diff --git a/internal/app/playlist/share.go b/internal/app/playlist/share.go
new file mode 100644
index 0000000..e097cde
--- /dev/null
+++ b/internal/app/playlist/share.go
@@ -0,0 +1,73 @@
+package playlist
+
+import (
+	"context"
+	"crypto/rand"
+	"encoding/base64"
+	"errors"
+
+	"github.com/dihemoe/alisten/internal/app/port"
+)
+
+// 分享 ID 是 128 位随机值，拿到链接的人才能打开，无法猜出别人的分享。
+const shareIDBytes = 16
+
+var shareIDLength = base64.RawURLEncoding.EncodedLen(shareIDBytes)
+
+// 开启分享和重新生成是同一个操作：每次都换一个新的分享 ID，旧链接随即失效。
+func (s *Service) Share(ctx context.Context, memberID, id string) (string, error) {
+	if err := s.registered(ctx, memberID); err != nil {
+		return "", err
+	}
+	b := make([]byte, shareIDBytes)
+	// crypto/rand.Read 不会返回错误。
+	rand.Read(b)
+	shareID := base64.RawURLEncoding.EncodeToString(b)
+	if err := s.playlists.SetPlaylistShare(ctx, memberID, id, shareID); err != nil {
+		return "", err
+	}
+	return shareID, nil
+}
+
+func (s *Service) Unshare(ctx context.Context, memberID, id string) error {
+	if err := s.registered(ctx, memberID); err != nil {
+		return err
+	}
+	return s.playlists.SetPlaylistShare(ctx, memberID, id, "")
+}
+
+// 不需要身份。
+func (s *Service) Shared(ctx context.Context, shareID string) (port.SharedPlaylist, error) {
+	if !validShareID(shareID) {
+		return port.SharedPlaylist{}, port.ErrShareNotFound
+	}
+	return s.playlists.SharedPlaylist(ctx, shareID)
+}
+
+func (s *Service) CopyShared(ctx context.Context, memberID, shareID, name string) (port.PlaylistRecord, error) {
+	if err := s.registered(ctx, memberID); err != nil {
+		return port.PlaylistRecord{}, err
+	}
+	name, err := validName(name)
+	if err != nil {
+		return port.PlaylistRecord{}, err
+	}
+	if !validShareID(shareID) {
+		return port.PlaylistRecord{}, port.ErrShareNotFound
+	}
+	// 来源歌单本身不超过曲目上限，复制出的也不会超过。
+	p, err := s.playlists.CopySharedPlaylist(ctx, memberID, shareID, name, maxPlaylists)
+	if errors.Is(err, port.ErrPlaylistLimit) {
+		return port.PlaylistRecord{}, ErrTooManyPlaylists
+	}
+	return p, err
+}
+
+// 分享 ID 取自地址，可能是任意文本：不像生成出来的就不必查存储，NUL 之类的字符也就不会到达数据库。
+func validShareID(id string) bool {
+	if len(id) != shareIDLength {
+		return false
+	}
+	_, err := base64.RawURLEncoding.DecodeString(id)
+	return err == nil
+}
diff --git a/internal/app/playlist/share_test.go b/internal/app/playlist/share_test.go
new file mode 100644
index 0000000..11c868f
--- /dev/null
+++ b/internal/app/playlist/share_test.go
@@ -0,0 +1,186 @@
+package playlist
+
+import (
+	"context"
+	"errors"
+	"reflect"
+	"regexp"
+	"slices"
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/dihemoe/alisten/internal/app/port"
+	"github.com/dihemoe/alisten/internal/app/port/porttest"
+	"github.com/dihemoe/alisten/internal/domain/track"
+)
+
+var shareIDPattern = regexp.MustCompile(`^[A-Za-z0-9_-]{22}$`)
+
+func TestGuestCannotShareOrCopy(t *testing.T) {
+	f := newFixture(t)
+	guest := f.member(t, "")
+	alice := f.member(t, "alice")
+	favorites := f.favorites(t, alice.ID)
+	if _, err := f.svc.Share(t.Context(), guest.ID, favorites); !errors.Is(err, ErrNotRegistered) {
+		t.Errorf("游客开启分享 = %v，期望 ErrNotRegistered", err)
+	}
+	if err := f.svc.Unshare(t.Context(), guest.ID, favorites); !errors.Is(err, ErrNotRegistered) {
+		t.Errorf("游客停止分享 = %v，期望 ErrNotRegistered", err)
+	}
+	shareID, err := f.svc.Share(t.Context(), alice.ID, favorites)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.svc.CopyShared(t.Context(), guest.ID, shareID, "抄来的"); !errors.Is(err, ErrNotRegistered) {
+		t.Errorf("游客复制 = %v，期望 ErrNotRegistered", err)
+	}
+	if _, err := f.svc.Shared(t.Context(), shareID); err != nil {
+		t.Errorf("不需要身份就能读取：%v", err)
+	}
+	if len(f.quota.keys) != 0 {
+		t.Errorf("分享和复制不应消耗额度：%v", f.quota.keys)
+	}
+}
+
+func TestShareGeneratesANewLinkEachTime(t *testing.T) {
+	f := newFixture(t)
+	alice := f.member(t, "alice")
+	bob := f.member(t, "bob")
+	favorites := f.favorites(t, alice.ID)
+	item, err := f.svc.Add(t.Context(), alice.ID, favorites, sunny)
+	if err != nil {
+		t.Fatal(err)
+	}
+	first, err := f.svc.Share(t.Context(), alice.ID, favorites)
+	if err != nil || !shareIDPattern.MatchString(first) {
+		t.Fatalf("分享 ID = %q, %v，期望 22 个 base64url 字符", first, err)
+	}
+	shared, err := f.svc.Shared(t.Context(), first)
+	want := port.SharedPlaylist{Name: "我喜欢", Favorites: true, OwnerName: alice.Name, Items: []port.PlaylistItem{item}}
+	if err != nil || !reflect.DeepEqual(shared, want) {
+		t.Errorf("分享的歌单 = %#v, %v，期望 %#v", shared, err, want)
+	}
+	if lists, err := f.svc.List(t.Context(), alice.ID); err != nil || lists[0].ShareID != first {
+		t.Errorf("歌单 = %#v, %v，期望带上分享 ID", lists, err)
+	}
+
+	second, err := f.svc.Share(t.Context(), alice.ID, favorites)
+	if err != nil || !shareIDPattern.MatchString(second) || second == first {
+		t.Fatalf("重新生成 = %q, %v，期望换一个新的分享 ID", second, err)
+	}
+	if _, err := f.svc.Shared(t.Context(), first); !errors.Is(err, port.ErrShareNotFound) {
+		t.Errorf("旧链接 = %v，期望 ErrShareNotFound", err)
+	}
+	if _, err := f.svc.Shared(t.Context(), second); err != nil {
+		t.Errorf("新链接 = %v", err)
+	}
+
+	if _, err := f.svc.Share(t.Context(), bob.ID, favorites); !errors.Is(err, port.ErrPlaylistNotFound) {
+		t.Errorf("分享别人的歌单 = %v，期望 ErrPlaylistNotFound", err)
+	}
+	if err := f.svc.Unshare(t.Context(), bob.ID, favorites); !errors.Is(err, port.ErrPlaylistNotFound) {
+		t.Errorf("停止别人的分享 = %v，期望 ErrPlaylistNotFound", err)
+	}
+	if err := f.svc.Unshare(t.Context(), alice.ID, favorites); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.svc.Shared(t.Context(), second); !errors.Is(err, port.ErrShareNotFound) {
+		t.Errorf("停止分享之后 = %v，期望 ErrShareNotFound", err)
+	}
+}
+
+type shareLookups struct {
+	*porttest.Store
+	lookups int
+}
+
+func (s *shareLookups) SharedPlaylist(ctx context.Context, shareID string) (port.SharedPlaylist, error) {
+	s.lookups++
+	return s.Store.SharedPlaylist(ctx, shareID)
+}
+
+func (s *shareLookups) CopySharedPlaylist(ctx context.Context, ownerID, shareID, name string, limit int) (port.PlaylistRecord, error) {
+	s.lookups++
+	return s.Store.CopySharedPlaylist(ctx, ownerID, shareID, name, limit)
+}
+
+func TestMalformedShareIDsAreNotLookedUp(t *testing.T) {
+	f := newFixture(t)
+	alice := f.member(t, "alice")
+	lookups := &shareLookups{Store: f.store}
+	f.svc.playlists = lookups
+	valid := strings.Repeat("a", 21)
+	for _, id := range []string{"", "abc", valid, valid + "aa", valid + "\x00", valid + "=", valid + "+", valid + "/", "\xff" + valid} {
+		if _, err := f.svc.Shared(t.Context(), id); !errors.Is(err, port.ErrShareNotFound) {
+			t.Errorf("读取 %q = %v，期望 ErrShareNotFound", id, err)
+		}
+		if _, err := f.svc.CopyShared(t.Context(), alice.ID, id, "抄来的"); !errors.Is(err, port.ErrShareNotFound) {
+			t.Errorf("复制 %q = %v，期望 ErrShareNotFound", id, err)
+		}
+	}
+	if lookups.lookups != 0 {
+		t.Errorf("格式不对的分享 ID 查了 %d 次存储，期望不查", lookups.lookups)
+	}
+	if _, err := f.svc.Shared(t.Context(), valid+"a"); !errors.Is(err, port.ErrShareNotFound) || lookups.lookups != 1 {
+		t.Errorf("格式正确却不存在的分享 = %v，查了 %d 次，期望查一次后 ErrShareNotFound", err, lookups.lookups)
+	}
+}
+
+func TestCopySharedPlaylist(t *testing.T) {
+	f := newFixture(t)
+	alice := f.member(t, "alice")
+	bob := f.member(t, "bob")
+	rain := netease("185811")
+	f.netease.Add(rain.ID, track.Meta{Title: "七里香", Artist: "周杰伦", PageURL: "https://music.163.com/#/song?id=185811"}, "", nil)
+	commute, err := f.svc.Create(t.Context(), alice.ID, "通勤")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, song := range []track.Track{sunny, rain} {
+		if _, err := f.svc.Add(t.Context(), alice.ID, commute.ID, song); err != nil {
+			t.Fatal(err)
+		}
+	}
+	shareID, err := f.svc.Share(t.Context(), alice.ID, commute.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	keys := len(f.quota.keys)
+
+	copied, err := f.svc.CopyShared(t.Context(), bob.ID, shareID, "  抄来的  ")
+	if err != nil || copied.Name != "抄来的" || copied.TrackCount != 2 || copied.ShareID != "" {
+		t.Fatalf("复制 = %#v, %v，期望去掉名称首尾的空白、带 2 首、没有分享", copied, err)
+	}
+	got, err := f.svc.Items(t.Context(), bob.ID, copied.ID)
+	original, _ := f.svc.Items(t.Context(), alice.ID, commute.ID)
+	if err != nil || !slices.Equal(got, original) {
+		t.Errorf("复制的曲目 = %#v, %v，期望与原歌单相同 %#v", got, err, original)
+	}
+	if len(f.quota.keys) != keys {
+		t.Errorf("复制不应消耗额度：%v", f.quota.keys[keys:])
+	}
+	if _, err := f.svc.CopyShared(t.Context(), bob.ID, shareID, "   "); !errors.Is(err, ErrInvalidName) {
+		t.Errorf("名称为空白 = %v，期望 ErrInvalidName", err)
+	}
+
+	if err := f.svc.Unshare(t.Context(), alice.ID, commute.ID); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.svc.CopyShared(t.Context(), bob.ID, shareID, "抄来的"); !errors.Is(err, port.ErrShareNotFound) {
+		t.Errorf("停止分享之后复制 = %v，期望 ErrShareNotFound", err)
+	}
+
+	shareID, err = f.svc.Share(t.Context(), alice.ID, commute.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := range 48 {
+		if _, err := f.svc.Create(t.Context(), bob.ID, "歌单"+strconv.Itoa(i)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if _, err := f.svc.CopyShared(t.Context(), bob.ID, shareID, "第 51 个"); !errors.Is(err, ErrTooManyPlaylists) {
+		t.Errorf("第 51 个歌单 = %v，期望 ErrTooManyPlaylists", err)
+	}
+}