Implement git-lfs-authenticate

1 files changed, 123 insertions, 32 deletions

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)
 }