On Fri, Feb 29, 2008 at 02:59:43PM +0000, Johannes Schindelin wrote:
> When encrypting, gpg uses a random element (to make the encryption harder
> to break, I guess). So when I update netrc with "git add" (and nothing
> was changed), git will have a _different_ blob.
This is probably due to two things:
1. random salting of the passphrase when generating a key
To turn a passphrase into a key, you usually do something like
salt = some random data
K = hash(salt + passphrase)
and then include the salt in your message (since the decrypter
needs to know it). The point is to avoid dictionary attacks against
common passphrases (IOW, if "foobar" always becomes 0xabcdef, then
I can just build a table lookup to make brute forcing faster).
So you can turn this off at the price of lessened security against
dictionary attacks.
2. CBC mode with a random IV
Most symmetric algorithms are block ciphers. There are many "modes"
for encrypting a stream; a common one is CBC, which works like
this:
C[0] = random initial vector
C[i] = E(K, P[i] ^ C[i-1])
so each block depends on the block before, and the first block
depends on some randomly selected data.
You can switch to ECB mode, where C[i] = E(P[i]), but it can reveal
patterns in the data (there is a nice graphical example on the
wikipedia page:
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation
You can also use the same IV over and over again, but that does
leak some information: you can tell up to which block two messages
encrypted with the same key and IV are the same.
In the case of your netrc, either of these is probably OK. You are
only trying to keep secret one fixed string within the file.
I don't think gpg's command-line interface is flexible enough to change
any of these options, but I might be wrong. You can definitely use
openssl like this:
openssl aes-256-ecb -nosalt
If you wanted to implement higher quality encryption in git, you could
just encrypt/decrypt objects going into the object database (like how we
do zlib compression), but still name them by hash. The downside, though,
is that if objects are named by their contents, there is an obvious
"guessing" attack where I can see if your repo contains an object with
particular content. There might be a way around that, but I'd have to
give it some thought.
-Peff
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