biscuiti.js

const key = 'VIGENERECODEVIGENERECODEVIGENERECODEVIGENER';

const table = {
    'A': 'ABCDEFGHIJKLMNOPQRSTUVWXYZ{}',
    'B': 'BCDEFGHIJKLMNOPQRSTUVWXYZ{}A',
    'C': 'CDEFGHIJKLMNOPQRSTUVWXYZ{}AB',
    'D': 'DEFGHIJKLMNOPQRSTUVWXYZ{}ABC',
    'E': 'EFGHIJKLMNOPQRSTUVWXYZ{}ABCD',
    'F': 'FGHIJKLMNOPQRSTUVWXYZ{}ABCDE',
    'G': 'GHIJKLMNOPQRSTUVWXYZ{}ABCDEF',
    'H': 'HIJKLMNOPQRSTUVWXYZ{}ABCDEFG',
    'I': 'IJKLMNOPQRSTUVWXYZ{}ABCDEFGH',
    'J': 'JKLMNOPQRSTUVWXYZ{}ABCDEFGHI',
    'K': 'KLMNOPQRSTUVWXYZ{}ABCDEFGHIJ',
    'L': 'LMNOPQRSTUVWXYZ{}ABCDEFGHIJK',
    'M': 'MNOPQRSTUVWXYZ{}ABCDEFGHIJKL',
    'N': 'NOPQRSTUVWXYZ{}ABCDEFGHIJKLM',
    'O': 'OPQRSTUVWXYZ{}ABCDEFGHIJKLMN',
    'P': 'PQRSTUVWXYZ{}ABCDEFGHIJKLMNO',
    'Q': 'QRSTUVWXYZ{}ABCDEFGHIJKLMNOP',
    'R': 'RSTUVWXYZ{}ABCDEFGHIJKLMNOPQ',
    'S': 'STUVWXYZ{}ABCDEFGHIJKLMNOPQR',
    'T': 'TUVWXYZ{}ABCDEFGHIJKLMNOPQRS',
    'U': 'UVWXYZ{}ABCDEFGHIJKLMNOPQRST',
    'V': 'VWXYZ{}ABCDEFGHIJKLMNOPQRSTU',
    'W': 'WXYZ{}ABCDEFGHIJKLMNOPQRSTUV',
    'X': 'XYZ{}ABCDEFGHIJKLMNOPQRSTUVW',
    'Y': 'YZ{}ABCDEFGHIJKLMNOPQRSTUVWX',
    'Z': 'Z{}ABCDEFGHIJKLMNOPQRSTUVWXY',
    '{': '{}ABCDEFGHIJKLMNOPQRSTUVWXYZ',
    '}': '}ABCDEFGHIJKLMNOPQRSTUVWXYZ{',
    '?': '????????????????????????????',
};

const plane = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ{}';

const  ci = 'LMIG}RPEDOEEWKJIQIWKJWMNDTSR}TFVUFWYOCBAJBQ';

let result = '';

for (let i = 0; i < ci.length; i++) {
    const k = key[i];
    const c = ci[i];
    const p = plane[table[k].search(c)];
    console.log(k, c, table[k].search(c), p);
    result += p === undefined ? '?' : p;
}

console.log(result);