Crypt::CBC losing its RandomIV

Solostian has asked for the wisdom of the Perl Monks concerning the following question:

I'm currently putting the finishing touch on a massive file transfer utility. The files are ZIPped, encrypted and attached to email then sent. On reception, the files are detached, decrypted then unZIPped.

I'm using the combo Crypt:CBC and Crypt::Twofish for the encryption.

Sending the files works flawlessly. Partway trough reception, I get the following error:

Cipher stream did not contain IV or salt, and you did not specify thes
+e values in new() at (...)
[download]

The file's beginning looks like this:

RandomIV$gÑ“ç
[download]

So, I can decrypt a couple of files and then the programs fails even though the "RandomIV" is there...

The hard part is handled within a custom object.

sub _symmetric {    #Applies symmetric cipher
    caller eq __PACKAGE__ or die "_symmetric is a private method\n";
    die "instance method only" unless ref (my $self = shift);
    my $ifname = shift;
    my ($tfname, $ofname, $fh_plain, $fh_cipher);
    my $cipher = Crypt::CBC->new($self->{SYMMKEY}, 'Twofish')    # Usi
+ng AES-level cipher
        or die "Unable to create cipher: $@\n";

    if ($self->{MODE} eq TRANSMIT) {
        $tfname = $ifname . ZIP;
        $ofname = $ifname . ENC;
        # A - compress
        $self->_compressor($ifname, $tfname);
        die "Unable to delete ${ifname}: $!\n" unless (unlink $ifname)
+;
        # B - encrypt
        $fh_plain = IO::File->new($tfname, O_RDONLY) or die "Unable to
+ open ${tfname}: $!\n";
        $fh_plain->binmode;
        $fh_cipher = IO::File->new($ofname, O_WRONLY | O_CREAT) or die
+ "Unable to create ${ofname}: $!\n";
        $fh_cipher->binmode;
        $cipher->start('encrypting');
        $fh_cipher->write($cipher->crypt($fh_plain->getline)) until ($
+fh_plain->eof);
        $fh_cipher->write($cipher->finish);
        $fh_cipher->flush;
        $fh_plain->close;
        $fh_cipher->close;
        die "Unable to delete ${tfname}: $!\n" unless (unlink $tfname)
+;
    }
    else {
        my @name_parts = split(/\./, $ifname);
        $ofname = shift(@name_parts);
        $tfname = $ofname . ZIP;
        # A - decrypt
        if ( (stat($ifname))[7] ) {
            $fh_cipher = IO::File->new($ifname, O_RDONLY) or die "Unab
+le to open ${ifname}: $!\n";
            $fh_cipher->binmode;
            $fh_plain = IO::File->new($tfname, O_WRONLY | O_CREAT) or 
+die "Unable to create ${tfname}: $!\n";
            $fh_plain->binmode;
#
# It fails on the following line
#
            $cipher->start('decrypting');
            $fh_plain->write($cipher->crypt($fh_cipher->getline)) unti
+l ($fh_cipher->eof);
            $fh_plain->write($cipher->finish);
            $fh_plain->flush;
            $fh_cipher->close;
            $fh_plain->close;
        # B - restore
            if ( (stat($tfname))[7] ) {
                $self->_compressor($tfname, $ofname);
            }
            die "Unable to delete ${tfname}: $!\n" unless (unlink $tfn
+ame);
        }
        die "Unable to delete ${ifname}: $!\n" unless (unlink $ifname)
+;
    }

    return $ofname;
}
[download]

Does anyone has an idea why it happens?

Solostian

Readmore tags added by GrandFather

Comment on Crypt::CBC losing its RandomIV Select or Download Code

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Re: Crypt::CBC losing its RandomIV by jimbojones (Friar) on Mar 17, 2006 at 21:07 UTC
Hi Haven't had time to get deep into this problem, but one thought: you have both filehandles in binmode and you are reading encrypted data that may not have "\n" characters. Perhaps you should do a `$fh_cipher->read` instead of `$fh_cipher->getline`? HTH, J	[reply] [d/l] [select]
Re^2: Crypt::CBC losing its RandomIV by Solostian (Beadle) on Mar 20, 2006 at 16:50 UTC
I just got to a point where I don't get errors anymore. Here's what I did. First, I modified the way I create the Crypt::CBC object to: `my $cipher = Crypt::CBC->new(-key => $self->{SYMMKEY}, -cipher => 'Twofish', -salt => 1) or die "Unable to create cipher:\n$@\n";` My undertsanding is that CBC is now using a random Salt instead of a random IV. Correct me if I am wrong. Doing that solved the decryption error I was getting. It was replaced by an EOCD error during the unzip operation. I then followed your suggestion and replaced the getline with read: `# $fh_cipher->write($cipher->crypt($fh_plain->getline)) until ($fh_pla +in->eof); do { $fh_plain->read($buf, length($self->{SYMMKEY})); $fh_cipher->write($cipher->crypt($buf)) } until ($fh_plain->eof);` [download] and: `# $fh_plain->write($cipher->crypt($fh_cipher->getline)) until ($fh_cip +her->eof); do { $fh_cipher->read($buf, length($self->{SYMMKEY})); $fh_plain->write($cipher->crypt($buf)) } until ($fh_cipher->eof);` [download] That got rid of the EOCD error. My app now runs as expected. Thanks for the tip!	[reply] [d/l] [select]


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