Substitution0

A message has come in but it seems to be all scrambled. Luckily it seems to have the key at the beginning. Can you crack this substitution cipher?

So, we are told that the key is at the beginning! Let's see what we have:

ZGSOCXPQUYHMILERVTBWNAFJDK 

Qctcnrel Mcptzlo ztebc, fuwq z ptzac zlo bwzwcmd zut, zlo gtenpqw ic wqc gccwmc
xtei z pmzbb szbc ul fqusq uw fzb clsmebco. Uw fzb z gcznwuxnm bsztzgzcnb, zlo, zw
wqzw wuic, nlhlefl we lzwntzmubwb—ex sentbc z ptczw rtukc ul z bsuclwuxus reulw
ex aucf. Wqctc fctc wfe tenlo gmzsh brewb lczt elc cjwtciuwd ex wqc gzsh, zlo z
melp elc lczt wqc ewqct. Wqc bszmcb fctc cjsccoulpmd qzto zlo pmebbd, fuwq zmm wqc
zrrcztzlsc ex gntlubqco pemo. Wqc fcupqw ex wqc ulbcsw fzb actd tcizthzgmc, zlo,
wzhulp zmm wqulpb ulwe selbuoctzwuel, U senmo qztomd gmzic Ynruwct xet qub eruluel
tcbrcswulp uw.

Wqc xmzp ub: ruseSWX{5NG5717N710L_3A0MN710L_357GX9XX}

So, the first line appears to be the "key". Well, if we write the alphabet under it:

ZGSOCXPQUYHMILERVTBWNAFJDK
ABCDEFGHIJKLMNOPQRSTUVWXYZ

There is a perfect correspondence! So, any instance of the character in the top list is meant to be substituted using the bottom list. We can easily make a python program to do it for us:

from string import ascii_uppercase, ascii_lowercase

alphabet = 'ZGSOCXPQUYHMILERVTBWNAFJDK'

text = '''
Qctcnrel Mcptzlo ztebc, fuwq z ptzac zlo bwzwcmd zut, zlo gtenpqw ic wqc gccwmc
xtei z pmzbb szbc ul fqusq uw fzb clsmebco. Uw fzb z gcznwuxnm bsztzgzcnb, zlo, zw
wqzw wuic, nlhlefl we lzwntzmubwb—ex sentbc z ptczw rtukc ul z bsuclwuxus reulw
ex aucf. Wqctc fctc wfe tenlo gmzsh brewb lczt elc cjwtciuwd ex wqc gzsh, zlo z
melp elc lczt wqc ewqct. Wqc bszmcb fctc cjsccoulpmd qzto zlo pmebbd, fuwq zmm wqc
zrrcztzlsc ex gntlubqco pemo. Wqc fcupqw ex wqc ulbcsw fzb actd tcizthzgmc, zlo,
wzhulp zmm wqulpb ulwe selbuoctzwuel, U senmo qztomd gmzic Ynruwct xet qub eruluel
tcbrcswulp uw.

Wqc xmzp ub: ruseSWX{5NG5717N710L_3A0MN710L_357GX9XX}
'''

dec = ''

for c in text:
    if c in ascii_uppercase:
        dec += ascii_uppercase[alphabet.index(c)]
    elif c in ascii_lowercase:
        dec += ascii_lowercase[alphabet.index(c.upper())]
    else:
        dec += c

print(dec)

# picoCTF{5UB5717U710N_3V0LU710N_357BF9FF}

We get a text from The Gold-Bug, by Edgar Allen Poe and the flag too:

Hereupon Legrand arose, with a grave and stately air, and brought me the beetle
from a glass case in which it was enclosed. It was a beautiful scarabaeus, and, at
that time, unknown to naturalists—of course a great prize in a scientific point
of view. There were two round black spots near one extremity of the back, and a
long one near the other. The scales were exceedingly hard and glossy, with all the
appearance of burnished gold. The weight of the insect was very remarkable, and,
taking all things into consideration, I could hardly blame Jupiter for his opinion
respecting it.

The flag is: picoCTF{5UB5717U710N_3V0LU710N_357BF9FF}