Add offline verification (#220)

* Add offline verification This adds the ability for gitsign commits to include rekor log entry details to enable offline verification of commits. - Adds new unauthenticated attributes corresponding to Rekor log entry values. - Adds support for offline commit verification along side existing SHA based online verification. - Adds config option for users to select offline or online storage options. Defaults to existing online behavior to allow users to opt-in, this will be changed to offline after a release. - Adds e2e test for offline verification. Signed-off-by: Billy Lynch <billy@chainguard.dev> * Refactor Sign func into 2 separate functions, rework verification doc. Signed-off-by: Billy Lynch <billy@chainguard.dev> --------- Signed-off-by: Billy Lynch <billy@chainguard.dev>
sigstore · May 22, 2023 · 5dd6092 · 5dd6092
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diff --git a/.github/workflows/e2e.yaml b/.github/workflows/e2e.yaml
@@ -84,6 +84,28 @@ jobs:
           echo "========== git verify-commit =========="
           git verify-commit HEAD
 
+          echo "========== gitsign verify =========="
+          gitsign verify \
+            --certificate-github-workflow-repository=${{ github.repository }} \
+            --certificate-github-workflow-sha=${{ github.sha }} \
+            --certificate-oidc-issuer="https://token.actions.githubusercontent.com" \
+            --certificate-identity="https://github.com/${{ github.workflow_ref }}"
+
+          # Extra debug info
+          git cat-file commit HEAD | sed -n '/BEGIN/, /END/p' | sed 's/^ //g' | sed 's/gpgsig //g' | sed 's/SIGNED MESSAGE/PKCS7/g' | openssl pkcs7 -print -print_certs -text
+      - name: Test Sign and Verify commit - offline verification
+        env:
+          GITSIGN_REKOR_MODE: "offline"
+        run: |
+          set -e
+
+          # Sign commit
+          git commit --allow-empty -S --message="Signed commit"
+
+          # Verify commit
+          echo "========== git verify-commit =========="
+          git verify-commit HEAD
+
           echo "========== gitsign verify =========="
           gitsign verify \
             --certificate-github-workflow-repository=${{ github.repository }} \

diff --git a/README.md b/README.md
diff --git a/docs/verification.md b/docs/verification.md
@@ -0,0 +1,264 @@
+# Verification
+
+## Offline Verification
+
+### How we sign
+
+In offline Rekor storage mode Gitsign will store a HashedRekord in Rekor
+corresponding to the commit content.
+
+Unfortunately this is a bit complex to query manually. Roughly this is:
+
+```
+sha256(der(sort(system time | commit data | content type)))
+```
+
+The resulting Rekor log entry fields and inclusion proof will be stored in the
+PKCS7 object as unauthenticated (i.e. not included in the cryptographic
+signature) attributes.
+
+### How we verify
+
+1. Recompute and compare commit content checksum from commit.
+2. Get Rekor LogEntry from signature.
+3. Verify Certificate against commit content checksum (ignoring cert NotAfter time).
+4. (if present) Verify signature against TSA cert.
+5. Verify Rekor LogEntry inclusion (offline).
+
+### What's stored in the commit signature
+
+- Commit content checksum (sha256)
+- Commit signing time (untrusted system time)
+- Protobuf encoded [Rekor TransparencyLogEntry](https://github.com/sigstore/protobuf-specs/blob/91485b44360d343dadd98fb7297a500f05e0b5b1/protos/sigstore_rekor.proto#L91)
+- (optional) TSA signature + cert
+
+Sample encoded TransparencyLogEntry:
+
+```
+unauth_attr:
+    object: Rekor TransparencyLogEntry proto (1.3.6.1.4.1.57264.3.1)
+    value.set:
+      OCTET STRING:
+        0000 - 08 af d5 d6 08 12 22 0a-20 c0 d2 3d 6a   ......". ..=j
+        000d - d4 06 97 3f 95 59 f3 ba-2d 1c a0 1f 84   ...?.Y..-....
+        001a - 14 7d 8f fc 5b 84 45 c2-24 f9 8b 95 91   .}..[.E.$....
+        0027 - 80 1d 1a 15 0a 0c 68 61-73 68 65 64 72   ......hashedr
+        0034 - 65 6b 6f 72 64 12 05 30-2e 30 2e 31 20   ekord..0.0.1 
+        0041 - a1 fc f5 a1 06 2a 49 0a-47 30 45 02 21   .....*I.G0E.!
+        004e - 00 fd ab 1a 0d 0b 39 fe-d5 0f f2 4d 87   ......9....M.
+        005b - 40 06 bd 2d 84 e8 ca d8-a2 39 99 e5 d9   @..-.....9...
+        0068 - 8a 3e b2 48 04 44 67 02-20 15 a5 02 7a   .>.H.Dg. ...z
+        0075 - 61 0b d1 58 46 81 b1 ff-53 e8 46 be b3   a..XF...S.F..
+        0082 - 70 9b f1 55 07 0c e8 32-bb 61 4e aa ce   p..U...2.aN..
+        008f - 61 16 32 81 05 08 c8 c6-d8 06 12 20 3f   a.2........ ?
+        009c - 5f bc 03 da 94 4e 17 05-44 a8 c2 1b e9   _....N..D....
+        00a9 - a7 6c 84 7d 39 66 4b 07-2f c2 7b 49 3d   .l.}9fK./.{I=
+        00b6 - 2b da 9a 84 30 18 c9 c6-d8 06 22 20 34   +...0....." 4
+        00c3 - 8d 79 2a f5 5b 0d e8 8f-6e 6b 3f 39 8e   .y*.[...nk?9.
+        00d0 - 43 02 2a d3 b3 c3 6b d5-d1 c6 84 cd 7f   C.*...k......
+        00dd - 08 24 2f a6 6e 22 20 64-47 c9 39 2b 77   .$/.n" dG.9+w
+        00ea - ba 3b b5 36 7f bd ea 8f-36 ef 32 33 14   .;.6....6.23.
+        00f7 - 2a e2 ec 2d 57 51 a6 4b-8f 00 59 d2 5e   *..-WQ.K..Y.^
+        0104 - 22 20 c0 d8 57 e5 d0 82-b2 b8 cf 26 b0   " ..W......&.
+        0111 - 58 e3 85 e5 71 ba 34 ab-5c 1b 49 5a 5e   X...q.4.\.IZ^
+        011e - c4 20 7b 7a 47 d6 02 0b-22 20 21 52 30   . {zG..." !R0
+        012b - e1 48 37 62 5c 39 56 bc-78 a6 84 d5 c3   .H7b\9V.x....
+        0138 - df 3d ea e4 75 80 07 a3-25 b9 c9 42 e6   .=..u...%..B.
+        0145 - 34 8e 49 22 20 4a 88 54-e3 e8 ed dd f0   4.I" J.T.....
+        0152 - 4b f4 e2 95 55 da a8 44-be 87 85 e6 d9   K...U..D.....
+        015f - 57 52 8f 97 b3 3a d3 d7-96 32 f9 22 20   WR...:...2." 
+        016c - 35 b2 b6 5b 9f 02 a8 bc-7d d2 f8 64 30   5..[....}..d0
+        0179 - d5 04 b1 c4 bb 2e 0c c8-bd 00 18 52 bb   ...........R.
+        0186 - 40 ad 84 6c 2d 68 22 20-4c 82 cf f1 63   @..l-h" L...c
+        0193 - 90 df b5 b4 3a 8b 0f bf-04 43 3e 52 0e   ....:....C>R.
+        01a0 - ef f6 d0 0e d3 c0 01 31-b1 8f 1b 68 82   .......1...h.
+        01ad - 74 22 20 ec 4c 65 15 56-3a 67 6a 41 1e   t" .Le.V:gjA.
+        01ba - 44 ad 06 b2 df 2d ff da-2c 03 77 87 ee   D....-..,.w..
+        01c7 - ba 00 c9 5b c3 b5 34 59-55 22 20 d6 30   ...[..4YU" .0
+        01d4 - 92 c2 27 78 05 dc b4 cb-36 1b ea 6e 09   ..'x....6..n.
+        01e1 - ac 7e d9 e9 e9 19 27 24-b8 f5 1e 57 e5   .~....'$...W.
+        01ee - 4b df 35 31 22 20 9e 04-00 66 df e5 f0   K.51" ...f...
+        01fb - 20 04 65 83 86 ac 66 cf-0b b6 ff e8 57    .e...f.....W
+        0208 - ed 71 cb 33 7c 7f 55 45-ec f4 55 8b 2a   .q.3|.UE..U.*
+        0215 - fe 01 0a fb 01 72 65 6b-6f 72 2e 73 69   .....rekor.si
+        0222 - 67 73 74 6f 72 65 2e 64-65 76 20 2d 20   gstore.dev - 
+        022f - 32 36 30 35 37 33 36 36-37 30 39 37 32   2605736670972
+        023c - 37 39 34 37 34 36 0a 31-34 30 33 33 37   794746.140337
+        0249 - 33 37 0a 50 31 2b 38 41-39 71 55 54 68   37.P1+8A9qUTh
+        0256 - 63 46 52 4b 6a 43 47 2b-6d 6e 62 49 52   cFRKjCG+mnbIR
+        0263 - 39 4f 57 5a 4c 42 79 2f-43 65 30 6b 39   9OWZLBy/Ce0k9
+        0270 - 4b 39 71 61 68 44 41 3d-0a 54 69 6d 65   K9qahDA=.Time
+        027d - 73 74 61 6d 70 3a 20 31-36 38 31 37 35   stamp: 168175
+        028a - 31 35 38 35 32 37 34 35-37 36 37 30 31   1585274576701
+        0297 - 0a 0a e2 80 94 20 72 65-6b 6f 72 2e 73   ..... rekor.s
+        02a4 - 69 67 73 74 6f 72 65 2e-64 65 76 20 77   igstore.dev w
+        02b1 - 4e 49 39 61 6a 42 45 41-69 42 31 56 4a   NI9ajBEAiB1VJ
+        02be - 48 46 6e 34 47 4e 63 32-65 38 65 42 78   HFn4GNc2e8eBx
+        02cb - 48 6f 4b 41 6c 56 6f 77-44 77 4a 51 72   HoKAlVowDwJQr
+        02d8 - 34 32 53 50 56 37 64 2f-6e 72 73 47 34   42SPV7d/nrsG4
+        02e5 - 77 49 67 4c 49 73 36 77-2b 59 75 39 42   wIgLIs6w+Yu9B
+        02f2 - 2f 35 2b 73 6b 6e 72 51-65 36 58 33 72   /5+sknrQe6X3r
+        02ff - 68 6e 6b 41 65 6a 6d 76-55 6d 4d 5a 5a   hnkAejmvUmMZZ
+        030c - 69 51 75 4d 53 49 59 3d-0a               iQuMSIY=.
+```
+
+This OID are defined by [Rekor](https://github.com/sigstore/rekor) and are
+used during verification to reconstruct the Rekor log entry and verify the
+commit signature.
+
+### What's stored in Rekor
+
+HashedRekord containing:
+
+- Commit content checksum
+- Fulcio certificate
+  - Public Key
+  - [Signer Identity info](https://github.com/sigstore/fulcio/blob/main/docs/oidc.md)
+
+## Online Verification
+
+Note: Gitsign is in the process of migrating clients to offline verification, but this section
+explains how verification used to work.
+
+### How we sign
+
+In online Rekor storage mode Gitsign will store the Git commit SHA in Rekor
+rather that persisting the Rekor log details in the commit itself. This works by:
+
+1. Get Fulcio Cert
+2. Sign the commit body using cert
+3. Generate commit SHA (commit doesn't actually exist yet because the commit includes the signature)
+4. Sign the commit SHA using the same cert
+5. Upload HashedRekord of commit SHA to Rekor
+6. Store the signed commit body signature in commit
+
+### How we verify
+
+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.
+
+This is done by:
+
+1. Recompute and compare commit content checksum from commit.
+2. Validate the checksum signature using the public key in the signature's cert (ignoring cert NotAfter time).
+3. (if present) Verify signature against TSA cert.
+4. Search Rekor for an entry matching the commit SHA + cert. (this is what makes the process online)
+5. Verify Rekor LogEntry inclusion (offline).
+
+We can manually validate that the commit exists in the transparency log by
+running:
+
+```sh
+$ 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 includes 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.
+
+### What's stored in the commit signature
+
+- Commit content checksum (sha256)
+- Commit signing time (untrusted system time)
+- (optional) TSA signature + cert
+
+### What's stored in Rekor
+
+HashedRekord containing:
+
+- Commit SHA checksum
+- Fulcio certificate
+  - Public Key
+  - [Signer Identity info](https://github.com/sigstore/fulcio/blob/main/docs/oidc.md)
diff --git a/go.mod b/go.mod
@@ -19,13 +19,15 @@ require (
 	github.com/secure-systems-lab/go-securesystemslib v0.6.0
 	github.com/sigstore/cosign/v2 v2.0.2
 	github.com/sigstore/fulcio v1.3.1
+	github.com/sigstore/protobuf-specs v0.1.0
 	github.com/sigstore/rekor v1.1.1
 	github.com/sigstore/sigstore v1.6.4
 	github.com/spf13/cobra v1.7.0
 	github.com/spf13/pflag v1.0.5
 	golang.org/x/crypto v0.9.0
 	golang.org/x/oauth2 v0.8.0
 	golang.org/x/xerrors v0.0.0-20220907171357-04be3eba64a2
+	google.golang.org/protobuf v1.30.0
 )
 
 require (
@@ -205,7 +207,6 @@ require (
 	google.golang.org/appengine v1.6.7 // indirect
 	google.golang.org/genproto v0.0.0-20230410155749-daa745c078e1 // indirect
 	google.golang.org/grpc v1.54.0 // indirect
-	google.golang.org/protobuf v1.30.0 // indirect
 	gopkg.in/go-jose/go-jose.v2 v2.6.0 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect

diff --git a/go.sum b/go.sum
@@ -676,6 +676,8 @@ github.com/sigstore/cosign/v2 v2.0.2 h1:Ttaj/OkJAy+ummhnHG2F+JSFeZQj8i0P6o8j2RY9
 github.com/sigstore/cosign/v2 v2.0.2/go.mod h1:yJXtRmWrumyQA/XPjTTjOufnNckI87mmmVxv9rtEqgE=
 github.com/sigstore/fulcio v1.3.1 h1:0ntW9VbQbt2JytoSs8BOGB84A65eeyvGSavWteYp29Y=
 github.com/sigstore/fulcio v1.3.1/go.mod h1:/XfqazOec45ulJZpyL9sq+OsVQ8g2UOVoNVi7abFgqU=
+github.com/sigstore/protobuf-specs v0.1.0 h1:X0l/E2C2c79t/rI/lmSu8WAoKWsQtMqDzAMiDdEMGr8=
+github.com/sigstore/protobuf-specs v0.1.0/go.mod h1:5shUCxf82hGnjUEFVWiktcxwzdtn6EfeeJssxZ5Q5HE=
 github.com/sigstore/rekor v1.1.1 h1:JCeSss+qUHnCATmwAZh4zT9k0Frdyq0BjmRwewSfEy4=
 github.com/sigstore/rekor v1.1.1/go.mod h1:x/xK+HK08MiuJv+v4OxY/Oo3bhuz1DtJXNJrV7hrzvs=
 github.com/sigstore/sigstore v1.6.4 h1:jH4AzR7qlEH/EWzm+opSpxCfuUcjHL+LJPuQE7h40WE=

diff --git a/internal/commands/root/sign.go b/internal/commands/root/sign.go
@@ -88,18 +88,23 @@ func commandSign(o *options, s *gsio.Streams, args ...string) error {
 		opts.UserName = o.Config.CommitterName
 		opts.UserEmail = o.Config.CommitterEmail
 	}
-	sig, cert, tlog, err := git.Sign(ctx, rekor, userIdent, dataBuf.Bytes(), opts)
+
+	var fn git.SignFunc = git.LegacySHASign
+	if o.Config.RekorMode == "offline" {
+		fn = git.Sign
+	}
+	resp, err := fn(ctx, rekor, userIdent, dataBuf.Bytes(), opts)
 	if err != nil {
 		return fmt.Errorf("failed to sign message: %w", err)
 	}
 
-	if tlog != nil && tlog.LogIndex != nil {
+	if tlog := resp.LogEntry; tlog != nil && tlog.LogIndex != nil {
 		fmt.Fprintf(s.TTYOut, "tlog entry created with index: %d\n", *tlog.LogIndex)
 	}
 
-	gpgout.EmitSigCreated(cert, o.FlagDetachedSignature)
+	gpgout.EmitSigCreated(resp.Cert, o.FlagDetachedSignature)
 
-	if _, err := s.Out.Write(sig); err != nil {
+	if _, err := s.Out.Write(resp.Signature); err != nil {
 		return errors.New("failed to write signature")
 	}