When we download and look at the file (`challenge.txt`), we see a 3x3 matrix of numbers and some other stuff. This reminds us of the Hill Cipher, which uses matrices and linear algebra to encrypt things. Also, the title mentions "Hill," so it's a reasonable guess. Below the matrix, we see `A8FC7A{H7_ZODCEND_8F_CCQV_RTTVHD}`, which looks like the encrypted flag. We also see `ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_{}`, which we can guess is the alphabet. Decrypting it using any method ([https://www.dcode.fr/hill-cipher](https://www.dcode.fr/hill-cipher) works pretty well), we find that the flag is `LITCTF{B3_RUNN1NG_UP_TH4T_H1LLLL}`.