♻ Rewrite decryptChunk

src/ssn/decryption/decryptChunk.ts

@@ -1,6 +1,6 @@
 import updateKeys from './lib/updateKeys';
 
-export default function decryptChunk(encryptedChunk: Buffer, [key0, key1, key2]: [number, number, number]): {decryptedChunk: Buffer, keys: [number, number, number]} {
+/*export default function decryptChunk(encryptedChunk: Buffer, [key0, key1, key2]: [number, number, number]): {decryptedChunk: Buffer, keys: [number, number, number]} {
   const decryptedChunk = Buffer.alloc(encryptedChunk.length);
   const dvOut = new DataView(decryptedChunk.buffer);
 
@@ -17,4 +17,35 @@ export default function decryptChunk(encryptedChunk: Buffer, [key0, key1, key2]:
   }
 
   return { decryptedChunk, keys: [key0, key1, key2] };
+}*/
+
+export default function getDecryptor([key0, key1, key2]: [number, number, number]) {
+  let position = 0;
+
+  return (encryptedChunk: Buffer) => {
+    const decryptedChunk = Buffer.alloc(encryptedChunk.length);
+    const dvOut = new DataView(decryptedChunk.buffer);
+
+    for (let i = 0; i < encryptedChunk.length; i += 1) {
+      //read and decrypt byte
+      let curChar = encryptedChunk.readUInt8(i);
+      const keyPart = (key2 | 2) & 0xFFFF;
+      const decryptedByte = (keyPart * (keyPart ^ 1)) >>> 8;
+      curChar ^= decryptedByte & 0xFF;
+      if (position + i < 12) {
+        //do nothing
+      } else if (position + i >= 12 && position < 12) {
+        dvOut.setUint8(position + i - 12, curChar);
+      } else {
+        dvOut.setUint8(i, curChar);
+      }
+
+      //update keys
+      [key0, key1, key2] = updateKeys([key0, key1, key2], curChar);
+    }
+
+    position += encryptedChunk.length;
+
+    return decryptedChunk;
+  };
 }

src/ssn/streams/decryptStream.ts

@@ -1,27 +1,18 @@
 import * as stream from 'stream';
-import decryptChunk from '../decryption/decryptChunk';
+import getDecryptor from '../decryption/decryptChunk';
 
 export default function decryptStream(inputStream: stream.Readable, [key0, key1, key2]: [number, number, number]): stream.Readable {
-  let skippedRandomHeader = false;
+  const decryptor = getDecryptor([key0, key1, key2]);
 
   const outStream = new stream.Readable({
     read(size) {
-      //There are 12 random bytes at the beginning, we need to use them to initialize the decryption keys, but we can ignore the decrypted bytes.
-      if (!skippedRandomHeader) {
-        const encryptedHeader: Buffer = inputStream.read(12);
-        const { keys } = decryptChunk(encryptedHeader, [key0, key1, key2]);
-        [key0, key1, key2] = keys;
-        skippedRandomHeader = true;
-      }
-
       //Decrypt chunk
       const encryptedChunk: Buffer = inputStream.read(size);
       if (encryptedChunk === null) {
         //If end has been reached, stop
         this.push(null);
       } else {
-        const { decryptedChunk, keys } = decryptChunk(encryptedChunk, [key0, key1, key2]);
+        const decryptedChunk = decryptor(encryptedChunk);
-        [key0, key1, key2] = keys;
         this.push(decryptedChunk);
       }
     },