diff --git a/scripts/decrypt_bios.ts b/scripts/decrypt_bios.ts
index 3126187..e779cb0 100644
--- a/scripts/decrypt_bios.ts
+++ b/scripts/decrypt_bios.ts
@@ -189,48 +189,14 @@ function InsydeSolver(serial: string): string[] {
 function FSI20DecOldSolver(serial: string): string[] {
     let ord = (str: string) => str.charCodeAt(0);
 
-    function copy_array(ar: number[]): number[] {
-        let temp_arr: number[] = [];
-        for (let i = 0; i < ar.length; i++) {
-            temp_arr[i] = ar[i];
-        }
-        return temp_arr;
-    }
-
-    function byteToChar(symbol_byte: number) {
-        if (symbol_byte > 9 ) {
-            return String.fromCharCode(ord("a") + symbol_byte - 10);
-        } else {
-            return String.fromCharCode(ord("0") + symbol_byte);
-        }
-    }
-
-    function bytesToString(bytes: number[]): string {
-        let str = "";
-        for (let i = 0; i < bytes.length; i++) {
-            str += byteToChar(bytes[i]);
-        }
-        return str;
-    }
-
-    function codeToBytes(code: string) {
-        let numbers = [
-            parseInt(code.slice(0, 5), 10),
-            parseInt(code.slice(5, 10), 10),
-            parseInt(code.slice(10, 15), 10),
-            parseInt(code.slice(15, 20), 10)
-        ];
-        let bytes: number[] = [];
-        for (let i = 0; i < numbers.length; i++) {
-            bytes.push(numbers[i] % 256);
-            bytes.push(Math.floor(numbers[i] / 256));
-        }
-        return bytes;
-    }
-
-/* op_arr - array with that operations do, ar1,ar2 - numbers */
+    function swap<T> (arr: T[], i1: number, i2: number): void {
+        let temp = arr[i1];
+        arr[i1] = arr[i2];
+        arr[i2] = temp;
+    };
+    // op_arr - array with that operations do, ar1,ar2 - numbers */
     function interleave(op_arr: number[], ar1: number[], ar2: number[]) {
-        let arr = copy_array(op_arr);
+        let arr = op_arr.slice(0); // copy array
         arr[ar1[0]] = ((op_arr[ar2[0]] >> 4) | (op_arr[ar2[3]] << 4)) & 0xFF;
         arr[ar1[1]] = ((op_arr[ar2[0]] & 0x0F) | (op_arr[ar2[3]] & 0xF0));
         arr[ar1[2]] = ((op_arr[ar2[1]] >> 4) | (op_arr[ar2[2]] << 4) & 0xFF);
@@ -238,33 +204,31 @@ function FSI20DecOldSolver(serial: string): string[] {
         return arr;
     }
 
-    function convert_to_remainder(arr: number[]) {
-        let temp_arr: number[] = [];
-        for (let i = 0; i < arr.length; i++) {
-            temp_arr[i] = arr[i] % 36;
-        }
-        return temp_arr;
+    function codeToBytes(code: string): number[] {
+        let numbers = [
+            parseInt(code.slice(0, 5), 10),
+            parseInt(code.slice(5, 10), 10),
+            parseInt(code.slice(10, 15), 10),
+            parseInt(code.slice(15, 20), 10)
+        ];
+        return numbers.reduce((acc, val) => {
+            acc.push(val % 256);
+            acc.push(Math.floor(val / 256));
+            return acc;
+        }, []);
     }
 
     function decryptCode_old(bytes: number[]) {
         const XORkey = ":3-v@e4i";
-        function swap<T> (arr: T[], i1: number, i2: number): void {
-            let temp = arr[i1];
-            arr[i1] = arr[i2];
-            arr[i2] = temp;
-        };
         // apply XOR key
-        for (let i = 0; i < bytes.length; i++) {
-            bytes[i] = bytes[i] ^ ord(XORkey.charAt(i));
-        }
-
+        bytes.forEach((val, i, arr) => {
+            arr[i] = val ^ ord(XORkey.charAt(i));
+        });
         // swap two bytes
         swap(bytes, 2, 6);
         swap(bytes, 3, 7);
-
         bytes = interleave(bytes, [0, 1, 2, 3], [0, 1, 2, 3]);
         bytes = interleave(bytes, [4, 5, 6, 7], [6, 7, 4, 5]);
-
         // final rotations
         bytes[0] = ((bytes[0] << 3) & 0xFF) | (bytes[0] >> 5);
         bytes[1] = ((bytes[1] << 5) & 0xFF) | (bytes[1] >> 3);
@@ -273,11 +237,12 @@ function FSI20DecOldSolver(serial: string): string[] {
         bytes[5] = ((bytes[5] << 6) & 0xFF) | (bytes[5] >> 2);
         bytes[6] = ((bytes[6] << 1) & 0xFF) | (bytes[6] >> 7);
         bytes[7] = ((bytes[7] << 2) & 0xFF) | (bytes[7] >> 6);
-
         // len(solution space) = 10 + 26
-        bytes = convert_to_remainder(bytes);
-
-        return bytesToString(bytes);
+        bytes = bytes.map(b => b % 36);
+        return bytes.map(sbyte => (sbyte > 9 )
+            ? String.fromCharCode(ord("a") + sbyte - 10)
+            : String.fromCharCode(ord("0") + sbyte)
+        ).join("");
     }
 
     return [decryptCode_old(codeToBytes(serial))];
diff --git a/spec/bios.spec.js b/spec/bios.spec.js
index 502b6df..d79008a 100644
--- a/spec/bios.spec.js
+++ b/spec/bios.spec.js
@@ -51,5 +51,6 @@ describe("Decryp BIOS", function() {
 
     it("FSI 20 dec old key for 1234-4321-1234-4321-1234 is 10cphf0b", function() {
         expect(bios.FSI20DecOld.solve("12344321123443211234")).toEqual(["10cphf0b"]);
+        expect(bios.FSI20DecOld.solve("12344321123443211236")).toEqual(["100phf0b"]);
     })
 });