Add support for checking cert email against user config before signing.

This change adds a new config option: gitsign.matchCommitter. This
option checks whether the certificate fetched matches the user
configured email/name.

For human users, this generally means that the SAN email in the cert
matches the `user.email` Git config option.

For non-email based identities (e.g.  machine users), the SAN URI can be
specified as the user name (since the URI isn't a valid email).

Gitsign requires at least one condition to match for the check to
succeed.

This change does *not* enforce any constraints on verification, since
this requires additional checking to know what IdP is considered valid.

docs/committer-verification.md

@@ -0,0 +1,34 @@
+# Committer Verification
+
+Gitsign can be optionally configured to verify that the committer user identity
+matches the git user configuration (i.e. `user.name` and `user.email`)
+
+To enable committer verification, run `git config gitsign.matchCommitter true`.
+
+Committer verification is done by matching the certificate Subject Alternative
+Name (SAN) against the issued Fulcio certificate in the following order:
+
+1. An `EmailAddresses` cert value matches the committer `user.email`. This
+   should be used for most human committer verification.
+2. A `URI` cert value matches the committer `user.name`. This should be used for
+   most automated user committer verification.
+
+In the event that multiple SAN values are provided, verification will succeed if
+at least one value matches.
+
+## Configuring Automated Users
+
+If running in an environment where the authenticated user does **not** have a
+user email to bind against (i.e. GitHub Actions, other workload identity
+workflows), users are expected to be identified by the SAN URI instead.
+
+See https://github.com/sigstore/fulcio/blob/main/docs/oidc.md for more details
+
+### GitHub Actions
+
+```sh
+# This configures the SAN URI for the expected identity in the Fulcio cert.
+$ git config user.name "https://myorg/myrepo/path/to/workflow"
+# This configures GitHub UI to recognize the commit as coming from a GitHub Action.
+$ git config user.email 1234567890+github-actions@users.noreply.github.com
+```

internal/commands/root/sign.go

@@ -78,12 +78,17 @@ func commandSign(o *options, s *gsio.Streams, args ...string) error {
 		return fmt.Errorf("failed to create rekor client: %w", err)
 	}
 
-	sig, cert, tlog, err := git.Sign(ctx, rekor, userIdent, dataBuf.Bytes(), signature.SignOptions{
+	opts := signature.SignOptions{
 		Detached:           o.FlagDetachedSignature,
 		TimestampAuthority: o.Config.TimestampURL,
 		Armor:              o.FlagArmor,
 		IncludeCerts:       o.FlagIncludeCerts,
-	})
+	}
+	if o.Config.MatchCommitter {
+		opts.UserName = o.Config.CommitterName
+		opts.UserEmail = o.Config.CommitterEmail
+	}
+	sig, cert, tlog, err := git.Sign(ctx, rekor, userIdent, dataBuf.Bytes(), opts)
 	if err != nil {
 		return fmt.Errorf("failed to sign message: %w", err)
 	}

internal/config/config.go

@@ -58,6 +58,11 @@ type Config struct {
 
 	// Path to log status output. Helpful for debugging when no TTY is available in the environment.
 	LogPath string
+
+	// Committer details
+	CommitterName  string
+	CommitterEmail string
+	MatchCommitter bool
 }
 
 // Get fetches the gitsign config options for the repo in the current working
@@ -112,7 +117,7 @@ func Get() (*Config, error) {
 // doesn't support deprecated subsection syntaxes
 // (https://github.com/sigstore/gitsign/issues/142).
 func realExec() (io.Reader, error) {
-	cmd := exec.Command("git", "config", "--get-regexp", `^gitsign\.`)
+	cmd := exec.Command("git", "config", "--get-regexp", `.*`)
 	stdout := new(bytes.Buffer)
 	stderr := new(bytes.Buffer)
 	cmd.Stdout = stdout
@@ -147,6 +152,10 @@ func parseConfig(r io.Reader) map[string]string {
 func applyGitOptions(out *Config, cfg map[string]string) {
 	for k, v := range cfg {
 		switch {
+		case strings.EqualFold(k, "user.name"):
+			out.CommitterName = v
+		case strings.EqualFold(k, "user.email"):
+			out.CommitterEmail = v
 		case strings.EqualFold(k, "gitsign.fulcio"):
 			out.Fulcio = v
 		case strings.EqualFold(k, "gitsign.fulcioRoot"):
@@ -167,6 +176,8 @@ func applyGitOptions(out *Config, cfg map[string]string) {
 			out.TimestampURL = v
 		case strings.EqualFold(k, "gitsign.timestampCertChain"):
 			out.TimestampCert = v
+		case strings.EqualFold(k, "gitsign.matchCommitter"):
+			out.MatchCommitter = strings.EqualFold(v, "true")
 		}
 	}
 }

internal/signature/sign.go

@@ -21,6 +21,7 @@ import (
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
+	"strings"
 
 	cms "github.com/github/smimesign/ietf-cms"
 )
@@ -40,6 +41,14 @@ type SignOptions struct {
 	// 1 includes leaf cert.
 	// >1 includes n from the leaf.
 	IncludeCerts int
+
+	// UserName specifies the email to match against. If present, signing
+	// will fail if the Fulcio identity SAN URI does not match the git committer name.
+	UserName string
+
+	// UserEmail specifies the email to match against. If present, signing
+	// will fail if the Fulcio identity SAN email does not match the git committer email.
+	UserEmail string
 }
 
 // Identity is a copy of smimesign.Identity to allow for compatibility without
@@ -63,8 +72,23 @@ type Identity interface {
 func Sign(ident Identity, body []byte, opts SignOptions) ([]byte, *x509.Certificate, error) {
 	cert, err := ident.Certificate()
 	if err != nil {
-		return nil, nil, fmt.Errorf("failed to get idenity certificate: %w", err)
+		return nil, nil, fmt.Errorf("failed to get identity certificate: %w", err)
 	}
+
+	// If specified, check if retrieved identity matches the expected identity.
+	if opts.UserName != "" || opts.UserEmail != "" {
+		if !matchSAN(cert, opts.UserName, opts.UserEmail) {
+			san := []string{}
+			if len(cert.EmailAddresses) > 0 {
+				san = append(san, fmt.Sprintf("email: %v", cert.EmailAddresses))
+			}
+			if len(cert.URIs) > 0 {
+				san = append(san, fmt.Sprintf("uri: %v", cert.URIs))
+			}
+			return nil, nil, fmt.Errorf("gitsign.matchCommitter: certificate identity does not match config - want %s <%s>, got %s", opts.UserName, opts.UserEmail, strings.Join(san, ","))
+		}
+	}
+
 	signer, err := ident.Signer()
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to get idenity signer: %w", err)
@@ -164,3 +188,24 @@ func chainWithoutRoot(chain []*x509.Certificate) []*x509.Certificate {
 
 	return chain
 }
+
+// matchSAN checks whether a given cert SAN matches the given user name/email.
+// At least 1 of the following needs to match to be considered successful:
+//
+// 1. SAN email == user email (typical for most human users)
+// 2. SAN URI == user name (for non-email based identities like CI)
+func matchSAN(cert *x509.Certificate, name, email string) bool {
+	for _, e := range cert.EmailAddresses {
+		if e == email {
+			return true
+		}
+	}
+
+	for _, u := range cert.URIs {
+		if u.String() == name {
+			return true
+		}
+	}
+
+	return false
+}

internal/signature/sign_test.go

@@ -0,0 +1,72 @@
+// Copyright 2023 The Sigstore Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package signature
+
+import (
+	"crypto/x509"
+	"net/url"
+	"testing"
+)
+
+func TestMatchSAN(t *testing.T) {
+	for _, tc := range []struct {
+		testname string
+		cert     *x509.Certificate
+		name     string
+		email    string
+		want     bool
+	}{
+		{
+			testname: "email match",
+			cert: &x509.Certificate{
+				EmailAddresses: []string{"foo@example.com"},
+			},
+			name:  "Foo Bar",
+			email: "foo@example.com",
+			want:  true,
+		},
+		{
+			testname: "uri match",
+			cert: &x509.Certificate{
+				URIs: []*url.URL{parseURL("https://github.com/foo/bar")},
+			},
+			name:  "https://github.com/foo/bar",
+			email: "foo@example.com",
+			want:  true,
+		},
+		{
+			testname: "no match",
+			cert:     &x509.Certificate{},
+			name:     "https://github.com/foo/bar",
+			email:    "foo@example.com",
+			want:     false,
+		},
+	} {
+		t.Run(tc.testname, func(t *testing.T) {
+			got := matchSAN(tc.cert, tc.name, tc.email)
+			if got != tc.want {
+				t.Fatalf("got %t, want %t", got, tc.want)
+			}
+		})
+	}
+}
+
+func parseURL(raw string) *url.URL {
+	u, err := url.Parse(raw)
+	if err != nil {
+		panic(err)
+	}
+	return u
+}