Sigstore allows software developers to quickly sign and verify the software they release. Many of the bigger projects use hardware-based OpenPGP keys to sign and release. But the steps used to make sure that the end-users are correctly verifying those signatures are long, and people make mistakes. Also, not every project has access to hardware smartcards, air-gapped private keys etc. Sigstore solves (or at least makes it way easier) these steps for most developers. It uses existing known (right now only 3) big OIDC providers using which one can sign and verify any data/software.

For this blog post, I will use the python tool called sigstore-python.

The first step is to create a virtual environment and then install the tool.

$ python3 -m venv .venv
$ source .venv/bin/activate
$ python -m pip install -r install/requirements.txt

Next, we create a file called message.txt with the data. This can be our actual release source code tarball.

$ echo "Kushal loves Python!" > message.txt

Signing the data

The next step is to actually sign the file.

$ python -m sigstore sign message.txt 
Waiting for browser interaction...
Using ephemeral certificate:
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

Transparency log entry created at index: 2844439
Signature written to file message.txt.sig
Certificate written to file message.txt.crt

The command will open up the default browser, and we will have the choice to select one of the 3 following OIDC providers.

This will also create message.txt.crt & message.txt.sig files in the same directory.

We can use the openssl command to see the contents of the certificate file.

$ openssl x509 -in message.txt.crt -noout -text
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            3a:c4:2d:19:20:f0:bf:85:37:a6:01:0f:49:d1:b6:39:20:06:fd:77
        Signature Algorithm: ecdsa-with-SHA384
        Issuer: O = sigstore.dev, CN = sigstore-intermediate
        Validity
            Not Before: Jul  5 14:45:23 2022 GMT
            Not After : Jul  5 14:55:23 2022 GMT
        Subject: 
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (384 bit)
                pub:
                    04:12:aa:88:fd:c7:1f:9e:62:78:46:2a:48:63:d3:
                    b6:92:8b:51:a4:eb:59:18:fb:18:a0:13:54:ac:d0:
                    a4:d8:20:ab:a3:f3:5e:f5:86:aa:34:9b:30:db:59:
                    1b:5c:3d:29:b1:5a:40:ff:55:2c:26:fc:42:58:95:
                    53:d6:23:e5:66:90:3c:32:8c:82:b7:fc:fd:f8:28:
                    2b:53:2d:5c:cb:df:2f:17:d0:f3:bc:26:d2:42:3d:
                    c0:b1:55:61:50:ff:18
                ASN1 OID: secp384r1
                NIST CURVE: P-384
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature
            X509v3 Extended Key Usage: 
                Code Signing
            X509v3 Subject Key Identifier: 
                6C:F0:C0:63:B8:3D:BB:08:90:C3:03:45:FF:55:92:43:7D:47:19:38
            X509v3 Authority Key Identifier: 
                DF:D3:E9:CF:56:24:11:96:F9:A8:D8:E9:28:55:A2:C6:2E:18:64:3F
            X509v3 Subject Alternative Name: critical
                email:mail@kushaldas.in
            1.3.6.1.4.1.57264.1.1: 
                https://github.com/login/oauth
            CT Precertificate SCTs: 
                Signed Certificate Timestamp:
                    Version   : v1 (0x0)
                    Log ID    : 08:60:92:F0:28:52:FF:68:45:D1:D1:6B:27:84:9C:45:
                                67:18:AC:16:3D:C3:38:D2:6D:E6:BC:22:06:36:6F:72
                    Timestamp : Jul  5 14:45:23.112 2022 GMT
                    Extensions: none
                    Signature : ecdsa-with-SHA256
                                30:46:02:21:00:AB:A6:ED:59:3E:B7:C4:79:11:6A:92:
                                29:92:BF:54:45:6A:B6:1F:6F:1C:63:7C:D9:89:26:D4:
                                6B:EF:E3:3E:9F:02:21:00:AD:87:A7:BA:BA:7C:61:D2:
                                53:34:E0:D0:C4:BF:6A:6E:28:B4:02:82:AA:F8:FD:0B:
                                FB:3A:CD:B9:33:3D:F4:36
    Signature Algorithm: ecdsa-with-SHA384
    Signature Value:
        30:65:02:30:17:89:76:ef:a1:0e:97:5b:a3:fe:c0:34:13:36:
        3f:6f:2a:ba:e9:cd:bd:f2:74:d9:8c:13:2a:88:c9:96:b2:72:
        de:34:44:95:41:f8:b0:69:5b:f0:86:a7:05:cf:81:7f:02:31:
        00:d8:3a:12:89:39:4b:2c:ad:ff:5a:23:85:d9:c0:73:f0:b1:
        db:5c:65:f9:5d:ee:7a:bb:b8:08:01:44:7a:2e:9f:ba:2b:4b:
        df:6a:93:08:e9:44:2c:23:88:66:2c:f7:8f

Verifying the signature

We can verify the signature, just make sure that the certificate & signature files are in the same directory.

$ python -m sigstore verify message.txt 
OK: message.txt

Now, to test this with some real software releases, we will download the cosign RPM package and related certificate & signature files. The certificate in this case, is base64 encoded, so we decode that file first.

$ curl -sOL https://github.com/sigstore/cosign/releases/download/v1.9.0/cosign-1.9.0.x86_64.rpm
$ curl -sOL https://github.com/sigstore/cosign/releases/download/v1.9.0/cosign-1.9.0.x86_64.rpm-keyless.sig
$ curl -sOL https://github.com/sigstore/cosign/releases/download/v1.9.0/cosign-1.9.0.x86_64.rpm-keyless.pem
$ base64 -d cosign-1.9.0.x86_64.rpm-keyless.pem > cosign-1.9.0.x86_64.rpm.pem

Now let us verify the downloaded RPM package along with the email address and signing OIDC issuer URL. We are also printing the debug statements, so that we can see what is actually happening for verification.

$ SIGSTORE_LOGLEVEL=debug python -m sigstore verify --certificate cosign-1.9.0.x86_64.rpm.pem --signature cosign-1.9.0.x86_64.rpm-keyless.sig --cert-email keyless@projectsigstore.iam.gserviceaccount.com --cert-oidc-issuer https://accounts.google.com  cosign-1.9.0.x86_64.rpm

DEBUG:sigstore._cli:parsed arguments Namespace(subcommand='verify', certificate=PosixPath('cosign-1.9.0.x86_64.rpm.pem'), signature=PosixPath('cosign-1.9.0.x86_64.rpm-keyless.sig'), cert_email='keyless@projectsigstore.iam.gserviceaccount.com', cert_oidc_issuer='https://accounts.google.com', rekor_url='https://rekor.sigstore.dev', staging=False, files=[PosixPath('cosign-1.9.0.x86_64.rpm')])
DEBUG:sigstore._cli:Using certificate from: cosign-1.9.0.x86_64.rpm.pem
DEBUG:sigstore._cli:Using signature from: cosign-1.9.0.x86_64.rpm-keyless.sig
DEBUG:sigstore._cli:Verifying contents from: cosign-1.9.0.x86_64.rpm
DEBUG:sigstore._verify:Successfully verified signing certificate validity...
DEBUG:sigstore._verify:Successfully verified signature...
DEBUG:urllib3.connectionpool:Starting new HTTPS connection (1): rekor.sigstore.dev:443
DEBUG:urllib3.connectionpool:https://rekor.sigstore.dev:443 "POST /api/v1/index/retrieve/ HTTP/1.1" 200 69
DEBUG:urllib3.connectionpool:https://rekor.sigstore.dev:443 "GET /api/v1/log/entries/9ee91f2c5444e4ff77a3a18885f46fa2b6f7e629450904d67b5920333327b90d HTTP/1.1" 200 None
DEBUG:sigstore._verify:Successfully verified Rekor entry...
OK: cosign-1.9.0.x86_64.rpm

Oh, one more important thing. The maintainers of the tool are amazing about feedback. I had some trouble initially (a few weeks ago). They sat down with me to make sure that they could understand the problem & also solved the issue I had. You can talk to the team (and other users, including me) in the slack room.