diff --git a/packages/backend/src/models/entities/meta.ts b/packages/backend/src/models/entities/meta.ts
index 0afb7c7fc..e026f04f7 100644
--- a/packages/backend/src/models/entities/meta.ts
+++ b/packages/backend/src/models/entities/meta.ts
@@ -77,6 +77,21 @@ export class Meta {
 	})
 	public blockedHosts: string[];
 
+	@Column('boolean', {
+		default: false
+	})
+	public secureMode: boolean;
+
+	@Column('boolean', {
+		default: false
+	})
+	public privateMode: boolean;
+
+	@Column('varchar', {
+		length: 256, array: true, default: '{}'
+	})
+	public allowedHosts: string[];
+
 	@Column('varchar', {
 		length: 512, array: true, default: '{/featured,/channels,/explore,/pages,/about-foundkey}',
 	})
diff --git a/packages/backend/src/queue/processors/deliver.ts b/packages/backend/src/queue/processors/deliver.ts
index 108379aa8..b937b18d5 100644
--- a/packages/backend/src/queue/processors/deliver.ts
+++ b/packages/backend/src/queue/processors/deliver.ts
@@ -28,6 +28,10 @@ export default async (job: Bull.Job<DeliverJobData>) => {
 		return 'skip (blocked)';
 	}
 
+	if (meta.privateMode && !meta.allowedHosts.includes(toPuny(host))) {
+		return 'skip (not allowed)';
+	}
+
 	// isSuspendedなら中断
 	let suspendedHosts = suspendedHostsCache.get(null);
 	if (suspendedHosts == null) {
diff --git a/packages/backend/src/queue/processors/inbox.ts b/packages/backend/src/queue/processors/inbox.ts
index 426713358..44b9e3f4e 100644
--- a/packages/backend/src/queue/processors/inbox.ts
+++ b/packages/backend/src/queue/processors/inbox.ts
@@ -39,6 +39,11 @@ export default async (job: Bull.Job<InboxJobData>): Promise<string> => {
 		return `Blocked request: ${host}`;
 	}
 
+	// Only permitted instances if in private mode.
+	if (meta.privateMode && !meta.allowedHosts.includes(host)) {
+		return `Blocked request: ${host}`;
+	}
+
 	const keyIdLower = signature.keyId.toLowerCase();
 	if (keyIdLower.startsWith('acct:')) {
 		return `Old keyId is no longer supported. ${keyIdLower}`;
diff --git a/packages/backend/src/remote/activitypub/check-fetch.ts b/packages/backend/src/remote/activitypub/check-fetch.ts
new file mode 100644
index 000000000..63103aa64
--- /dev/null
+++ b/packages/backend/src/remote/activitypub/check-fetch.ts
@@ -0,0 +1,70 @@
+import config from '@/config/index.js';
+import { IncomingMessage } from 'http';
+import { fetchMeta } from '@/misc/fetch-meta.js';
+import httpSignature from '@peertube/http-signature';
+import { URL } from 'url';
+import { toPuny } from '@/misc/convert-host.js';
+import DbResolver from '@/remote/activitypub/db-resolver.js';
+import { getApId } from '@/remote/activitypub/type.js';
+
+
+export default async function checkFetch(req: IncomingMessage): Promise<number> {
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		let signature;
+
+		try {
+			signature = httpSignature.parseRequest(req, { 'headers': [] });
+		} catch (e) {
+			return 401;
+		}
+
+		const keyId = new URL(signature.keyId);
+		const host = toPuny(keyId.hostname);
+
+		if (meta.blockedHosts.includes(host)) {
+			return 403;
+		}
+
+		if (meta.privateMode && host !== config.host && !meta.allowedHosts.includes(host)) {
+			return 403;
+		}
+
+		const keyIdLower = signature.keyId.toLowerCase();
+		if (keyIdLower.startsWith('acct:')) {
+			// Old keyId is no longer supported.
+			return 401;
+		}
+
+		const dbResolver = new DbResolver();
+
+		// Get user from database based on HTTP-Signature keyId
+		let authUser = await dbResolver.getAuthUserFromKeyId(signature.keyId);
+
+		// If keyid is unknown, try resolving it
+		if (authUser == null) {
+			try {
+				keyId.hash = '';
+				authUser = await dbResolver.getAuthUserFromApId(getApId(keyId.toString()));
+			} catch (e) {
+				return 403;
+			}
+		}
+
+		if (authUser?.key == null) {
+			return 403;
+		}
+
+		if (authUser.user.host !== host) {
+			return 403;
+		}
+
+		// HTTP-Signature validation
+		const httpSignatureValidated = httpSignature.verifySignature(signature, authUser.key.keyPem);
+
+		if (!httpSignatureValidated) {
+			return 403;
+		}
+	}
+	return 200;
+}
diff --git a/packages/backend/src/remote/activitypub/resolver.ts b/packages/backend/src/remote/activitypub/resolver.ts
index 3cea4c44e..26cf4c6df 100644
--- a/packages/backend/src/remote/activitypub/resolver.ts
+++ b/packages/backend/src/remote/activitypub/resolver.ts
@@ -72,7 +72,11 @@ export default class Resolver {
 			throw new Error('Instance is blocked');
 		}
 
-		if (!this.user) {
+		if (meta.privateMode && config.host !== host && !meta.allowedHosts.includes(host)) {
+			throw new Error('Instance is not allowed');
+		}
+
+		if (config.signToActivityPubGet && !this.user) {
 			this.user = await getInstanceActor();
 		}
 
diff --git a/packages/backend/src/server/activitypub.ts b/packages/backend/src/server/activitypub.ts
index f1a8f4914..dd66f29c4 100644
--- a/packages/backend/src/server/activitypub.ts
+++ b/packages/backend/src/server/activitypub.ts
@@ -9,11 +9,14 @@ import renderKey from '@/remote/activitypub/renderer/key.js';
 import { renderPerson } from '@/remote/activitypub/renderer/person.js';
 import renderEmoji from '@/remote/activitypub/renderer/emoji.js';
 import { inbox as processInbox } from '@/queue/index.js';
-import { isSelfHost } from '@/misc/convert-host.js';
+import { isSelfHost, toPuny } from '@/misc/convert-host.js';
 import { Notes, Users, Emojis, NoteReactions } from '@/models/index.js';
 import { ILocalUser, User } from '@/models/entities/user.js';
 import { renderLike } from '@/remote/activitypub/renderer/like.js';
 import { getUserKeypair } from '@/misc/keypair-store.js';
+import checkFetch from '@/remote/activitypub/check-fetch.js';
+import { getInstanceActor } from '@/services/instance-actor.js';
+import { fetchMeta } from '@/misc/fetch-meta.js';
 import renderFollow from '@/remote/activitypub/renderer/follow.js';
 import Outbox, { packActivity } from './activitypub/outbox.js';
 import Followers from './activitypub/followers.js';
@@ -66,6 +69,12 @@ router.post('/users/:user/inbox', json(), inbox);
 router.get('/notes/:note', async (ctx, next) => {
 	if (!isActivityPubReq(ctx)) return await next();
 
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const note = await Notes.findOneBy({
 		id: ctx.params.note,
 		visibility: In(['public' as const, 'home' as const]),
@@ -88,12 +97,24 @@ router.get('/notes/:note', async (ctx, next) => {
 	}
 
 	ctx.body = renderActivity(await renderNote(note, false));
-	ctx.set('Cache-Control', 'public, max-age=180');
+
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 	setResponseType(ctx);
 });
 
 // note activity
 router.get('/notes/:note/activity', async ctx => {
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const note = await Notes.findOneBy({
 		id: ctx.params.note,
 		userHost: IsNull(),
@@ -107,7 +128,12 @@ router.get('/notes/:note/activity', async ctx => {
 	}
 
 	ctx.body = renderActivity(await packActivity(note));
-	ctx.set('Cache-Control', 'public, max-age=180');
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 	setResponseType(ctx);
 });
 
@@ -125,6 +151,20 @@ router.get('/users/:user/collections/featured', Featured);
 
 // publickey
 router.get('/users/:user/publickey', async ctx => {
+	const instanceActor = await getInstanceActor();
+	if (ctx.params.user === instanceActor.id) {
+		ctx.body = renderActivity(renderKey(instanceActor, await getUserKeypair(instanceActor.id)));
+		ctx.set('Cache-Control', 'public, max-age=180');
+		setResponseType(ctx);
+		return;
+	}
+
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const userId = ctx.params.user;
 
 	const user = await Users.findOneBy({
@@ -141,7 +181,12 @@ router.get('/users/:user/publickey', async ctx => {
 
 	if (Users.isLocalUser(user)) {
 		ctx.body = renderActivity(renderKey(user, keypair));
-		ctx.set('Cache-Control', 'public, max-age=180');
+		const meta = await fetchMeta();
+		if (meta.secureMode || meta.privateMode) {
+			ctx.set('Cache-Control', 'no-store');
+		} else {
+			ctx.set('Cache-Control', 'public, max-age=180');
+		}
 		setResponseType(ctx);
 	} else {
 		ctx.status = 400;
@@ -156,13 +201,30 @@ async function userInfo(ctx: Router.RouterContext, user: User | null) {
 	}
 
 	ctx.body = renderActivity(await renderPerson(user as ILocalUser));
-	ctx.set('Cache-Control', 'public, max-age=180');
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 	setResponseType(ctx);
 }
 
 router.get('/users/:user', async (ctx, next) => {
 	if (!isActivityPubReq(ctx)) return await next();
 
+	const instanceActor = await getInstanceActor();
+	if (ctx.params.user === instanceActor.id) {
+		await userInfo(ctx, instanceActor);
+		return;
+	}
+
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const userId = ctx.params.user;
 
 	const user = await Users.findOneBy({
@@ -177,6 +239,18 @@ router.get('/users/:user', async (ctx, next) => {
 router.get('/@:user', async (ctx, next) => {
 	if (!isActivityPubReq(ctx)) return await next();
 
+	if (ctx.params.user === 'instance.actor') {
+		const instanceActor = await getInstanceActor();
+		await userInfo(ctx, instanceActor);
+		return;
+	}
+
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const user = await Users.findOneBy({
 		usernameLower: ctx.params.user.toLowerCase(),
 		host: IsNull(),
@@ -185,6 +259,11 @@ router.get('/@:user', async (ctx, next) => {
 
 	await userInfo(ctx, user);
 });
+
+router.get('/actor', async (ctx, next) => {
+	const instanceActor = await getInstanceActor();
+	await userInfo(ctx, instanceActor);
+});
 //#endregion
 
 // emoji
@@ -200,12 +279,23 @@ router.get('/emojis/:emoji', async ctx => {
 	}
 
 	ctx.body = renderActivity(await renderEmoji(emoji));
-	ctx.set('Cache-Control', 'public, max-age=180');
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 	setResponseType(ctx);
 });
 
 // like
 router.get('/likes/:like', async ctx => {
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const note = await Notes.findOneBy({
 		id: reaction.noteId,
 		visibility: In(['public' as const, 'home' as const]),
@@ -224,12 +314,22 @@ router.get('/likes/:like', async ctx => {
 	}
 
 	ctx.body = renderActivity(await renderLike(reaction, note));
-	ctx.set('Cache-Control', 'public, max-age=180');
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 	setResponseType(ctx);
 });
 
 // follow
 router.get('/follows/:follower/:followee', async ctx => {
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
 	// This may be used before the follow is completed, so we do not
 	// check if the following exists.
 
@@ -250,7 +350,12 @@ router.get('/follows/:follower/:followee', async ctx => {
 	}
 
 	ctx.body = renderActivity(renderFollow(follower, followee));
-	ctx.set('Cache-Control', 'public, max-age=180');
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 	setResponseType(ctx);
 });
 
diff --git a/packages/backend/src/server/activitypub/featured.ts b/packages/backend/src/server/activitypub/featured.ts
index 09906250f..6d3680798 100644
--- a/packages/backend/src/server/activitypub/featured.ts
+++ b/packages/backend/src/server/activitypub/featured.ts
@@ -6,8 +6,17 @@ import renderOrderedCollection from '@/remote/activitypub/renderer/ordered-colle
 import renderNote from '@/remote/activitypub/renderer/note.js';
 import { Users, Notes, UserNotePinings } from '@/models/index.js';
 import { setResponseType } from '../activitypub.js';
+import { IsNull } from 'typeorm';
+import checkFetch from '@/remote/activitypub/check-fetch.js';
+import { fetchMeta } from '@/misc/fetch-meta.js';
 
 export default async (ctx: Router.RouterContext) => {
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const userId = ctx.params.user;
 
 	const user = await Users.findOneBy({
@@ -36,6 +45,12 @@ export default async (ctx: Router.RouterContext) => {
 	);
 
 	ctx.body = renderActivity(rendered);
-	ctx.set('Cache-Control', 'public, max-age=180');
+
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 	setResponseType(ctx);
 };
diff --git a/packages/backend/src/server/activitypub/followers.ts b/packages/backend/src/server/activitypub/followers.ts
index beb48713a..3c8ea9458 100644
--- a/packages/backend/src/server/activitypub/followers.ts
+++ b/packages/backend/src/server/activitypub/followers.ts
@@ -9,8 +9,16 @@ import renderFollowUser from '@/remote/activitypub/renderer/follow-user.js';
 import { Users, Followings, UserProfiles } from '@/models/index.js';
 import { Following } from '@/models/entities/following.js';
 import { setResponseType } from '../activitypub.js';
+import checkFetch from '@/remote/activitypub/check-fetch.js';
+import { fetchMeta } from '@/misc/fetch-meta.js';
 
 export default async (ctx: Router.RouterContext) => {
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const userId = ctx.params.user;
 
 	const cursor = ctx.request.query.cursor;
@@ -89,7 +97,12 @@ export default async (ctx: Router.RouterContext) => {
 		// index page
 		const rendered = renderOrderedCollection(partOf, user.followersCount, `${partOf}?page=true`);
 		ctx.body = renderActivity(rendered);
-		ctx.set('Cache-Control', 'public, max-age=180');
 		setResponseType(ctx);
 	}
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 };
diff --git a/packages/backend/src/server/activitypub/following.ts b/packages/backend/src/server/activitypub/following.ts
index 3a25a6316..836cd4d26 100644
--- a/packages/backend/src/server/activitypub/following.ts
+++ b/packages/backend/src/server/activitypub/following.ts
@@ -9,8 +9,16 @@ import renderFollowUser from '@/remote/activitypub/renderer/follow-user.js';
 import { Users, Followings, UserProfiles } from '@/models/index.js';
 import { Following } from '@/models/entities/following.js';
 import { setResponseType } from '../activitypub.js';
+import checkFetch from '@/remote/activitypub/check-fetch.js';
+import { fetchMeta } from '@/misc/fetch-meta.js';
 
 export default async (ctx: Router.RouterContext) => {
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const userId = ctx.params.user;
 
 	const cursor = ctx.request.query.cursor;
@@ -89,7 +97,12 @@ export default async (ctx: Router.RouterContext) => {
 		// index page
 		const rendered = renderOrderedCollection(partOf, user.followingCount, `${partOf}?page=true`);
 		ctx.body = renderActivity(rendered);
-		ctx.set('Cache-Control', 'public, max-age=180');
 		setResponseType(ctx);
 	}
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 };
diff --git a/packages/backend/src/server/activitypub/outbox.ts b/packages/backend/src/server/activitypub/outbox.ts
index d88b6c899..5dfaff366 100644
--- a/packages/backend/src/server/activitypub/outbox.ts
+++ b/packages/backend/src/server/activitypub/outbox.ts
@@ -14,8 +14,16 @@ import { Note } from '@/models/entities/note.js';
 import { isPureRenote } from '@/misc/renote.js';
 import { makePaginationQuery } from '../api/common/make-pagination-query.js';
 import { setResponseType } from '../activitypub.js';
+import checkFetch from '@/remote/activitypub/check-fetch.js';
+import { fetchMeta } from '@/misc/fetch-meta.js';
 
 export default async (ctx: Router.RouterContext) => {
+	const verify = await checkFetch(ctx.req);
+	if (verify != 200) {
+		ctx.status = verify;
+		return;
+	}
+
 	const userId = ctx.params.user;
 
 	const sinceId = ctx.request.query.since_id;
@@ -90,9 +98,15 @@ export default async (ctx: Router.RouterContext) => {
 			`${partOf}?page=true&since_id=000000000000000000000000`,
 		);
 		ctx.body = renderActivity(rendered);
-		ctx.set('Cache-Control', 'public, max-age=180');
+
 		setResponseType(ctx);
 	}
+	const meta = await fetchMeta();
+	if (meta.secureMode || meta.privateMode) {
+		ctx.set('Cache-Control', 'no-store');
+	} else {
+		ctx.set('Cache-Control', 'public, max-age=180');
+	}
 };
 
 /**
diff --git a/packages/backend/src/server/api/endpoints.ts b/packages/backend/src/server/api/endpoints.ts
index e7563555b..c9052ed9a 100644
--- a/packages/backend/src/server/api/endpoints.ts
+++ b/packages/backend/src/server/api/endpoints.ts
@@ -692,6 +692,12 @@ export interface IEndpointMeta {
 	 */
 	readonly secure?: boolean;
 
+	/**
+	 * If in private mode, whether credentials are required when making a request to this endpoint.
+	 * If omitted, this is interpreted as false.
+	 */
+	readonly requireCredentialPrivateMode?: boolean;
+
 	/**
 	 * エンドポイントの種類
 	 * パーミッションの実現に利用されます。
diff --git a/packages/backend/src/server/api/endpoints/meta.ts b/packages/backend/src/server/api/endpoints/meta.ts
index cbf74ef04..205d0916f 100644
--- a/packages/backend/src/server/api/endpoints/meta.ts
+++ b/packages/backend/src/server/api/endpoints/meta.ts
@@ -249,6 +249,16 @@ export const meta = {
 					},
 				},
 			},
+			secureMode: {
+				type: 'boolean',
+				optional: true, nullable: false,
+				default: false,
+			},
+			privateMode: {
+				type: 'boolean',
+				optional: true, nullable: false,
+				default: false,
+			},
 		},
 	},
 } as const;