diff --git a/src/ssn/decryption/modifyPassword.ts b/src/ssn/decryption/modifyPassword.ts
index a5417d6..0687018 100644
--- a/src/ssn/decryption/modifyPassword.ts
+++ b/src/ssn/decryption/modifyPassword.ts
@@ -1,11 +1,11 @@
 /** Takes a password as it is included in the extra field, and returns a password that can be used to decode the file. */
-export default function modifyPassword(passwordIn: Uint8Array) {
+export default function modifyPassword(passwordIn: Uint8Array): Uint8Array {
   const passwordLength = passwordIn.byteLength;
   const passwordOut = new Uint8Array(passwordLength);
 
   for (let i = 0; i < passwordLength; i += 1) {
     if (passwordIn[i] === 0) { break; }
-    let curChar = passwordIn[i] + ((1 << (i % 32)) & 0xFF);
+    let curChar = (passwordIn[i] + ((1 << (i % 32)) & 0xFF)) & 0xFF;
     if (curChar > 0x7E) {
       if (curChar === 0xFF || curChar === 0x7F) {
         curChar = 0x3F;
@@ -14,7 +14,7 @@ export default function modifyPassword(passwordIn: Uint8Array) {
       }
     }
     if (curChar < 0x21) {
-      curChar = (curChar | (1 << ((curChar % 3) + 5))) + 1;
+      curChar = ((curChar | (1 << ((curChar % 3) + 5))) + 1) & 0xFF;
     }
     passwordOut[i] = curChar;
   }