## Python solution script

import string
instr="rgnoDVD{O0NU_WQ3_G1G3O3T3_A1AH3S_2951c89f}"
key="CYLAB"
outstr=""
offset_lower=ord('a')
offset_upper=ord('A')
lower=string.ascii_lowercase
upper=string.ascii_uppercase


dicts_l={}
for k in key:
    key_loc=ord(k)-offset_upper
    dicts_l[k]={}
    for l in lower:
        cipher=chr((ord(l)-offset_lower+key_loc)%26+offset_lower)
        dicts_l[k][cipher]=l #dicts_l[key][cipher]=[plain]


dicts_u = {}
for k in key:
    key_loc = ord(k) - offset_upper
    dicts_u[k] = {}
    for u in upper:
        dicts_u[k][chr((ord(u) - offset_upper + key_loc) % 26 + offset_upper)] = u  # dicts_u[key][cipher]=[plain]

count=0
for c in instr:
    if c.isalpha():
        if c.islower():
            outstr+=dicts_l[key[count%len(key)]][c]
        elif c.isupper():
            outstr+=dicts_u[key[count%len(key)]][c]
        count+=1
    else:
        outstr+=c
print(outstr)

=>`picoCTF{D0NT_US3_V1G3N3R3_C1PH3R_2951a89h}`

Note that we have to use an indepedent `count` (increase only after processing a letter) to decode the cipher. If we use `i` from `enumerate`, the output is going to be weird, like this:
`picoCTF{O0LW_WP3_V1F3Q3T3_C1AG3U_2951r89d}`
## Easier solution
https://www.dcode.fr/vigenere-cipher

## REFs
- https://i.ytimg.com/vi/SkJcmCaHqS0/maxresdefault.jpg
- Chart at the bottom of https://www.dcode.fr/vigenere-cipher