---
id: 69bc6cb30c1d112a2e110a07
title: "Challenge 250: Hidden Key"
challengeType: 29
dashedName: challenge-250
---

# --description--

Welcome to the 250th daily challenge!

Given an encoded string, decode it using an encryption key and return it.

To find the key:

- Look at all daily challenges up to today whose challenge number is a multiple of 25 (including this one).
- Take the first letter from each of those challenge titles and combine them into a string. If the title starts with a non-letter, find its first letter.

To decode the message, go over each letter in the encoded message and:

  1. Look at the corresponding letter in the key (repeat the key if the message is longer than the key).
  2. Convert the key letter to its corresponding number: `"A"` = 1, `"B"` = 2, ..., `"Z"` = 26.
  3. Shift the encoded letter backward in the alphabet by that number.
  4. If the shift goes before `"A"`, wrap around to `"Z"`.

For example, if the encoded message starts with `"Y"` and the first key letter is `"V"` (22), shift `"Y"` back 22 places to get `"C"`. Repeat this process for each letter to decode the full message.

- Only letters are shifted, spaces are returned as-is.
- All given and returned letters are uppercase.

# --hints--

`decode("YAVJYNXE")` should return `"CONGRATS"`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(decode("YAVJYNXE"), "CONGRATS")`)
}})
```

`decode("YALLUT PQUMJP")` should return `"CODING LEGEND"`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(decode("YALLUT PQUMJP"), "CODING LEGEND")`)
}})
```

`decode("UAC DYR EISAKYM")` should return `"YOU ARE AWESOME"`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(decode("UAC DYR EISAKYM"), "YOU ARE AWESOME")`)
}})
```

`decode("GQMS NBMZU")` should return `"KEEP GOING"`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(decode("GQMS NBMZU"), "KEEP GOING")`)
}})
```

`decode("W IQQURV UG I ZDMDTRV IVW JQDHY TMHSA QB")` should return `"A WINNER IS A DREAMER WHO NEVER GIVES UP"`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(decode("W IQQURV UG I ZDMDTRV IVW JQDHY TMHSA QB"), "A WINNER IS A DREAMER WHO NEVER GIVES UP")`)
}})
```

# --seed--

## --seed-contents--

```py
def decode(message):

    return message
```

# --solutions--

```py
def decode(message):
    key = "VLHCGMDLNH"
    result = []
    key_index = 0
    for char in message:
        if char == ' ':
            result.append(char)
        else:
            shift = ord(key[key_index % len(key)]) - 64
            key_index += 1
            result.append(chr((ord(char) - 65 - shift) % 26 + 65))
    return ''.join(result)
```