photog/auth.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"net/http"
	"os"
	"strings"

	"golang.org/x/crypto/bcrypt"
	"golang.org/x/time/rate"
)

// Authenticator checks HTTP Basic credentials against bcrypt hashes loaded from
// an Apache-style htpasswd file. Only bcrypt entries are supported — generate
// them with `htpasswd -B`. This replaces nginx's basic_auth so the app can run
// behind a plain TLS-terminating reverse proxy with no auth of its own.
type Authenticator struct {
	creds map[string][]byte // username -> bcrypt hash

	// A single bcrypt comparison runs on every failed lookup so that a request
	// for an unknown user costs the same as one for a known user with a wrong
	// password, closing a user-enumeration timing side channel.
	dummyHash []byte
}

// LoadHtpasswd reads user:hash lines from path. Blank lines and #-comments are
// skipped, as is any entry whose hash is not bcrypt (with a warning). It is an
// error for the file to yield no usable entries.
func LoadHtpasswd(path string) (*Authenticator, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	a := &Authenticator{creds: make(map[string][]byte)}

	sc := bufio.NewScanner(f)
	for sc.Scan() {
		line := strings.TrimSpace(sc.Text())
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		user, hash, ok := strings.Cut(line, ":")
		if !ok || user == "" {
			continue
		}
		// bcrypt hashes start $2a$, $2b$ or $2y$; everything else (MD5 $apr1$,
		// SHA {SHA}, crypt) is unsupported here.
		if !strings.HasPrefix(hash, "$2") {
			log.Printf("auth: skipping non-bcrypt entry for %q — regenerate with `htpasswd -B`", user)
			continue
		}
		a.creds[user] = []byte(hash)
	}
	if err := sc.Err(); err != nil {
		return nil, err
	}
	if len(a.creds) == 0 {
		return nil, fmt.Errorf("no bcrypt entries in %s (use `htpasswd -B`)", path)
	}

	// Cost 10 matches htpasswd -B's default, keeping the dummy comparison the
	// same price as a real one.
	a.dummyHash, _ = bcrypt.GenerateFromPassword([]byte("dummy"), 10)
	return a, nil
}

// check reports whether user/pass matches a stored credential.
func (a *Authenticator) check(user, pass string) bool {
	hash, ok := a.creds[user]
	if !ok {
		// Spend the same time as a real comparison to avoid leaking which
		// usernames exist.
		bcrypt.CompareHashAndPassword(a.dummyHash, []byte(pass))
		return false
	}
	return bcrypt.CompareHashAndPassword(hash, []byte(pass)) == nil
}

// basicAuth wraps next, demanding valid HTTP Basic credentials. On a failed
// attempt it consumes a token from limiter; once the limiter is exhausted,
// further failures get 429 instead of 401, throttling brute-force guessing.
// The limiter is only touched on failure, so legitimate (even bursty, bulk)
// uploads are never throttled.
func (a *Authenticator) basicAuth(limiter *rate.Limiter, next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		user, pass, ok := r.BasicAuth()
		if ok && a.check(user, pass) {
			next(w, r)
			return
		}
		if !limiter.Allow() {
			http.Error(w, "too many failed attempts", http.StatusTooManyRequests)
			return
		}
		w.Header().Set("WWW-Authenticate", `Basic realm="photog"`)
		http.Error(w, "unauthorized", http.StatusUnauthorized)
	}
}