Service Worker를 활용한 스트리밍 방식 파일 복호화 구현

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)
+  );
+};