diff --git a/README.md b/README.md
index ce5ef3e..c76073b 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 A private notebook in the spirit of Obsidian and Notion, for people who would rather run their own.
 Pages that nest inside each other, `[[links]]` between them with backlinks, a journal page for each
-day, tags, and quick search, all kept as plain markdown files. One small binary, no database, no
+day, tags, and quick search, all kept as encrypted markdown files. One small binary, no database, no
 dependencies. You sign in with **Authentik** and nothing else: there are no passwords in Mako.
 
 Named for the mako shark, the fastest in the sea, because a notebook should keep up with you.
@@ -116,23 +116,42 @@ written to it:
 | `oidc.client_id` | `MAKO_OIDC_CLIENT_ID` | |
 | `oidc.client_secret` | `MAKO_OIDC_CLIENT_SECRET` | |
 | `allowed` | `MAKO_ALLOWED` | optional; emails, usernames or Authentik ids, comma separated |
+| `key` | `MAKO_KEY` | encrypts your pages; made for you, see Encryption below |
 | `listen` | `MAKO_LISTEN` | default `127.0.0.1:8490` |
 | `data_dir` | `MAKO_DATA` | default `data` |
 
 `mako.json` also holds a `secret` that signs session cookies. It is made on first run and written
 with mode `0600`. Changing it signs everyone out.
 
+## Encryption
+
+Every page is encrypted on disk with AES-256-GCM. Each person has a key of their own, made from the
+`key` in `mako.json` together with who they are in Authentik (the issuer and their subject id). So one
+person's files cannot be opened with another's key, and a file copied over another page's name stops
+opening. Pages are decrypted in the server's memory while it runs, and your downloads are plain
+markdown.
+
+- **The key is made on first run** and written to `mako.json` (mode `0600`), or set `MAKO_KEY` (at least
+  32 characters, for example `openssl rand -base64 32`) to keep it out of the file.
+- **Back the key up, apart from the notes.** Without it the pages cannot be read, and Mako refuses to
+  start a notebook it cannot decrypt rather than overwrite it. Do not change it. Unlike `secret`,
+  changing `key` does not just sign people out.
+- **What it protects:** a stolen disk or backup of `data/`, and anyone who can read the notes folder but
+  not the settings. **What it does not:** someone who controls the running server, or who can read both
+  `mako.json` and `data/`. Who you are in Authentik is not secret, so it cannot be the key by itself.
+- **Upgrading from 0.1.0:** existing plain files are read and saved again encrypted the first time
+  that person signs in, and a `key` is added to your settings file.
+
 ## Where your notes are
 
 ```
 data/users/<hash of who you are>/
-  owner.txt                            your name and email, so the folder can be matched to a person
-  notes/20261031-143205-a1b2c3.md      one file per page
-  notes/daily-2026-10-31.md            the journal page for a day
+  notes/20261031-143205-a1b2c3         one encrypted file per page (named .md)
+  notes/daily-2026-10-31               the journal page for a day
   trash/                               deleted pages, until the trash is emptied
 ```
 
-A page file looks like this, so you can edit it with anything:
+Inside, a page is markdown with a small header (shown here decrypted, and what the zip export holds):
 
 ```
 ---
@@ -146,8 +165,8 @@ pinned: true
 Flights and hotels, see [[Packing list]] #travel
 ```
 
-A plain `.md` file dropped into `notes/` is picked up too, with its file time as its date (name it with
-lower-case letters, numbers and dashes). Restart Mako to see files added by hand.
+A plain `.md` file dropped into `notes/` is picked up when that person's notebook next opens, with its
+file time as its date, and is then encrypted (name it with lower-case letters, numbers and dashes).
 
 ## Security notes
 
@@ -161,7 +180,8 @@ lower-case letters, numbers and dashes). Restart Mako to see files added by hand
   person's requests can only ever reach their own folder.
 - Every change from the browser needs a header a page on another site can't send. The page has a strict
   Content-Security-Policy, and pages are never rendered as HTML.
-- Pages are **not encrypted** on disk. Anyone who can read the server's disk can read them.
+- Pages are encrypted on disk (see Encryption), with a key per person. The server itself, while it
+  runs, can read them: this is not end-to-end encryption.
 
 ## Honest limits
 
diff --git a/cmd/mako/main.go b/cmd/mako/main.go
index 654de99..c5a528c 100644
--- a/cmd/mako/main.go
+++ b/cmd/mako/main.go
@@ -99,7 +99,7 @@ func serve(args []string) error {
 	if err != nil {
 		return configError{err}
 	}
-	if !h.Exists() { // writing it now is what makes the generated secret stick
+	if h.NeedsSave() { // writing it now is what makes the generated secret and key stick
 		if err := h.Save(); err != nil {
 			return fmt.Errorf("could not write %s: %w", *path, err)
 		}
diff --git a/deploy/docker-compose.yml b/deploy/docker-compose.yml
index 615d9db..87d5027 100644
--- a/deploy/docker-compose.yml
+++ b/deploy/docker-compose.yml
@@ -14,4 +14,4 @@ services:
       MAKO_OIDC_CLIENT_SECRET: change-me
       # MAKO_ALLOWED: you@example.com   # optional: limit who may sign in
     volumes:
-      - ./config:/config       # mako.json (the signing secret) and data/ (your pages) live here
+      - ./config:/config       # mako.json (signing secret and note key; back it up apart from data/) and data/ live here
diff --git a/deploy/install.sh b/deploy/install.sh
index ea2d687..5d5991b 100644
--- a/deploy/install.sh
+++ b/deploy/install.sh
@@ -155,7 +155,8 @@ MAKO_OIDC_CLIENT_SECRET=
 # everyone Authentik lets into the application gets a private notebook of their own.
 #MAKO_ALLOWED=you@example.com
 
-# Where the settings file is (it holds the signing secret).
+# Where the settings file is. It holds the signing secret and the key that encrypts your notes:
+# back it up, apart from $DATA_DIR. Without that key the notes cannot be read.
 MAKO_CONFIG=$CONF_FILE
 
 # Optional. These override what the settings file says.
diff --git a/internal/config/config.go b/internal/config/config.go
index 539f9e8..17b0166 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -13,6 +13,8 @@ import (
 	"path/filepath"
 	"strings"
 	"sync"
+
+	"git.veillax.com/Veillax/mako/internal/vault"
 )
 
 // OIDC is the OAuth client Authentik gives you for Mako.
@@ -37,6 +39,10 @@ type Config struct {
 
 	// Secret signs session cookies, so a restart does not sign you out. It is made on first run.
 	Secret string `json:"secret,omitempty"`
+
+	// Key encrypts the pages on disk (each person's key is made from it and who they are). It is made
+	// on first run. Unlike Secret it must never change or be lost: without it the pages cannot be read.
+	Key string `json:"key,omitempty"`
 }
 
 // Defaults fills in what is missing.
@@ -52,6 +58,11 @@ func (c *Config) Defaults() {
 		rand.Read(b)
 		c.Secret = base64.RawURLEncoding.EncodeToString(b)
 	}
+	if c.Key == "" {
+		b := make([]byte, 32)
+		rand.Read(b)
+		c.Key = base64.RawURLEncoding.EncodeToString(b)
+	}
 	c.PublicURL = strings.TrimRight(strings.TrimSpace(c.PublicURL), "/")
 	c.OIDC.Issuer = strings.TrimSpace(c.OIDC.Issuer)
 	c.OIDC.ClientID = strings.TrimSpace(c.OIDC.ClientID)
@@ -83,6 +94,9 @@ func checkURL(name, raw string) error {
 
 // Check says what is still missing before Mako can sign anyone in, in words a person can act on.
 func (c Config) Check() error {
+	if len(c.Key) < vault.MinKey {
+		return fmt.Errorf("the encryption key (key, or MAKO_KEY) must be at least %d characters; leave it out to have one made, or make one with: openssl rand -base64 32", vault.MinKey)
+	}
 	var missing []string
 	if c.PublicURL == "" {
 		missing = append(missing, "  public_url           (MAKO_PUBLIC_URL)          where you open Mako, such as https://notes.example.com")
@@ -121,6 +135,7 @@ type Holder struct {
 	path string
 	mu   sync.RWMutex
 	file Config
+	made bool // the file is missing, or lacked a secret or key that was just made
 }
 
 // Load reads the file. A missing file gives the defaults (and is created by the first Update).
@@ -136,10 +151,21 @@ func Load(path string) (*Holder, error) {
 			return nil, fmt.Errorf("%s: %w", path, err)
 		}
 	}
+	h.made = err != nil || h.file.Secret == "" || (h.file.Key == "" && !keyInEnv())
+	keyless := h.file.Key == "" && keyInEnv() // a key in the environment is used instead, so none is made
 	h.file.Defaults()
+	if keyless {
+		h.file.Key = ""
+	}
 	return h, nil
 }
 
+func keyInEnv() bool { return strings.TrimSpace(os.Getenv("MAKO_KEY")) != "" }
+
+// NeedsSave says whether the file should be written now: it is missing, or a secret or key was
+// made that is only in memory. Writing it is what makes them stick.
+func (h *Holder) NeedsSave() bool { return h.made }
+
 func (h *Holder) Path() string { return h.path }
 
 // Exists says whether the file is on disk yet.
@@ -162,6 +188,7 @@ func (h *Holder) Get() Config {
 	env("MAKO_OIDC_ISSUER", &c.OIDC.Issuer)
 	env("MAKO_OIDC_CLIENT_ID", &c.OIDC.ClientID)
 	env("MAKO_OIDC_CLIENT_SECRET", &c.OIDC.ClientSecret)
+	env("MAKO_KEY", &c.Key)
 	if v := os.Getenv("MAKO_ALLOWED"); strings.TrimSpace(v) != "" {
 		c.Allowed = nil
 		for _, a := range strings.FieldsFunc(v, func(r rune) bool { return r == ',' || r == ';' || r == '\n' }) {
diff --git a/internal/store/store.go b/internal/store/store.go
index 590a6e6..ddac9f8 100644
--- a/internal/store/store.go
+++ b/internal/store/store.go
@@ -1,5 +1,5 @@
-// Package store keeps pages as plain markdown files, one per page, so they stay readable and
-// portable with or without Mako. An in-memory index makes listing, searching and linking quick.
+// Package store keeps pages as markdown files, one per page, encrypted at rest (see package vault)
+// and exportable as plain markdown. An in-memory index makes listing, searching and linking quick.
 package store
 
 import (
@@ -20,6 +20,8 @@ import (
 	"time"
 	"unicode"
 	"unicode/utf8"
+
+	"git.veillax.com/Veillax/mako/internal/vault"
 )
 
 // MaxBody is the longest page, in bytes. MaxTitle is the longest title, in characters.
@@ -60,6 +62,7 @@ type Note struct {
 
 type Store struct {
 	dir string
+	box *vault.Cipher
 	mu  sync.RWMutex
 	by  map[string]*Note
 }
@@ -72,11 +75,16 @@ var (
 // ValidID says whether s can be used as a page id (and so as a file name).
 func ValidID(s string) bool { return idRe.MatchString(s) }
 
-// Open reads every page under dir, creating the folders if needed.
-func Open(dir string) (*Store, error) {
-	s := &Store{dir: dir, by: map[string]*Note{}}
+// Open reads every page under dir, creating the folders if needed. Pages are encrypted with box. A
+// plain markdown file found there is still read, and is saved again encrypted.
+func Open(dir string, box *vault.Cipher) (*Store, error) {
+	if box == nil {
+		return nil, errors.New("a notebook needs an encryption key")
+	}
+	s := &Store{dir: dir, box: box, by: map[string]*Note{}}
+	var plain []*Note
 	for _, sub := range []string{"notes", "trash"} {
-		if err := os.MkdirAll(filepath.Join(dir, sub), 0o755); err != nil {
+		if err := os.MkdirAll(filepath.Join(dir, sub), 0o700); err != nil {
 			return nil, err
 		}
 		entries, err := os.ReadDir(filepath.Join(dir, sub))
@@ -97,12 +105,26 @@ func Open(dir string) (*Store, error) {
 			if err != nil {
 				return nil, err
 			}
+			sealed := vault.IsSealed(b)
+			if sealed {
+				if b, err = box.Open(b, id); err != nil {
+					return nil, fmt.Errorf("%s: %w; put the original key back (key in the settings file, or MAKO_KEY)", path, err)
+				}
+			}
 			n := parse(id, string(b), info.ModTime())
 			n.Trashed = sub == "trash"
 			s.by[id] = n
+			if !sealed {
+				plain = append(plain, n)
+			}
 		}
 	}
 	s.mend()
+	for _, n := range plain {
+		if err := s.write(n); err != nil {
+			return nil, err
+		}
+	}
 	return s, nil
 }
 
@@ -339,7 +361,7 @@ func isNumber(s string) bool {
 func (s *Store) write(n *Note) error {
 	path := s.path(n)
 	tmp := filepath.Join(filepath.Dir(path), "."+n.ID+".tmp")
-	if err := os.WriteFile(tmp, format(n), 0o644); err != nil {
+	if err := os.WriteFile(tmp, s.box.Seal(format(n), n.ID), 0o600); err != nil {
 		return err
 	}
 	return os.Rename(tmp, path)
diff --git a/internal/store/store_test.go b/internal/store/store_test.go
index a0cc9e6..4190414 100644
--- a/internal/store/store_test.go
+++ b/internal/store/store_test.go
@@ -10,12 +10,14 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"git.veillax.com/Veillax/mako/internal/vault"
 )
 
 func open(t *testing.T) (*Store, string) {
 	t.Helper()
 	dir := t.TempDir()
-	s, err := Open(dir)
+	s, err := Open(dir, testBox(t))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -100,7 +102,7 @@ func TestCreateRoundTripsThroughDisk(t *testing.T) {
 	if _, err := os.Stat(filepath.Join(dir, "notes", n.ID+".md")); err != nil {
 		t.Fatal(err)
 	}
-	s2, err := Open(dir)
+	s2, err := Open(dir, testBox(t))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -126,7 +128,7 @@ func TestPlainMarkdownFileIsAPage(t *testing.T) {
 	os.WriteFile(path, []byte("# Dropped in\r\nby hand #manual\r\n"), 0o644)
 	mod := time.Date(2025, 3, 4, 5, 6, 7, 0, time.UTC)
 	os.Chtimes(path, mod, mod)
-	s2, err := Open(dir)
+	s2, err := Open(dir, testBox(t))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -209,7 +211,7 @@ func TestRenameRewritesLinks(t *testing.T) {
 	if other, _ := s.Get(b.ID); other.Body != b.Body || !other.Updated.Equal(b.Updated) {
 		t.Fatal("unrelated pages must not be touched")
 	}
-	s2, _ := Open(s.dir)
+	s2, _ := Open(s.dir, testBox(t))
 	if again, _ := s2.Get(a.ID); again.Body != want {
 		t.Fatal("rewrite was not saved")
 	}
@@ -255,7 +257,7 @@ func TestTreeParentsAndCycles(t *testing.T) {
 	os.WriteFile(filepath.Join(dir, "notes", "x1.md"), []byte("---\nparent: x2\n---\none"), 0o644)
 	os.WriteFile(filepath.Join(dir, "notes", "x2.md"), []byte("---\nparent: x1\n---\ntwo"), 0o644)
 	os.WriteFile(filepath.Join(dir, "notes", "x3.md"), []byte("---\nparent: ghost\n---\nthree"), 0o644)
-	s2, err := Open(dir)
+	s2, err := Open(dir, testBox(t))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -299,7 +301,7 @@ func TestTrashRestorePurge(t *testing.T) {
 	if _, err := s.Create(NewNote{Title: "Child"}); err != nil {
 		t.Fatal(err)
 	}
-	s2, _ := Open(dir)
+	s2, _ := Open(dir, testBox(t))
 	back, err := s2.Restore(n.ID)
 	if err != nil || back.Title != "Child 2" || back.Parent != parent.ID {
 		t.Fatalf("restore: %v %+v", err, back)
@@ -355,7 +357,7 @@ func TestDaily(t *testing.T) {
 		t.Fatalf("a day can be linked by its date: %+v", got)
 	}
 	// reload keeps the title from the date, and an empty day stays out of navigation
-	s2, _ := Open(s.dir)
+	s2, _ := Open(s.dir, testBox(t))
 	if n, _ := s2.Get(d1.ID); n.Title != "2026-10-31" {
 		t.Fatalf("title after reload: %q", n.Title)
 	}
@@ -465,7 +467,7 @@ func TestIDsCannotEscape(t *testing.T) {
 	}
 	_, dir := open(t)
 	os.WriteFile(filepath.Join(dir, "notes", "Bad Name.md"), []byte("ignored"), 0o644)
-	s2, _ := Open(dir)
+	s2, _ := Open(dir, testBox(t))
 	if list, _ := s2.List(Query{}); len(list) != 0 {
 		t.Fatal("a file whose name is not a valid id should be ignored")
 	}
@@ -502,3 +504,75 @@ func TestZipIsAnObsidianVault(t *testing.T) {
 		}
 	}
 }
+
+func testBox(t *testing.T) *vault.Cipher {
+	t.Helper()
+	b, err := vault.New("0123456789abcdef0123456789abcdef", "https://auth.example.com/", "alice")
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}
+
+func TestFilesAreEncrypted(t *testing.T) {
+	s, dir := open(t)
+	n := mk(t, s, "Diary", "meet at the #harbour")
+	b, err := os.ReadFile(filepath.Join(dir, "notes", n.ID+".md"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !vault.IsSealed(b) || bytes.Contains(b, []byte("harbour")) || bytes.Contains(b, []byte("Diary")) {
+		t.Fatalf("the page is on disk in the clear: %q", b)
+	}
+	// trashing keeps it encrypted, and everything reads back
+	if _, err := s.Trash(n.ID); err != nil {
+		t.Fatal(err)
+	}
+	b, _ = os.ReadFile(filepath.Join(dir, "trash", n.ID+".md"))
+	if !vault.IsSealed(b) {
+		t.Fatalf("a trashed page is in the clear: %q", b)
+	}
+	s2, err := Open(dir, testBox(t))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, err := s2.Get(n.ID); err != nil || got.Body != "meet at the #harbour" || !got.Trashed {
+		t.Fatalf("reload: %+v %v", got, err)
+	}
+}
+
+func TestPlainFilesAreEncryptedWhenOpened(t *testing.T) {
+	_, dir := open(t)
+	path := filepath.Join(dir, "notes", "old-page.md")
+	os.WriteFile(path, []byte("---\ntitle: \"Old\"\n---\n\nwritten before encryption\n"), 0o644)
+	s, err := Open(dir, testBox(t))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n, err := s.Get("old-page"); err != nil || n.Title != "Old" {
+		t.Fatalf("%+v %v", n, err)
+	}
+	b, _ := os.ReadFile(path)
+	if !vault.IsSealed(b) || bytes.Contains(b, []byte("before encryption")) {
+		t.Fatalf("still in the clear: %q", b)
+	}
+}
+
+func TestWrongKeyRefusesInsteadOfLosingPages(t *testing.T) {
+	s, dir := open(t)
+	n := mk(t, s, "Keep", "me")
+	other, _ := vault.New("zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz", "https://auth.example.com/", "alice")
+	if _, err := Open(dir, other); !errors.Is(err, vault.ErrWrongKey) {
+		t.Fatalf("want ErrWrongKey, got %v", err)
+	}
+	// a page file copied over another page's name does not open either
+	src := filepath.Join(dir, "notes", n.ID+".md")
+	b, _ := os.ReadFile(src)
+	os.WriteFile(filepath.Join(dir, "notes", "other-id.md"), b, 0o600)
+	if _, err := Open(dir, testBox(t)); !errors.Is(err, vault.ErrWrongKey) {
+		t.Fatalf("a swapped file opened: %v", err)
+	}
+	if _, err := Open(dir, nil); err == nil {
+		t.Fatal("opened with no key")
+	}
+}
diff --git a/internal/vault/vault.go b/internal/vault/vault.go
new file mode 100644
index 0000000..5468ba9
--- /dev/null
+++ b/internal/vault/vault.go
@@ -0,0 +1,78 @@
+// Package vault encrypts pages at rest. Every person gets a key of their own, made from the server's
+// master key and who they are in Authentik, so one person's files cannot be opened (or swapped in)
+// with another's key. Who they are is not secret; the master key is what keeps the files private.
+package vault
+
+import (
+	"bytes"
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/hkdf"
+	"crypto/rand"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+)
+
+// ErrWrongKey means a file is encrypted but this key cannot open it: the master key was changed or
+// lost, or the file was altered.
+var ErrWrongKey = errors.New("these notes were saved with a different encryption key")
+
+// magic starts every encrypted file, so a plain markdown file can still be told apart and read.
+var magic = []byte("MAKO1\n")
+
+// MinKey is the shortest master key accepted, in characters.
+const MinKey = 32
+
+// Cipher seals and opens the files of one person.
+type Cipher struct{ aead cipher.AEAD }
+
+// New makes the cipher for one person from the master key, the Authentik issuer and their subject.
+func New(master, issuer, sub string) (*Cipher, error) {
+	if len(master) < MinKey {
+		return nil, fmt.Errorf("the encryption key must be at least %d characters", MinKey)
+	}
+	if sub == "" {
+		return nil, errors.New("no user to make an encryption key for")
+	}
+	key, err := hkdf.Key(sha256.New, []byte(master), nil, "mako notes v1|"+issuer+"|"+sub, 32)
+	if err != nil {
+		return nil, err
+	}
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+	return &Cipher{aead}, nil
+}
+
+// IsSealed says whether data is an encrypted file.
+func IsSealed(data []byte) bool { return bytes.HasPrefix(data, magic) }
+
+// Seal encrypts a file. The name (the page id) is signed into it, so a file copied over another
+// page's name no longer opens.
+func (c *Cipher) Seal(plain []byte, name string) []byte {
+	nonce := make([]byte, c.aead.NonceSize())
+	rand.Read(nonce)
+	out := make([]byte, 0, len(magic)+len(nonce)+len(plain)+c.aead.Overhead())
+	out = append(out, magic...)
+	out = append(out, nonce...)
+	return c.aead.Seal(out, nonce, plain, []byte(name))
+}
+
+// Open decrypts a file made by Seal.
+func (c *Cipher) Open(data []byte, name string) ([]byte, error) {
+	n := len(magic) + c.aead.NonceSize()
+	if !IsSealed(data) || len(data) < n+c.aead.Overhead() {
+		return nil, ErrWrongKey
+	}
+	plain, err := c.aead.Open(nil, data[len(magic):n], data[n:], []byte(name))
+	if err != nil {
+		return nil, ErrWrongKey
+	}
+	return plain, nil
+}
diff --git a/internal/vault/vault_test.go b/internal/vault/vault_test.go
new file mode 100644
index 0000000..a179c6b
--- /dev/null
+++ b/internal/vault/vault_test.go
@@ -0,0 +1,71 @@
+package vault
+
+import (
+	"bytes"
+	"errors"
+	"strings"
+	"testing"
+)
+
+const master = "0123456789abcdef0123456789abcdef"
+
+func must(t *testing.T, m, iss, sub string) *Cipher {
+	t.Helper()
+	c, err := New(m, iss, sub)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return c
+}
+
+func TestRoundTrip(t *testing.T) {
+	c := must(t, master, "https://a/", "alice")
+	plain := []byte("---\ntitle: \"Secret\"\n---\n\ndear diary")
+	sealed := c.Seal(plain, "page-1")
+	if !IsSealed(sealed) || bytes.Contains(sealed, []byte("diary")) || bytes.Contains(sealed, []byte("Secret")) {
+		t.Fatalf("not encrypted: %q", sealed)
+	}
+	got, err := c.Open(sealed, "page-1")
+	if err != nil || !bytes.Equal(got, plain) {
+		t.Fatalf("open: %q %v", got, err)
+	}
+	if bytes.Equal(sealed, c.Seal(plain, "page-1")) {
+		t.Fatal("two seals of the same text must differ")
+	}
+}
+
+func TestWrongKeyNameOrDamage(t *testing.T) {
+	c := must(t, master, "https://a/", "alice")
+	sealed := c.Seal([]byte("hello"), "p1")
+
+	cases := map[string]*Cipher{
+		"another person":     must(t, master, "https://a/", "bob"),
+		"another issuer":     must(t, master, "https://b/", "alice"),
+		"another master key": must(t, strings.Repeat("z", 40), "https://a/", "alice"),
+	}
+	for name, other := range cases {
+		if _, err := other.Open(sealed, "p1"); !errors.Is(err, ErrWrongKey) {
+			t.Errorf("%s: want ErrWrongKey, got %v", name, err)
+		}
+	}
+	if _, err := c.Open(sealed, "p2"); !errors.Is(err, ErrWrongKey) {
+		t.Errorf("a file moved to another page name opened: %v", err)
+	}
+	bad := append([]byte{}, sealed...)
+	bad[len(bad)-1] ^= 1
+	if _, err := c.Open(bad, "p1"); !errors.Is(err, ErrWrongKey) {
+		t.Errorf("a damaged file opened: %v", err)
+	}
+	if _, err := c.Open([]byte("plain text"), "p1"); !errors.Is(err, ErrWrongKey) {
+		t.Errorf("plain text opened: %v", err)
+	}
+}
+
+func TestNewChecks(t *testing.T) {
+	if _, err := New("short", "i", "s"); err == nil {
+		t.Error("a short key was accepted")
+	}
+	if _, err := New(master, "i", ""); err == nil {
+		t.Error("an empty user was accepted")
+	}
+}
diff --git a/internal/web/server.go b/internal/web/server.go
index c869da8..a45688b 100644
--- a/internal/web/server.go
+++ b/internal/web/server.go
@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"io/fs"
+	"log"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -18,6 +19,7 @@ import (
 	"git.veillax.com/Veillax/mako/internal/config"
 	"git.veillax.com/Veillax/mako/internal/oidc"
 	"git.veillax.com/Veillax/mako/internal/store"
+	"git.veillax.com/Veillax/mako/internal/vault"
 )
 
 //go:embed static
@@ -63,16 +65,19 @@ func (s *Server) notebook(c config.Config, sub, name, email string) (*store.Stor
 	if st, ok := s.books[key]; ok {
 		return st, nil
 	}
-	dir := filepath.Join(c.DataDir, "users", key)
-	st, err := store.Open(dir)
+	box, err := vault.New(c.Key, c.OIDC.Issuer, sub)
 	if err != nil {
 		return nil, err
 	}
-	// A note for whoever looks after the server, so a folder can be matched to a person.
-	who := fmt.Sprintf("%s <%s>\n", name, email)
-	if b, _ := os.ReadFile(filepath.Join(dir, "owner.txt")); string(b) != who {
-		os.WriteFile(filepath.Join(dir, "owner.txt"), []byte(who), 0o644)
+	dir := filepath.Join(c.DataDir, "users", key)
+	st, err := store.Open(dir, box)
+	if err != nil {
+		log.Printf("could not open the notebook of %s <%s>: %v", name, email, err)
+		return nil, err
 	}
+	// Earlier versions left a plain note here naming the owner, which does not belong next to
+	// encrypted pages.
+	os.Remove(filepath.Join(dir, "owner.txt"))
 	s.books[key] = st
 	return st, nil
 }
@@ -142,7 +147,7 @@ func (s *Server) api(fn handlerFunc) http.HandlerFunc {
 		}
 		st, err := s.notebook(c, ses.Sub, ses.Name, ses.Email)
 		if err != nil {
-			fail(w, http.StatusInternalServerError, "could not open your notes: "+err.Error())
+			fail(w, http.StatusInternalServerError, "could not open your notes; the server's log says why")
 			return
 		}
 		r.Body = http.MaxBytesReader(w, r.Body, maxRequestBody)
diff --git a/internal/web/web_test.go b/internal/web/web_test.go
index a1e5dad..79282dc 100644
--- a/internal/web/web_test.go
+++ b/internal/web/web_test.go
@@ -612,3 +612,68 @@ func TestFailCapitalizes(t *testing.T) {
 		t.Fatalf("%q", out["error"])
 	}
 }
+
+func TestNothingIsOnDiskInTheClear(t *testing.T) {
+	e := setup(t, nil)
+	a := e.signIn(alice)
+	e.do("POST", "/api/notes", map[string]string{"title": "Harbour plans", "body": "meet at midnight"}, a)
+	e.do("POST", "/api/notes", map[string]string{"body": "a quick thought"}, a)
+	files := 0
+	filepath.WalkDir(filepath.Join(e.dir, "data"), func(p string, d os.DirEntry, err error) error {
+		if err != nil || d.IsDir() {
+			return nil
+		}
+		files++
+		b, _ := os.ReadFile(p)
+		for _, s := range []string{"Harbour", "midnight", "quick thought", "Alice", "alice@"} {
+			if bytes.Contains(b, []byte(s)) {
+				t.Errorf("%s holds %q in the clear", p, s)
+			}
+		}
+		return nil
+	})
+	if files < 2 {
+		t.Fatalf("only %d files written", files)
+	}
+}
+
+func TestKeyIsMadeOnceAndKept(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "mako.json")
+	h, err := config.Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !h.NeedsSave() {
+		t.Fatal("a new file must be written")
+	}
+	h.Save()
+	key := h.Get().Key
+	if len(key) < 32 {
+		t.Fatalf("key %q", key)
+	}
+	h2, _ := config.Load(path)
+	if h2.NeedsSave() || h2.Get().Key != key {
+		t.Fatal("the key changed or was made again")
+	}
+	// a file from before encryption has a secret but no key: one is made and must be written
+	os.WriteFile(path, []byte(`{"secret":"old-secret"}`), 0o600)
+	h3, _ := config.Load(path)
+	if !h3.NeedsSave() || len(h3.Get().Key) < 32 {
+		t.Fatal("an old file gets no key")
+	}
+	// a key from the environment wins and is never written to the file
+	t.Setenv("MAKO_KEY", strings.Repeat("k", 40))
+	os.WriteFile(path, []byte(`{"secret":"old-secret"}`), 0o600)
+	h4, _ := config.Load(path)
+	h4.Save()
+	if h4.Get().Key != strings.Repeat("k", 40) {
+		t.Fatal("the environment key was not used")
+	}
+	if b, _ := os.ReadFile(path); strings.Contains(string(b), "kkkk") || strings.Contains(string(b), `"key"`) {
+		t.Fatalf("the environment key was written to the file: %s", b)
+	}
+	t.Setenv("MAKO_KEY", "too short")
+	if err := h4.Get().Check(); err == nil || !strings.Contains(err.Error(), "encryption key") {
+		t.Fatalf("a short key was accepted: %v", err)
+	}
+}