[source] c8-20251031_002439|065_vigenere_decrypt_flag.txt

TIME: 2025-10-31 00:53:52
CMD : $cipher = 'LCTC5618{HCB_SXIUQ_CVL_OXL}'
$key = 'JOHN'
$keyCodes = $key.ToCharArray() | ForEach-Object { ([int][char]$_) - [int][char]'A' }
$keyLen = $keyCodes.Length
$index = 0

$plainChars = foreach ($ch in $cipher.ToCharArray()) {
if ($ch -match '[A-Z]') {
$shift = $keyCodes[$index % $keyLen]
$code = ([int][char]$ch) - [int][char]'A'
$dec = ($code - $shift + 26) % 26
$index++
[char]($dec + [int][char]'A')
} else {
$ch
}
}

$plain = -join $plainChars
$plain | Tee-Object 'D:\work\challenge8_native\flag_plain.txt'
---- OUTPUT ----
COMP5618{YOU_FOUND_THE_BOX}
---- END (Status=True) ----