how to create a symmetric cipher

[Wei Wu [H]]

Thanks to all for offering help. I think I did not describe clearly what I
need (actually for one of my friends). 

Let me know if what I intend to do make no sense.

The data to be protected resides on a fixed harddisk in a Windows computer.
I have a tool on Windows platform that does encryption using either a
passphrase or a key file. Use of a key file is recommended as it is more
secure (assuming passphrases can be cracked relatively easily). The key file
is expected to be stored separately in a removable disk. So I need a tool to
create a key. 

I checked a few key tools such as java keytool and gpg, but their genkey
option does not support the generation of a symmetric key/cipher.

I read the script you (Henry) sent me. Though it does show how to do
symmetric encryption, I don't see how it produce a symmetric cipher. 

Basically, I need to do the job in two steps:
1. create a symmetric key
2. encrypt/decrypt using the key

Regards,
Wei


-----Original Message-----
From: gnupg-users-bounces at gnupg.org [mailto:gnupg-users-bounces at gnupg.org]
On Behalf Of Henry Hertz Hobbit
Sent: Tuesday, November 14, 2006 7:00 PM
To: gnupg-users at gnupg.org
Subject: Re: how to create a symmetric cipher

On Tue, 2006-11-14 at 16:01 -0600, wvu at dls.net wrote:

> Thanks.
> 
> Well, my ultimate goal is to encrypt data, but I don't want to use
> passphrase to do it as I believe it is not secure enough.
> 
> So my questions are:
> 
> 1. How to create a symmetric key or cipher? With that, I may use another
> tool to encrypt/decrypt.
> 
> 2. Can gpg be used to do key based encryption? Not passphrase based.
> 
> Regards,
> WW

I already sent you some scripts to do it off-group.  I am puzzled about
this though.  If you looked at the mail archives, there is Seahorse for
Gnome users:

http://www.gnome.org/projects/seahorse/

I must confess I am baffled by your statements.

1. Creating a key, any key, without a pass-phrase is less safe (I am
   thinking of something like SSH or SSL) than with a pass-phrase.
   If you use the default key with SSH or SSL, all that is necessary
   is to get the key you are using.  Once I can do that I can pretend
   to be you.  But if you have a pass-phrase, even if they steal your
   key, they still need the pass-phrase to pretend to be you.  How is
   that less secure?

2. A key is just a key.  Hypothetically, if gpg didn't complain about
   you not having a key you could hypothetically use gpg to do all
   of the symmetric encryption you want, but even there, a pass-phrase
   (which SHOULD be different than the pass-phrase bound with your key)
   is a good thing, not a bad thing.  If you encrypt a file without a
   pass-phrase, then I can use gpg to decrypt your file and do the same
   thing you did - don't use a passphrase and voila, the file you
   encrypted is decrypted for me without me having to type anything
   other than the decrypt command.

3. Your key is primarily used to sign things and for ASYMMETRIC
   encryption, not symmetric encryption.  By asymmetric encryption
   I mean stuff you send to others in email, using their public key
   to encrypt a message you send to them. Then only they can decrypt
   it using their secret key (which again requires they use their
   pass-phrase).  When you sign a file or a  message you send to
   them, you again must use the pass-phrase bound to your secret
   key when you to achieve the signing.  Would you want me to steal
   your key, sign a message to your boss and send it to him telling
   him that he is a dirty scum-bag?  Without a pass-phrase, that is
   entirely possible.  With a well designed pass-phrase, even if I
   steal your key it makes it very hard if not impossible for me
   to pretend to be you (and get you in hot water).  I can't do it
   without knowing the pass-phrase that must be used with your key.

I am trying to understand how a machine key that is used with something
like SSL used without a password is SAFER than something encrypted WITH
a password.  Every extra thing you can add for verification (and a 
pass-phrase is one of them), security is enhanced, not downgraded.

HHH

PS  Even the NSA has big problems with a symmetric cipher like TWOFISH
    or AES256.  The weakness isn't the algorithm.  The weakness if there
    is one is a BAD or even worse NO encryption PASS-PHRASE.  In fact,
    if you use no pass-phrase with symmetric encryption, you may as well
    not even encrypt the file at all.



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