libjcat
This library allows reading and writing gzip-compressed JSON catalog files, which can be used to store GPG, PKCS-7 and SHA-256 checksums for each file.
This provides equivalent functionality to the catalog files supported in Microsoft Windows.
Design
Each JSON file is gzipped, and is logically divided into three structures:
JcatBlob
The 'signature' blob, which can be a proper detached sigature like PKCS-7 or just a checksum like SHA-256.
JcatItem
Items roughly approximate single files, and can have multiple JcatBlobs assigned.
In a typical firmware archive you would have two items, with IDs firmware.bin
and firmware.metainfo.xml
JcatFile
The container which contains one or multiple JcatItems.
Self Signing
Jcat files can be signed using a certificate and key that are automatically generated on your local computer. This means you can only verify the Jcat archive on the same computer (and probably the same user) that you use to sign the archive.
It does however mean you can skip manually generating a secret key and public key pair. If you do upload the public certificate up to a web service (for instance the LVFS) it does mean it can verify your signatures.
$ jcat-tool --appstream-id localhost self-sign firmware.jcat firmware.bin
$ jcat-tool info firmware.jcat
JcatFile:
Version: 0.1
JcatItem:
ID: firmware.bin
JcatBlob:
Kind: pkcs7
Flags: is-utf8
AppstreamId: localhost
Timestamp: 2020-03-05T12:06:42Z
Size: 0x2d9
Data: -----BEGIN PKCS7-----
MIIB9wYJKoZIhvcNAQcCoIIB6DCCAeQCAQExDTALBglghkgBZQMEAgEwCwYJKoZI
...
oDd2UcfqgdQnihpYf0NaPDYhpcP5r7dmH1XN
-----END PKCS7-----
Public Key Signing
Jcat can of course sign the archive with proper keys too. Here we will generate a private and public key ourselves, but you should probably talk to your IT department security team and ask them how to get a user certificate that's been signed by the corporate CA certificate.
Lets create our own certificate authority (CA) and issue a per-user key for local testing. Never use these in any kind of production system!
$ ../contrib/build-certs.py
$ ls ACME-CA.* rhughes*
ACME-CA.key ACME-CA.pem rhughes.csr rhughes.key rhughes.pem rhughes_signed.pem
Then we can actually use both files:
$ jcat-tool --appstream-id com.redhat.rhughes sign firmware.jcat firmware.bin rhughes_signed.pem rhughes.key
JcatFile:
Version: 0.1
JcatItem:
ID: firmware.bin
JcatBlob:
Kind: pkcs7
Flags: is-utf8
AppstreamId: com.redhat.rhughes
Timestamp: 2020-03-05T12:16:30Z
Size: 0x373
Data: -----BEGIN PKCS7-----
MIICZwYJKoZIhvcNAQcCoIICWDCCAlQCAQExDTALBglghkgBZQMEAgEwCwYJKoZI
...
8jggo0FbhDSs8frXhr1BHKBktOPKEbA3sETxlbHViYt6oldpi1uszV0kHA==
-----END PKCS7-----
Lets verify this new signature:
$ jcat-tool --appstream-id com.redhat.rhughes verify firmware.jcat
firmware.bin:
FAILED pkcs7: failed to verify data for O=ACME Corp.,CN=ACME CA: Public key signature verification has failed. [-89]
FAILED: Validation failed
Validation failed
Ahh, of course; we need to tell Jcat to load our generated CA certificate:
$ jcat-tool --appstream-id com.redhat.rhughes verify firmware.jcat --public-key ACME-CA.pem
firmware.bin:
PASSED pkcs7: O=ACME Corp.,CN=ACME CA
We can then check the result using
$ jcat-tool export firmware.jcat
Wrote ./firmware.bin-com.redhat.rhughes.p7b
$ certtool --p7-verify --infile firmware.bin-com.redhat.rhughes.p7b --load-data firmware.bin --load-ca-certificate=ACME-CA.pem
Loaded CAs (1 available)
eContent Type: 1.2.840.113549.1.7.1
Signers:
Signer's issuer DN: O=ACME Corp.,CN=ACME CA
Signer's serial: 4df758978d0601c6500ab6f266963916d8b7ab33
Signature Algorithm: RSA-SHA256
Signature status: ok
Testing
Download a firmware from the LVFS, and decompress with gcab -x -- we can now
validate the signatures are valid:
certtool --p7-verify --infile=firmware.bin.p7b --load-ca-certificate=/etc/pki/fwupd/LVFS-CA.pem --load-data=firmware.bin
Lets create a Jcat file with a single checksum:
$ jcat-tool sign test.jcat firmware.bin sha256
$ jcat-tool info test.jcat
JcatFile:
Version: 0.1
JcatItem:
ID: firmware.bin
JcatBlob:
Kind: sha256
Flags: is-utf8
Timestamp: 2020-03-04T13:59:57Z
Size: 0x40
Data: bd598c9019baee65373da1963fbce7478d6e9e8963bd837d12896f53b03be83e
Now we can import both existing signatures into a Jcat file, and then validate it again.
$ jcat-tool import test.jcat firmware.bin firmware.bin.asc
$ jcat-tool import test.jcat firmware.bin firmware.bin.p7b
$ jcat-tool info test.jcat
JcatFile:
Version: 0.1
JcatItem:
ID: firmware.bin
JcatBlob:
Kind: sha256
Flags: is-utf8
Timestamp: 2020-03-04T13:59:57Z
Size: 0x40
Data: bd598c9019baee65373da1963fbce7478d6e9e8963bd837d12896f53b03be83e
JcatBlob:
Kind: gpg
Flags: is-utf8
Timestamp: 2020-03-04T14:00:30Z
Size: 0x1ea
Data: -----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.22 (GNU/Linux)
iQEcBAABAgAGBQJeVoylAAoJEEim2A5FOLrCagQIAIb6uDCzwUBBoZRqRzekxf0E
...
=0GGy
-----END PGP SIGNATURE-----
JcatBlob:
Kind: pkcs7
Flags: is-utf8
Timestamp: 2020-03-04T14:00:34Z
Size: 0x8c0
Data: -----BEGIN PKCS7-----
MIIGUgYJKoZIhvcNAQcCoIIGQzCCBj8CAQExDTALBglghkgBZQMEAgEwCwYJKoZI
...
EYOqoEV8PaVQZW3ndWEaQfyo6MgZ/WqpO6Gv2zTx1CXk0APIGG8=
-----END PKCS7-----
$ jcat-tool verify test.jcat --public-keys /etc/pki/fwupd
firmware.bin:
PASSED sha256: OK
PASSED gpg: 3FC6B804410ED0840D8F2F9748A6D80E4538BAC2
PASSED pkcs7: O=Linux Vendor Firmware Project,CN=LVFS CA
Security
Unlike Microsoft catalog files which are a signed manifest of hashes, a Jcat file
is a manifest of signatures. This means it's possible (and positively encouraged)
to modify the .jcat file to add new signatures or replace existing ones.
This means Jcat does not verify that the set of file has not been modified, only that the individual files and signatures themselves have not been changed.
If you require some trust in that file A was signed at the same time, or by the same person as file B then then best way to do this is to embed a checksum (e.g SHA-256) into one file and then verify it in the client software.
For instance, when installing firmware we need to know if a metadata file was
provided by the LVFS with the vendor firmware file. To do this, we add the
SHA-256 checksum of the firmware.bin in the firmware.metainfo.xml file itself,
and then add both files to a Jcat archive.
The client software (e.g. fwupd) then needs to check the firmware checksum as
an additional step of verifying the signatures in the Jcat file.