Inspecting Gitsign Commit Signatures

Git commit signatures use CMS/PKCS7 signatures. You can inspect the underlying data and certificate associated with a project’s HEAD commit by running:

git cat-file commit HEAD | sed -n '/BEGIN/, /END/p' | sed 's/^ //g' | sed 's/gpgsig //g' | sed 's/SIGNED MESSAGE/PKCS7/g' | openssl pkcs7 -print -print_certs -text

You should receive output similar to this:

PKCS7:
  type: pkcs7-signedData (1.2.840.113549.1.7.2)
  d.sign:
    version: 1
    md_algs:
        algorithm: sha256 (2.16.840.1.101.3.4.2.1)
        parameter: <ABSENT>
    contents:
      type: pkcs7-data (1.2.840.113549.1.7.1)
      d.data: <ABSENT>
    cert:
        cert_info:
          version: 2
          serialNumber: 4061203728062639434060493046878247211328523247
          signature:
            algorithm: ecdsa-with-SHA384 (1.2.840.10045.4.3.3)
            parameter: <ABSENT>
          issuer: O=sigstore.dev, CN=sigstore
          validity:
            notBefore: May  2 20:51:47 2022 GMT
            notAfter: May  2 21:01:46 2022 GMT
          subject:
          key:
            algor:
              algorithm: id-ecPublicKey (1.2.840.10045.2.1)
              parameter: OBJECT:prime256v1 (1.2.840.10045.3.1.7)
            public_key:  (0 unused bits)
              0000 - 04 ec 60 4b 67 aa 28 d9-34 f3 83 9c 17 a5   ..`Kg.(.4.....
              000e - c8 a5 87 5e de db c2 65-c8 8b e6 dc c4 6f   ...^...e.....o
              001c - 9c 87 60 05 42 18 f7 b7-0d 8f 06 f1 62 ce   ..`.B.......b.
              002a - 9a 59 9d 71 12 55 1b c3-d4 c7 90 a5 f6 0a   .Y.q.U........
              0038 - b4 1a b3 0e a7 3d 4e 12-38                  .....=N.8
          issuerUID: <ABSENT>
          subjectUID: <ABSENT>
          extensions:
              object: X509v3 Key Usage (2.5.29.15)
              critical: TRUE
              value:
                0000 - 03 02 07 80                              ....

              object: X509v3 Extended Key Usage (2.5.29.37)
              critical: BOOL ABSENT
              value:
                0000 - 30 0a 06 08 2b 06 01 05-05 07 03 03      0...+.......

              object: X509v3 Basic Constraints (2.5.29.19)
              critical: TRUE
              value:
                0000 - 30                                       0
                0002 - <SPACES/NULS>

              object: X509v3 Subject Key Identifier (2.5.29.14)
              critical: BOOL ABSENT
              value:
                0000 - 04 14 a0 b1 ea 03 c5 08-4a 70 93 21 da   ........Jp.!.
                000d - a3 a0 0b 4c 55 49 d3 06-3d               ...LUI..=

              object: X509v3 Authority Key Identifier (2.5.29.35)
              critical: BOOL ABSENT
              value:
                0000 - 30 16 80 14 58 c0 1e 5f-91 45 a5 66 a9   0...X.._.E.f.
                000d - 7a cc 90 a1 93 22 d0 2a-c5 c5 fa         z....".*...

              object: X509v3 Subject Alternative Name (2.5.29.17)
              critical: TRUE
              value:
                0000 - 30 16 81 14 62 69 6c 6c-79 40 63 68 61   0...billy@cha
                000d - 69 6e 67 75 61 72 64 2e-64 65 76         inguard.dev

              object: undefined (1.3.6.1.4.1.57264.1.1)
              critical: BOOL ABSENT
              value:
                0000 - 68 74 74 70 73 3a 2f 2f-67 69 74 68 75   https://githu
                000d - 62 2e 63 6f 6d 2f 6c 6f-67 69 6e 2f 6f   b.com/login/o
                001a - 61 75 74 68                              auth
        sig_alg:
          algorithm: ecdsa-with-SHA384 (1.2.840.10045.4.3.3)
          parameter: <ABSENT>
        signature:  (0 unused bits)
          0000 - 30 65 02 31 00 af be f5-bf e7 05 f5 15 e2 07   0e.1...........
          000f - 85 48 00 ce 81 1e 3e ba-7b 21 d3 e4 a4 8a e6   .H....>.{!.....
          001e - e5 38 9f 01 8a 16 6c 4c-d3 94 af 77 f0 7d 6e   .8....lL...w.}n
          002d - b1 9c f9 29 f9 2c b5 13-02 30 74 eb a5 5a 8a   ...).,...0t..Z.
          003c - 77 a0 95 7f 42 8e df 6a-ed 26 96 cc b4 30 29   w...B..j.&...0)
          004b - b7 94 18 32 c6 8d a5 a4-06 88 e2 01 51 c4 61   ...2........Q.a
          005a - 36 1a 1a 55 ec df a4 0a-84 b5 03 6e 12         6..U.......n.
    crl:
      <EMPTY>
    signer_info:
        version: 1
        issuer_and_serial:
          issuer: O=sigstore.dev, CN=sigstore
          serial: 4061203728062639434060493046878247211328523247
        digest_alg:
          algorithm: sha256 (2.16.840.1.101.3.4.2.1)
          parameter: <ABSENT>
        auth_attr:
            object: contentType (1.2.840.113549.1.9.3)
            value.set:
              OBJECT:pkcs7-data (1.2.840.113549.1.7.1)

            object: signingTime (1.2.840.113549.1.9.5)
            value.set:
              UTCTIME:May  2 20:51:49 2022 GMT

            object: messageDigest (1.2.840.113549.1.9.4)
            value.set:
              OCTET STRING:
                0000 - 66 4e 98 f6 29 46 31 f6-ca 8f 21 44 06   fN..)F1...!D.
                000d - 34 07 2a 8a b2 dd 64 29-4a e9 74 71 d0   4.*...d)J.tq.
                001a - a1 84 ec d5 03 3f                        .....?
        digest_enc_alg:
          algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
          parameter: <ABSENT>
        enc_digest:
          0000 - 30 45 02 20 58 02 c6 8c-30 51 df 4b 14 5e ff   0E. X...0Q.K.^.
          000f - 54 a8 b3 44 0e 32 25 3a-2d 5b cf d9 e4 4e 4c   T..D.2%:-[...NL
          001e - 37 47 af 6e d4 17 02 21-00 81 d9 4c fc b7 e3   7G.n...!...L...
          002d - 92 7e cd a7 c8 84 d6 ae-47 93 88 bd 17 c2 92   .~......G......
          003c - a3 d4 a3 00 ec f6 c9 5b-8b 81 9a               .......[...
        unauth_attr:
          <EMPTY>
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            b6:1c:55:19:85:4a:99:bd:57:12:0d:ec:75:bb:9a:1a:4e:cb:ef
    Signature Algorithm: ecdsa-with-SHA384
        Issuer: O=sigstore.dev, CN=sigstore
        Validity
            Not Before: May  2 20:51:47 2022 GMT
            Not After : May  2 21:01:46 2022 GMT
        Subject:
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:ec:60:4b:67:aa:28:d9:34:f3:83:9c:17:a5:c8:
                    a5:87:5e🇩🇪db:c2:65:c8:8b:e6:dc:c4:6f:9c:87:
                    60:05:42:18:f7:b7:0d:8f:06:f1:62:ce:9a:59:9d:
                    71:12:55:1b:c3:d4:c7:90:a5:f6:0a:b4:1a:b3:0e:
                    a7:3d:4e:12:38
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature
            X509v3 Extended Key Usage:
                Code Signing
            X509v3 Basic Constraints: critical
                CA:FALSE
            X509v3 Subject Key Identifier:
                A0:B1:EA:03:C5:08:4A:70:93:21:DA:A3:A0:0B:4C:55:49:D3:06:3D
            X509v3 Authority Key Identifier:
                keyid:58:C0:1E:5F:91:45:A5:66:A9:7A:CC:90:A1:93:22:D0:2A:C5:C5:FA

            X509v3 Subject Alternative Name: critical
                email:billy@chainguard.dev
            1.3.6.1.4.1.57264.1.1:
                https://github.com/login/oauth
    Signature Algorithm: ecdsa-with-SHA384
         30:65:02:31:00:af:be:f5:bf:e7:05:f5:15:e2:07:85:48:00:
         ce:81:1e:3e:ba:7b:21:d3:e4:a4:8a:e6:e5:38:9f:01:8a:16:
         6c:4c:d3:94:af:77:f0:7d:6e:b1:9c:f9:29:f9:2c:b5:13:02:
         30:74:eb:a5:5a:8a:77:a0:95:7f:42:8e:df:6a:ed:26:96:cc:
         b4:30:29:b7:94:18:32:c6:8d:a5:a4:06:88:e2:01:51:c4:61:
         36:1a:1a:55:ec:df:a4:0a:84:b5:03:6e:12
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Verifying the Transparency Log

As part of signature verification, Gitsign not only checks that the given signature matches the commit, but also that the commit exists within the Rekor transparency log.

To manually validate that the commit exists in the transparency log, first you should obtain the relevant UUID from your Git history and then use rekor-cli to query for this entry. You can do this with the following commands:

uuid=$(rekor-cli search --artifact <(git rev-parse HEAD | tr -d '\n') | tail -n 1)
rekor-cli get --uuid=$uuid --format=json | jq .

If the commit was properly signed and stored in Rekor, you should obtain output similar to this:

LogID: c0d23d6ad406973f9559f3ba2d1ca01f84147d8ffc5b8445c224f98b9591801d
Index: 2212633
IntegratedTime: 2022-05-02T20:51:49Z
UUID: d0444ed9897f31fefc820ade9a706188a3bb030055421c91e64475a8c955ae2c
Body: {
  "HashedRekordObj": {
    "data": {
      "hash": {
        "algorithm": "sha256",
        "value": "05b4f02a24d1c4c2c95dacaee30de2a6ce4b5b88fa981f4e7b456b76ea103141"
      }
    },
    "signature": {
      "content": "MEYCIQCeZwhnq9dgS7ZvU2K5m785V6PqqWAsmkNzAOsf8F++gAIhAKfW2qReBZL34Xrzd7r4JzUlJbf5eoeUZvKT+qsbbskL",
      "publicKey": {
        "content": "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"
      }
    }
  }
}

Notice that this will only check that Rekor has an entry with the provided UUID.

Verifying the Commit Signature

To verify the commit signature and cross-validate it with Rekor, you should use the cosign verify-blob command, providing the signature and the certificate used to sign the commit.

The following commands will:

  1. set up a variable named $sig containing the signature content;
  2. set up a variable named $cert with the certificate’s public key;
  3. run cosign verify-blob using these variables as parameters.
sig=$(rekor-cli get --uuid=$uuid --format=json | jq -r .Body.HashedRekordObj.signature.content)
cert=$(rekor-cli get --uuid=$uuid --format=json | jq -r .Body.HashedRekordObj.signature.publicKey.content)
cosign verify-blob --cert <(echo $cert | base64 --decode) --signature <(echo $sig | base64 --decode) <(git rev-parse HEAD | tr -d '\n')

You should get output similar to this, verifying that the signature is valid for the provided UUID:

tlog entry verified with uuid: d0444ed9897f31fefc820ade9a706188a3bb030055421c91e64475a8c955ae2c index: 2212633
Verified OK

To inspect the certificate, you can use the following command:

echo $cert | base64 --decode | openssl x509 -text

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            b6:1c:55:19:85:4a:99:bd:57:12:0d:ec:75:bb:9a:1a:4e:cb:ef
    Signature Algorithm: ecdsa-with-SHA384
        Issuer: O=sigstore.dev, CN=sigstore
        Validity
            Not Before: May  2 20:51:47 2022 GMT
            Not After : May  2 21:01:46 2022 GMT
        Subject:
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:ec:60:4b:67:aa:28:d9:34:f3:83:9c:17:a5:c8:
                    a5:87:5e🇩🇪db:c2:65:c8:8b:e6:dc:c4:6f:9c:87:
                    60:05:42:18:f7:b7:0d:8f:06:f1:62:ce:9a:59:9d:
                    71:12:55:1b:c3:d4:c7:90:a5:f6:0a:b4:1a:b3:0e:
                    a7:3d:4e:12:38
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature
            X509v3 Extended Key Usage:
                Code Signing
            X509v3 Basic Constraints: critical
                CA:FALSE
            X509v3 Subject Key Identifier:
                A0:B1:EA:03:C5:08:4A:70:93:21:DA:A3:A0:0B:4C:55:49:D3:06:3D
            X509v3 Authority Key Identifier:
                keyid:58:C0:1E:5F:91:45:A5:66:A9:7A:CC:90:A1:93:22:D0:2A:C5:C5:FA

            X509v3 Subject Alternative Name: critical
                email:billy@chainguard.dev
            1.3.6.1.4.1.57264.1.1:
                https://github.com/login/oauth
    Signature Algorithm: ecdsa-with-SHA384
         30:65:02:31:00:af:be:f5:bf:e7:05:f5:15:e2:07:85:48:00:
         ce:81:1e:3e:ba:7b:21:d3:e4:a4:8a:e6:e5:38:9f:01:8a:16:
         6c:4c:d3:94:af:77:f0:7d:6e:b1:9c:f9:29:f9:2c:b5:13:02:
         30:74:eb:a5:5a:8a:77:a0:95:7f:42:8e:df:6a:ed:26:96:cc:
         b4:30:29:b7:94:18:32:c6:8d:a5:a4:06:88:e2:01:51:c4:61:
         36:1a:1a:55:ec:df:a4:0a:84:b5:03:6e:12
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Notice that the Rekor entry uses the same certificate that was used to generate the Git commit signature. This can be used to correlate the two messages, even though they signed different content!

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