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

Replies are listed 'Best First'.
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]
      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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