MIME::Decoder::NBit

Section: User Contributed Perl Documentation (3)
Updated: 2021-01-27
Page Index

NAME

MIME::Decoder::NBit - encode/decode a "7bit" or "8bit" stream

SYNOPSIS

A generic decoder object; see MIME::Decoder for usage.

DESCRIPTION

This is a MIME::Decoder subclass for the "7bit" and "8bit" content transfer encodings. These are not ``encodings'' per se: rather, they are simply assertions of the content of the message. From RFC-2045 Section 6.2.:

   Three transformations are currently defined: identity, the "quoted-
   printable" encoding, and the "base64" encoding.  The domains are
   "binary", "8bit" and "7bit".

   The Content-Transfer-Encoding values "7bit", "8bit", and "binary" all
   mean that the identity (i.e. NO) encoding transformation has been
   performed.  As such, they serve simply as indicators of the domain of
   the body data, and provide useful information about the sort of
   encoding that might be needed for transmission in a given transport
   system.

In keeping with this: as of MIME-tools 4.x, this class does no modification of its input when encoding; all it does is attempt to detect violations of the 7bit/8bit assertion, and issue a warning (one per message) if any are found.

Legal 7bit data

RFC-2045 Section 2.7 defines legal "7bit" data:

   "7bit data" refers to data that is all represented as relatively
   short lines with 998 octets or less between CRLF line separation
   sequences [RFC-821].  No octets with decimal values greater than 127
   are allowed and neither are NULs (octets with decimal value 0).  CR
   (decimal value 13) and LF (decimal value 10) octets only occur as
   part of CRLF line separation sequences.

Legal 8bit data

RFC-2045 Section 2.8 defines legal "8bit" data:

   "8bit data" refers to data that is all represented as relatively
   short lines with 998 octets or less between CRLF line separation
   sequences [RFC-821]), but octets with decimal values greater than 127
   may be used.  As with "7bit data" CR and LF octets only occur as part
   of CRLF line separation sequences and no NULs are allowed.

How decoding is done

The decoder does a line-by-line pass-through from input to output, leaving the data unchanged except that an end-of-line sequence of CRLF is converted to a newline ``\n''. Given the line-oriented nature of 7bit and 8bit, this seems relatively sensible.

How encoding is done

The encoder does a line-by-line pass-through from input to output, and simply attempts to detect violations of the "7bit"/"8bit" domain. The default action is to warn once per encoding if violations are detected; the warnings may be silenced with the QUIET configuration of MIME::Tools.

AUTHOR

Eryq (eryq@zeegee.com), ZeeGee Software Inc (http://www.zeegee.com).