Testing for encryption. (fwd)

21 Feb 2001

      Yeah, yeah, yeah. Only problem is the constraint that GPG must be
specified as the allowed source encryption standard a priori. This was NOT
in the original problem description.

There is NO algorithm which will determine if an arbitrary piece of text
is encrypted by an arbitrary algorithm. You can tell, at least
statistically if a particular piece of arbitrary text ISN'T a particular
algorithm or language either. Again, not the same problem.

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 ---------- Forwarded message ----------
Date: 21 Feb 2001 19:44:33 -0000
From: Septic Remailer Admin <septic-admin@nym.alias.net>
To: Jim Choate <ravage@EINSTEIN.ssz.com>
Subject: Testing for encryption.

-----BEGIN PGP SIGNED MESSAGE-----

#!/usr/bin/perl

# name: encryption_check.pl
# purpose: Tests if STDIN is encrypted. Prints file to STDOUT if
# it is encrypted. Discards unencrypted files.
# requires: /bin/tempfile from debianutils, /usr/bin/gpg

# TODO:
# Does it need to detect Mixmaster messages?
# It should handle pings (i.e. plain text) to another remailer
# by checking rlist or some arbitrary whitelist.

# author: septic-admin@nym.alias.net
# date: 4 OCT 2000
# license: GPL

if (!-x "/bin/tempfile") { die "requires /bin/tempfile"; }
if (!-x "/usr/bin/gpg") { die "requires /usr/bin/gpg"; }

my $TMP = `tempfile` || die "cannot create temp file";
my $ENCRYPTED = 0;

# Store the file.
open(TEMPFILE, ">$TMP") || die "cannot open temp file";
while (<>) {
	print TEMPFILE;
}
close(TEMPFILE) || die "cannot close tempfile";

# Test the file.
open(GPG, "gpg --batch 2>&1 <$TMP|") || die "cannot open GPG pipe";
while (<GPG>) {
	if ($_ =~ /^gpg: encrypted/) {
		$ENCRYPTED = 1;
	}
}
close(GPG);

# Insert code here to test destination against whitelist (for pings).

# Print the file to STDOUT if it is encrypted.
if ($ENCRYPTED) {
	open(TEMPFILE, "<$TMP") || die "cannot reopen temp file";
	while (<TEMPFILE>) {
		print;
	}
	close(TEMPFILE);
}

unlink $TMP || die "cannot unlink temp file";

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