emailrelay-rot13.pl

#!/usr/bin/env perl
#
# SPDX-FileCopyrightText: 2026 Graeme Walker <graeme_walker@users.sourceforge.net>
# SPDX-License-Identifier: FSFAP
#
# Copyright (c) 2026 Graeme Walker <graeme_walker@users.sourceforge.net>
#
# Copying and distribution of this file, with or without modification,
# are permitted in any medium without royalty provided the copyright
# notice and this notice are preserved.  This file is offered as-is,
# without any warranty.
# ===
#
# emailrelay-rot13.pl
#
# An example E-MailRelay filter script that does rot-13
# masking.
#

use strict ;
use IO::File ;
$SIG{__DIE__} = sub { (my $e = join(" ",@_)) =~ s/\n/ /g ; print "<<error: $e>>\n" ; exit 99 } ;

my $content = $ARGV[0] ;
my $content_tmp = "$content.tmp" ;
my $fh_in = new IO::File( $content , "r" ) or die "cannot open content file [$content]: $!\n" ;
my $fh_out = new IO::File( "$content_tmp" , "w" ) or die "cannot open temporary file [$content_tmp]: $!\n" ;
my $boundary = "-----emailrelay-rot13-$$" ;
my $in_header = 1 ;
my @headers = () ;
while(<$fh_in>)
{
    chomp( my $line = $_ ) ;
    $line =~ s/\r$// ;

    if( $in_header && ( $line =~ m/^\s/ ) && scalar(@headers) ) # folding
    {
            $headers[-1] .= "\r\n$line" ;
    }
    elsif( $in_header && ( $line =~ m/^$/ ) )
    {
            $in_header = 0 ;
            for my $h ( @headers )
            {
                    if( $h =~ m/^(subject|to|from):/i )
                    {
                            print $fh_out $h , "\r\n" ;
                    }
            }
            print $fh_out "Content-Type: multipart/mixed; boundary=\"$boundary\"\r\n" ;
            print $fh_out "\r\n" ;
            print $fh_out "\r\n" ;
            print $fh_out "--$boundary\r\n" ;
            print $fh_out "Content-Type: text/plain; charset=us-ascii\r\n" ;
            print $fh_out "\r\n" ;
            print $fh_out "The original message has been masked...\r\n" ;
            print $fh_out "\r\n" ;
            print $fh_out "--$boundary\r\n" ;
            print $fh_out "Content-Type: text/plain\r\n" ;
            print $fh_out "Content-Transfer-Encoding: 8bit\r\n" ;
            print $fh_out "Content-Description: masked message\r\n" ;
            print $fh_out "\r\n" ;
            print $fh_out join( "\r\n" , map { rot13($_) } (@headers,"") ) ;
    }
    elsif( $in_header )
    {
            push @headers , $line ;
    }
    else
    {
            print $fh_out rot13($line) , "\r\n" ;
    }
}
print $fh_out "--$boundary--\r\n" ;
print $fh_out "\r\n" ;

$fh_in->close() or die ;
$fh_out->close() or die "cannot write new file [$content_tmp]: $!\n" ;
unlink( $content ) or die "cannot delete original file [$content]: $!\n" ;
rename( $content_tmp , $content ) or die "cannot rename [$content_tmp]: $!\n" ;
exit( 0 ) ;

sub rot13
{
    my ( $s ) = @_ ;
    $s =~ tr/[a-m][n-z][A-M][N-Z]/[n-z][a-m][N-Z][A-M]/ ;
    return $s ;
}