Re: Is this secure?

From: Jean-Luc Cooke (jlcooke_at_engsoc.org)
Date: 03/31/04 

Date: 31 Mar 2004 14:43:14 GMT

John Burton <john.burton@jbmail.com> wrote:
> If I want to encrypt and transmit some data from a program running on
> one computer to a remote one over the internet.

Sounds like an assignment to me. :)

> 1. Generate an RSA style public and private key pair (Using 1024 but
> modulus in my case but that's variable anyway) and put the public key on
> the machine I want to encrypt data on.

Make sure phi( phi( p * q ) ) is large. This is a common recomendation
to prevent Pollard p-1 attacks.

> 2. Generate a 128 bit random number (I know this has to be done properly
> and it's the point of my question)
> 3. Encrypt the data to be encrypted using AES with the 128 bit number as
> they using CFB mode.

Any reason why you chose this in stead of CBC or CTR? Why don't you add
a MAC to the end of that? OMAC is nice.

> 4. Turn the 128 bit random number into an integer and encrypt it using
> the RSA algorithm and the public key.

Make sure you use a good padding scheme (see PKCS#1.5 or whatever the
acronym for the new one is)

> 5. Send the encrypted key and encrypted data together as the encrypted
> file. (In this base by concatenating them and base64 encoding)

Sounds good... base64 is a "good thing" when using HTTP or other
protocols. But if you're just making a file or a socket stream, you can
just send the bytes raw and save 25% of the transmission overhead.

> I realise the general method is quite a standard way of encrypting
> things, I just want to make sure I've not missed anything and in
> particular that just sending the encrypted key and data in this form is
> *safe*.

Not really. You got most of it. Just add padding to your RSA
operation, add a MAC to your data. Optionally you can get ride of
base64 (depends on application) and make sure your modulus is strong
against p-1 attacks (which 999 times out of 1000 it is).

> I'm looking for this to be 'secure' as a general method so that if one
> message were cracked it wouldn't help with another, and I'm looking for
> it to be generally pretty secure against attacks taking place now rather
> than theoretical attacks taking place in the future.

> It's partly something I'm doing as a learning excercise as well as
> hopefully being something useful for myself in future so while I'd
> welcome any comments about products or other methods to use I'm
> particularly interested in what might be wrong if anything with this, or
> any information about where to look further.

Well written post. Hope you get your answer.

JLC 

--

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