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You don't really need to decompile this one, as you can quickly discover the algorithm by just playing around with the program. Here you give some input, then a key that contains every unique character from the input. The output is each character in the plaintext's index in the key field, separated with `:`. We are given the pattern in the text file, so this one is fairly easy.

print(''.join(["{qwertyuiopas_dfghjklzxcvbnm}"[i] for i in map(int,"4:11:14:11:4:0:8:26:14:3:22:13:9:15:13:27:11:19:3:13:12:27:11:20:20:13:23:4:6:10:5:9:4:28".split(':'))]))