[[meta title="MonkeySphere needs to be able to cleanly export passphrase-locked secret keys from the GPG keyring"]]
At the moment, the only tool we have to export passphrase-locked
secret keys from the GPG keyring is gpg itself (and gpg2, which
has roughly the same behavior).
As a result, we have the seckey2sshagent hack, which is unfriendly
and awkward to use.
Ideally, openpgp2ssh would be able to convert passphrase-locked
secret keys into clean subkeys. However, i've tried to do this via
GnuTLS, and that library is not ready for this.
OpenCDK, which is the component of GnuTLS which reads OpenPGP-style
keys, cannot cope with encrypted secret key material. I have had
some
success
in getting GnuTLS's OpenCDK to accept the existence of encrypted
secret key packets, i learned that OpenCDK as included in GnuTLS is
incapable of dealing with the encrypted packets
themselves.
Some possible resolutions:
If we can assume that the passphrase-encrypted key we want to use is
actually a subkey, and if we could fix GnuTLS to ignore the use of the
"gnu-dummy S2K" produced by gpg --export-secret-subkeys for the
primary key, then something like the following script should actually
work for reasonable values of $KEYID:
TMPDIR=$(mktemp -d)
umask 077
mkfifo "$TMPDIR/passphrase"
kname="MonkeySphere Key $KEYID"
mkfifo "$TMPDIR/$kname"
ssh-askpass "Please enter the passphrase for MonkeySphere key $KEYID" >"$TMPDIR/passphrase" &
gpg --passphrase-fd 3 3<"$TMPDIR/passphrase" \
--export-options export-reset-subkey-passwd,export-minimal,no-export-attributes \
--export-secret-subkeys "$KEYID"\! | openpgp2ssh "$KEYID" > "$TMPDIR/$kname" &
(cd "$TMPDIR" && ssh-add -c "$kname")
rm -rf "$TMPDIR"
Good news! I've crafted a patch for GnuTLS to enable it to read
exported subkeys using this GNU
extension,
so if we can get it incorporated into upstream (and/or into debian),
we have a possible solution, as long as the authentication key is a
subkey, and not a primary key.
As of version 0.11-1, monkeysphere subkey-to-ssh-agent implements
this particular strategy (and fails cleanly if the version of GnuTLS
present doesn't support the GNU dummy S2K extension).
Ben Laurie and Rachel Willmer's
OpenPGPSDK is a candidate: this is a
C-based library that intends to implement RFC 4880 functionality.
We could potentially re-write openpgp2ssh using this library, and it
should be able to handle everything we need from the OpenPGP side
(though it might need to be re-linked to OpenSSL to handle PEM-encoded
exports.
Concerns:
-
OpenPGPSDK is not in debian yet, and doesn't currently (2008-08-13)
build with gcc 4.2 or 4.3.
-
OpenPGPSDK uses the apache license and appears to link to OpenSSL,
which has a GPL-incompatible license. I think this would mean that
openpgp2ssh could not remain GPL (though the rest of the
monkeysphere could).
We could try to use perl. The last time i checked, the pure-perl
OpenPGP implementations all depended on Math::PARI, which is not in
debian. The most likely candidate is
Crypt::OpenPGP,
despite some
bugginess.
Concerns:
Other alternatives?
