Solution to October 16 challenge
The encrypted message is VG VF HC GB LBH GB QB N TBBQ ONPXHC. The technique used to encrypt this message was substitution.
There are lots of clues within the encrypted message. The single-letter word N can only be I or A when decrypted. The double-letter word TBBQ can only have a limited number of possibilities for the BB combination, such as OO, EE, etc. There are also five different two-letter words, which must be drawn from a list containing AN, OR, DO, AS, IF, TO, GO, UP, IS, etc.
Even with these clues, if the message is uncrackable, the encryption alphabet looks like this:
The decrypted message is IT IS UP TO YOU TO DO A GOOD BACKUP