This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
STD 11, RFC 822, defines a message representation protocol specifying considerable detail about US-ASCII message headers, and leaves the message content, or message body, as flat US-ASCII text. This set of documents, collectively called the Multipurpose Internet Mail Extensions, or MIME, redefines the format of messages to allow for
These documents are based on earlier work documented in RFC 934, STD 11, and RFC 1049, but extends and revises them. Because RFC 822 said so little about message bodies, these documents are largely orthogonal to (rather than a revision of) RFC 822.
This particular document is the third document in the series. It describes extensions to RFC 822 to allow non-US-ASCII text data in Internet mail header fields.
Other documents in this series include:
+ RFC 2045, which specifies the various headers used to describe
the structure of MIME messages. + RFC 2046, which defines the general structure of the MIME media
typing system and defines an initial set of media types, + RFC 2048, which specifies various IANA registration procedures
for MIME-related facilities, and + RFC 2049, which describes MIME conformance criteria and
provides some illustrative examples of MIME message formats,
acknowledgements, and the bibliography.These documents are revisions of RFCs 1521, 1522, and 1590, which themselves were revisions of RFCs 1341 and 1342. An appendix in RFC 2049 describes differences and changes from previous versions.
RFC 2045 describes a mechanism for denoting textual body parts which are coded in various character sets, as well as methods for encoding such body parts as sequences of printable US-ASCII characters. This memo describes similar techniques to allow the encoding of non-ASCII text in various portions of a RFC 822 [2] message header, in a manner which is unlikely to confuse existing message handling software.
Like the encoding techniques described in RFC 2045, the techniques outlined here were designed to allow the use of non-ASCII characters in message headers in a way which is unlikely to be disturbed by the quirks of existing Internet mail handling programs. In particular, some mail relaying programs are known to (a) delete some message header fields while retaining others, (b) rearrange the order of addresses in To or Cc fields, (c) rearrange the (vertical) order of header fields, and/or (d) "wrap" message headers at different places than those in the original message. In addition, some mail reading programs are known to have difficulty correctly parsing message headers which, while legal according to RFC 822, make use of backslash-quoting to "hide" special characters such as "<", ",", or ":", or which exploit other infrequently-used features of that specification.
While it is unfortunate that these programs do not correctly interpret RFC 822 headers, to "break" these programs would cause severe operational problems for the Internet mail system. The extensions described in this memo therefore do not rely on little- used features of RFC 822.
Instead, certain sequences of "ordinary" printable ASCII characters (known as "encoded-words") are reserved for use as encoded data. The syntax of encoded-words is such that they are unlikely to "accidentally" appear as normal text in message headers. Furthermore, the characters used in encoded-words are restricted to those which do not have special meanings in the context in which the encoded-word appears.
Generally, an "encoded-word" is a sequence of printable ASCII characters that begins with "=?", ends with "?=", and has two "?"s in between. It specifies a character set and an encoding method, and also includes the original text encoded as graphic ASCII characters, according to the rules for that encoding method.
A mail composer that implements this specification will provide a means of inputting non-ASCII text in header fields, but will translate these fields (or appropriate portions of these fields) into encoded-words before inserting them into the message header.
A mail reader that implements this specification will recognize encoded-words when they appear in certain portions of the message header. Instead of displaying the encoded-word "as is", it will reverse the encoding and display the original text in the designated character set.
This memo relies heavily on notation and terms defined RFC 822 and RFC 2045. In particular, the syntax for the ABNF used in this memo is defined in RFC 822, as well as many of the terminal or nonterminal symbols from RFC 822 are used in the grammar for the header extensions defined here. Among the symbols defined in RFC 822 and referenced in this memo are: 'addr-spec', 'atom', 'CHAR', 'comment', 'CTLs', 'ctext', 'linear-white-space', 'phrase', 'quoted-pair'. 'quoted-string', 'SPACE', and 'word'. Successful implementation of this protocol extension requires careful attention to the RFC 822 definitions of these terms.
When the term "ASCII" appears in this memo, it refers to the "7-Bit American Standard Code for Information Interchange", ANSI X3.4-1986. The MIME charset name for this character set is "US-ASCII". When not specifically referring to the MIME charset name, this document uses the term "ASCII", both for brevity and for consistency with RFC 822. However, implementors are warned that the character set name must be spelled "US-ASCII" in MIME message and body part headers.
This memo specifies a protocol for the representation of non-ASCII text in message headers. It specifically DOES NOT define any translation between "8-bit headers" and pure ASCII headers, nor is any such translation assumed to be possible.
An 'encoded-word' is defined by the following ABNF grammar. The notation of RFC 822 is used, with the exception that white space characters MUST NOT appear between components of an 'encoded-word'.
「encoded-word」は次の ABNF 文法で定義します。 RFC 822 の記法を使いますが、空白間隔文字が「encoded-word」 の部品間に現れてはいけません。
encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
charset = token ; see section 3
encoding = token ; see section 4
token = 1*<Any CHAR except SPACE, CTLs, and especials>
especials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "
<"> / "/" / "[" / "]" / "?" / "." / "=" encoded-text = 1*<Any printable ASCII character other than "?"
or SPACE>
; (but see "Use of encoded-words in message
; headers", section 5)Both 'encoding' and 'charset' names are case-independent. Thus the charset name "ISO-8859-1" is equivalent to "iso-8859-1", and the encoding named "Q" may be spelled either "Q" or "q".
「encoding」と「charset」の名前はどちらも大文字・小文字を 区別しません。ですから charset 名「ISO-8859-1」は 「iso-8859-1」と同じで、符号化方式名「Q」は「Q」とも「q」とも 綴ることが出来ます。
An 'encoded-word' may not be more than 75 characters long, including 'charset', 'encoding', 'encoded-text', and delimiters. If it is desirable to encode more text than will fit in an 'encoded-word' of 75 characters, multiple 'encoded-word's (separated by CRLF SPACE) may be used.
「encoded-word」は「charset」, 「encoding」, 「encoded-text」 と区切りを含めて75文字長より長くなってはなりません。 「encoded-word」で75文字を超える文字を符号化したい時は、 複数の「encoded-word」(CRLF SPACE で区切る) を使うことが出来ます。
While there is no limit to the length of a multiple-line header field, each line of a header field that contains one or more 'encoded-word's is limited to 76 characters.
複数行頭領域の長さの制限はないので、頭領域の各行は1つ以上の「encoded-word」 を含む頭領域の各行は76文字に制限します。
The length restrictions are included both to ease interoperability through internetwork mail gateways, and to impose a limit on the amount of lookahead a header parser must employ (while looking for a final ?= delimiter) before it can decide whether a token is an "encoded-word" or something else.
長さの制限をするはネットワーク間メイル関門での相互通信を容易にし、 頭解析器の (最後の ?= 区切りを探して) token が 「encoded-word」 か他の何かか決める前の先読みの量を制限するためです。
IMPORTANT: 'encoded-word's are designed to be recognized as 'atom's by an RFC 822 parser. As a consequence, unencoded white space characters (such as SPACE and HTAB) are FORBIDDEN within an 'encoded-word'. For example, the character sequence
重要: 「encoded-word」は RFC 822 解析器には「atom」と みなされるように設計されています。ですから、符号化されていない 空白間隔文字 (SPACE や HTAB) は「encoded-word」では禁止します。 例えば、文字列
=?iso-8859-1?q?this is some text?=
would be parsed as four 'atom's, rather than as a single 'atom' (by an RFC 822 parser) or 'encoded-word' (by a parser which understands 'encoded-words'). The correct way to encode the string "this is some text" is to encode the SPACE characters as well, e.g.
は (RFC 822 解析器には) 1つの「atom」や (「encoded-word」を理解する解析器には) 「encoded-word」ではなく、4つの「atom」と解析されます。 文字列「This is some text」を符号化する正しい方法は、 SPACE 文字を同様に符号化して次のようにするものです。
=?iso-8859-1?q?this=20is=20some=20text?=
The characters which may appear in 'encoded-text' are further restricted by the rules in section 5.
「encoded-text」に現れる文字は第5章の規則で更に制限します。
The 'charset' portion of an 'encoded-word' specifies the character set associated with the unencoded text. A 'charset' can be any of the character set names allowed in an MIME "charset" parameter of a "text/plain" body part, or any character set name registered with IANA for use with the MIME text/plain content-type.
Some character sets use code-switching techniques to switch between "ASCII mode" and other modes. If unencoded text in an 'encoded-word' contains a sequence which causes the charset interpreter to switch out of ASCII mode, it MUST contain additional control codes such that ASCII mode is again selected at the end of the 'encoded-word'. (This rule applies separately to each 'encoded-word', including adjacent 'encoded-word's within a single header field.)
幾つかの文字集合は「ASCII 状態」と他の状態を切り替えるのに 符号切り替え技術を使います。「encoded-word」中の符号化されていない文が、 charset 解釈者に ASCII 状態の他に切り替えさせる列である場合、 「encoded-word」の終わりで再び ASCII 状態に切り替える制御符号 を加えて入れなければなりません。 (この規則は各「encoded-word」 にそれぞれ適用されます。単一頭領域内の隣接した「encoded-word」も含みます。)
When there is a possibility of using more than one character set to represent the text in an 'encoded-word', and in the absence of private agreements between sender and recipients of a message, it is recommended that members of the ISO-8859-* series be used in preference to other character sets.
「encoded-word」中の文を表現する文字集合を複数使用できるときは、 メッセージの送信者と受信者の間の私的な合意が無い限り、 ISO-8859-* 系列のどれかを他の文字集合に優先して使用することを推奨します。
Initially, the legal values for "encoding" are "Q" and "B". These encodings are described below. The "Q" encoding is recommended for use when most of the characters to be encoded are in the ASCII character set; otherwise, the "B" encoding should be used. Nevertheless, a mail reader which claims to recognize 'encoded-word's MUST be able to accept either encoding for any character set which it supports.
Only a subset of the printable ASCII characters may be used in 'encoded-text'. Space and tab characters are not allowed, so that the beginning and end of an 'encoded-word' are obvious. The "?" character is used within an 'encoded-word' to separate the various portions of the 'encoded-word' from one another, and thus cannot appear in the 'encoded-text' portion. Other characters are also illegal in certain contexts. For example, an 'encoded-word' in a 'phrase' preceding an address in a From header field may not contain any of the "specials" defined in RFC 822. Finally, certain other characters are disallowed in some contexts, to ensure reliability for messages that pass through internetwork mail gateways.
The "B" encoding automatically meets these requirements. The "Q" encoding allows a wide range of printable characters to be used in non-critical locations in the message header (e.g., Subject), with fewer characters available for use in other locations.
The "B" encoding is identical to the "BASE64" encoding defined by RFC 2045.
「B」符号化方式は RFC 2045 で定義された "BASE64" 符号化方式と同じです。
The "Q" encoding is similar to the "Quoted-Printable" content- transfer-encoding defined in RFC 2045. It is designed to allow text containing mostly ASCII characters to be decipherable on an ASCII terminal without decoding.
(1) Any 8-bit value may be represented by a "=" followed by two
hexadecimal digits. For example, if the character set in use
were ISO-8859-1, the "=" character would thus be encoded as
"=3D", and a SPACE by "=20". (Upper case should be used for
hexadecimal digits "A" through "F".)(2) The 8-bit hexadecimal value 20 (e.g., ISO-8859-1 SPACE) may be represented as "_" (underscore, ASCII 95.). (This character may not pass through some internetwork mail gateways, but its use will greatly enhance readability of "Q" encoded data with mail readers that do not support this encoding.) Note that the "_" always represents hexadecimal 20, even if the SPACE character occupies a different code position in the character set in use.
(2) 8ビットの16進値で 20 (ISO-8859-1 では SPACE (間隔)) は 「_」 (下線, ASCII 95) えで表現できます。 (この文字は 幾つかのネットワーク間メイル関門を通過できないかもしれませんが、 この符号化法に対応していないメイル読者の「Q」符号化データ可読性を 非常に高めることになります。) なお、「_」は SPACE 文字が使われる 文字集合において他の符号位置にある場合であっても常に16進20を表現します。
(3) 8-bit values which correspond to printable ASCII characters other than "=", "?", and "_" (underscore), MAY be represented as those characters. (But see section 5 for restrictions.) In particular, SPACE and TAB MUST NOT be represented as themselves within encoded words.
(3) 「=」, 「?」, 「_」(下線) を除く印字可能な ASCII 文字に対応する 8ビット値はその文字で表現しても構いません。 (但し第5章の 制限も参照して下さい。) 特に、 SPACE と TAB は符号化語内で 自身を表現してはなりません。
An 'encoded-word' may appear in a message header or body part header according to the following rules:
(1) An 'encoded-word' may replace a 'text' token (as defined by RFC 822) in any Subject or Comments header field, any extension message header field, or any MIME body part field for which the field body is defined as '*text'. An 'encoded-word' may also appear in any user-defined ("X-") message or body part header field.
(1) 「encoded-word」は Subject (主題), Comments (注釈) 頭欄や欄本体が「*text」と定義された拡張メッセージ頭欄, MIME 本体部分欄の「text」 token (RFC 822 で定義) を置き換えることが出来ます。 「encoded-word」は利用者定義 (「X-」) メッセージ/本体部分頭領域 に出現することも出来ます。
訳注: 「拡張メッセージ頭欄, 欄本体が『*text』と定義された MIME 本体部分欄」ではありません。 ietf-822 の <mid:200210141709.g9EH9c015818@astro.cs.utk.edu> を参照。
Ordinary ASCII text and 'encoded-word's may appear together in the same header field. However, an 'encoded-word' that appears in a header field defined as '*text' MUST be separated from any adjacent 'encoded-word' or 'text' by 'linear-white-space'.
普通の ASCII 文と「encoded-word」は同じ頭欄に同時に出現出来ます。 しかし、「*text」と定義された頭欄内に出現する「encoded-word」 は他のどの隣接する「encoded-word」または「text」とも 「linear-white-space」で区切らなければなりません。
(2) An 'encoded-word' may appear within a 'comment' delimited by "(" and ")", i.e., wherever a 'ctext' is allowed. More precisely, the RFC 822 ABNF definition for 'comment' is amended as follows:
(2) 「encoded-word」は「(」と「)」で区切られた「comment」(注釈)内、 つまり「ctext」が認められる場所で出現出来ます。より正確に言うと、 「comment」の RFC 822 ABNF 定義は次のように改訂されます。
comment = "(" *(ctext / quoted-pair / comment / encoded-word) ")"A "Q"-encoded 'encoded-word' which appears in a 'comment' MUST NOT contain the characters "(", ")" or " 'encoded-word' that appears in a 'comment' MUST be separated from any adjacent 'encoded-word' or 'ctext' by 'linear-white-space'.
「comment」中の「Q」符号化された「encoded-word」では 文字「(」, 「)」, 「\」は使ってはいけません。 「comment」中に現れる「encoded-word」は他のどの「encoded-word」 や「ctext」とも「linear-white-space」で区切らなければなりません。
It is important to note that 'comment's are only recognized inside "structured" field bodies. In fields whose bodies are defined as '*text', "(" and ")" are treated as ordinary characters rather than comment delimiters, and rule (1) of this section applies. (See RFC 822, sections 3.1.2 and 3.1.3)
「comment」は「構造化」領域本体でのみ認識されることに 注意するのが重要です。本体が「*text」と定義された領域では 「(」や「)」は注釈区切りではなく普通の文字として扱われるので、 この節の規則(1)が適用されます。 (RFC 822 第3.1.2節, 第3.1.3節参照)
(3) As a replacement for a 'word' entity within a 'phrase', for example, one that precedes an address in a From, To, or Cc header. The ABNF definition for 'phrase' from RFC 822 thus becomes:
(3) 「phrase」中の「word」実体を置き換えます。例えば From (送信元), To (送信先), Cc (同報) 頭中のアドレスの前に 来るものです。 RFC 822 の「phrase」の ABNF 定義はですから次のようになります。
phrase = 1*( encoded-word / word )
In this case the set of characters that may be used in a "Q"-encoded 'encoded-word' is restricted to: <upper and lower case ASCII letters, decimal digits, "!", "*", "+", "-", "/", "=", and "_" (underscore, ASCII 95.)>. An 'encoded-word' that appears within a 'phrase' MUST be separated from any adjacent 'word', 'text' or 'special' by 'linear-white-space'.
この場合に「Q」符号化された「encoded-word」で使用可能な文字の集合は <ASCII の大文字・小文字, 10進数字, 「!」, 「*」, 「+」, 「+」, 「-」, 「/」, 「=」, 「_」(下線, ASCII 95)> に制限されます。 「phrase」中に現れる「encoded-word」は他のどの隣接する「word」, 「text」, 「special」とも「linear-white-space」で区切らなければ なりません。
These are the ONLY locations where an 'encoded-word' may appear. In particular: これだけが「encoded-word」が出現できる場所です。 特に、
The 'encoded-text' in an 'encoded-word' must be self-contained; 'encoded-text' MUST NOT be continued from one 'encoded-word' to another. This implies that the 'encoded-text' portion of a "B" 'encoded-word' will be a multiple of 4 characters long; for a "Q" 'encoded-word', any "=" character that appears in the 'encoded-text' portion will be followed by two hexadecimal characters.
Each 'encoded-word' MUST encode an integral number of octets. The 'encoded-text' in each 'encoded-word' must be well-formed according to the encoding specified; the 'encoded-text' may not be continued in the next 'encoded-word'. (For example, "=?charset?Q?=?= =?charset?Q?AB?=" would be illegal, because the two hex digits "AB" must follow the "=" in the same 'encoded-word'.)
Each 'encoded-word' MUST represent an integral number of characters. A multi-octet character may not be split across adjacent 'encoded- word's.
Only printable and white space character data should be encoded using this scheme. However, since these encoding schemes allow the encoding of arbitrary octet values, mail readers that implement this decoding should also ensure that display of the decoded data on the recipient's terminal will not cause unwanted side-effects.
Use of these methods to encode non-textual data (e.g., pictures or sounds) is not defined by this memo. Use of 'encoded-word's to represent strings of purely ASCII characters is allowed, but discouraged. In rare cases it may be necessary to encode ordinary text that looks like an 'encoded-word'.
A mail reader must parse the message and body part headers according to the rules in RFC 822 to correctly recognize 'encoded-word's.
'encoded-word's are to be recognized as follows:
(1) Any message or body part header field defined as '*text', or any
user-defined header field, should be parsed as follows: Beginning
at the start of the field-body and immediately following each
occurrence of 'linear-white-space', each sequence of up to 75
printable characters (not containing any 'linear-white-space')
should be examined to see if it is an 'encoded-word' according to
the syntax rules in section 2. Any other sequence of printable
characters should be treated as ordinary ASCII text.(1) 「*text」と定義されたメッセージ/本体部分頭領域, または利用者定義の頭領域 では次のように解析します: 領域本体の最初から初めて、 すぐに「linear-white-space」が各々続き、75字までの印字可能文字 (「linear-white-space」を含まない) の各列を第2章の構文規則に従った 「encoded-wprd」か調べます。他の印字可能な文字の列は普通の ASCII 文として扱います。
(2) Any header field not defined as '*text' should be parsed
according to the syntax rules for that header field. However,
any 'word' that appears within a 'phrase' should be treated as an
'encoded-word' if it meets the syntax rules in section 2.
Otherwise it should be treated as an ordinary 'word'.(2) 「*text」と定義されていない頭領域はその頭領域の構文規則に 従って解析します。しかし、「phrase」中に現れる「word」 は第2章の構文規則に合致すれば「encoded-word」として扱います。 そうでなければ普通の「word」として扱います。
(3) Within a 'comment', any sequence of up to 75 printable characters
(not containing 'linear-white-space'), that meets the syntax
rules in section 2, should be treated as an 'encoded-word'.
Otherwise it should be treated as normal comment text.(3) 「COMMENT」中では、75字までの印字可能な文字 (「linear-white-space」 を含まない) で第2章の構文規則に合致するものを 「encoded-word」として扱います。そうでなければ普通の comment 文として 扱います。
(4) A MIME-Version header field is NOT required to be present for
'encoded-word's to be interpreted according to this
specification. One reason for this is that the mail reader is
not expected to parse the entire message header before displaying
lines that may contain 'encoded-word's.(4) MIME-Version 頭領域は「encoded-word」をこの仕様書に従って 解釈するのにある必要はありません。この理由の一つは メイル読者がメッセージ頭全体を「encoded-word」を含む行を表示する前に 解析するとか限らないからです。
Any 'encoded-word's so recognized are decoded, and if possible, the resulting unencoded text is displayed in the original character set.
認識された「encoded-word」は復号し、可能ならその結果の 符号化されていない文を元の文字集合で表示します。
NOTE: Decoding and display of encoded-words occurs *after* a structured field body is parsed into tokens. It is therefore possible to hide 'special' characters in encoded-words which, when displayed, will be indistinguishable from 'special' characters in the surrounding text. For this and other reasons, it is NOT generally possible to translate a message header containing 'encoded-word's to an unencoded form which can be parsed by an RFC 822 mail reader.
参考: encoded-word の復号と表示は構造化領域本体が token に 解析された'後'にします。そうしなければ、 encoded-word 中に 周りの文中の「special」文字と区別がつかなくなる「special」文字が隠れて いるかもしれないのです。この理由や他の理由により、 「encoded-word」を含むメッセージ頭を RFC 822 メイル読者が 解析できる符号化されていないものに変換することは 通常出来ないのです。
When displaying a particular header field that contains multiple 'encoded-word's, any 'linear-white-space' that separates a pair of adjacent 'encoded-word's is ignored. (This is to allow the use of multiple 'encoded-word's to represent long strings of unencoded text, without having to separate 'encoded-word's where spaces occur in the unencoded text.)
複数の「encoded-word」を含む頭領域を表示する場合、 隣接する「encoded-word」を区切る「linear-white-space」は無視します。 (これは長い文字列の符号化されていない文を、 符号化されていない文中で空白間隔で区切った「encoded-word」を使わずに 表現するのに「encoded-word」を利用できるようにするためです。)
In the event other encodings are defined in the future, and the mail reader does not support the encoding used, it may either (a) display the 'encoded-word' as ordinary text, or (b) substitute an appropriate message indicating that the text could not be decoded.
将来定義される他の符号化方式であった時、メイル読者が対応していない 符号化法式が使われていたときは、 (a) 「encoded-word」を普通の文で 表示または (b) 復号できなかった文を示す適切なメッセージを 代わりに使うかのどちらかを採っても構いません。
If the mail reader does not support the character set used, it may (a) display the 'encoded-word' as ordinary text (i.e., as it appears in the header), (b) make a "best effort" to display using such characters as are available, or (c) substitute an appropriate message indicating that the decoded text could not be displayed.
メイル読者が対応していない文字集合が使われていた場合、 (a) 「encoded-word」を普通の文として表示 (頭中に出てくるままに), (b) 使われている文字が表示出来るように「最大の努力」を払う, (c) 復号できなかった文を示す適切なメッセージを 代わりに使うのいずれかを採っても構いません。
If the character set being used employs code-switching techniques, display of the encoded text implicitly begins in "ASCII mode". In addition, the mail reader must ensure that the output device is once again in "ASCII mode" after the 'encoded-word' is displayed.
符号切り替え技術を用いる文字集合が使われている場合、 符号化文は暗黙の内に「ASCII 状態」から始まります。 加えて、メイル読者は「encoded-word」が表示された後再び 「ASCII 状態」出力機器を念のため戻さなければなりません。
It is possible that an 'encoded-word' that is legal according to the syntax defined in section 2, is incorrectly formed according to the rules for the encoding being used. For example:
(1) An 'encoded-word' which contains characters which are not legal
for a particular encoding (for example, a "-" in the "B"
encoding, or a SPACE or HTAB in either the "B" or "Q" encoding),
is incorrectly formed.(1) その符号化方式で妥当でない文字 (例えば、「B」符号化方式の「-」 や「B」・「Q」いずれの符号化方式においても SPACE (間隔) や HTAB (水平タブ)) を含む「encoded-word」は不正確に形成されています。
(2) Any 'encoded-word' which encodes a non-integral number of
characters or octets is incorrectly formed.(2) 非整数個の文字やオクテットを符号化した「encoded-word」は不正確に 形成されています。
A mail reader need not attempt to display the text associated with an 'encoded-word' that is incorrectly formed. However, a mail reader MUST NOT prevent the display or handling of a message because an 'encoded-word' is incorrectly formed.
メイル読者は不正確に形成された「encoded-word」に関連付けられた文を表示 しようとする必要はありません。しかし、メイル読者は「encoded-word」が 不正確に形成されているからといってメッセージの表示や取り扱いを やめてはいけません。
A mail composing program claiming compliance with this specification
MUST ensure that any string of non-white-space printable ASCII
characters within a '*text' or '*ctext' that begins with "=?" and
ends with "?=" be a valid 'encoded-word'. ("begins" means: at the
start of the field-body, immediately following 'linear-white-space',
or immediately following a "(" for an 'encoded-word' within '*ctext';
"ends" means: at the end of the field-body, immediately preceding
'linear-white-space', or immediately preceding a ")" for an
'encoded-word' within '*ctext'.) In addition, any 'word' within a
'phrase' that begins with "=?" and ends with "?=" must be a valid
'encoded-word'.この使用に適合すると主張するメイル作成プログラムは 「*text」や「*ctext」中の「=?」で始まり「?=」で終わる 非空白間隔印字可能 ASCII 文字から成る文字列を妥当な「encoded-word」 としなければなりません。 (「始まる」の意味: 領域本体の開始時に、 直ぐに「linear-white-space」が来るか 「*ctext」中の「encoded-word」の場合直ぐに「(」が来る。 「終わる」の意味: 領域本体の終わりで、すぐに「linear-white-space」 が続くか「*ctext」中の「encoded-word」の場合直ぐに「)」が来る。) 加えて、「phrase」中の「word」で「=?」で始まり「?=」で終わるものを全て 妥当な「encoded-word」としなければなりません。
A mail reading program claiming compliance with this specification must be able to distinguish 'encoded-word's from 'text', 'ctext', or 'word's, according to the rules in section 6, anytime they appear in appropriate places in message headers. It must support both the "B" and "Q" encodings for any character set which it supports. The program must be able to display the unencoded text if the character set is "US-ASCII". For the ISO-8859-* character sets, the mail reading program must at least be able to display the characters which are also in the ASCII set.
この仕様書に適合すると主張するメイルを読むプログラムは 「encoded-word」を第6章の規則に従い「text」, 「ctext」, 「word」から 区別できなければなりません。また、「B」符号化方式と 「Q」符号化方式の両方を対応しているどの文字集合に就いても 対応しなければなりません。プログラムは、文字集合が「US-ASCII」なら 符号化されていない文を表示できなければなりません。ISO-8859-* 文字集合については、メイルを読むプログラムは少なくても ASCII 集合と同じ部分は表示できなければなりません。
The following are examples of message headers containing 'encoded-word's:
次に挙げるのは「encoded-word」を含むメッセージ頭の例です。
From: =?US-ASCII?Q?Keith_Moore?= <moore@cs.utk.edu>
To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <keld@dkuug.dk>
CC: =?ISO-8859-1?Q?Andr=E9?= Pirard <PIRARD@vm1.ulg.ac.be>
Subject: =?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?=
=?ISO-8859-2?B?dSB1bmRlcnN0YW5kIHRoZSBleGFtcGxlLg==?= Note: In the first 'encoded-word' of the Subject field above, the
last "=" at the end of the 'encoded-text' is necessary because each
'encoded-word' must be self-contained (the "=" character completes a
group of 4 base64 characters representing 2 octets). An additional
octet could have been encoded in the first 'encoded-word' (so that
the encoded-word would contain an exact multiple of 3 encoded
octets), except that the second 'encoded-word' uses a different
'charset' than the first one.参考: 上の Subject (主題) 領域の最初の「encoded-word」で、 「encoded-text」の最後の「=」は、各「encoded-word」が 自己完結していなければならない (「=」文字は2オクテットを表現する 4つの base64 文字の集団を完成させる) ので必要です。...
From: =?ISO-8859-1?Q?Olle_J=E4rnefors?= <ojarnef@admin.kth.se> To: ietf-822@dimacs.rutgers.edu, ojarnef@admin.kth.se Subject: Time for ISO 10646?
To: Dave Crocker <dcrocker@mordor.stanford.edu> Cc: ietf-822@dimacs.rutgers.edu, paf@comsol.se From: =?ISO-8859-1?Q?Patrik_F=E4ltstr=F6m?= <paf@nada.kth.se> Subject: Re: RFC-HDR care and feeding
From: Nathaniel Borenstein <nsb@thumper.bellcore.com>
(=?iso-8859-8?b?7eXs+SDv4SDp7Oj08A==?=)
To: Greg Vaudreuil <gvaudre@NRI.Reston.VA.US>, Ned Freed
<ned@innosoft.com>, Keith Moore <moore@cs.utk.edu>
Subject: Test of new header generator
MIME-Version: 1.0
Content-type: text/plain; charset=ISO-8859-1 The following examples illustrate how text containing 'encoded-word's
which appear in a structured field body. The rules are slightly
different for fields defined as '*text' because "(" and ")" are not
recognized as 'comment' delimiters. [Section 5, paragraph (1)]. In each of the following examples, if the same sequence were to occur
in a '*text' field, the "displayed as" form would NOT be treated as
encoded words, but be identical to the "encoded form". This is
because each of the encoded-words in the following examples is
adjacent to a "(" or ")" character.encoded form displayed as --------------------------------------------------------------------- (=?ISO-8859-1?Q?a?=) (a)
(=?ISO-8859-1?Q?a?= b) (a b)
Within a 'comment', white space MUST appear between an
'encoded-word' and surrounding text. [Section 5,
paragraph (2)]. However, white space is not needed between
the initial "(" that begins the 'comment', and the
'encoded-word'.(=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=) (ab)
White space between adjacent 'encoded-word's is not
displayed.(=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=) (ab)
Even multiple SPACEs between 'encoded-word's are ignored
for the purpose of display. (=?ISO-8859-1?Q?a?= (ab)
=?ISO-8859-1?Q?b?=) Any amount of linear-space-white between 'encoded-word's,
even if it includes a CRLF followed by one or more SPACEs,
is ignored for the purposes of display.(=?ISO-8859-1?Q?a_b?=) (a b)
In order to cause a SPACE to be displayed within a portion
of encoded text, the SPACE MUST be encoded as part of the
'encoded-word'.(=?ISO-8859-1?Q?a?= =?ISO-8859-2?Q?_b?=) (a b)
In order to cause a SPACE to be displayed between two strings
of encoded text, the SPACE MAY be encoded as part of one of
the 'encoded-word's. [RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982. [RFC 2049] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Examples",
RFC 2049, November 1996. [RFC 2045] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996. [RFC 2046] Borenstein N., and N. Freed, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996. [RFC 2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", RFC 2048, November 1996.Security issues are not discussed in this memo.
The author wishes to thank Nathaniel Borenstein, Issac Chan, Lutz Donnerhacke, Paul Eggert, Ned Freed, Andreas M. Kirchwitz, Olle Jarnefors, Mike Rosin, Yutaka Sato, Bart Schaefer, and Kazuhiko Yamamoto, for their helpful advice, insightful comments, and illuminating questions in response to earlier versions of this specification.
Keith Moore University of Tennessee 107 Ayres Hall Knoxville TN 37996-1301
EMail: moore@cs.utk.edu
+ explicitly state that the MIME-Version is not requried to use
'encoded-word's. + add explicit note that SPACEs and TABs are not allowed within
'encoded-word's, explaining that an 'encoded-word' must look like an
'atom' to an RFC822 parser.values, to be precise). + add examples from Olle Jarnefors (thanks!) which illustrate how
encoded-words with adjacent linear-white-space are displayed.+ explicitly list terms defined in RFC822 and referenced in this memo
+ fix transcription typos that caused one or two lines and a couple of
characters to disappear in the resulting text, due to nroff quirks. + clarify that encoded-words are allowed in '*text' fields in both
RFC822 headers and MIME body part headers, but NOT as parameter
values. + clarify the requirement to switch back to ASCII within the encoded
portion of an 'encoded-word', for any charset that uses code switching
sequences. + add a note about 'encoded-word's being delimited by "(" and ")"
within a comment, but not in a *text (how bizarre!). + fix the Andre Pirard example to get rid of the trailing "_" after
the =E9. (no longer needed post-1342). + clarification: an 'encoded-word' may appear immediately following
the initial "(" or immediately before the final ")" that delimits a
comment, not just adjacent to "(" and ")" *within* *ctext. + add a note to explain that a "B" 'encoded-word' will always have a
multiple of 4 characters in the 'encoded-text' portion.+ add note about the "=" in the examples
+ note that processing of 'encoded-word's occurs *after* parsing, and
some of the implications thereof. + explicitly state that you can't expect to translate between
1522 and either vanilla 822 or so-called "8-bit headers". + explicitly state that 'encoded-word's are not valid within a
'quoted-string'.RFC 2047 provides support for non-US-ASCII character sets in RFC 822 message header comments, phrases, and any unstructured text field. This is done by defining an encoded word construct which can appear in any of these places. Given that these are fields intended for display, it is sometimes necessary to associate language information with encoded words as well as just the character set. This specification extends the definition of an encoded word to allow the inclusion of such information. This is simply done by suffixing the character set specification with an asterisk followed by the language tag. For example:
From: =?US-ASCII*EN?Q?Keith_Moore?= <moore@cs.utk.edu>
The ABNF given in RFC 2047 for encoded-words is:
encoded-word := "=?" charset "?" encoding "?" encoded-text "?="
This specification changes this ABNF to:
encoded-word := "=?" charset ["*" language] "?" encoded-text "?="