\- text/html

The HTML part and the plain text part have the identical content, except that the HTML part contains additional markup, in this case for displaying the word World in bold. Since those parts are in a multipart/alternative container, the message viewer application can freely choose which part it displays. Some users might prefer reading the message in HTML format, some might prefer reading the message in plain text format.

Of course, a HTML message could also consist only of a single text/html, without the multipart/alternative container and therefore without an alternative plain text part. However, people preferring the plain text version wouldn't like this, especially if their mail client has no HTML engine and they would see the HTML source including all tags only. Therefore, HTML messages should always include an alternative plain text part.

HTML messages can also contain attachments. In this case, the message contains both a multipart/alternative and a multipart/mixed node, for example with the following structure, for an HTML message that has an image attachment:

multipart/mixed
|- multipart/alternative
|  |- text/plain
|  \- text/html
\- image/png

The message itself would look like this:

From: Thomas McGuire <thomas@domain.com>
Subject: HTML message with an attachment
Date: Thu, 4 Mar 2010 15:20:26 +0100
MIME-Version: 1.0
Content-Type: Multipart/Mixed;
  boundary="Boundary-00=_qG8jLwWCwkUfJV1"

--Boundary-00=_qG8jLwWCwkUfJV1
Content-Type: multipart/alternative;
  boundary="Boundary-01=_qG8jLfs1FRmlOhl"
Content-Transfer-Encoding: 7bit

--Boundary-01=_qG8jLfs1FRmlOhl
Content-Type: text/plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Hello World

--Boundary-01=_qG8jLfs1FRmlOhl
Content-Type: text/html;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">
<html>
  <head></head>
  <body>
    Hello <b>World</b>
  </body>
</html>
--Boundary-01=_qG8jLfs1FRmlOhl--

--Boundary-00=_qG8jLwWCwkUfJV1
Content-Type: image/png;
  name="test.png"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
  filename="test.png"

iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAACXBIWXMAAA8SAAAPEgEhm/IzAAAC
[SNIP]
eFkXsFgBMG4fJhYlx+iyB3cLpNZwYr/iP7teTwNYa7DZAAAAAElFTkSuQmCC

--Boundary-00=_qG8jLwWCwkUfJV1--

HTML Messages with Inline Images

HTML has support for showing images, with the img tag. Such an image is shown at the place where the img tag occurs, which is called an inline image. Note that inline images are different from images that are just normal attachments: Normal attachments are always shown at the beginning or at the end of the message, while inline images are shown in-place. In HTML, the img tag points to an image file that is either a file on disk or a URL of an image on the Internet. To make inline images work with MIME messages, a different mechanism is needed, since the image is not a file on disk or on the Internet, but a MIME part somewhere in the same message. As specified in RFC 2557, the way this can be done is by referring to a Content-ID in the img tag, and marking the MIME part that is the image with that content ID as well.

An example will probably be more clear than this explanation:

From: Thomas McGuire <thomas@domain.com>
Subject: Inine Image Test
Date: Thu, 4 Mar 2010 16:54:53 +0100
MIME-Version: 1.0
Content-Type: multipart/related;
  boundary="Boundary-02=_Nf9jLpJ2aGp5RQK"
Content-Transfer-Encoding: 7bit

--Boundary-02=_Nf9jLpJ2aGp5RQK
Content-Type: multipart/alternative;
  boundary="Boundary-01=_Nf9jLZ6aPhm3WrN"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

--Boundary-01=_Nf9jLZ6aPhm3WrN
Content-Type: text/plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Text before image

Text after image

--Boundary-01=_Nf9jLZ6aPhm3WrN
Content-Type: text/html;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">
<html>
  <head></head>
  <body>
    Text before image<br>
    <img src="cid:547730348@KDE" /><br>
    Text after image
  </body>
</html>
--Boundary-01=_Nf9jLZ6aPhm3WrN--

--Boundary-02=_Nf9jLpJ2aGp5RQK
Content-Type: image/png;
  name="test.png"
Content-Transfer-Encoding: base64
Content-Id: <547730348@KDE>

iVBORw0KGgoAAAANSUhEUgAAAMgAAADICAIAAAAiOjnJAAAACXBIWXMAAA7EAAAOxAGVKw4bAAAg
[SNIP]
AABJRU5ErkJggg==
--Boundary-02=_Nf9jLpJ2aGp5RQK--

The first thing you'll notice in this example probably is that it has a multipart/related node with the following structure:

multipart/related
|- multipart/alternative
|  |- text/plain
|  \- text/html
\- image/png

When the HTML part has inline image, the HTML part and its image part both have to be children of a multipart/related container, like in this example. In this case, the img tag has the source cid:547730348@KDE, which is a placeholder that refers to the Content-Id header of another part. The image part contains exactly that value in its Content-Id header, and therefore a message viewer application can connect both.

The plain text part cannot have inline images, therefore its text might seem a bit confusing.

HTML messages with inline images can of course also have attachments, in which the message structure becomes a mix of multipart/related, multipart/alternative and multipart/mixed. The following example shows the structure of a message with two inline images and one .tar.gz attachment:

multipart/mixed
|- multipart/related
|  |- multipart/alternative
|  |  |- text/plain
|  |  \- text/html
|  |- image/png
|  \- image/png
\- application/x-compressed-tar

The structure of MIME messages can get arbitrarily complex, the above is just one relatively simple example. The nesting of multipart nodes can get much deeper, there is no restriction on nesting levels.

Encapsulated messages

Encapsulated messages are messages which are attachments to another message. The most common example is a forwarded mail, like in this example:

From: Frank <frank@domain.com>
To: Bob <bob@domain.com>
Subject: Fwd: Blub
MIME-Version: 1.0
Content-Type: Multipart/Mixed;
  boundary="Boundary-00=_sX+jLVPkV1bLFdZ"

--Boundary-00=_sX+jLVPkV1bLFdZ
Content-Type: text/plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Hi Bob,

hereby I forward you an interesting message from Greg.

--Boundary-00=_sX+jLVPkV1bLFdZ
Content-Type: message/rfc822;
  name="forwarded message"
Content-Transfer-Encoding: 7bit
Content-Description: Forwarded Message
Content-Disposition: inline

From: Greg <greg@domain.com>
To: Frank <frank@domain.com>
Subject: Blub
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Bla Bla Bla

--Boundary-00=_sX+jLVPkV1bLFdZ--


multipart/mixed
|- text/plain
\- message/rfc822
\- text/plain

The attached message is treated like any other attachment, and therefore the top-level content type is multipart/mixed. The most interesting part is the message/rfc822 MIME part. As usual, it has some MIME headers, like Content-Type or Content-Disposition, followed by the MIME body. The MIME body in this case is the attached message. Since it is a message, it consists of a header and a body itself. Therefore, the message/rfc822 MIME part appears to have two headers; in reality, it is the normal MIME header and the message header of the encapsulated message. The message header and the message body are both in the MIME body of the message/rfc822 MIME part.

Signed and Encrypted Messages

MIME messages can be cryptographically signed and/or encrypted. The format for those messages is defined in RFC 1847, which specifies two new multipart subtypes, multipart/signed and multipart/encrypted. The crypto format of these new security multiparts is defined in additional RFCs; the most common formats are OpenPGP and S/MIME. Both formats use the principle of public-key cryptography. OpenPGP uses keys, and S/MIME uses certificates. For easier text flow, only the term key will be used for both keys and certificates in the text below.

Security multiparts only sign or encrypt a specific MIME part. The consequence is that the message headers can not be signed or encrypted. Also this means that it is possible to sign or encrypt only some of the MIME parts of a message, while leaving other MIME parts unsigned or unencrypted. Furthermore, it is possible to sign or encrypt different MIME parts with different crypto formats. As you can see, security multiparts are very flexible.

Security multiparts are not supported by KMime. However, it is possible for applications to use KMime when providing support for crypto messages. For example, the messageviewer component in KDE PIM's messagelib supports signed and encrypted MIME parts, and the messagecomposer library can create such messages.

Signed MIME parts are signed with the private key of the sender, and everybody who has the public key of the sender can verify the signature. Encrypted MIME parts are encrypted with the public key of the receiver, and only the receiver, who is the sole person possessing the private key, can decrypt it. Sending an encrypted message to multiple recipients therefore means that the message has to be sent multiple times, once for each receiver, as each message needs to be encrypted with a different key.

Signed MIME parts

A multipart/signed MIME part has exactly two children: The first child is the content that is signed, and the second child is the signature.

From: Thomas McGuire <thomas@domain.com>
Subject: My Subject
Date: Mon, 15 Mar 2010 12:20:16 +0100
MIME-Version: 1.0
Content-Type: multipart/signed;
  boundary="nextPart2567247.O5e8xBmMpa";
  protocol="application/pgp-signature";
  micalg=pgp-sha1
Content-Transfer-Encoding: 7bit

--nextPart2567247.O5e8xBmMpa
Content-Type: Text/Plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

Simple message

--nextPart2567247.O5e8xBmMpa
Content-Type: application/pgp-signature; name=signature.asc
Content-Description: This is a digitally signed message part.

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.14 (GNU/Linux)

iEYEABECAAYFAkueF/UACgkQKglv3sO8a1MdTACgnBEP6ZUal931Vwu7PyiXT1bn
Zr0Anj4bAI9JhHEDiwA/iwrWGfSC+Nlz
=d2ol
-----END PGP SIGNATURE-----
--nextPart2567247.O5e8xBmMpa--


multipart/signed
|- text/plain
\- application/pgp-signature

The example here uses the OpenPGP format to sign a simply plain text message. Here, the text/plain MIME part is signed, and the application/pgp-signature MIME part contains the signature data, which in this case is ASCII-armored.

As said above, it is possible to sign only some MIME parts. A message which has a image/jpeg attachment that is signed, but a main text part is not signed, has the following MIME structure:

multipart/mixed
|- text/plain
\- multipart/signed
|- image/jpeg
\- application/pgp-signature

It is possible to sign multipart parts as well. Consider the above example that has a plain text part and an image attachment. Those two parts can be signed together, with the following structure:

multipart/signed
|- multipart/mixed
|  |- text/plain
|  \- image/jpeg
\- application/pgp-signature

Signed messages in the S/MIME format use a different content type for the signature data, like here:

multipart/signed
|- text/plain
\- application/x-pkcs7-signature

Encrypted MIME parts

Multipart/encrypted MIME parts also have exactly two children: The first child contains metadata about the encrypted data, such as a version number. The second child then contains the actual encrypted data.

From: someone@domain.com
To: Thomas McGuire <thomas@domain.com>
Subject: Encrypted message
Date: Mon, 15 Mar 2010 12:50:16 +0100
MIME-Version: 1.0
Content-Type: multipart/encrypted;
  boundary="nextPart2726747.j47xUGTWKg";
  protocol="application/pgp-encrypted"
Content-Transfer-Encoding: 7bit

--nextPart2726747.j47xUGTWKg
Content-Type: application/pgp-encrypted
Content-Disposition: attachment

Version: 1
--nextPart2726747.j47xUGTWKg
Content-Type: application/octet-stream
Content-Disposition: inline; filename="msg.asc"

-----BEGIN PGP MESSAGE-----
Version: GnuPG v2.0.14 (GNU/Linux)

hQIOA8p5rdC5CBNfEAf+NZVzVq48C1r5opOOiWV96+FUzIWuMQ6u8fzFgI7YVyCn
[SNIP]
=reNr
--nextPart2726747.j47xUGTWKg--
-----END PGP MESSAGE-----


multipart/encrypted
|- application/pgp-encrypted
\- application/octet-stream

The encrypted data is contained in the application/octet-stream MIME part. Without decrypting the data, it is unknown what the original content type of the encrypted MIME data is! The encrypted data could be a simple text/plain MIME part, an image attachment, or a multipart part. The encrypted data contains both the MIME header and the MIME body of the original MIME part, as the header is needed to know the content type of the data. The data could as well be of content type multipart/signed, in which case the message would be both signed and encrypted.

Inline Crypto Formats

Although using the security multiparts multipart/signed and multipart/encrypted is the recommended standard, there are other possibilities to sign or encrypt a message. The most common methods are Inline OpenPGP and S/MIME Opaque.

For inline OpenPGP messages, the crypto data is contained inlined in the actual MIME part. For example, a message with a signed text/plain part might look like this:

From: someone@domain.com
To: someoneelse@domain.com
Subject: Inline OpenPGP test
MIME-Version: 1.0
Content-Type: text/plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Inline OpenPGP signed example.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.14 (GNU/Linux)

iEYEARECAAYFAkueJ2EACgkQKglv3sO8a1MS3QCfcsYnJG7uYQxzxz6J5cPF7lHz
WIoAn3PjVPlWibu02dfdFObwd2eJ1jAW
=p3uO
-----END PGP SIGNATURE-----

Encrypted inline OpenPGP works in a similar way. Opaque S/MIME messages are also similar: For signed MIME parts, both the signature and the signed data are contained in a single MIME part with a content type of application/pkcs7-mime.

As security multiparts are preferred over inline OpenPGP and over opaque S/MIME, I won't go into more detail here.

Miscellaneous Points about Messages

Line Breaks

Each line in a MIME message has to end with a CRLF, which is a carriage return followed by a newline, which is the escape sequence \\r\\n. CR and LF may not appear in other places in a MIME message. Special care needs to be taken with encoded line breaks in binary data, and with distinguishing soft and hard line breaks when converting between different content transfer encodings. For more details, have a look at the RFCs.

While the official format is to have a CRLF at the end of each line, KMime only expects a single LF for its in-memory storage. Therefore, when loading a message from disk or from a server into KMime, the CRLFs need to be converted to LFs first, for example with KMime::CRLFtoLF(). The opposite needs to be done when storing a KMime message somewhere.

Lines should not be longer than 78 characters and may not be longer than 998 characters.

Header Folding and CFWS

Header fields can span multiple lines, which was already shown in some of the examples above where the parameters of the header field value were in the next line. The header field is said to be folded in this case. In general, header fields can be folded whenever whitespace (WS) occurs.

Header field values can contain comments; these comments are semantically invisible and have no meaning. Comments are surrounded by parentheses.

Date: Thu, 13
      Feb 1969 23:32 -0330 (Newfoundland Time)

This example shows a folded header that also has a comment (Newfoundland Time). The date header is a structured header field, and therefore it has to obey to a defined syntax; however, adding comments and whitespace is allowed almost anywhere, and they are ignored when parsing the message. Comments and whitespace where folding is allowed is sometimes referred to as CFWS. Any occurrence of CFWS is semantically regarded as a single space.

The two in-memory representations of messages # {#string-broken-down }

There are two representations of messages in memory. The first is called string representation and the other one is called broken-down representation.

String representation is somewhat misnamed, a better term would be "byte array representation". The string representation is just a big array of bytes in memory, and those bytes make up the encoded mail. The string representation is what is stored on disk or what is received from an IMAP server, for example.

With the broken-down representation, the mail is broken down into smaller structures. For example, instead of having a single byte array for all headers, the broken-down structure has a list of individual headers, and each header in that list is again broken down into a structure. While the string representation is just an array of 7 bit characters that might be encoded, the broken-down representations contain the decoded text strings.

As an example, consider the byte array

"Hugo Maier" <hugo.maier@mailer.domain>

Although this is just a bunch of 7 bit characters, a human immediately recognizes the broken-down structure and sees that the display name is "Hugo Maier" and that the localpart of the email address is "hugo.maier". To illustrate, the broken-down structure could be stored in a structure like this:

struct Mailbox
{
    QString displayName;
    QByteArray addressSpec;
};

The address spec actually could be broken down further into a localpart and a domain. The process of converting the string representation to a broken-down representation is called parsing, and the reverse is called assembling. Parsing a message is necessary when wanting to access or modify the broken-down structure. For example, when sending a mail, the address spec of a mailbox needs to be passed to the SMTP server, which means that the recipient headers need to be parsed in order to access that information. Another example is the message list in an mail application, where the broken-down structure of an email is needed to display information like subject, sender and date in the list. On the other hand, assembling a message is for example done in the composer of a mail application, where the mail information is available in a broken-down form in the composer window, and is then assembled into a final MIME message that is then sent with SMTP.

Parsing is often quite tricky. You should always use the methods from KMime instead of writing parsing routines yourself. Even the simple mailbox example above is in practice difficult to parse, as many things like comments and escaped characters need to be taken into consideration. The same is true for assembling: In the above case, one could be tempted to assemble the mailbox by simply writing code like this:

QByteArray stringRepresentation = '"' + displayName + "\" <" + addressSpec + ">";

However, just like with parsing, you shouldn't be doing assembling yourself. In the above case, for example, the display name might contain non-ASCII characters, and RFC2047 encoding would need to be applied. So use KMime for assembling in all cases.

When parsing a message and assembling it afterwards, the result might not be the same as the original byte array. For example, comments in header fields are ignored during parsing and not stored in the broken-down structure, therefore the assembled message will also not contain comments.

Messages in memory are usually stored in a broken-down structure so that it is easy to to access and manipulate the message. On disk and on servers, messages are stored in string representation.

Overview of KMime classes

KMime has basically two sets of classes: Classes for headers and classes for MIME parts. A MIME part is represented by KMime::Content. A Content can be parsed from a string representation and also be assembled from the broken-down representation again. If parsed, it has a list of sub-contents (in case of multipart contents) and a list of headers. If the Content is not parsed, it stores the headers and the body in a byte array, which can be accessed with head() and body(). There is also a class KMime::Message, which basically is a thin wrapper around Content for the top-level MIME part. Message also contains convenience methods to access the message headers.

For headers, there is a class hierarchy, with KMime::Headers::Base as the base class, and KMime::Headers::Generics::Structured and KMime::Headers::Generics::Unstructured in the next levels. Unstructured is for headers that don't have a defined structure, like Subject, whereas Structured headers have a specific structure, like Date. The header classes have methods to parse headers, like from7BitString(), and to assemble them, like as7BitString(). Once a header is parsed, the classes provide access to the broken-down structures; for example the Date header has a method dateTime(). The parsing in from7BitString() is usually handled by a protected parse() function, which in turn call parsing functions for different types, like parseAddressList() or parseAddrSpec() from the KMime::HeaderParsing namespace.

When modifying messages, the message is first parsed into a broken-down representation. This broken-down representation can then be accessed and modified with the appropriate functions. After changing the broken-down structure, it needs to be assembled again to get the modified string representation.

KMime also comes with some codes for handling base64 and quoted-printable encoding, with KMime::Codec as the base class.

RFCs

• RFC 5322: Internet Message Format
• RFC 5536: Netnews Article Format
• RFC 2045: Multipurpose Internet Mail Extensions (MIME), Part 1: Format of Internet Message Bodies
• RFC 2046: Multipurpose Internet Mail Extensions (MIME), Part 2: Media Types
• RFC 2047: Multipurpose Internet Mail Extensions (MIME), Part 3: Message Header Extensions for Non-ASCII Text
• RFC 2048: Multipurpose Internet Mail Extensions (MIME), Part 4: Registration Procedures
• RFC 2049: Multipurpose Internet Mail Extensions (MIME), Part 5: Conformance Criteria and Examples
• RFC 2231: MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations
• RFC 2183: Communicating Presentation Information in Internet Message: The Content-Disposition Header Field
• RFC 2557: MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)
• RFC 1847: Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted
• RFC 3851: S/MIME Version 3 Message Specification
• RFC 3156: MIME Security with OpenPGP
• RFC 2298: An Extensible Message Format for Message Disposition Notifications
• RFC 2646: The Text/Plain Format Parameter (not supported by KMime)

Further Reading

• Wikipedia article on MIME
• The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)
• A tutorial on character code issues
• Online Base64 encoder and decoder
• Online quoted-printable encoder
• Onlinw quota reached
• Online charset converter
• Wikipedia article on public-key cryptography