RFC 2152 (rfc2152) - Page 1 of 15
UTF-7 A Mail-Safe Transformation Format of Unicode
Alternative Format: Original Text Document
Network Working Group D. Goldsmith Request for Comments: 2152 Apple Computer, Inc. Obsoletes: RFC 1642 M. Davis Category: Informational Taligent, Inc. May 1997 UTF-7 A Mail-Safe Transformation Format of Unicode Status of this Memo This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Abstract The Unicode Standard, version 2.0, and ISO/IEC 10646-1:1993(E) (as amended) jointly define a character set (hereafter referred to as Unicode) which encompasses most of the world's writing systems. However, Internet mail (STD 11, RFC 822) currently supports only 7- bit US ASCII as a character set. MIME (RFC 2045 through 2049) extends Internet mail to support different media types and character sets, and thus could support Unicode in mail messages. MIME neither defines Unicode as a permitted character set nor specifies how it would be encoded, although it does provide for the registration of additional character sets over time. This document describes a transformation format of Unicode that contains only 7-bit ASCII octets and is intended to be readable by humans in the limiting case that the document consists of characters from the US-ASCII repertoire. It also specifies how this transformation format is used in the context of MIME and RFC 1641, "Using Unicode with MIME". Motivation Although other transformation formats of Unicode exist and could conceivably be used in this context (most notably UTF-8, also known as UTF-2 or UTF-FSS), they suffer the disadvantage that they use octets in the range decimal 128 through 255 to encode Unicode characters outside the US-ASCII range. Thus, in the context of mail, those octets must themselves be encoded. This requires putting text through two successive encoding processes, and leads to a significant expansion of characters outside the US-ASCII range, putting non- English speakers at a disadvantage. For example, using UTF-8 together Goldsmith & Davis Informational
