On 1/31/22 15:24, Eric Biggers wrote:
On Mon, Jan 31, 2022 at 02:29:19PM -0500, Stefan Berger wrote:
don't think I realized there was a more direct, PKCS#7-less way to do it and
that IMA used that way.) However, it would be better to use this as an
opportunity to move people off of the built-in signatures entirely, either by
switching to a full userspace solution or by switching to IMA.
If what we sign remains the same, then we could support multiple
methods and use a selector to let fsverity_verify_signature() know
how it should verify the signature. I don't know what would be a
proper place for the selector.
PKCS#7 seems ok, as it is used for kernel modules. IMA would be
also ok, as it can verify the signature more directly. I would also
be interested in supporting PGP, to avoid the requirement for
Linux distributions to manage a secondary key. I have a small
extension for rpmsign, that I would like to test in the Fedora
infrastructure.
Both the PKCS#7 and the PGP methods don't require additional
support from outside, the functions verify_pkcs7_signature()
and verify_pgp_signature() (proposed, not yet in the upstream
kernel) would be sufficient.
FYI: An empty file signed with pkcs7 and an ecc key for NIST p256 generates
a signature of size 817 bytes. If an RPM needs to carry such signatures on a
per-file basis we are back to the size increase of nearly an RSA signature.
I would say for packages this is probably too much size increase.. and this
is what drove the implementation of ECC support.
I am getting 256 bytes for an ECC signature in PKCS#7 format:
cd src/fsverity-utils
make
openssl ecparam -name prime256v1 -genkey -noout -out key.pem
openssl req -new -x509 -key key.pem -out cert.pem -days 360
touch file
./fsverity sign file file.sig --key=key.pem --cert=cert.pem
stat -c %s file.sig
Probably you accidentally included the whole certificate in the PKCS#7 message.
That's not required here.
There are definitely problems with PKCS#7, and it does have space overhead. But
the space overhead is not as bad as you state.
You are right. I used openssl cms without -nocerts and -noattr
(unintentionately). Though 256 bytes is RSA 2048 signature size again.
ECDSA with NIST p256 key is around 70 bytes.
- Eric
