using same symmetric key for multiple files

[Peter Lebbing]

On 05/03/13 19:52, Phillip Gardner wrote:
> gpg2 --symmetric --force-mdc --cipher-algo AES256 backup20130405.tar

> Is it a problem using the same key when encrypting multiple files which will
> all be stored together? These files were very similar in content prior to
> being encrypted.

1) It is irrelevant that the files were similar. The passphrase you choose is
used to encrypt a random session key, and the random session key encrypts the
data. So the passphrase never "comes into direct contact" with the data
cryptographically.

2) While in theory there are attacks thinkable that can exploit the fact that
the passphrase is the same each time, the passphrase is also salted before being
used as keying material, so the actual key used changes each time. The material
being encrypted also changes each time (the random session key with an algorithm
specifier prepended), and is very small.

I doubt an attacker would gain something by having multiple Symmetric-Key
Encrypted Session Key packets all created with the same passphrase. But some of
the experts here might know a sweet attack. Or some of those nasty experts that
don't share it here but rather go after your data.

I think you're safe. But why don't you just create a keypair and encrypt to
yourself? It does obviously mean you should have a good backup of it somewhere,
outside Florida, even though you love in Florida. Spread the love! ;)

There is the slight thingy that someone can replace your encrypted data with
other, also correctly decrypting data, since your public key is usually public.
So you should sign then too if you're worried about that.

HTH,

Peter.

-- 
I use the GNU Privacy Guard (GnuPG) in combination with Enigmail.
You can send me encrypted mail if you want some privacy.
My key is available at <http://digitalbrains.com/2012/openpgp-key-peter>