• use https://github.com/Ganapati/RsaCtfTool

after installing the tool run command:

python3 ~/RsaCtfTool/RsaCtfTool.py --publickey ./key.pub --uncipherfile mystery.txt

in challenge directory and it'll spit the flag

flag

kqctf{y0uv3_6r4du473d_fr0m_r54_m1ddl3_5ch00l_abe7e79e244a9686efc0}