smimecosign

README

THIS PROJECT HAS MOVED! See https://github.com/sigstore/gitsign 🎉

smimecosign

Keyless Git signing with cosign!

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

Installation

$ go install github.com/wlynch/smimecosign@latest

Configuration

Single Repository:

$ cd /path/to/my/repository
$ git config --local gpg.x509.program smimecosign
$ git config --local gpg.format x509

All respositories:

$ git config --global gpg.x509.program smimecosign
$ git config --global gpg.format x509

Cosign configuration options

Environment Variable	Default	Description
COSIGN_FULCIO_URL	https://fulcio.sigstore.dev	Address of Fulcio server
COSIGN_OIDC_CLIENT_ID	sigstore	OIDC client ID for application
COSIGN_OIDC_ISSUER	https://oauth2.sigstore.dev/auth	OIDC provider to be used to issue ID token
COSIGN_REKOR_URL	https://rekor.sigstore.dev	Address of Rekor server

Usage

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"
[main cb6eee1] Signed commit

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

Commits can then be verified using git log:

$ git --no-pager log --show-signature -1
commit 227e796042fdd170e58b7e3b7627a1badd320224 (HEAD -> main)
searching tlog for commit: 227e796042fdd170e58b7e3b7627a1badd320224
tlog index: 2212633
smimesign: Signature made using certificate ID 0x815ada5516906a862af8f528d69d3c86e4774b4f | CN=sigstore,O=sigstore.dev
smimesign: Good signature from "" ([billy@chainguard.dev])
Author: Billy Lynch <billy@chainguard.dev>
Date:   Mon May 2 16:51:44 2022 -0400

    Signed commit

Limitations

• Due to how Git invokes signing tools, smimecosign does not work in device mode - a browser session is required to sign commits.

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, smimecosign 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)
$ 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!