chore: Add additional git attestor output.

When the git binary is used to for status we also include
the path to the binary and the hash of the binary files.

Also by default the git tool used to generate the attestation
data is added as contextual information.

Signed-off-by: Matthias Glastra <[email protected]>

attestation/git/git.go

@@ -79,6 +79,9 @@ type Tag struct {
 }
 
 type Attestor struct {
+	GitTool        string               `json:"gittool"`
+	GitBinPath     string               `json:"gitbinpath,omitempty"`
+	GitBinHash     string               `json:"gitbinhash,omitempty"`
 	CommitHash     string               `json:"commithash"`
 	Author         string               `json:"author"`
 	AuthorEmail    string               `json:"authoremail"`
@@ -222,11 +225,25 @@ func (a *Attestor) Attest(ctx *attestation.AttestationContext) error {
 	a.TreeHash = commit.TreeHash.String()
 
 	if GitExists() {
+		a.GitTool = "go-git+git-bin"
+
+		a.GitBinPath, err = GitGetBinPath()
+		if err != nil {
+			return err
+		}
+
+		a.GitBinHash, err = GitGetBinHash(ctx)
+		if err != nil {
+			return err
+		}
+
 		a.Status, err = GitGetStatus(ctx.WorkingDir())
 		if err != nil {
 			return err
 		}
 	} else {
+		a.GitTool = "go-git"
+
 		a.Status, err = GoGitGetStatus(repo)
 		if err != nil {
 			return err

attestation/git/git_bin.go

@@ -15,10 +15,13 @@
 package git
 
 import (
+	"fmt"
 	"os/exec"
 	"strings"
 
 	"github.com/go-git/go-git/v5"
+	"github.com/in-toto/go-witness/attestation"
+	"github.com/in-toto/go-witness/cryptoutil"
 )
 
 // GitExists checks if the git binary is available.
@@ -33,6 +36,40 @@ func GitExists() bool {
 	}
 }
 
+// GitGetBinPath retrieves the path to the git binary that is used by the attestor.
+func GitGetBinPath() (string, error) {
+	path, err := exec.LookPath("git")
+	if err != nil {
+		return "", err
+	} else {
+		return path, nil
+	}
+}
+
+// GitGetBinHash retrieves a sha256 hash of the git binary that is located on the system.
+// The path is determined based on exec.LookPath().
+func GitGetBinHash(ctx *attestation.AttestationContext) (string, error) {
+	path, err := exec.LookPath("git")
+	if err != nil {
+		return "", err
+	}
+
+	gitBinDigest, err := cryptoutil.CalculateDigestSetFromFile(path, ctx.Hashes())
+	fmt.Printf("%s", gitBinDigest)
+	if err != nil {
+		return "", err
+	}
+
+	digestMap, err := gitBinDigest.ToNameMap()
+	if err != nil {
+		return "", err
+	}
+
+	return fmt.Sprintf("sha256:%s", digestMap["sha256"]), nil
+}
+
+// GitGetStatus retrieves the status of staging and worktree
+// from the git status --porcelain output
 func GitGetStatus(workDir string) (map[string]Status, error) {
 
 	// Execute the git status --porcelain command