diff --git a/src/hooks.client.ts b/src/hooks.client.ts
index 99e11c9..a677d9f 100644
--- a/src/hooks.client.ts
+++ b/src/hooks.client.ts
@@ -1,7 +1,6 @@
 import type { ClientInit } from "@sveltejs/kit";
 import { cleanupDanglingInfos, getClientKey, getMasterKeys, getHmacSecrets } from "$lib/indexedDB";
 import { prepareFileCache } from "$lib/modules/file";
-import { prepareOpfs } from "$lib/modules/opfs";
 import { clientKeyStore, masterKeyStore, hmacSecretStore } from "$lib/stores";
 
 const requestPersistentStorage = async () => {
@@ -46,7 +45,6 @@ export const init: ClientInit = async () => {
     prepareClientKeyStore(),
     prepareMasterKeyStore(),
     prepareHmacSecretStore(),
-    prepareOpfs(),
   ]);
 
   cleanupDanglingInfos(); // Intended
diff --git a/src/lib/constants/index.ts b/src/lib/constants/index.ts
index ab6125a..4983846 100644
--- a/src/lib/constants/index.ts
+++ b/src/lib/constants/index.ts
@@ -1 +1,2 @@
+export * from "./serviceWorker";
 export * from "./upload";
diff --git a/src/lib/constants/serviceWorker.ts b/src/lib/constants/serviceWorker.ts
new file mode 100644
index 0000000..8c09d05
--- /dev/null
+++ b/src/lib/constants/serviceWorker.ts
@@ -0,0 +1 @@
+export const DECRYPTED_FILE_URL_PREFIX = "/_internal/decrypted-file/";
diff --git a/src/lib/constants/upload.ts b/src/lib/constants/upload.ts
index 337700d..99d94bb 100644
--- a/src/lib/constants/upload.ts
+++ b/src/lib/constants/upload.ts
@@ -1,5 +1,6 @@
-export const CHUNK_SIZE = 4 * 1024 * 1024;
-
 export const AES_GCM_IV_SIZE = 12;
 export const AES_GCM_TAG_SIZE = 16;
 export const ENCRYPTION_OVERHEAD = AES_GCM_IV_SIZE + AES_GCM_TAG_SIZE;
+
+export const CHUNK_SIZE = 4 * 1024 * 1024;
+export const ENCRYPTED_CHUNK_SIZE = CHUNK_SIZE + ENCRYPTION_OVERHEAD;
diff --git a/src/lib/modules/crypto/aes.ts b/src/lib/modules/crypto/aes.ts
index 67f6a9f..fe11afb 100644
--- a/src/lib/modules/crypto/aes.ts
+++ b/src/lib/modules/crypto/aes.ts
@@ -9,7 +9,7 @@ import {
 
 export const generateMasterKey = async () => {
   return {
-    masterKey: await window.crypto.subtle.generateKey(
+    masterKey: await crypto.subtle.generateKey(
       {
         name: "AES-KW",
         length: 256,
@@ -22,7 +22,7 @@ export const generateMasterKey = async () => {
 
 export const generateDataKey = async () => {
   return {
-    dataKey: await window.crypto.subtle.generateKey(
+    dataKey: await crypto.subtle.generateKey(
       {
         name: "AES-GCM",
         length: 256,
@@ -35,9 +35,9 @@ export const generateDataKey = async () => {
 };
 
 export const makeAESKeyNonextractable = async (key: CryptoKey) => {
-  return await window.crypto.subtle.importKey(
+  return await crypto.subtle.importKey(
     "raw",
-    await window.crypto.subtle.exportKey("raw", key),
+    await crypto.subtle.exportKey("raw", key),
     key.algorithm,
     false,
     key.usages,
@@ -45,12 +45,12 @@ export const makeAESKeyNonextractable = async (key: CryptoKey) => {
 };
 
 export const wrapDataKey = async (dataKey: CryptoKey, masterKey: CryptoKey) => {
-  return encodeToBase64(await window.crypto.subtle.wrapKey("raw", dataKey, masterKey, "AES-KW"));
+  return encodeToBase64(await crypto.subtle.wrapKey("raw", dataKey, masterKey, "AES-KW"));
 };
 
 export const unwrapDataKey = async (dataKeyWrapped: string, masterKey: CryptoKey) => {
   return {
-    dataKey: await window.crypto.subtle.unwrapKey(
+    dataKey: await crypto.subtle.unwrapKey(
       "raw",
       decodeFromBase64(dataKeyWrapped),
       masterKey,
@@ -63,12 +63,12 @@ export const unwrapDataKey = async (dataKeyWrapped: string, masterKey: CryptoKey
 };
 
 export const wrapHmacSecret = async (hmacSecret: CryptoKey, masterKey: CryptoKey) => {
-  return encodeToBase64(await window.crypto.subtle.wrapKey("raw", hmacSecret, masterKey, "AES-KW"));
+  return encodeToBase64(await crypto.subtle.wrapKey("raw", hmacSecret, masterKey, "AES-KW"));
 };
 
 export const unwrapHmacSecret = async (hmacSecretWrapped: string, masterKey: CryptoKey) => {
   return {
-    hmacSecret: await window.crypto.subtle.unwrapKey(
+    hmacSecret: await crypto.subtle.unwrapKey(
       "raw",
       decodeFromBase64(hmacSecretWrapped),
       masterKey,
@@ -84,8 +84,8 @@ export const unwrapHmacSecret = async (hmacSecretWrapped: string, masterKey: Cry
 };
 
 export const encryptData = async (data: BufferSource, dataKey: CryptoKey) => {
-  const iv = window.crypto.getRandomValues(new Uint8Array(12));
-  const ciphertext = await window.crypto.subtle.encrypt(
+  const iv = crypto.getRandomValues(new Uint8Array(12));
+  const ciphertext = await crypto.subtle.encrypt(
     {
       name: "AES-GCM",
       iv,
@@ -101,7 +101,7 @@ export const decryptData = async (
   iv: string | BufferSource,
   dataKey: CryptoKey,
 ) => {
-  return await window.crypto.subtle.decrypt(
+  return await crypto.subtle.decrypt(
     {
       name: "AES-GCM",
       iv: typeof iv === "string" ? decodeFromBase64(iv) : iv,
diff --git a/src/lib/modules/crypto/rsa.ts b/src/lib/modules/crypto/rsa.ts
index 13dfd46..11e136f 100644
--- a/src/lib/modules/crypto/rsa.ts
+++ b/src/lib/modules/crypto/rsa.ts
@@ -1,7 +1,7 @@
 import { encodeString, encodeToBase64, decodeFromBase64 } from "./util";
 
 export const generateEncryptionKeyPair = async () => {
-  const keyPair = await window.crypto.subtle.generateKey(
+  const keyPair = await crypto.subtle.generateKey(
     {
       name: "RSA-OAEP",
       modulusLength: 4096,
@@ -18,7 +18,7 @@ export const generateEncryptionKeyPair = async () => {
 };
 
 export const generateSigningKeyPair = async () => {
-  const keyPair = await window.crypto.subtle.generateKey(
+  const keyPair = await crypto.subtle.generateKey(
     {
       name: "RSA-PSS",
       modulusLength: 4096,
@@ -37,7 +37,7 @@ export const generateSigningKeyPair = async () => {
 export const exportRSAKey = async (key: CryptoKey) => {
   const format = key.type === "public" ? ("spki" as const) : ("pkcs8" as const);
   return {
-    key: await window.crypto.subtle.exportKey(format, key),
+    key: await crypto.subtle.exportKey(format, key),
     format,
   };
 };
@@ -54,14 +54,14 @@ export const importEncryptionKeyPairFromBase64 = async (
     name: "RSA-OAEP",
     hash: "SHA-256",
   };
-  const encryptKey = await window.crypto.subtle.importKey(
+  const encryptKey = await crypto.subtle.importKey(
     "spki",
     decodeFromBase64(encryptKeyBase64),
     algorithm,
     true,
     ["encrypt", "wrapKey"],
   );
-  const decryptKey = await window.crypto.subtle.importKey(
+  const decryptKey = await crypto.subtle.importKey(
     "pkcs8",
     decodeFromBase64(decryptKeyBase64),
     algorithm,
@@ -79,14 +79,14 @@ export const importSigningKeyPairFromBase64 = async (
     name: "RSA-PSS",
     hash: "SHA-256",
   };
-  const signKey = await window.crypto.subtle.importKey(
+  const signKey = await crypto.subtle.importKey(
     "pkcs8",
     decodeFromBase64(signKeyBase64),
     algorithm,
     true,
     ["sign"],
   );
-  const verifyKey = await window.crypto.subtle.importKey(
+  const verifyKey = await crypto.subtle.importKey(
     "spki",
     decodeFromBase64(verifyKeyBase64),
     algorithm,
@@ -98,17 +98,11 @@ export const importSigningKeyPairFromBase64 = async (
 
 export const makeRSAKeyNonextractable = async (key: CryptoKey) => {
   const { key: exportedKey, format } = await exportRSAKey(key);
-  return await window.crypto.subtle.importKey(
-    format,
-    exportedKey,
-    key.algorithm,
-    false,
-    key.usages,
-  );
+  return await crypto.subtle.importKey(format, exportedKey, key.algorithm, false, key.usages);
 };
 
 export const decryptChallenge = async (challenge: string, decryptKey: CryptoKey) => {
-  return await window.crypto.subtle.decrypt(
+  return await crypto.subtle.decrypt(
     {
       name: "RSA-OAEP",
     } satisfies RsaOaepParams,
@@ -119,7 +113,7 @@ export const decryptChallenge = async (challenge: string, decryptKey: CryptoKey)
 
 export const wrapMasterKey = async (masterKey: CryptoKey, encryptKey: CryptoKey) => {
   return encodeToBase64(
-    await window.crypto.subtle.wrapKey("raw", masterKey, encryptKey, {
+    await crypto.subtle.wrapKey("raw", masterKey, encryptKey, {
       name: "RSA-OAEP",
     } satisfies RsaOaepParams),
   );
@@ -131,7 +125,7 @@ export const unwrapMasterKey = async (
   extractable = false,
 ) => {
   return {
-    masterKey: await window.crypto.subtle.unwrapKey(
+    masterKey: await crypto.subtle.unwrapKey(
       "raw",
       decodeFromBase64(masterKeyWrapped),
       decryptKey,
@@ -146,7 +140,7 @@ export const unwrapMasterKey = async (
 };
 
 export const signMessageRSA = async (message: BufferSource, signKey: CryptoKey) => {
-  return await window.crypto.subtle.sign(
+  return await crypto.subtle.sign(
     {
       name: "RSA-PSS",
       saltLength: 32, // SHA-256
@@ -161,7 +155,7 @@ export const verifySignatureRSA = async (
   signature: BufferSource,
   verifyKey: CryptoKey,
 ) => {
-  return await window.crypto.subtle.verify(
+  return await crypto.subtle.verify(
     {
       name: "RSA-PSS",
       saltLength: 32, // SHA-256
diff --git a/src/lib/modules/crypto/sha.ts b/src/lib/modules/crypto/sha.ts
index 3acb258..9bf2dea 100644
--- a/src/lib/modules/crypto/sha.ts
+++ b/src/lib/modules/crypto/sha.ts
@@ -1,10 +1,10 @@
 export const digestMessage = async (message: BufferSource) => {
-  return await window.crypto.subtle.digest("SHA-256", message);
+  return await crypto.subtle.digest("SHA-256", message);
 };
 
 export const generateHmacSecret = async () => {
   return {
-    hmacSecret: await window.crypto.subtle.generateKey(
+    hmacSecret: await crypto.subtle.generateKey(
       {
         name: "HMAC",
         hash: "SHA-256",
@@ -16,5 +16,5 @@ export const generateHmacSecret = async () => {
 };
 
 export const signMessageHmac = async (message: BufferSource, hmacSecret: CryptoKey) => {
-  return await window.crypto.subtle.sign("HMAC", hmacSecret, message);
+  return await crypto.subtle.sign("HMAC", hmacSecret, message);
 };
diff --git a/src/lib/server/modules/http.ts b/src/lib/modules/http.ts
similarity index 100%
rename from src/lib/server/modules/http.ts
rename to src/lib/modules/http.ts
diff --git a/src/lib/modules/opfs.ts b/src/lib/modules/opfs.ts
index 41f1f72..a367aae 100644
--- a/src/lib/modules/opfs.ts
+++ b/src/lib/modules/opfs.ts
@@ -1,13 +1,5 @@
-let rootHandle: FileSystemDirectoryHandle | null = null;
-
-export const prepareOpfs = async () => {
-  rootHandle = await navigator.storage.getDirectory();
-};
-
 const getFileHandle = async (path: string, create = true) => {
-  if (!rootHandle) {
-    throw new Error("OPFS not prepared");
-  } else if (path[0] !== "/") {
+  if (path[0] !== "/") {
     throw new Error("Path must be absolute");
   }
 
@@ -17,7 +9,7 @@ const getFileHandle = async (path: string, create = true) => {
   }
 
   try {
-    let directoryHandle = rootHandle;
+    let directoryHandle = await navigator.storage.getDirectory();
     for (const part of parts.slice(0, -1)) {
       if (!part) continue;
       directoryHandle = await directoryHandle.getDirectoryHandle(part, { create });
@@ -34,12 +26,15 @@ const getFileHandle = async (path: string, create = true) => {
   }
 };
 
-export const readFile = async (path: string) => {
+export const getFile = async (path: string) => {
   const { fileHandle } = await getFileHandle(path, false);
   if (!fileHandle) return null;
 
-  const file = await fileHandle.getFile();
-  return await file.arrayBuffer();
+  return await fileHandle.getFile();
+};
+
+export const readFile = async (path: string) => {
+  return (await getFile(path))?.arrayBuffer() ?? null;
 };
 
 export const writeFile = async (path: string, data: ArrayBuffer) => {
@@ -61,9 +56,7 @@ export const deleteFile = async (path: string) => {
 };
 
 const getDirectoryHandle = async (path: string) => {
-  if (!rootHandle) {
-    throw new Error("OPFS not prepared");
-  } else if (path[0] !== "/") {
+  if (path[0] !== "/") {
     throw new Error("Path must be absolute");
   }
 
@@ -73,7 +66,7 @@ const getDirectoryHandle = async (path: string) => {
   }
 
   try {
-    let directoryHandle = rootHandle;
+    let directoryHandle = await navigator.storage.getDirectory();
     let parentHandle;
     for (const part of parts.slice(1)) {
       if (!part) continue;
diff --git a/src/lib/serviceWorker/client.ts b/src/lib/serviceWorker/client.ts
new file mode 100644
index 0000000..771c15e
--- /dev/null
+++ b/src/lib/serviceWorker/client.ts
@@ -0,0 +1,39 @@
+import { DECRYPTED_FILE_URL_PREFIX } from "$lib/constants";
+import type { FileMetadata, ServiceWorkerMessage, ServiceWorkerResponse } from "./types";
+
+const PREPARE_TIMEOUT_MS = 5000;
+
+const getServiceWorker = async () => {
+  const registration = await navigator.serviceWorker.ready;
+  const sw = registration.active;
+  if (!sw) {
+    throw new Error("Service worker not activated");
+  }
+  return sw;
+};
+
+export const prepareFileDecryption = async (id: number, metadata: FileMetadata) => {
+  const sw = await getServiceWorker();
+  return new Promise<void>((resolve, reject) => {
+    const timeout = setTimeout(
+      () => reject(new Error("Service worker timeout")),
+      PREPARE_TIMEOUT_MS,
+    );
+    const handler = (event: MessageEvent<ServiceWorkerResponse>) => {
+      if (event.data.type === "decryption-ready" && event.data.fileId === id) {
+        clearTimeout(timeout);
+        navigator.serviceWorker.removeEventListener("message", handler);
+        resolve();
+      }
+    };
+    navigator.serviceWorker.addEventListener("message", handler);
+
+    sw.postMessage({
+      type: "decryption-prepare",
+      fileId: id,
+      ...metadata,
+    } satisfies ServiceWorkerMessage);
+  });
+};
+
+export const getDecryptedFileUrl = (id: number) => `${DECRYPTED_FILE_URL_PREFIX}${id}`;
diff --git a/src/lib/serviceWorker/index.ts b/src/lib/serviceWorker/index.ts
new file mode 100644
index 0000000..d2ec230
--- /dev/null
+++ b/src/lib/serviceWorker/index.ts
@@ -0,0 +1,2 @@
+export * from "./client";
+export * from "./types";
diff --git a/src/lib/serviceWorker/types.ts b/src/lib/serviceWorker/types.ts
new file mode 100644
index 0000000..97edd6d
--- /dev/null
+++ b/src/lib/serviceWorker/types.ts
@@ -0,0 +1,19 @@
+export interface FileMetadata {
+  isLegacy: boolean;
+  dataKey: CryptoKey;
+  encContentSize: number;
+  contentType: string;
+}
+
+export interface DecryptionPrepareMessage extends FileMetadata {
+  type: "decryption-prepare";
+  fileId: number;
+}
+
+export interface DecryptionReadyMessage {
+  type: "decryption-ready";
+  fileId: number;
+}
+
+export type ServiceWorkerMessage = DecryptionPrepareMessage;
+export type ServiceWorkerResponse = DecryptionReadyMessage;
diff --git a/src/routes/(fullscreen)/file/[id]/+page.svelte b/src/routes/(fullscreen)/file/[id]/+page.svelte
index 4aa6b42..674bc22 100644
--- a/src/routes/(fullscreen)/file/[id]/+page.svelte
+++ b/src/routes/(fullscreen)/file/[id]/+page.svelte
@@ -17,6 +17,7 @@
     requestFileDownload,
     requestThumbnailUpload,
     requestFileAdditionToCategory,
+    requestVideoStream,
   } from "./service";
   import TopBarMenu from "./TopBarMenu.svelte";
 
@@ -37,6 +38,7 @@
   let viewerType: "image" | "video" | undefined = $state();
   let fileBlob: Blob | undefined = $state();
   let fileBlobUrl: string | undefined = $state();
+  let videoStreamUrl: string | undefined = $state();
   let videoElement: HTMLVideoElement | undefined = $state();
 
   const updateViewer = async (buffer: ArrayBuffer, contentType: string) => {
@@ -95,12 +97,27 @@
       untrack(() => {
         if (!downloadState && !isDownloadRequested) {
           isDownloadRequested = true;
-          requestFileDownload(data.id, info!.dataKey!.key, info!.isLegacy!).then(async (buffer) => {
-            const blob = await updateViewer(buffer, contentType);
-            if (!viewerType) {
-              FileSaver.saveAs(blob, info!.name);
-            }
-          });
+
+          if (viewerType === "video" && !info!.isLegacy) {
+            requestVideoStream(data.id, info!.dataKey!.key, contentType).then((streamUrl) => {
+              if (streamUrl) {
+                videoStreamUrl = streamUrl;
+              } else {
+                requestFileDownload(data.id, info!.dataKey!.key, info!.isLegacy!).then((buffer) =>
+                  updateViewer(buffer, contentType),
+                );
+              }
+            });
+          } else {
+            requestFileDownload(data.id, info!.dataKey!.key, info!.isLegacy!).then(
+              async (buffer) => {
+                const blob = await updateViewer(buffer, contentType);
+                if (!viewerType) {
+                  FileSaver.saveAs(blob, info!.name);
+                }
+              },
+            );
+          }
         }
       });
     }
@@ -159,9 +176,10 @@
             {@render viewerLoading("이미지를 불러오고 있어요.")}
           {/if}
         {:else if viewerType === "video"}
-          {#if fileBlobUrl}
+          {#if videoStreamUrl || fileBlobUrl}
             <div class="flex flex-col space-y-2">
-              <video bind:this={videoElement} src={fileBlobUrl} controls muted></video>
+              <video bind:this={videoElement} src={videoStreamUrl ?? fileBlobUrl} controls muted
+              ></video>
               <IconEntryButton
                 icon={IconCamera}
                 onclick={() => updateThumbnail(info?.dataKey?.key!, info?.dataKey?.version!)}
diff --git a/src/routes/(fullscreen)/file/[id]/service.ts b/src/routes/(fullscreen)/file/[id]/service.ts
index 09ec86f..ea3e49c 100644
--- a/src/routes/(fullscreen)/file/[id]/service.ts
+++ b/src/routes/(fullscreen)/file/[id]/service.ts
@@ -1,11 +1,32 @@
 import { encryptData } from "$lib/modules/crypto";
 import { storeFileThumbnailCache } from "$lib/modules/file";
+import { prepareFileDecryption, getDecryptedFileUrl } from "$lib/serviceWorker";
 import { requestFileThumbnailUpload } from "$lib/services/file";
 import { trpc } from "$trpc/client";
 
 export { requestCategoryCreation, requestFileRemovalFromCategory } from "$lib/services/category";
 export { requestFileDownload } from "$lib/services/file";
 
+export const requestVideoStream = async (
+  fileId: number,
+  dataKey: CryptoKey,
+  contentType: string,
+) => {
+  const res = await fetch(`/api/file/${fileId}/download`, { method: "HEAD" });
+  if (!res.ok) return null;
+
+  const encContentSize = parseInt(res.headers.get("Content-Length") ?? "0", 10);
+  if (encContentSize <= 0) return null;
+
+  try {
+    await prepareFileDecryption(fileId, { isLegacy: false, dataKey, encContentSize, contentType });
+    return getDecryptedFileUrl(fileId);
+  } catch {
+    // TODO: Error Handling
+    return null;
+  }
+};
+
 export const requestThumbnailUpload = async (
   fileId: number,
   thumbnail: Blob,
diff --git a/src/routes/api/file/[id]/download/+server.ts b/src/routes/api/file/[id]/download/+server.ts
index 974dd54..68191ef 100644
--- a/src/routes/api/file/[id]/download/+server.ts
+++ b/src/routes/api/file/[id]/download/+server.ts
@@ -1,7 +1,7 @@
 import { error } from "@sveltejs/kit";
 import { z } from "zod";
 import { authorize } from "$lib/server/modules/auth";
-import { parseRangeHeader, getContentRangeHeader } from "$lib/server/modules/http";
+import { parseRangeHeader, getContentRangeHeader } from "$lib/modules/http";
 import { getFileStream } from "$lib/server/services/file";
 import type { RequestHandler } from "./$types";
 
diff --git a/src/routes/api/file/[id]/thumbnail/download/+server.ts b/src/routes/api/file/[id]/thumbnail/download/+server.ts
index 70d4cd3..4fc7c1a 100644
--- a/src/routes/api/file/[id]/thumbnail/download/+server.ts
+++ b/src/routes/api/file/[id]/thumbnail/download/+server.ts
@@ -1,7 +1,7 @@
 import { error } from "@sveltejs/kit";
 import { z } from "zod";
 import { authorize } from "$lib/server/modules/auth";
-import { parseRangeHeader, getContentRangeHeader } from "$lib/server/modules/http";
+import { parseRangeHeader, getContentRangeHeader } from "$lib/modules/http";
 import { getFileThumbnailStream } from "$lib/server/services/file";
 import type { RequestHandler } from "./$types";
 
diff --git a/src/service-worker/handlers/decryptFile.ts b/src/service-worker/handlers/decryptFile.ts
new file mode 100644
index 0000000..e374e5d
--- /dev/null
+++ b/src/service-worker/handlers/decryptFile.ts
@@ -0,0 +1,117 @@
+import { DECRYPTED_FILE_URL_PREFIX, CHUNK_SIZE, ENCRYPTED_CHUNK_SIZE } from "../modules/constants";
+import { decryptChunk, getEncryptedRange, getDecryptedSize } from "../modules/crypto";
+import { parseRangeHeader, getContentRangeHeader } from "../modules/http";
+import { getFile } from "../modules/opfs";
+import { fileMetadataStore } from "../stores";
+import type { FileMetadata } from "../types";
+
+const createResponse = (
+  stream: ReadableStream<Uint8Array>,
+  isRangeRequest: boolean,
+  range: { start: number; end: number; total: number },
+  contentType?: string,
+) => {
+  return new Response(stream, {
+    status: isRangeRequest ? 206 : 200,
+    headers: {
+      "Accept-Ranges": "bytes",
+      "Content-Length": String(range.end - range.start + 1),
+      "Content-Type": contentType ?? "application/octet-stream",
+      ...(isRangeRequest ? getContentRangeHeader(range) : {}),
+    },
+  });
+};
+
+const streamFromOpfs = async (
+  file: File,
+  metadata?: FileMetadata,
+  range?: { start?: number; end?: number },
+) => {
+  const start = range?.start ?? 0;
+  const end = range?.end ?? file.size - 1;
+  if (start > end || start < 0 || end >= file.size) {
+    return new Response("Invalid range", { status: 416 });
+  }
+
+  return createResponse(
+    file.slice(start, end + 1).stream(),
+    !!range,
+    { start, end, total: file.size },
+    metadata?.contentType,
+  );
+};
+
+const streamFromServer = async (
+  id: number,
+  metadata: FileMetadata,
+  range?: { start?: number; end?: number },
+) => {
+  const totalSize = getDecryptedSize(metadata.encContentSize, metadata.isLegacy);
+  const start = range?.start ?? 0;
+  const end =
+    range?.end ??
+    (range && !metadata.isLegacy ? Math.min(start + CHUNK_SIZE, totalSize) : totalSize) - 1;
+  if (start > end || start < 0 || end >= totalSize) {
+    return new Response("Invalid range", { status: 416 });
+  }
+
+  const encryptedRange = getEncryptedRange(start, end, metadata.encContentSize, metadata.isLegacy);
+  const apiResponse = await fetch(`/api/file/${id}/download`, {
+    headers: { Range: `bytes=${encryptedRange.start}-${encryptedRange.end}` },
+  });
+  if (apiResponse.status !== 206) {
+    return new Response("Failed to fetch encrypted file", { status: 502 });
+  }
+
+  const fileEncrypted = await apiResponse.arrayBuffer();
+  return createResponse(
+    new ReadableStream<Uint8Array>({
+      async start(controller) {
+        if (metadata.isLegacy) {
+          const decrypted = await decryptChunk(fileEncrypted, metadata.dataKey);
+          controller.enqueue(new Uint8Array(decrypted.slice(start, end + 1)));
+          controller.close();
+          return;
+        }
+
+        const chunks = encryptedRange.lastChunkIndex - encryptedRange.firstChunkIndex + 1;
+
+        for (let i = 0; i < chunks; i++) {
+          const chunk = await decryptChunk(
+            fileEncrypted.slice(i * ENCRYPTED_CHUNK_SIZE, (i + 1) * ENCRYPTED_CHUNK_SIZE),
+            metadata.dataKey,
+          );
+          const sliceStart = i === 0 ? start % CHUNK_SIZE : 0;
+          const sliceEnd = i === chunks - 1 ? (end % CHUNK_SIZE) + 1 : chunk.byteLength;
+          controller.enqueue(new Uint8Array(chunk.slice(sliceStart, sliceEnd)));
+        }
+
+        controller.close();
+      },
+    }),
+    !!range,
+    { start, end, total: totalSize },
+    metadata.contentType,
+  );
+};
+
+const decryptFileHandler = async (request: Request) => {
+  const url = new URL(request.url);
+  const fileId = parseInt(url.pathname.slice(DECRYPTED_FILE_URL_PREFIX.length), 10);
+  if (isNaN(fileId)) {
+    throw new Response("Invalid file id", { status: 400 });
+  }
+
+  const metadata = fileMetadataStore.get(fileId);
+  const range = parseRangeHeader(request.headers.get("Range"));
+  const cache = await getFile(`/cache/${fileId}`);
+  if (cache) {
+    return streamFromOpfs(cache, metadata, range);
+  } else if (metadata) {
+    return streamFromServer(fileId, metadata, range);
+  } else {
+    return new Response("Decryption not prepared", { status: 400 });
+  }
+};
+
+export default decryptFileHandler;
diff --git a/src/service-worker/handlers/index.ts b/src/service-worker/handlers/index.ts
new file mode 100644
index 0000000..fe5b0f9
--- /dev/null
+++ b/src/service-worker/handlers/index.ts
@@ -0,0 +1 @@
+export { default as decryptFile } from "./decryptFile";
diff --git a/src/service-worker/index.ts b/src/service-worker/index.ts
new file mode 100644
index 0000000..051f8d9
--- /dev/null
+++ b/src/service-worker/index.ts
@@ -0,0 +1,43 @@
+/// <reference no-default-lib="true"/>
+/// <reference lib="esnext" />
+/// <reference lib="webworker" />
+/// <reference types="@sveltejs/kit" />
+
+import { DECRYPTED_FILE_URL_PREFIX } from "./modules/constants";
+import { decryptFile } from "./handlers";
+import { fileMetadataStore } from "./stores";
+import type { ServiceWorkerMessage, ServiceWorkerResponse } from "./types";
+
+const self = globalThis.self as unknown as ServiceWorkerGlobalScope;
+
+self.addEventListener("message", (event) => {
+  const message: ServiceWorkerMessage = event.data;
+  switch (message.type) {
+    case "decryption-prepare":
+      fileMetadataStore.set(message.fileId, message);
+      event.source?.postMessage({
+        type: "decryption-ready",
+        fileId: message.fileId,
+      } satisfies ServiceWorkerResponse);
+      break;
+    default: {
+      const exhaustive: never = message.type;
+      return exhaustive;
+    }
+  }
+});
+
+self.addEventListener("fetch", (event) => {
+  const url = new URL(event.request.url);
+  if (url.pathname.startsWith(DECRYPTED_FILE_URL_PREFIX)) {
+    event.respondWith(decryptFile(event.request));
+  }
+});
+
+self.addEventListener("install", () => {
+  self.skipWaiting();
+});
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(self.clients.claim());
+});
diff --git a/src/service-worker/modules/constants.ts b/src/service-worker/modules/constants.ts
new file mode 100644
index 0000000..cca093e
--- /dev/null
+++ b/src/service-worker/modules/constants.ts
@@ -0,0 +1 @@
+export * from "../../lib/constants";
diff --git a/src/service-worker/modules/crypto.ts b/src/service-worker/modules/crypto.ts
new file mode 100644
index 0000000..1afee74
--- /dev/null
+++ b/src/service-worker/modules/crypto.ts
@@ -0,0 +1,40 @@
+import { ENCRYPTION_OVERHEAD, CHUNK_SIZE, ENCRYPTED_CHUNK_SIZE } from "./constants";
+
+export * from "../../lib/modules/crypto";
+
+export const getEncryptedRange = (
+  start: number,
+  end: number,
+  totalEncryptedSize: number,
+  isLegacy: boolean,
+) => {
+  if (isLegacy) {
+    return {
+      firstChunkIndex: 0,
+      lastChunkIndex: 0,
+      start: 0,
+      end: totalEncryptedSize - 1,
+    };
+  }
+
+  const firstChunkIndex = Math.floor(start / CHUNK_SIZE);
+  const lastChunkIndex = Math.floor(end / CHUNK_SIZE);
+  return {
+    firstChunkIndex,
+    lastChunkIndex,
+    start: firstChunkIndex * ENCRYPTED_CHUNK_SIZE,
+    end: Math.min((lastChunkIndex + 1) * ENCRYPTED_CHUNK_SIZE - 1, totalEncryptedSize - 1),
+  };
+};
+
+export const getDecryptedSize = (encryptedSize: number, isLegacy: boolean) => {
+  if (isLegacy) {
+    return encryptedSize - ENCRYPTION_OVERHEAD;
+  }
+
+  const fullChunks = Math.floor(encryptedSize / ENCRYPTED_CHUNK_SIZE);
+  const lastChunkEncSize = encryptedSize % ENCRYPTED_CHUNK_SIZE;
+  return (
+    fullChunks * CHUNK_SIZE + (lastChunkEncSize > 0 ? lastChunkEncSize - ENCRYPTION_OVERHEAD : 0)
+  );
+};
diff --git a/src/service-worker/modules/http.ts b/src/service-worker/modules/http.ts
new file mode 100644
index 0000000..0d1bf5e
--- /dev/null
+++ b/src/service-worker/modules/http.ts
@@ -0,0 +1 @@
+export * from "../../lib/modules/http";
diff --git a/src/service-worker/modules/opfs.ts b/src/service-worker/modules/opfs.ts
new file mode 100644
index 0000000..0ef5769
--- /dev/null
+++ b/src/service-worker/modules/opfs.ts
@@ -0,0 +1 @@
+export * from "../../lib/modules/opfs";
diff --git a/src/service-worker/stores.ts b/src/service-worker/stores.ts
new file mode 100644
index 0000000..22d899e
--- /dev/null
+++ b/src/service-worker/stores.ts
@@ -0,0 +1,3 @@
+import type { FileMetadata } from "./types";
+
+export const fileMetadataStore = new Map<number, FileMetadata>();
diff --git a/src/service-worker/types.ts b/src/service-worker/types.ts
new file mode 100644
index 0000000..f04ed39
--- /dev/null
+++ b/src/service-worker/types.ts
@@ -0,0 +1 @@
+export * from "../lib/serviceWorker/types";