/gitsign

Keyless Git signing using Sigstore

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Gitsign

CI E2E

Gitsign logo

Keyless Git signing with Sigstore!

This is heavily inspired by https://github.com/github/smimesign, but uses keyless Sigstore to sign Git commits with your own GitHub / OIDC identity.

Installation

Using Homebrew:

brew install sigstore/tap/gitsign

Using Go:

go install github.com/sigstore/gitsign@latest

Configuration

Single Repository:

cd /path/to/my/repository
git config --local commit.gpgsign true  # Sign all commits
git config --local tag.gpgsign true  # Sign all tags
git config --local gpg.x509.program gitsign  # Use gitsign for signing
git config --local gpg.format x509  # gitsign expects x509 args

All respositories:

git config --global commit.gpgsign true  # Sign all commits
git config --global tag.gpgsign true  # Sign all tags
git config --global gpg.x509.program gitsign  # Use gitsign for signing
git config --global gpg.format x509  # gitsign expects x509 args

To learn more about these options, see git-config.

File config

Gitsign can be configured with a standard git-config file. For example, to set the Fulcio option for a single repo:

$ git config --local gitsign.fulcio https://fulcio.example.com

The following config options are supported:

Option Default Description
fulcio https://fulcio.sigstore.dev Address of Fulcio server
logPath Path to log status output. Helpful for debugging when no TTY is available in the environment.
clientID sigstore OIDC client ID for application
issuer https://oauth2.sigstore.dev/auth OIDC provider to be used to issue ID token
redirectURL OIDC Redirect URL
rekor https://rekor.sigstore.dev Address of Rekor server
connectorID Optional Connector ID to auto-select to pre-select auth flow to use. For the public sigstore instance, valid values are:
- https://github.com/login/oauth
- https://accounts.google.com
- https://login.microsoftonline.com
timestampServerURL Address of timestamping authority. If set, a trusted timestamp will be included in the signature.
timestampCertChain Path to PEM encoded certificate chain for RFC3161 Timestamp Authority verification.

Environment Variables

Environment Variable Sigstore
Prefix		 Default Description
GITSIGN_CREDENTIAL_CACHE ❌ Optional path to gitsign-credential-cache socket.
GITSIGN_CONNECTOR_ID ✅ Optional Connector ID to auto-select to pre-select auth flow to use. For the public sigstore instance, valid values are:
- https://github.com/login/oauth
- https://accounts.google.com
- https://login.microsoftonline.com
GITSIGN_FULCIO_URL ✅ https://fulcio.sigstore.dev Address of Fulcio server
GITSIGN_LOG ❌ Path to log status output. Helpful for debugging when no TTY is available in the environment.
GITSIGN_OIDC_CLIENT_ID ✅ sigstore OIDC client ID for application
GITSIGN_OIDC_ISSUER ✅ https://oauth2.sigstore.dev/auth OIDC provider to be used to issue ID token
GITSIGN_OIDC_REDIRECT_URL ✅ OIDC Redirect URL
GITSIGN_REKOR_URL ✅ https://rekor.sigstore.dev Address of Rekor server
GITSIGN_TIMESTAMP_SERVER_URL ✅ Address of timestamping authority. If set, a trusted timestamp will be included in the signature.
GITSIGN_TIMESTAMP_CERT_CHAIN ✅ Path to PEM encoded certificate chain for RFC3161 Timestamp Authority verification.
GITSIGN_FULCIO_ROOT ✅ Path to PEM encoded certificate for Fulcio CA (additional alias: SIGSTORE_ROOT_FILE)

For environment variables that support Sigstore Prefix, the values may be provided with either a GITSIGN_ or SIGSTORE_ prefix - e.g. GITSIGN_CONNECTOR_ID or SIGSTORE_CONNECTOR_ID. If both environment variables are set, GITSIGN_ prefix takes priority.

Other environment variables

Environment Variable Description
SIGSTORE_REKOR_PUBLIC_KEY This specifies an out of band PEM-encoded public key to use for a custom Rekor.

Usage

Signing Commits

Once configured, you can sign commits as usual with git commit -S (or git config --global commit.gpgsign true to enable signing for all commits).

$ git commit --allow-empty --message="Signed commit"
Your browser will now be opened to:
https://oauth2.sigstore.dev/auth/auth?access_type=online&client_id=sigstore&...
[main 040b9af] Signed commit

This will redirect you through the Sigstore Keyless flow to authenticate and sign the commit.

Commits can then be verified using git verify-commit:

$ git verify-commit HEAD
tlog index: 2801760
gitsign: Signature made using certificate ID 0xf805288664f2e851dcb34e6a03b1a5232eb574ae | CN=sigstore-intermediate,O=sigstore.dev
gitsign: Good signature from [billy@chainguard.dev]
Validated Git signature: true
Validated Rekor entry: true

Signing Tags

Once configured, you can sign commits as usual with git tag -s (or git config --global tag.gpgsign true to enable signing for all tags).

$ git tag v0.0.1
Your browser will now be opened to:
https://oauth2.sigstore.dev/auth/auth?access_type=online&client_id=sigstore&...

This will redirect you through the Sigstore Keyless flow to authenticate and sign the tag.

Tags can then be verified using git verify-tag:

$ git verify-tag v0.0.1
tlog index: 2802961
gitsign: Signature made using certificate ID 0xe56a5a962ed59f9e3730d6696137eceb8b4ee8ea | CN=sigstore-intermediate,O=sigstore.dev
gitsign: Good signature from [billy@chainguard.dev]
Validated Git signature: true
Validated Rekor entry: true

Private Sigstore

Gitsign is compatible with other Sigstore tools cosign for running against other Sigstore instances besides the default public instance. See cosign documentation for how to configure and use another instance.

FAQ

Is there any way to bypass the browser flow?

A browser window is needed to get an OAuth token, since gitsign aims to not store refresh tokens or other cryptographic material on disk, but there are some things you can do to make this process a bit easier!

  1. Set the connectorID config option - This preselects the identity provider to use. Assuming you're already signed in, in most cases you'll bounce directly to the auth success screen! (and you can clean up the browser tabs later)
  2. Use the Credential Cache. This uses an in-memory credential cache over a file socket that allows you to persist keys and certificates for their full lifetime (meaning you only need to auth once every 10 minutes).

Why doesn't GitHub show commits as verified?

GitHub doesn't recognize Gitsign signatures as verified at the moment:

  1. The sigstore CA root is not a part of GitHub's trust root.
  2. Because Gitsign's ephemeral keys are only valid for a short time, using standard x509 verification would consider the certificate invalid after expiration. Verification needs to include validation via Rekor to verify the cert was valid at the time it was used.

We hope to work with GitHub to get these types of signatures recognized as verified in the future!

Debugging

Configuration

If gitsign is running with unexpected configs, you can validate the config values that are being ran by running gitsign --version:

$ gitsign --version
gitsign version v0.5.2
parsed config:
{
  "Fulcio": "https://fulcio.sigstore.dev",
  "FulcioRoot": "",
  "Rekor": "https://rekor.sigstore.dev",
  "ClientID": "sigstore",
  "RedirectURL": "",
  "Issuer": "https://oauth2.sigstore.dev/auth",
  "ConnectorID": "",
  "TimestampURL": "",
  "TimestampCert": "",
  "LogPath": ""
}

Signing

If there is an error during signing, you may see an error like:

error: gpg failed to sign the data
fatal: failed to write commit object

When Git invokes signing tools, both stdout and stderr are captured which means gitsign cannot push back messages to shells interactively. If a TTY is available, gitsign will output information to the TTY output directly. If a TTY is not available (e.g. in CI runners, etc.), you can use the GITSIGN_LOG environment variable to tee logs into a readable location for debugging.

Verification

  • failed to verify detached signature: x509: certificate signed by unknown authority

    This usually means the TUF root used to verify the commit is not the same as the root that was used to sign it. This can happen if you use multiple sigstore instances frequently (e.g. if you're a sigstore developer - sigstore staging). You can double check relevant environment variables by running gitsign --version.

Privacy

What data does Gitsign store?

Gitsign stores data in 2 places:

  1. Within the Git commit

    The commit itself contains a signed digest of the user commit content (e.g. author, committer, message, parents, etc.) along with the code signing certificate. This data is stored within the commit itself as part of your repository. See Inspecting the Git commit signature for more details.

  2. Within the Rekor transparency log

    To be able to verify signatures for ephemeral certs past their Not After time, Gitsign records commits and the code signing certificates to Rekor. This data is a HashedRekord containing a SHA256 hash of the commit SHA, as well as the code signing certificate. See Verifying the Transparency Log for more details.

    By default, data is written to the public Rekor instance. In particular, users and organizations may be sensitive to the data contained within code signing certificates returned by Fulcio, which may include user emails or repo identifiers. See OIDC usage in Fulcio for more details for what data is contained in the code signing certs, and Deploy a Rekor Server Manually for how to run your own Rekor instance.

Security

Should you discover any security issues, please refer to the security process

Advanced

Inspecting the Git commit signature

Git commit signatures use CMS/PKCS7 signatures. We can inspect the underlying data / certificate used 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
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:de: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.

We can manually validate that the commit exists in the transparency log by running:

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

$ 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')
tlog entry verified with uuid: d0444ed9897f31fefc820ade9a706188a3bb030055421c91e64475a8c955ae2c index: 2212633
Verified OK
$ 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:de: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 cert that was used to generate the git commit signature. This can be used to correlate the 2 messages, even though they signed different content!

Note that for Git tags, the annotated tag object SHA is what is used (i.e. the output of git rev-parse <tag>), not the SHA of the underlying tagged commit.