OpenPGP and usability

Werner Koch wk at
Sun Aug 12 12:51:32 CEST 2007

On Fri, 10 Aug 2007 18:34, seh at said:

> Was there some change in this prescription? If so, from where? I hadn't
> heard about "X-" falling from use.

The current standard, RFC2822 does not mention it anymore:

   3.6.8. Optional fields

   Fields may appear in messages that are otherwise unspecified in this
   standard.  They MUST conform to the syntax of an optional-field.
   This is a field name, made up of the printable US-ASCII characters
   except SP and colon, followed by a colon, followed by any text which
   conforms to unstructured.

   The field names of any optional-field MUST NOT be identical to any
   field name specified elsewhere in this standard.

and the change notices say:

   11. Extension header fields no longer specifically called out.

In contrast the old rfc822 has very detailed information about extension

     4.7.4.  EXTENSION-FIELD

             A limited number of common fields have  been  defined  in
        this  document.   As  network mail requirements dictate, addi-
        tional fields may be standardized.   To  provide  user-defined
        fields  with  a  measure  of  safety,  in name selection, such
        extension-fields will never have names  that  begin  with  the
        string "X-".

             Names of Extension-fields are registered with the Network
        Information Center, SRI International, Menlo Park, California.


             Individual users of network mail are free to  define  and
        use  additional  header  fields.   Such fields must have names
        which are not already used in the current specification or  in
        any definitions of extension-fields, and the overall syntax of
        these user-defined-fields must conform to this specification's
        rules   for   delimiting  and  folding  fields.   Due  to  the
        extension-field  publishing  process,  the  name  of  a  user-
        defined-field may be pre-empted

        Note:  The prefatory string "X-" will never  be  used  in  the
               names  of Extension-fields.  This provides user-defined
               fields with a protected set of names.

My conclusion is that X- was never required by the standard and that
after the 19 years the IETF realized that there was no need for it.
There is an obvious advantage of not using "X-": If the use of such an
optional field later makes it into a standard, the software does not
need to be enhanced to cope with the then standardized field



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