Open the binary in IDA, I noticed that it just:
- check input length;
- permute input into ch1;
- compare ch1 with ch2="t_nssiamwp_lsei_hatt{_gaalllF}"
This give me a quick solution.
- debugging the program using gdb;
- set a breakpoint at call to strcmp(ch1,ch2);
- start the program and feed input "ABCDEFGHIJKLMNOabcdefghijklmno"
- continue excution to breakpoint and check ch1, got "OaNbMcLdKeJfIgHhGiFjEkDlCmBnAo"
- perform reverse permutation on ch2:
python -c 'print "".join(x for _,x in sorted(zip("OaNbMcLdKeJfIgHhGiFjEkDlCmBnAo","t_nssiamwp_lsei_hatt{_gaalllF}")))'
=> Flag{this_wasnt_simple_at_all}