feat: add xiami crypto & have a working decryption workflow (#2, #9)

src/decrypt-worker/crypto/CryptoBase.ts

@@ -0,0 +1,6 @@
+export interface CryptoBase {
+  isSupported(blob: Blob): Promise<boolean>;
+  decrypt(blob: Blob): Promise<Blob>;
+}
+
+export type CryptoFactory = () => CryptoBase;

src/decrypt-worker/crypto/xiami/xiami.ts

@@ -0,0 +1,47 @@
+// Xiami file header
+// offset  description
+//   0x00  "ifmt"
+//   0x04  Format name, e.g. "FLAC".
+//   0x08  0xfe, 0xfe, 0xfe, 0xfe
+//   0x0C  (3 bytes) Little-endian, size of data to copy without modification.
+//         e.g. [ 8a 19 00 ] = 6538 bytes of plaintext data.
+//   0x0F  (1 byte) File key, applied to
+//   0x10  Plaintext data
+//   ????  Encrypted data
+
+import type { CryptoBase } from '../CryptoBase';
+
+const XIAMI_FILE_MAGIC = new Uint8Array('ifmt'.split('').map((x) => x.charCodeAt(0)));
+const XIAMI_EXPECTED_PADDING = new Uint8Array([0xfe, 0xfe, 0xfe, 0xfe]);
+
+const u8Sub = (a: number, b: number) => {
+  if (a > b) {
+    return a - b;
+  }
+
+  return a + 0xff - b;
+};
+
+export class XiamiCrypto implements CryptoBase {
+  async isSupported(blob: Blob): Promise<boolean> {
+    const headerBuffer = await blob.slice(0, 0x10).arrayBuffer();
+    const header = new Uint8Array(headerBuffer);
+
+    return (
+      header.slice(0x00, 0x04).every((b, i) => b === XIAMI_FILE_MAGIC[i]) &&
+      header.slice(0x08, 0x0c).every((b, i) => b === XIAMI_EXPECTED_PADDING[i])
+    );
+  }
+
+  async decrypt(blob: Blob): Promise<Blob> {
+    const headerBuffer = await blob.slice(0, 0x10).arrayBuffer();
+    const header = new Uint8Array(headerBuffer);
+    const key = u8Sub(header[0x0f], 1);
+    const plainTextSize = header[0x0c] | (header[0x0d] << 8) | (header[0x0e] << 16);
+    const decrypted = new Uint8Array(await blob.slice(0x10).arrayBuffer());
+    for (let i = decrypted.byteLength - 1; i >= plainTextSize; i--) {
+      decrypted[i] = u8Sub(key, decrypted[i]);
+    }
+    return new Blob([decrypted]);
+  }
+}