the provided text file shows really big numbers
lotta_numbers.txt
and if you haven't noticed the challenge name and description give off that this is a RSA algorithm challenge
now we can easily compute $n = p * q$ and we'll have the public key (e, n)
we can compute phi from $\phi(n) = (p-1) * (q-1)$
and lastly $d$ the modular inverse of $e$ from this calculator
the decrypted message can be acquired from the following formula:
$$ m = c^d \enspace mod \enspace n $$
we have all the required parameters to crack the cipher so plug them into [this calculator](<https://www.dcode.fr/rsa-cipher>)
and u get the flag
kqctf{y0uv3_6r4du473d_fr0m_r54_3l3m3n74ry_5ch00l_ac8770bdcebc}