diff --git a/cmd/lazysecrets/main.go b/cmd/lazysecrets/main.go
index 1b2b63d..38afd40 100644
--- a/cmd/lazysecrets/main.go
+++ b/cmd/lazysecrets/main.go
@@ -317,7 +317,7 @@ Flags:
 	if shareArgs != nil {
 		ctx, cancel := context.WithTimeout(context.Background(), *timeout)
 		defer cancel()
-		return shareFiles(ctx, providers, st, cfg.Share.Key, shareArgs, os.Stdout)
+		return shareFiles(ctx, providers, st, cfg.Share.Key, shareRules(cfg), shareArgs, os.Stdout)
 	}
 
 	// Links opened elsewhere (lazysecrets launch) can come here.
@@ -391,6 +391,7 @@ Flags:
 		Changelog:      lazysecrets.Changelog,
 		Dir:            placeDir,
 		ShareKey:       cfg.Share.Key,
+		ShareRules:     shareRules(cfg),
 		Open:           open,
 		Styles:         &theme,
 		DefaultFg:      defaultFg,
diff --git a/cmd/lazysecrets/share.go b/cmd/lazysecrets/share.go
index baae319..0d851f8 100644
--- a/cmd/lazysecrets/share.go
+++ b/cmd/lazysecrets/share.go
@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 
+	"git.kebcloud.no/keb/lazysecrets/internal/config"
 	"git.kebcloud.no/keb/lazysecrets/internal/provider"
 	"git.kebcloud.no/keb/lazysecrets/internal/provider/repo"
 	"git.kebcloud.no/keb/lazysecrets/internal/share"
@@ -19,7 +20,7 @@ import (
 // the --key secret's value, else the repo's or the config's share.key:
 //
 //	lazysecrets share .env apps/api/.env
-func shareFiles(ctx context.Context, providers []provider.Provider, st *store.Store, general string, args []string, out io.Writer) error {
+func shareFiles(ctx context.Context, providers []provider.Provider, st *store.Store, general string, rules share.Rules, args []string, out io.Writer) error {
 	fs := flag.NewFlagSet("share", flag.ContinueOnError)
 	keyLink := fs.String("key", "", "the secret to encrypt with: its lazysecrets:// link, Key Vault URL or reference (default: share.key)")
 	if err := fs.Parse(args); err != nil {
@@ -78,10 +79,15 @@ func shareFiles(ctx context.Context, providers []provider.Provider, st *store.St
 	if err != nil {
 		return fmt.Errorf("reading the key %s: %w", key, err)
 	}
-	link, err := share.Seal(share.Header{Key: share.Pin(key, version), Repo: id}, []byte(v.Reveal()), files)
+	link, err := share.Seal(share.Header{Key: share.Pin(key, version), Repo: id}, []byte(v.Reveal()), files, rules)
 	if err != nil {
 		return err
 	}
 	_, err = fmt.Fprintln(out, link)
 	return err
 }
+
+// shareRules are the config's share.keyRules.
+func shareRules(cfg config.Config) share.Rules {
+	return share.Rules{MinLength: cfg.Share.KeyRules.MinLength, MinClasses: cfg.Share.KeyRules.MinClasses}
+}
diff --git a/cmd/lazysecrets/share_test.go b/cmd/lazysecrets/share_test.go
index dbcdc24..7dffd0e 100644
--- a/cmd/lazysecrets/share_test.go
+++ b/cmd/lazysecrets/share_test.go
@@ -16,6 +16,9 @@ import (
 	"git.kebcloud.no/keb/lazysecrets/internal/store"
 )
 
+// demoRules take the demo key, made up to be remembered.
+var demoRules = share.Rules{MinLength: 16, MinClasses: 1}
+
 func TestShareFiles(t *testing.T) {
 	root, _ := filepath.EvalSymlinks(t.TempDir())
 	for _, args := range [][]string{{"init", "-q"}, {"remote", "add", "origin", "git@git.example.com:team/app.git"}} {
@@ -31,7 +34,7 @@ func TestShareFiles(t *testing.T) {
 	key := "lazysecrets://azure/kv-app-prod.vault.azure.net/secrets/db-password"
 
 	var out bytes.Buffer
-	if err := shareFiles(ctx, providers, st, key, []string{filepath.Join(root, ".env")}, &out); err != nil {
+	if err := shareFiles(ctx, providers, st, key, demoRules, []string{filepath.Join(root, ".env")}, &out); err != nil {
 		t.Fatal(err)
 	}
 	r, err := ref.Parse(strings.TrimSpace(out.String()))
@@ -57,7 +60,7 @@ func TestShareFiles(t *testing.T) {
 		{key, nil, "usage"},
 		{"", []string{"--key", "lazysecrets://azure/kv-app-prod.vault.azure.net", filepath.Join(root, ".env")}, "isn't a secret"},
 	} {
-		if err := shareFiles(ctx, providers, st, c.general, c.args, &out); err == nil || !strings.Contains(err.Error(), c.want) {
+		if err := shareFiles(ctx, providers, st, c.general, demoRules, c.args, &out); err == nil || !strings.Contains(err.Error(), c.want) {
 			t.Errorf("%v: %v, want %q", c.args, err, c.want)
 		}
 	}
diff --git a/docs/config.md b/docs/config.md
index 3a6cb9c..0aeb16d 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -37,6 +37,9 @@ providers:
     exclude: ["**/testdata/**"]
 share:                 # sharing env files as encrypted links
   key: https://kv-team.vault.azure.net/secrets/env-share  # or its lazysecrets:// link; for repos naming none
+  keyRules:            # what a key's value must be
+    minLength: 32      # characters, 16 at least
+    minClasses: 2      # kinds: lowercase, uppercase, digits, symbols
 ```
 
 More, each on its own page: [repos](repos.md) (`providers.repos`),
diff --git a/docs/links.md b/docs/links.md
index b8b9bfc..4f7e629 100644
--- a/docs/links.md
+++ b/docs/links.md
@@ -42,8 +42,7 @@ lazysecrets://share/AQ…
 The files are encrypted with a secret's value, so whoever can read that
 secret (in Key Vault, Databricks or a repo's SOPS file) can open the link,
 and nobody else: who may open it is up to the vault's access, as for
-everything else. Make a secret for it once, random and at least 16
-characters:
+everything else. Make a secret for it once, random:
 
 ```sh
 az keyvault secret set --vault-name kv-team --name env-share --value "$(openssl rand -base64 32)"
@@ -63,6 +62,19 @@ With neither, sharing asks which secret, in [Go to](keys.md#go-to): it
 lists only secrets, and `enter` picks one (pasting a reference picks it
 too).
 
+Anyone with a link can try to guess the key, so sharing refuses one that
+isn't long and varied enough, saying how (never showing it). By default
+that's 32 characters of two kinds; your config can ask for more, or less:
+
+```yaml
+share:
+  keyRules:
+    minLength: 32    # characters; 16 at least
+    minClasses: 2    # kinds of character: lowercase, uppercase, digits, symbols
+```
+
+Opening a link doesn't check them.
+
 On the command line, `lazysecrets share [--key <secret>] <env file>...`
 prints the link.
 
diff --git a/internal/config/config.go b/internal/config/config.go
index fa32d82..a1eb082 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -90,6 +90,16 @@ type Share struct {
 	// lazysecrets:// link, Key Vault URL or reference), for repos whose
 	// .lazysecrets.yaml doesn't name one.
 	Key string `yaml:"key"`
+	// KeyRules are what a key's value must be to share with it.
+	KeyRules KeyRules `yaml:"keyRules"`
+}
+
+// KeyRules default to 32 characters of 2 kinds (share.DefaultRules).
+type KeyRules struct {
+	MinLength int `yaml:"minLength"` // at least 16
+	// MinClasses is how many kinds of character: lowercase, uppercase,
+	// digits, symbols.
+	MinClasses int `yaml:"minClasses"`
 }
 
 type Providers struct {
@@ -236,6 +246,12 @@ func (c Config) validate(path string) error {
 			return fmt.Errorf("%s: share.key: %q is not a secret's lazysecrets:// link, Key Vault URL or reference", path, k)
 		}
 	}
+	if n := c.Share.KeyRules.MinLength; n != 0 && n < 16 {
+		return fmt.Errorf("%s: share.keyRules.minLength: %d is under 16", path, n)
+	}
+	if n := c.Share.KeyRules.MinClasses; n < 0 || n > 4 {
+		return fmt.Errorf("%s: share.keyRules.minClasses: %d is not between 1 and 4", path, n)
+	}
 	for i, v := range c.Providers.Azure.Vaults {
 		if !strings.HasPrefix(v, "https://") {
 			return fmt.Errorf("%s: providers.azure.vaults[%d]: %q is not an https:// vault URL", path, i, v)
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index 27a285b..d2e86c7 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -72,6 +72,9 @@ func TestLoadRejectsMistakes(t *testing.T) {
 		"view":       "sourcesView: tree\n",
 		"releases":   "releases: [git.example.com/team/lazysecrets]\n",
 		"http":       "releases: [http://git.example.com/team/lazysecrets]\n",
+		"share key":  "share:\n  key: https://kv.vault.azure.net/\n",
+		"short keys": "share:\n  keyRules:\n    minLength: 8\n",
+		"classes":    "share:\n  keyRules:\n    minClasses: 5\n",
 	} {
 		if _, err := Load(write(t, content)); err == nil {
 			t.Errorf("%s: no error", name)
diff --git a/internal/share/rules.go b/internal/share/rules.go
new file mode 100644
index 0000000..90abe80
--- /dev/null
+++ b/internal/share/rules.go
@@ -0,0 +1,59 @@
+package share
+
+import (
+	"fmt"
+	"unicode"
+)
+
+// Rules are what a key's value must be to share with it: random enough
+// that guessing it is out of reach, since anyone with a link can try.
+type Rules struct {
+	MinLength  int // characters
+	MinClasses int // kinds of character: lowercase, uppercase, digits, symbols
+}
+
+// DefaultRules take a random base64 or hex value (openssl rand -base64
+// 32, or -hex 32) and refuse passwords made up to be remembered.
+var DefaultRules = Rules{MinLength: 32, MinClasses: 2}
+
+// Check reports what key lacks, without showing it.
+func (r Rules) Check(key []byte) error {
+	r = r.withDefaults()
+	s := string(key)
+	var lower, upper, digit, symbol bool
+	for _, c := range s {
+		switch {
+		case unicode.IsLower(c):
+			lower = true
+		case unicode.IsUpper(c):
+			upper = true
+		case unicode.IsDigit(c):
+			digit = true
+		default:
+			symbol = true
+		}
+	}
+	kinds := 0
+	for _, has := range []bool{lower, upper, digit, symbol} {
+		if has {
+			kinds++
+		}
+	}
+	n := len([]rune(s))
+	if n >= r.MinLength && kinds >= r.MinClasses {
+		return nil
+	}
+	return fmt.Errorf("the key's value is %d characters of %d kinds, and share.keyRules wants at least %d characters of %d kinds (lowercase, uppercase, digits, symbols): use a random one, like openssl rand -base64 32 makes",
+		n, kinds, r.MinLength, r.MinClasses)
+}
+
+func (r Rules) withDefaults() Rules {
+	if r.MinLength == 0 {
+		r.MinLength = DefaultRules.MinLength
+	}
+	if r.MinClasses == 0 {
+		r.MinClasses = DefaultRules.MinClasses
+	}
+	r.MinLength = max(r.MinLength, MinKey)
+	return r
+}
diff --git a/internal/share/share.go b/internal/share/share.go
index f8f4948..92d831d 100644
--- a/internal/share/share.go
+++ b/internal/share/share.go
@@ -40,7 +40,7 @@ const (
 	// MaxLink keeps a link short enough for chat and tickets (Teams takes
 	// about 28 KB) and for passing it to a terminal window.
 	MaxLink = 16 << 10
-	// MinKey is the shortest secret value accepted as a key.
+	// MinKey is the shortest key the rules can ask for.
 	MinKey = 16
 	// maxFiles caps what opening a link unpacks.
 	maxFiles = 1 << 20
@@ -85,13 +85,14 @@ type file struct {
 	Data string `json:"data"`
 }
 
-// Seal encrypts files with key, the value of h.Key, into a share link.
-func Seal(h Header, key []byte, files []File) (ref.Ref, error) {
+// Seal encrypts files with key, the value of h.Key, into a share link,
+// if key keeps to rules.
+func Seal(h Header, key []byte, files []File, rules Rules) (ref.Ref, error) {
 	if _, err := ParseKey(h.Key.String()); err != nil {
 		return ref.Ref{}, fmt.Errorf("the key: %w", err)
 	}
-	if len(key) < MinKey {
-		return ref.Ref{}, fmt.Errorf("the key's value is %d characters: use a random one of at least %d", len(key), MinKey)
+	if err := rules.Check(key); err != nil {
+		return ref.Ref{}, err
 	}
 	if len(files) == 0 {
 		return ref.Ref{}, errors.New("no files to share")
diff --git a/internal/share/share_test.go b/internal/share/share_test.go
index 72044f8..2fc7dc7 100644
--- a/internal/share/share_test.go
+++ b/internal/share/share_test.go
@@ -23,7 +23,7 @@ func TestSealOpen(t *testing.T) {
 		{Path: ".env", Data: []byte("DB_PASSWORD=hunter2\n")},
 		{Path: "apps/api/.env.local", Data: []byte("TOKEN=\"a b\" # quoted\n")},
 	}
-	link, err := Seal(Header{Key: key, Repo: "git.example.com/team/app"}, pass, files)
+	link, err := Seal(Header{Key: key, Repo: "git.example.com/team/app"}, pass, files, Rules{})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -62,7 +62,7 @@ func TestSealRefuses(t *testing.T) {
 		files []File
 		want  string
 	}{
-		"short key":    {Header{Key: key}, []byte("short"), ok, "at least 16"},
+		"short key":    {Header{Key: key}, []byte("short"), ok, "keyRules wants"},
 		"vault as key": {Header{Key: ref.Ref{Kind: provider.KindAzure, Host: "kv.vault.azure.net"}}, pass, ok, "isn't a secret"},
 		"no files":     {Header{Key: key}, pass, nil, "no files"},
 		"not env":      {Header{Key: key}, pass, []File{{Path: "main.go", Data: []byte("x")}}, "isn't an env file"},
@@ -70,7 +70,7 @@ func TestSealRefuses(t *testing.T) {
 		"binary":       {Header{Key: key}, pass, []File{{Path: ".env", Data: []byte{0xff, 0xfe}}}, "isn't text"},
 		"too big":      {Header{Key: key}, pass, []File{{Path: ".env", Data: []byte(random(2 * MaxLink))}}, "more than the 16 KB"},
 	} {
-		if _, err := Seal(c.h, c.pass, c.files); err == nil || !strings.Contains(err.Error(), c.want) {
+		if _, err := Seal(c.h, c.pass, c.files, Rules{}); err == nil || !strings.Contains(err.Error(), c.want) {
 			t.Errorf("%s: %v, want %q", name, err, c.want)
 		}
 	}
@@ -89,8 +89,31 @@ func random(n int) string {
 	return b.String()
 }
 
+func TestRules(t *testing.T) {
+	for _, c := range []struct {
+		r    Rules
+		key  string
+		pass bool
+	}{
+		{Rules{}, "q2Hd8bT4Lw1sZp9Kf6Vx3Nc7Rm0Ya5Ue", true},                                 // base64-like, 32
+		{Rules{}, "9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08", true}, // hex
+		{Rules{}, "correct-horse-battery-staple", false},                                    // 28: short
+		{Rules{}, strings.Repeat("a", 40), false},                                           // one kind
+		{Rules{MinLength: 16, MinClasses: 1}, strings.Repeat("a", 16), true},
+		{Rules{MinLength: 4}, "abc123", false}, // never under MinKey
+		{Rules{MinClasses: 4}, "q2Hd8bT4Lw1sZp9Kf6Vx3Nc7Rm0Ya5Ue", false},
+		{Rules{MinClasses: 4}, "q2Hd8bT4Lw1sZp9Kf6Vx3Nc7Rm0Ya5U+", true},
+	} {
+		if err := c.r.Check([]byte(c.key)); (err == nil) != c.pass {
+			t.Errorf("%+v %q: %v", c.r, c.key, err)
+		} else if err != nil && strings.Contains(err.Error(), c.key) {
+			t.Errorf("the error shows the key: %v", err)
+		}
+	}
+}
+
 func TestReadGarbled(t *testing.T) {
-	link, err := Seal(Header{Key: key}, pass, []File{{Path: ".env", Data: []byte("A=1")}})
+	link, err := Seal(Header{Key: key}, pass, []File{{Path: ".env", Data: []byte("A=1")}}, Rules{})
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/internal/ui/app/app.go b/internal/ui/app/app.go
index 2594352..c410c57 100644
--- a/internal/ui/app/app.go
+++ b/internal/ui/app/app.go
@@ -24,6 +24,7 @@ import (
 	"git.kebcloud.no/keb/lazysecrets/internal/commands"
 	"git.kebcloud.no/keb/lazysecrets/internal/provider"
 	"git.kebcloud.no/keb/lazysecrets/internal/ref"
+	"git.kebcloud.no/keb/lazysecrets/internal/share"
 	"git.kebcloud.no/keb/lazysecrets/internal/store"
 	"git.kebcloud.no/keb/lazysecrets/internal/ui/icons"
 	"git.kebcloud.no/keb/lazysecrets/internal/ui/keys"
@@ -84,6 +85,8 @@ type Options struct {
 	// ShareKey is the config's share.key: the secret that encrypts shared
 	// env files, for repos whose .lazysecrets.yaml names none.
 	ShareKey string
+	// ShareRules are what a key's value must be to share with it.
+	ShareRules share.Rules
 	// Dir is the folder lazysecrets runs in: it goes back to where it was
 	// left there, unless it was started on a link. "" doesn't.
 	Dir string
@@ -273,7 +276,8 @@ type Model struct {
 	linked     []provider.Container // vaults opened from links, this session
 	open       *ref.Ref             // reference to open at startup
 
-	shareKey string // the config's share.key
+	shareKey   string      // the config's share.key
+	shareRules share.Rules // and share.keyRules
 
 	// A lazysecrets:// link found on the clipboard, offered in the footer.
 	dismissedLink string // the last link offered: not offered again
@@ -460,7 +464,7 @@ func New(providers []provider.Provider, opts Options) Model {
 	m.spinning = true
 
 	m.cachePath, m.placeDir, m.sourcesAt = opts.CacheFile, opts.Dir, map[string]time.Time{}
-	m.shareKey = opts.ShareKey
+	m.shareKey, m.shareRules = opts.ShareKey, opts.ShareRules
 	m.configFiles, m.statePath, m.changelog = opts.ConfigFiles, opts.StatePath, changelogFields(opts.Changelog)
 	if m.cachePath != "" {
 		m.loadCacheFile()
diff --git a/internal/ui/app/sharefiles.go b/internal/ui/app/sharefiles.go
index a25388c..23d7089 100644
--- a/internal/ui/app/sharefiles.go
+++ b/internal/ui/app/sharefiles.go
@@ -158,14 +158,14 @@ func (m *Model) pickPasted(text string) tea.Cmd {
 
 // seal reads the key and encrypts the files with it.
 func (m *Model) seal(s sharing) tea.Cmd {
-	read := m.keyReader()
+	read, rules := m.keyReader(), m.shareRules
 	m.flash = flash{text: "encrypting with " + s.key.Name + "…"}
 	return func() tea.Msg {
 		v, version, err := read(s.key)
 		if err != nil {
 			return sealedMsg{err: fmt.Errorf("reading the key %s: %w", s.key.Name, err)}
 		}
-		link, err := share.Seal(share.Header{Key: share.Pin(s.key, version), Repo: s.repo.Detail}, []byte(v.Reveal()), s.files)
+		link, err := share.Seal(share.Header{Key: share.Pin(s.key, version), Repo: s.repo.Detail}, []byte(v.Reveal()), s.files, rules)
 		return sealedMsg{link: link, n: len(s.files), err: err}
 	}
 }
diff --git a/internal/ui/app/sharefiles_test.go b/internal/ui/app/sharefiles_test.go
index 5f99097..213c03c 100644
--- a/internal/ui/app/sharefiles_test.go
+++ b/internal/ui/app/sharefiles_test.go
@@ -12,12 +12,16 @@ import (
 
 	"git.kebcloud.no/keb/lazysecrets/internal/provider/fake"
 	"git.kebcloud.no/keb/lazysecrets/internal/provider/repo"
+	"git.kebcloud.no/keb/lazysecrets/internal/share"
 )
 
 // scryptWait is how long the driver waits for encrypting and decrypting,
 // which take about a second on purpose.
 const scryptWait = 20 * time.Second
 
+// demoRules take the demo key, made up to be remembered.
+var demoRules = share.Rules{MinLength: 16, MinClasses: 1}
+
 // cloneWith makes a clone of git.example.com/team/app holding files.
 func cloneWith(t *testing.T, files map[string]string) string {
 	t.Helper()
@@ -35,7 +39,7 @@ func cloneWith(t *testing.T, files map[string]string) string {
 func shareFrom(t *testing.T, root string, paste bool) string {
 	t.Helper()
 	clip := &fakeClipboard{}
-	d := newDriverOpts(t, Options{Clipboard: clip}, fake.Azure(0), repo.New(repo.Settings{Dir: root}))
+	d := newDriverOpts(t, Options{Clipboard: clip, ShareRules: demoRules}, fake.Azure(0), repo.New(repo.Settings{Dir: root}))
 	d.wait = scryptWait
 	if src, ok := d.m.selectedSource(); !ok || src.ID != root {
 		t.Fatalf("selected %v, want the repo", src.ID)