使用 encoded-words 的 MIME headers 中的换行是否合法？

Question

RFC 2047 定义了 encoded-words 机制，用于对 MIME 文档中的 non-ASCII 字符进行编码。它指定 encoded-word.

中不允许使用白色 space 字符（space 和制表符）

但是，RFC 5322 用于解析电子邮件 MIME 文档指定应“折叠”较长的 header 行。这种折叠应该发生在 encoded-words 解码之前还是之后？

我最近收到一封电子邮件，其中 encoded-text 部分 header 中有换行符，如下所示：

Header: =?UTF-8?Q?=C3=A5
 =C3=A4?=

这有效吗？

当然，电子邮件可能以许多令人兴奋的方式无效，解析器需要处理它，但了解“正确”方式很有趣。 :)

Answer 1

我看错了问题，回答的好像是另一种白人space。在这种情况下，白色 space 出现在 MIME 字内，而不是由白色 space.

分隔的多个字

明确禁止这种事情。从RFC2047中对格式的介绍：

2. Syntax of encoded-words

   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'.

稍后在同一部分：

   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

      =?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.

      =?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.

较早的回答

这种事情是明确允许的。 Headers with MIME words 应为 76 个字符或更少，并在需要时折叠。 RFC822 folded headers 缩进第二行和任何其他行。 RFC2047 headers 应该只缩进一个 space。第一行 ?= 和 =? 之间的 whitespace应该从输出中抑制。

参见 RFC 第 12 页底部的示例：

encoded form                                displayed as
---------------------------------------------------------------------
(=?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.

使用 encoded-words 的 MIME headers 中的换行是否合法？

Are newlines in MIME headers using encoded-words legal?

email

mime

rfc

较早的回答