uncensored-send/internal/auth/tokens.go

// Package auth manages the named upload tokens and resolves the effective
// limits for a request.
package auth

import (
	"crypto/pbkdf2"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"slices"
	"sort"
	"sync"
	"time"

	"send/internal/config"
)

// tokenFilePerm is deliberately stricter than the rest of the data directory:
// this is the one file holding credential material.
const tokenFilePerm fs.FileMode = 0o600

var (
	ErrNotFound = errors.New("no such token")
	ErrExists   = errors.New("a token with that name already exists")
)

// Key derivation kinds. A generated token is 256 bits of randomness, so a
// plain digest is all it needs: there is no smaller space to search than the
// key space itself. A chosen one is a passphrase, and passphrases are guessable
// and reused elsewhere, so those get a deliberately slow derivation.
const (
	KDFSHA256 = "sha256" // implied when the field is absent
	KDFPBKDF2 = "pbkdf2-sha256"

	// PBKDF2Iterations follows the current OWASP guidance for PBKDF2-HMAC-SHA256.
	PBKDF2Iterations = 600_000
	minIterations    = 100_000

	// MinChosenLength is the floor for a token someone picks themselves.
	// Shorter than this and the throttle on failed logins is the only thing
	// standing between a guesser and the account.
	MinChosenLength = 4
	maxTokenLength  = 256
)

var (
	ErrTokenTooShort = fmt.Errorf("a chosen token must be at least %d characters", MinChosenLength)
	ErrTokenTooLong  = fmt.Errorf("a token must be at most %d characters", maxTokenLength)
)

// Token is one named credential. The pointer fields distinguish "not set, so
// inherit the server default" from "set to zero, meaning unlimited".
type Token struct {
	Name string `json:"name"`

	// KDF is empty for a generated token and KDFPBKDF2 for a chosen one.
	KDF           string    `json:"kdf,omitempty"`
	Salt          string    `json:"salt,omitempty"`
	Iter          int       `json:"iter,omitempty"`
	Hash          string    `json:"hash"`
	MaxSize       *string   `json:"max_size,omitempty"`
	MaxExpiry     *string   `json:"max_expiry,omitempty"`
	DefaultExpiry *string   `json:"default_expiry,omitempty"`
	AllowVanity   bool      `json:"allow_vanity"`
	Admin         bool      `json:"admin"`
	Created       time.Time `json:"created"`

	maxSize       *int64
	maxExpiry     *time.Duration
	defaultExpiry *time.Duration
	salt          []byte
}

// Chosen reports whether this credential is a passphrase somebody picked
// rather than a generated secret.
func (t *Token) Chosen() bool { return t.KDF == KDFPBKDF2 }

// Verify checks a presented secret against this token.
func (t *Token) Verify(secret string) bool {
	if secret == "" || len(secret) > maxTokenLength {
		return false
	}
	switch t.KDF {
	case "", KDFSHA256:
		return EqualHash(t.Hash, HashSecret(secret))
	case KDFPBKDF2:
		sum, err := pbkdf2.Key(sha256.New, secret, t.salt, t.Iter, sha256.Size)
		if err != nil {
			return false
		}
		return EqualHash(t.Hash, hex.EncodeToString(sum))
	default:
		return false
	}
}

// resolve parses the human-written limit strings once, at load time, so a
// malformed token file is rejected at startup rather than mid-upload.
func (t *Token) resolve() error {
	if t.Name == "" {
		return errors.New("token has no name")
	}
	if _, err := hex.DecodeString(t.Hash); err != nil || len(t.Hash) != sha256.Size*2 {
		return fmt.Errorf("token %q: hash is not a sha256 hex digest", t.Name)
	}
	switch t.KDF {
	case "", KDFSHA256:
		if t.Salt != "" || t.Iter != 0 {
			return fmt.Errorf("token %q: salt and iter belong only to %s", t.Name, KDFPBKDF2)
		}
	case KDFPBKDF2:
		salt, err := hex.DecodeString(t.Salt)
		if err != nil || len(salt) < 16 {
			return fmt.Errorf("token %q: salt must be at least 16 random bytes in hex", t.Name)
		}
		if t.Iter < minIterations {
			return fmt.Errorf("token %q: iter is %d, below the %d minimum", t.Name, t.Iter, minIterations)
		}
		t.salt = salt
	default:
		return fmt.Errorf("token %q: unknown kdf %q", t.Name, t.KDF)
	}
	if t.MaxSize != nil {
		n, err := config.ParseSize(*t.MaxSize)
		if err != nil {
			return fmt.Errorf("token %q: max_size: %w", t.Name, err)
		}
		t.maxSize = &n
	}
	if t.MaxExpiry != nil {
		d, err := config.ParseDuration(*t.MaxExpiry)
		if err != nil {
			return fmt.Errorf("token %q: max_expiry: %w", t.Name, err)
		}
		t.maxExpiry = &d
	}
	if t.DefaultExpiry != nil {
		d, err := config.ParseDuration(*t.DefaultExpiry)
		if err != nil {
			return fmt.Errorf("token %q: default_expiry: %w", t.Name, err)
		}
		t.defaultExpiry = &d
	}
	return nil
}

// Limits is the effective permission set for one request.
type Limits struct {
	Name          string // "" for an anonymous caller
	MaxSize       int64
	MaxExpiry     time.Duration
	DefaultExpiry time.Duration
	AllowVanity   bool
	Admin         bool
}

func (l Limits) Anonymous() bool { return l.Name == "" }

// Anonymous returns the limits applied to a caller presenting no credentials.
func Anonymous(c *config.Config) Limits {
	return Limits{
		MaxSize:       c.MaxSize,
		MaxExpiry:     c.MaxExpiry,
		DefaultExpiry: c.DefaultExpiry,
	}
}

// Limits resolves a token's permissions against the server defaults. A field
// the token does not set is inherited, so "the same as anonymous unless
// configured otherwise" needs no special casing.
func (t *Token) Limits(c *config.Config) Limits {
	l := Anonymous(c)
	l.Name = t.Name
	l.AllowVanity = t.AllowVanity
	l.Admin = t.Admin
	if t.maxSize != nil {
		l.MaxSize = *t.maxSize
	}
	if t.maxExpiry != nil {
		l.MaxExpiry = *t.maxExpiry
	}
	if t.defaultExpiry != nil {
		l.DefaultExpiry = *t.defaultExpiry
	}
	// An inherited default longer than an explicitly widened maximum would be
	// surprising; clamp rather than reject, since the token file is trusted.
	if l.MaxExpiry != config.Unlimited &&
		(l.DefaultExpiry == config.Unlimited || l.DefaultExpiry > l.MaxExpiry) {
		l.DefaultExpiry = l.MaxExpiry
	}
	return l
}

// HashSecret is the fast one-way transform, used for generated tokens and for
// per-object delete tokens. Both are 256-bit random values, so a plain digest
// is sufficient - there is nothing to brute force - and lookup by digest
// reveals nothing through timing. Chosen passphrases never go through here;
// see Token.Verify.
func HashSecret(s string) string {
	sum := sha256.Sum256([]byte(s))
	return hex.EncodeToString(sum[:])
}

// EqualHash compares two digests without an early exit.
func EqualHash(a, b string) bool {
	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
}

// maxVerifyCache bounds the memo below. It is cleared wholesale when full,
// which costs one extra derivation per live session and needs no bookkeeping.
const maxVerifyCache = 4096

// File is the token store, backed by a JSON file and reloadable at runtime.
type File struct {
	path string

	mu     sync.RWMutex
	byHash map[string]*Token // generated tokens, found in one step
	byName map[string]*Token
	chosen []*Token // passphrases, each needing its own derivation

	// verified memoises derivation results, negative ones included, so a
	// passphrase costs its full price once rather than on every request.
	// Cleared whenever the file is reloaded.
	verified map[string]*Token

	modTime time.Time
	size    int64
}

// Load reads the token file. A missing file is not an error: the service simply
// starts with no credentials and only the anonymous tier available.
func Load(path string) (*File, error) {
	f := &File{
		path:     path,
		byHash:   map[string]*Token{},
		byName:   map[string]*Token{},
		verified: map[string]*Token{},
	}
	if err := f.Reload(); err != nil {
		return nil, err
	}
	return f, nil
}

func (f *File) Path() string { return f.path }

func (f *File) read() ([]*Token, os.FileInfo, error) {
	info, err := os.Stat(f.path)
	if errors.Is(err, fs.ErrNotExist) {
		return nil, nil, nil
	}
	if err != nil {
		return nil, nil, err
	}
	// Refuse to use credentials the rest of the system can read.
	if perm := info.Mode().Perm(); perm&0o077 != 0 {
		return nil, nil, fmt.Errorf("%s has mode %#o; it must not be group- or world-accessible (chmod 600)", f.path, perm)
	}
	b, err := os.ReadFile(f.path)
	if err != nil {
		return nil, nil, err
	}
	var tokens []*Token
	if err := json.Unmarshal(b, &tokens); err != nil {
		return nil, nil, fmt.Errorf("%s: %w", f.path, err)
	}
	for _, t := range tokens {
		if err := t.resolve(); err != nil {
			return nil, nil, fmt.Errorf("%s: %w", f.path, err)
		}
	}
	return tokens, info, nil
}

// Reload re-reads the token file unconditionally.
func (f *File) Reload() error {
	tokens, info, err := f.read()
	if err != nil {
		return err
	}
	byHash := make(map[string]*Token, len(tokens))
	byName := make(map[string]*Token, len(tokens))
	var chosen []*Token
	for _, t := range tokens {
		if _, dup := byName[t.Name]; dup {
			return fmt.Errorf("%s: duplicate token name %q", f.path, t.Name)
		}
		byName[t.Name] = t
		if t.Chosen() {
			chosen = append(chosen, t)
		} else {
			byHash[t.Hash] = t
		}
	}

	f.mu.Lock()
	defer f.mu.Unlock()
	f.byHash, f.byName, f.chosen = byHash, byName, chosen
	clear(f.verified)
	if info != nil {
		f.modTime, f.size = info.ModTime(), info.Size()
	} else {
		f.modTime, f.size = time.Time{}, 0
	}
	return nil
}

// MaybeReload re-reads the file only if it looks changed. It is cheap enough to
// call on every authenticated request.
func (f *File) MaybeReload() error {
	info, err := os.Stat(f.path)
	if errors.Is(err, fs.ErrNotExist) {
		f.mu.RLock()
		empty := len(f.byHash) == 0
		f.mu.RUnlock()
		if empty {
			return nil
		}
		return f.Reload()
	}
	if err != nil {
		return err
	}
	f.mu.RLock()
	unchanged := info.ModTime().Equal(f.modTime) && info.Size() == f.size
	f.mu.RUnlock()
	if unchanged {
		return nil
	}
	return f.Reload()
}

// Resolved reports whether Lookup can answer for this secret without running a
// key derivation. Callers use it to decide whether the work needs rate limiting.
func (f *File) Resolved(secret string) bool {
	if secret == "" {
		return true
	}
	h := HashSecret(secret)
	f.mu.RLock()
	defer f.mu.RUnlock()
	if _, ok := f.byHash[h]; ok {
		return true
	}
	if _, ok := f.verified[h]; ok {
		return true
	}
	return len(f.chosen) == 0 // nothing slow to try, so the answer is already in
}

// Lookup resolves a presented secret to its token, or nil.
//
// Generated tokens are found by digest in one step. A chosen passphrase has a
// salt of its own, so there is no index to look it up in: each candidate has to
// be derived and compared. That is why the result is memoised, and why callers
// should check Resolved first when the secret came from an untrusted source.
func (f *File) Lookup(secret string) *Token {
	if secret == "" {
		return nil
	}
	h := HashSecret(secret)

	f.mu.RLock()
	if t, ok := f.byHash[h]; ok && EqualHash(t.Hash, h) {
		f.mu.RUnlock()
		return t
	}
	if t, ok := f.verified[h]; ok {
		f.mu.RUnlock()
		return t
	}
	chosen := f.chosen
	f.mu.RUnlock()

	var found *Token
	for _, t := range chosen {
		if t.Verify(secret) {
			found = t
			break
		}
	}

	f.mu.Lock()
	if len(f.verified) >= maxVerifyCache {
		clear(f.verified)
	}
	f.verified[h] = found
	f.mu.Unlock()
	return found
}

// List returns the tokens, name-sorted, for the CLI.
func (f *File) List() []*Token {
	f.mu.RLock()
	defer f.mu.RUnlock()
	out := make([]*Token, 0, len(f.byName))
	for _, t := range f.byName {
		out = append(out, t)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
	return out
}

// Add appends a token and rewrites the file.
func (f *File) Add(t *Token) error {
	if err := t.resolve(); err != nil {
		return err
	}
	f.mu.Lock()
	defer f.mu.Unlock()
	if _, dup := f.byName[t.Name]; dup {
		return ErrExists
	}
	f.byName[t.Name] = t
	if t.Chosen() {
		f.chosen = append(f.chosen, t)
	} else {
		f.byHash[t.Hash] = t
	}
	clear(f.verified)
	return f.saveLocked()
}

// Remove deletes a token by name and rewrites the file.
func (f *File) Remove(name string) error {
	f.mu.Lock()
	defer f.mu.Unlock()
	t, ok := f.byName[name]
	if !ok {
		return ErrNotFound
	}
	delete(f.byName, name)
	delete(f.byHash, t.Hash)
	f.chosen = slices.DeleteFunc(f.chosen, func(c *Token) bool { return c == t })
	clear(f.verified)
	return f.saveLocked()
}

// saveLocked writes the token file atomically, with owner-only permissions.
func (f *File) saveLocked() error {
	tokens := make([]*Token, 0, len(f.byName))
	for _, t := range f.byName {
		tokens = append(tokens, t)
	}
	sort.Slice(tokens, func(i, j int) bool { return tokens[i].Name < tokens[j].Name })

	b, err := json.MarshalIndent(tokens, "", "  ")
	if err != nil {
		return err
	}
	b = append(b, '\n')

	dir := filepath.Dir(f.path)
	if err := os.MkdirAll(dir, 0o775); err != nil {
		return err
	}
	tmp, err := os.CreateTemp(dir, "."+filepath.Base(f.path)+".*")
	if err != nil {
		return err
	}
	defer os.Remove(tmp.Name())

	if err := tmp.Chmod(tokenFilePerm); err != nil {
		tmp.Close()
		return err
	}
	if _, err := tmp.Write(b); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	if err := os.Rename(tmp.Name(), f.path); err != nil {
		return err
	}
	info, err := os.Stat(f.path)
	if err == nil {
		f.modTime, f.size = info.ModTime(), info.Size()
	}
	return nil
}

// NewGenerated builds a credential from a fresh 256-bit secret, which it also
// returns: this is the only time the secret exists.
func NewGenerated(name string) (*Token, string, error) {
	var b [32]byte
	if _, err := rand.Read(b[:]); err != nil {
		return nil, "", err
	}
	secret := hex.EncodeToString(b[:])
	return &Token{Name: name, Hash: HashSecret(secret), Created: now()}, secret, nil
}

// NewChosen builds a credential from a passphrase somebody picked.
//
// Unlike a generated secret this one is guessable and, realistically, reused
// somewhere else, so it is stored under a slow derivation with a salt of its
// own. Cracking the file should not hand an attacker a password that opens
// something that matters more than this service.
func NewChosen(name, secret string) (*Token, error) {
	switch {
	case len(secret) < MinChosenLength:
		return nil, ErrTokenTooShort
	case len(secret) > maxTokenLength:
		return nil, ErrTokenTooLong
	}
	salt := make([]byte, 16)
	if _, err := rand.Read(salt); err != nil {
		return nil, err
	}
	sum, err := pbkdf2.Key(sha256.New, secret, salt, PBKDF2Iterations, sha256.Size)
	if err != nil {
		return nil, err
	}
	return &Token{
		Name:    name,
		KDF:     KDFPBKDF2,
		Salt:    hex.EncodeToString(salt),
		Iter:    PBKDF2Iterations,
		Hash:    hex.EncodeToString(sum),
		Created: now(),
		// Set here as well as in resolve, so a token is usable the moment it is
		// built rather than only after a round trip through the file.
		salt: salt,
	}, nil
}

func now() time.Time { return time.Now().UTC().Truncate(time.Second) }