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package main
import (
"bytes"
"crypto/hmac"
"crypto/sha256"
"encoding/binary"
"encoding/hex"
"encoding/json"
"errors"
"fmt"
"io"
"io/fs"
"os"
"path"
"strings"
"time"
)
func die(msg string, args ...any) {
fmt.Fprint(os.Stderr, "Fatal: ")
fmt.Fprintf(os.Stderr, msg, args...)
fmt.Fprint(os.Stderr, "\n")
os.Exit(1)
}
type authenticateResponse struct {
// When providing href, the Git LFS client will use href as the base URL
// instead of building the base URL using the Service Discovery mechanism.
// It should end with /info/lfs. See
// https://github.com/git-lfs/git-lfs/blob/baf40ac99850a62fe98515175d52df5c513463ec/docs/api/server-discovery.md#ssh
HRef string `json:"href,omitempty"`
Header map[string]string `json:"header"`
// In seconds.
ExpiresIn int64 `json:"expires_in,omitempty"`
// The expires_at (RFC3339) property could also be used, but we leave it
// out since we don't use it. The Git LFS docs recommend using expires_in
// instead (???)
}
func wipe(b []byte) {
for i := range b {
b[i] = 0
}
}
const usage = "Usage: git-lfs-authenticate <REPO> upload/download"
func main() {
// 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 {
fmt.Println(usage)
os.Exit(1)
}
repo := strings.TrimPrefix(path.Clean(strings.TrimSuffix(os.Args[1], ".git")), "/")
operation := os.Args[2]
if operation != "download" && operation != "upload" {
fmt.Println(usage)
os.Exit(1)
}
if repo == ".." || strings.HasPrefix(repo, "../") {
die("highly illegal repo name (Anzeige ist raus)")
}
repoDir := path.Join(repo + ".git")
finfo, err := os.Stat(repoDir)
if err != nil {
if errors.Is(err, fs.ErrNotExist) {
die("repo not found")
}
die("could not stat repo: %s", err)
}
if !finfo.IsDir() {
die("repo not found")
}
hrefBase := os.Getenv("GITOLFS3_HREF_BASE")
if hrefBase == "" {
die("incomplete configuration: base URL not provided")
}
if !strings.HasSuffix(hrefBase, "/") {
hrefBase += "/"
}
keyPath := os.Getenv("GITOLFS3_KEY_PATH")
if keyPath == "" {
die("incomplete configuration: key path not provided")
}
keyStr, err := os.ReadFile(keyPath)
if err != nil {
wipe(keyStr)
die("cannot read key")
}
keyStr = bytes.TrimSpace(keyStr)
defer wipe(keyStr)
if hex.DecodedLen(len(keyStr)) != 64 {
die("bad key length")
}
key := make([]byte, 64)
defer wipe(key)
if _, err = hex.Decode(key, keyStr); err != nil {
die("cannot decode key")
}
expiresIn := time.Minute * 5
expiresAtUnix := time.Now().Add(expiresIn).Unix()
tag := hmac.New(sha256.New, key)
io.WriteString(tag, "git-lfs-authenticate")
tag.Write([]byte{0})
io.WriteString(tag, repo)
tag.Write([]byte{0})
io.WriteString(tag, operation)
tag.Write([]byte{0})
binary.Write(tag, binary.BigEndian, &expiresAtUnix)
tagStr := hex.EncodeToString(tag.Sum(nil))
response := authenticateResponse{
Header: map[string]string{
"Authorization": "Tag " + tagStr,
},
ExpiresIn: int64(expiresIn.Seconds()),
HRef: fmt.Sprintf("%s%s?p=1&te=%d",
hrefBase,
path.Join(repo+".git", "/info/lfs"),
expiresAtUnix,
),
}
json.NewEncoder(os.Stdout).Encode(response)
}
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