types: make pre auth key use bcrypt (#2853)

hscontrol/db/preauth_keys.go

@@ -1,8 +1,6 @@
 package db
 
 import (
-	"crypto/rand"
-	"encoding/hex"
 	"errors"
 	"fmt"
 	"slices"
@@ -10,6 +8,8 @@ import (
 	"time"
 
 	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/juanfont/headscale/hscontrol/util"
+	"golang.org/x/crypto/bcrypt"
 	"gorm.io/gorm"
 	"tailscale.com/util/set"
 )
@@ -28,12 +28,18 @@ func (hsdb *HSDatabase) CreatePreAuthKey(
 	ephemeral bool,
 	expiration *time.Time,
 	aclTags []string,
-) (*types.PreAuthKey, error) {
-	return Write(hsdb.DB, func(tx *gorm.DB) (*types.PreAuthKey, error) {
+) (*types.PreAuthKeyNew, error) {
+	return Write(hsdb.DB, func(tx *gorm.DB) (*types.PreAuthKeyNew, error) {
 		return CreatePreAuthKey(tx, uid, reusable, ephemeral, expiration, aclTags)
 	})
 }
 
+const (
+	authKeyPrefix       = "hskey-auth-"
+	authKeyPrefixLength = 12
+	authKeyLength       = 64
+)
+
 // CreatePreAuthKey creates a new PreAuthKey in a user, and returns it.
 func CreatePreAuthKey(
 	tx *gorm.DB,
@@ -42,7 +48,7 @@ func CreatePreAuthKey(
 	ephemeral bool,
 	expiration *time.Time,
 	aclTags []string,
-) (*types.PreAuthKey, error) {
+) (*types.PreAuthKeyNew, error) {
 	user, err := GetUserByID(tx, uid)
 	if err != nil {
 		return nil, err
@@ -65,14 +71,43 @@ func CreatePreAuthKey(
 	}
 
 	now := time.Now().UTC()
-	// TODO(kradalby): unify the key generations spread all over the code.
-	kstr, err := generateKey()
+
+	prefix, err := util.GenerateRandomStringURLSafe(authKeyPrefixLength)
+	if err != nil {
+		return nil, err
+	}
+
+	// Validate generated prefix (should always be valid, but be defensive)
+	if len(prefix) != authKeyPrefixLength {
+		return nil, fmt.Errorf("%w: generated prefix has invalid length: expected %d, got %d", ErrPreAuthKeyFailedToParse, authKeyPrefixLength, len(prefix))
+	}
+
+	if !isValidBase64URLSafe(prefix) {
+		return nil, fmt.Errorf("%w: generated prefix contains invalid characters", ErrPreAuthKeyFailedToParse)
+	}
+
+	toBeHashed, err := util.GenerateRandomStringURLSafe(authKeyLength)
+	if err != nil {
+		return nil, err
+	}
+
+	// Validate generated hash (should always be valid, but be defensive)
+	if len(toBeHashed) != authKeyLength {
+		return nil, fmt.Errorf("%w: generated hash has invalid length: expected %d, got %d", ErrPreAuthKeyFailedToParse, authKeyLength, len(toBeHashed))
+	}
+
+	if !isValidBase64URLSafe(toBeHashed) {
+		return nil, fmt.Errorf("%w: generated hash contains invalid characters", ErrPreAuthKeyFailedToParse)
+	}
+
+	keyStr := authKeyPrefix + prefix + "-" + toBeHashed
+
+	hash, err := bcrypt.GenerateFromPassword([]byte(toBeHashed), bcrypt.DefaultCost)
 	if err != nil {
 		return nil, err
 	}
 
 	key := types.PreAuthKey{
-		Key:        kstr,
 		UserID:     user.ID,
 		User:       *user,
 		Reusable:   reusable,
@@ -80,13 +115,24 @@ func CreatePreAuthKey(
 		CreatedAt:  &now,
 		Expiration: expiration,
 		Tags:       aclTags,
+		Prefix:     prefix, // Store prefix
+		Hash:       hash,   // Store hash
 	}
 
 	if err := tx.Save(&key).Error; err != nil {
 		return nil, fmt.Errorf("failed to create key in the database: %w", err)
 	}
 
-	return &key, nil
+	return &types.PreAuthKeyNew{
+		ID:         key.ID,
+		Key:        keyStr,
+		Reusable:   key.Reusable,
+		Ephemeral:  key.Ephemeral,
+		Tags:       key.Tags,
+		Expiration: key.Expiration,
+		CreatedAt:  key.CreatedAt,
+		User:       key.User,
+	}, nil
 }
 
 func (hsdb *HSDatabase) ListPreAuthKeys(uid types.UserID) ([]types.PreAuthKey, error) {
@@ -110,6 +156,107 @@ func ListPreAuthKeysByUser(tx *gorm.DB, uid types.UserID) ([]types.PreAuthKey, e
 	return keys, nil
 }
 
+var ErrPreAuthKeyFailedToParse = errors.New("failed to parse AuthKey")
+
+func findAuthKey(tx *gorm.DB, keyStr string) (*types.PreAuthKey, error) {
+	var pak types.PreAuthKey
+
+	// Validate input is not empty
+	if keyStr == "" {
+		return nil, ErrPreAuthKeyFailedToParse
+	}
+
+	_, prefixAndHash, found := strings.Cut(keyStr, authKeyPrefix)
+
+	if !found {
+		// Legacy format (plaintext) - backwards compatibility
+		err := tx.Preload("User").First(&pak, "key = ?", keyStr).Error
+		if err != nil {
+			return nil, ErrPreAuthKeyNotFound
+		}
+
+		return &pak, nil
+	}
+
+	// New format: hskey-auth-{12-char-prefix}-{64-char-hash}
+	// Expected minimum length: 12 (prefix) + 1 (separator) + 64 (hash) = 77
+	const expectedMinLength = authKeyPrefixLength + 1 + authKeyLength
+	if len(prefixAndHash) < expectedMinLength {
+		return nil, fmt.Errorf(
+			"%w: key too short, expected at least %d chars after prefix, got %d",
+			ErrPreAuthKeyFailedToParse,
+			expectedMinLength,
+			len(prefixAndHash),
+		)
+	}
+
+	// Use fixed-length parsing instead of separator-based to handle dashes in base64 URL-safe
+	prefix := prefixAndHash[:authKeyPrefixLength]
+
+	// Validate separator at expected position
+	if prefixAndHash[authKeyPrefixLength] != '-' {
+		return nil, fmt.Errorf(
+			"%w: expected separator '-' at position %d, got '%c'",
+			ErrPreAuthKeyFailedToParse,
+			authKeyPrefixLength,
+			prefixAndHash[authKeyPrefixLength],
+		)
+	}
+
+	hash := prefixAndHash[authKeyPrefixLength+1:]
+
+	// Validate hash length
+	if len(hash) != authKeyLength {
+		return nil, fmt.Errorf(
+			"%w: hash length mismatch, expected %d chars, got %d",
+			ErrPreAuthKeyFailedToParse,
+			authKeyLength,
+			len(hash),
+		)
+	}
+
+	// Validate prefix contains only base64 URL-safe characters
+	if !isValidBase64URLSafe(prefix) {
+		return nil, fmt.Errorf(
+			"%w: prefix contains invalid characters (expected base64 URL-safe: A-Za-z0-9_-)",
+			ErrPreAuthKeyFailedToParse,
+		)
+	}
+
+	// Validate hash contains only base64 URL-safe characters
+	if !isValidBase64URLSafe(hash) {
+		return nil, fmt.Errorf(
+			"%w: hash contains invalid characters (expected base64 URL-safe: A-Za-z0-9_-)",
+			ErrPreAuthKeyFailedToParse,
+		)
+	}
+
+	// Look up key by prefix
+	err := tx.Preload("User").First(&pak, "prefix = ?", prefix).Error
+	if err != nil {
+		return nil, ErrPreAuthKeyNotFound
+	}
+
+	// Verify hash matches
+	err = bcrypt.CompareHashAndPassword(pak.Hash, []byte(hash))
+	if err != nil {
+		return nil, fmt.Errorf("invalid auth key: %w", err)
+	}
+
+	return &pak, nil
+}
+
+// isValidBase64URLSafe checks if a string contains only base64 URL-safe characters.
+func isValidBase64URLSafe(s string) bool {
+	for _, c := range s {
+		if (c < 'A' || c > 'Z') && (c < 'a' || c > 'z') && (c < '0' || c > '9') && c != '-' && c != '_' {
+			return false
+		}
+	}
+
+	return true
+}
+
 func (hsdb *HSDatabase) GetPreAuthKey(key string) (*types.PreAuthKey, error) {
 	return GetPreAuthKey(hsdb.DB, key)
 }
@@ -117,12 +264,7 @@ func (hsdb *HSDatabase) GetPreAuthKey(key string) (*types.PreAuthKey, error) {
 // GetPreAuthKey returns a PreAuthKey for a given key. The caller is responsible
 // for checking if the key is usable (expired or used).
 func GetPreAuthKey(tx *gorm.DB, key string) (*types.PreAuthKey, error) {
-	pak := types.PreAuthKey{}
-	if err := tx.Preload("User").First(&pak, "key = ?", key).Error; err != nil {
-		return nil, ErrPreAuthKeyNotFound
-	}
-
-	return &pak, nil
+	return findAuthKey(tx, key)
 }
 
 // DestroyPreAuthKey destroys a preauthkey. Returns error if the PreAuthKey
@@ -159,13 +301,3 @@ func ExpirePreAuthKey(tx *gorm.DB, k *types.PreAuthKey) error {
 	now := time.Now()
 	return tx.Model(&types.PreAuthKey{}).Where("id = ?", k.ID).Update("expiration", now).Error
 }
-
-func generateKey() (string, error) {
-	size := 24
-	bytes := make([]byte, size)
-	if _, err := rand.Read(bytes); err != nil {
-		return "", err
-	}
-
-	return hex.EncodeToString(bytes), nil
-}