From f6c92c5e2d87ab1334648b0d1293771de7aae4a5 Mon Sep 17 00:00:00 2001
From: Rutger Broekhoff
Date: Sat, 30 Dec 2023 14:00:34 +0100
Subject: Implement git-lfs-authenticate

---
 cmd/git-lfs-authenticate/main.go | 155 +++++++++++++++++++++++++++++++--------
 1 file changed, 123 insertions(+), 32 deletions(-)

(limited to 'cmd/git-lfs-authenticate')

diff --git a/cmd/git-lfs-authenticate/main.go b/cmd/git-lfs-authenticate/main.go
index 41e2dbc..56b9d78 100644
--- a/cmd/git-lfs-authenticate/main.go
+++ b/cmd/git-lfs-authenticate/main.go
@@ -1,8 +1,18 @@
 package main
 
 import (
+	"bytes"
+	"crypto/ed25519"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
+	"os/exec"
+	"strings"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
 )
 
 func die(msg string, args ...any) {
@@ -12,37 +22,118 @@ func die(msg string, args ...any) {
 	os.Exit(1)
 }
 
+func getGitoliteAccess(path, user, gitolitePerm string) bool {
+	// gitolite access -q: returns only exit code
+	cmd := exec.Command("gitolite", "access", "-q", path, user, gitolitePerm)
+	err := cmd.Run()
+	permGranted := err == nil
+	var exitErr *exec.ExitError
+	if err != nil && !errors.As(err, &exitErr) {
+		die("failed to query access information")
+	}
+	return permGranted
+}
+
+type gitolfs3Claims struct {
+	Repository string `json:"repository"`
+	Permission string `json:"permission"`
+}
+
+type customClaims struct {
+	Gitolfs3 gitolfs3Claims `json:"gitolfs3"`
+	*jwt.RegisteredClaims
+}
+
+type authenticateResponse struct {
+	Header map[string]string `json:"header"`
+	// In seconds.
+	ExpiresIn int64 `json:"expires_in,omitempty"`
+	// expires_at (RFC3339) could also be used, but we leave it out since we
+	// don't use it.
+}
+
+func wipe(b []byte) {
+	for i := range b {
+		b[i] = 0
+	}
+}
+
 func main() {
-	//	if len(os.Args) != 3 {
-	//		die("expected 2 arguments [path, operation], got %d", len(os.Args)-1)
-	//	}
-	//
-	// path := strings.TrimPrefix(strings.TrimSuffix(os.Args[1], ".git"), "/")
-	// operation := os.Args[2]
-	//
-	//	if operation != "download" && operation != "upload" {
-	//		die("expected operation to be in {upload, download}, got %s", operation)
-	//	}
-	//
-	// user := os.Getenv("GL_USER")
-	//
-	//	if user == "" {
-	//		die("expected Gitolite user env (GL_USER) to be set")
-	//	}
-	//
-	// gitolitePerm := "R"
-	//
-	//	if operation == "upload" {
-	//		gitolitePerm = "W"
-	//	}
-	//
-	// // gitolite access -q: returns only exit code
-	// cmd := exec.Command("gitolite", "access", "-q", path, user, gitolitePerm)
-	// err := cmd.Run()
-	// permGranted := err == nil
-	// var exitErr *exec.ExitError
-	//
-	//	if err != nil && !errors.As(err, &exitErr) {
-	//		die("failed to query Gitolite access information")
-	//	}
+	// Even though not explicitly described in the Git LFS documentation, the
+	// git-lfs-authenticate command is expected to either exit succesfully with
+	// exit code 0 and to then print credentials in the prescribed JSON format
+	// to standard out. On errors, the command should exit with a non-zero exit
+	// code and print the error message in plain text to standard error. See
+	// https://github.com/git-lfs/git-lfs/blob/baf40ac99850a62fe98515175d52df5c513463ec/lfshttp/ssh.go#L76-L117
+
+	if len(os.Args) != 3 {
+		die("expected 2 arguments (path, operation), got %d", len(os.Args)-1)
+	}
+
+	path := strings.TrimPrefix(strings.TrimSuffix(os.Args[1], ".git"), "/")
+	operation := os.Args[2]
+	if operation != "download" && operation != "upload" {
+		die("expected operation to be upload or download, got %s", operation)
+	}
+
+	user := os.Getenv("GL_USER")
+	if user == "" {
+		die("internal error")
+	}
+	keyPath := os.Getenv("GITOLFS3_KEY_PATH")
+	if keyPath == "" {
+		die("internal error")
+	}
+	keyStr, err := os.ReadFile(keyPath)
+	if err != nil {
+		die("internal error")
+	}
+	keyStr = bytes.TrimSpace(keyStr)
+	defer wipe(keyStr)
+
+	if hex.DecodedLen(len(keyStr)) != ed25519.SeedSize {
+		die("internal error")
+	}
+
+	seed := make([]byte, ed25519.SeedSize)
+	defer wipe(seed)
+	if _, err = hex.Decode(seed, keyStr); err != nil {
+		die("internal error")
+	}
+	privateKey := ed25519.NewKeyFromSeed(seed)
+
+	if !getGitoliteAccess(path, user, "R") {
+		die("repository not found")
+	}
+	if operation == "upload" && !getGitoliteAccess(path, user, "W") {
+		// User has read access but not write access
+		die("forbidden")
+	}
+
+	expiresIn := time.Hour * 24
+	claims := customClaims{
+		Gitolfs3: gitolfs3Claims{
+			Repository: path,
+			Permission: operation,
+		},
+		RegisteredClaims: &jwt.RegisteredClaims{
+			Subject:   user,
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(expiresIn)),
+		},
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodEdDSA, claims)
+	ss, err := token.SignedString(privateKey)
+	if err != nil {
+		die("failed to generate token")
+	}
+
+	response := authenticateResponse{
+		Header: map[string]string{
+			"Authorization": "Bearer " + ss,
+		},
+		ExpiresIn: int64(expiresIn.Seconds()),
+	}
+	json.NewEncoder(os.Stdout).Encode(response)
 }
-- 
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