We get N, E, & C from the txt file given,
we get P & q (which are the same) from https://factordb.com when we input N in it. (( P & Q are factors of N))
and using p, q, e, c in https://www.dcode.fr/rsa-cipher we get the decoded text flag!flag: `DawgCTF{sm@ll_d_b1g_dr3am5}` 
For detailed answer: https://floatingbrij.medium.com/really-secure-algorithm-dawgctf-2021-709bc2e9322c