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UTF-7 (7-bit Unicode Transformation Format) is a variable-length character encoding that was proposed for representing Unicode text using a stream of ASCII characters. It was originally intended to provide a means of encoding Unicode text for use in Internet E-mail messages that was more efficient than the combination of UTF-8 with quoted-printable. == Motivation == MIME, the modern standard of E-mail format, forbids encoding of headers using byte values above the ASCII range. Although MIME allows encoding the message body in various character sets (broader than ASCII), the underlying transmission infrastructure (SMTP, the main E-mail transfer standard) is still not guaranteed to be 8-bit clean. Therefore, a non-trivial content transfer encoding has to be applied in case of doubt. Unfortunately base64 has a disadvantage of making even US-ASCII characters unreadable in non-MIME clients. On the other hand, UTF-8 combined with quoted-printable produces a very size-inefficient format requiring 6–9 bytes for non-ASCII characters from the BMP and 12 bytes for characters outside the BMP. Provided certain rules are followed during encoding, UTF-7 can be sent in e-mail without using an underlying MIME transfer encoding, but still must be explicitly identified as the text character set. In addition, if used within e-mail headers such as "Subject:", UTF-7 must be contained in MIME encoded words identifying the character set. Since encoded words force use of either quoted-printable or base64, UTF-7 was designed to avoid using the = sign as an escape character to avoid double escaping when it is combined with quoted-printable (or its variant, the RFC 2047/1522 ?Q?-encoding of headers). UTF-7 is generally not used as a native representation within applications as it is very awkward to process. Despite its size advantage over the combination of UTF-8 with either quoted-printable or base64, the Internet Mail Consortium recommends against its use.〔Internet Mail Consortium, (Using International Characters in Internet Mail ), 1 August 1998, retrieved 8 January 2009〕 8BITMIME has also been introduced, which reduces the need to encode message bodies in a 7-bit format. A modified form of UTF-7 is currently used in the IMAP e-mail retrieval protocol for mailbox names.〔RFC 3501 section 5.1.3〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「UTF-7」の詳細全文を読む スポンサード リンク
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