added optional skip buffer for decrypting only a part of a file

Author: larryrider

src/services/crypto.service.ts

@@ -116,8 +116,17 @@ export class CryptoService {
     return Buffer.concat([decipher.update(contentsToDecrypt), decipher.final()]).toString('utf8');
   };
 
-  public async decryptStream(inputSlices: ReadableStream<Uint8Array>[], key: Buffer, iv: Buffer) {
+  public async decryptStream(
+    inputSlices: ReadableStream<Uint8Array>[],
+    key: Buffer,
+    iv: Buffer,
+    skipOptions?: { total: number },
+  ) {
     const decipher = createDecipheriv('aes-256-ctr', key, iv);
+    if (skipOptions) {
+      const skipBuffer = Buffer.alloc(skipOptions.total, 0);
+      decipher.update(skipBuffer);
+    }
     const encryptedStream = StreamUtils.joinReadableBinaryStreams(inputSlices);
 
     let keepReading = true;

src/services/network/network-facade.service.ts

@@ -72,10 +72,15 @@ export class NetworkFacade {
     };
 
     const decryptFile: DecryptFileFunction = async (_, key, iv) => {
+      let skipOptions;
+      if (rangeOptions) {
+        skipOptions = { total: rangeOptions.parsed.start };
+      }
       fileStream = await this.cryptoService.decryptStream(
         encryptedContentStreams,
         Buffer.from(key as ArrayBuffer),
         Buffer.from(iv as ArrayBuffer),
+        skipOptions,
       );
 
       await fileStream.pipeTo(to);