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: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.
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:
- set up a variable named
$sigcontaining the signature content; - set up a variable named
$certwith the certificate’s public key; - run
cosign verify-blobusing these variables as parameters.
sig=$(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: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-----
MIICFTCCAZugAwIBAgIUALYcVRmFSpm9VxIN7HW7mhpOy+8wCgYIKoZIzj0EAwMw
KjEVMBMGA1UEChMMc2lnc3RvcmUuZGV2MREwDwYDVQQDEwhzaWdzdG9yZTAeFw0y
MjA1MDIyMDUxNDdaFw0yMjA1MDIyMTAxNDZaMAAwWTATBgcqhkjOPQIBBggqhkjO
PQMBBwNCAATsYEtnqijZNPODnBelyKWHXt7bwmXIi+bcxG+ch2AFQhj3tw2PBvFi
zppZnXESVRvD1MeQpfYKtBqzDqc9ThI4o4HIMIHFMA4GA1UdDwEB/wQEAwIHgDAT
BgNVHSUEDDAKBggrBgEFBQcDAzAMBgNVHRMBAf8EAjAAMB0GA1UdDgQWBBSgseoD
xQhKcJMh2qOgC0xVSdMGPTAfBgNVHSMEGDAWgBRYwB5fkUWlZql6zJChkyLQKsXF
+jAiBgNVHREBAf8EGDAWgRRiaWxseUBjaGFpbmd1YXJkLmRldjAsBgorBgEEAYO/
MAEBBB5odHRwczovL2dpdGh1Yi5jb20vbG9naW4vb2F1dGgwCgYIKoZIzj0EAwMD
aAAwZQIxAK++9b/nBfUV4geFSADOgR4+unsh0+SkiublOJ8BihZsTNOUr3fwfW6x
nPkp+Sy1EwIwdOulWop3oJV/Qo7fau0mlsy0MCm3lBgyxo2lpAaI4gFRxGE2GhpV
7N+kCoS1A24S
-----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!