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design wrapup

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Joey Hess 2011-04-17 11:27:24 -04:00
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doc/design/encryption.mdwn
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@ -22,8 +22,8 @@ The basis of this scheme was originally developed by Lars Wirzenius et al
[for Obnam](http://braawi.org/obnam/encryption/).
"""]]
Data is encrypted by gpg, using a symmetric cipher. The passphrase of the
cipher is itself checked into your git repository, encrypted using one or
Data is encrypted by gpg, using a symmetric cipher.
The cipher is itself checked into your git repository, encrypted using one or
more gpg public keys. This scheme allows new gpg private keys to be given
access to content that has already been stored in the remote.
@ -58,6 +58,33 @@ for each file in the repository, contact the encrypted remote to check
if it has the file. This can be done without enumeration, although it will
mean running gpg once per file fscked, to get the encrypted filename.
So, the files stored in the remote should be encrypted. But, it needs
to be a repeatable encryption, so they cannot just be gpg encrypted,
that would yeild a new name each time. Instead, HMAC is used. Any hash
could be used with HMAC; currently SHA1 is used.
It was suggested that it might not be wise to use the same cipher for both
gpg and HMAC. Being paranoid, it's best not to tie the security of one
to the security of the other. So, the encrypted cipher described above is
actually split in two; half is used for HMAC, and half for gpg.
----
Does the HMAC cipher need to be gpg encrypted? Imagine if it were
stored in plainext in the git repository. Anyone who can access
the git repository already knows the actual filenames, and typically also
the content hashes of annexed content. Having access to the HMAC cipher
could perhaps be said to only let them verify that data they already
know.
While this seems a pretty persuasive argument, I'm not 100% convinced, and
anyway, most times that the HMAC cipher is needed, the gpg cipher is also
needed. Keeping the HMAC cipher encrypted does slow down two things:
dropping content from encrypted remotes, and checking if encrypted remotes
really have content. If it's later determined to be safe to not encrypt the
HMAC cipher, the current design allows changing that, even for existing
remotes.
## risks
A risk of this scheme is that, once the symmetric cipher has been obtained, it