Cache successful basic auth credentials for a minute

This stores a hash of the username + password in map which is indexed by
the username. Indexing by username avoids accidentally introducing a
timing side-channel as a successful/failed lookup only provides
information on whether a cache entry exists for a username or not.

Hashing the username and password together makes it simple to get a
constant-time string comparison as we no longer have to worry about
string length differences.

Expriy is done by a goroutine which every few seconds checks for expired
cache entries and removes those.

htpasswd.go

@@ -26,6 +26,8 @@ THE SOFTWARE.
 
 import (
 	"crypto/sha1"
+	"crypto/sha256"
+	"crypto/subtle"
 	"encoding/base64"
 	"encoding/csv"
 	"log"
@@ -42,8 +44,18 @@ import (
 // CheckInterval represents how often we check for changes in htpasswd file.
 const CheckInterval = 30 * time.Second
 
+// PasswordCacheDuration represents how long authentication credentials are
+// cached in memory after they were successfully verified. This allows avoiding
+// repeatedly verifying the same authentication credentials.
+const PasswordCacheDuration = time.Minute
+
 // Lookup passwords in a htpasswd file.  The entries must have been created with -s for SHA encryption.
 
+type cacheEntry struct {
+	expiry   time.Time
+	verifier []byte
+}
+
 // HtpasswdFile is a map for usernames to passwords.
 type HtpasswdFile struct {
 	mutex    sync.Mutex
@@ -51,6 +63,7 @@ type HtpasswdFile struct {
 	stat     os.FileInfo
 	throttle chan struct{}
 	users    map[string]string
+	cache    map[string]cacheEntry
 }
 
 // NewHtpasswdFromFile reads the users and passwords from a htpasswd file and returns them.  If an error is encountered,
@@ -68,6 +81,7 @@ func NewHtpasswdFromFile(path string) (*HtpasswdFile, error) {
 		path:     path,
 		stat:     stat,
 		throttle: make(chan struct{}),
+		cache:    make(map[string]cacheEntry),
 	}
 
 	if err := h.Reload(); err != nil {
@@ -76,6 +90,7 @@ func NewHtpasswdFromFile(path string) (*HtpasswdFile, error) {
 
 	// Start a goroutine that limits reload checks to once per CheckInterval
 	go h.throttleTimer()
+	go h.expiryTimer()
 
 	go func() {
 		for range c {
@@ -100,6 +115,20 @@ func (h *HtpasswdFile) throttleTimer() {
 	}
 }
 
+func (h *HtpasswdFile) expiryTimer() {
+	for {
+		time.Sleep(5 * time.Second)
+		now := time.Now()
+		h.mutex.Lock()
+		for user, entry := range h.cache {
+			if entry.expiry.After(now) {
+				delete(h.cache, user)
+			}
+		}
+		h.mutex.Unlock()
+	}
+}
+
 var validUsernameRegexp = regexp.MustCompile(`^[\p{L}\d@._-]+$`)
 
 // Reload reloads the htpasswd file. If the reload fails, the Users map is not changed and the error is returned.
@@ -130,6 +159,7 @@ func (h *HtpasswdFile) Reload() error {
 
 	// Replace the Users map
 	h.mutex.Lock()
+	h.cache = make(map[string]cacheEntry)
 	h.users = users
 	h.mutex.Unlock()
 
@@ -177,30 +207,58 @@ func (h *HtpasswdFile) ReloadCheck() error {
 func (h *HtpasswdFile) Validate(user string, password string) bool {
 	_ = h.ReloadCheck()
 
+	hash := sha256.New()
+	// hash.Write can never fail
+	_, _ = hash.Write([]byte(user))
+	_, _ = hash.Write([]byte(":"))
+	_, _ = hash.Write([]byte(password))
+
 	h.mutex.Lock()
+	// avoid race conditions with cache replacements
+	cache := h.cache
 	realPassword, exists := h.users[user]
+	entry, cacheExists := h.cache[user]
 	h.mutex.Unlock()
 
 	if !exists {
 		return false
 	}
 
+	if cacheExists && subtle.ConstantTimeCompare(entry.verifier, hash.Sum(nil)) == 1 {
+		return true
+	}
+
 	var shaRe = regexp.MustCompile(`^{SHA}`)
 	var bcrRe = regexp.MustCompile(`^\$2b\$|^\$2a\$|^\$2y\$`)
 
+	isValid := false
+
 	switch {
 	case shaRe.MatchString(realPassword):
 		d := sha1.New()
 		_, _ = d.Write([]byte(password))
 		if realPassword[5:] == base64.StdEncoding.EncodeToString(d.Sum(nil)) {
-			return true
+			isValid = true
 		}
 	case bcrRe.MatchString(realPassword):
 		err := bcrypt.CompareHashAndPassword([]byte(realPassword), []byte(password))
 		if err == nil {
-			return true
+			isValid = true
 		}
 	}
-	log.Printf("Invalid htpasswd entry for %s.", user)
-	return false
+
+	if !isValid {
+		log.Printf("Invalid htpasswd entry for %s.", user)
+		return false
+	}
+
+	h.mutex.Lock()
+	// repurpose mutex to prevent concurrent cache updates
+	cache[user] = cacheEntry{
+		verifier: hash.Sum(nil),
+		expiry:   time.Now().Add(PasswordCacheDuration),
+	}
+	h.mutex.Unlock()
+
+	return true
 }