diff --git a/src/ssn/decryption/decryptChunk.ts b/src/ssn/decryption/decryptChunk.ts
index 3370756..f98ceee 100644
--- a/src/ssn/decryption/decryptChunk.ts
+++ b/src/ssn/decryption/decryptChunk.ts
@@ -19,11 +19,12 @@ import updateKeys from './lib/updateKeys';
   return { decryptedChunk, keys: [key0, key1, key2] };
 }*/
 
-export default function getDecryptor([key0, key1, key2]: [number, number, number]) {
+export default function getDecryptor(decryptionKeys: [number, number, number]) {
+  let [key0, key1, key2] = decryptionKeys;
   let position = 0;
 
   return (encryptedChunk: Buffer) => {
-    const decryptedChunk = Buffer.alloc(encryptedChunk.length);
+    const decryptedChunk = Buffer.alloc(encryptedChunk.length - Math.max(12 - position, 0));
 
     for (let i = 0; i < encryptedChunk.length; i += 1) {
       //read byte
@@ -34,6 +35,9 @@ export default function getDecryptor([key0, key1, key2]: [number, number, number
       const decryptedByte = (keyPart * (keyPart ^ 1)) >>> 8;
       curChar ^= decryptedByte & 0xFF;
 
+      //update keys
+      [key0, key1, key2] = updateKeys([key0, key1, key2], curChar);
+
       //write byte
       if (position + i < 12) {
         //do nothing
@@ -42,9 +46,6 @@ export default function getDecryptor([key0, key1, key2]: [number, number, number
       } else {
         decryptedChunk.writeUInt8(curChar, i);
       }
-
-      //update keys
-      [key0, key1, key2] = updateKeys([key0, key1, key2], curChar);
     }
 
     position += encryptedChunk.length;