Invented by French cryptographer Felix Delastelle, the four-square cipher uses a manual symmetric encryption technique. The technique encrypts pairs of letters, or digraphs. For this reason, this cipher is part of a category of ciphers known as polygraphic substitution ciphers. Compared to monographic substitution ciphers, which use single characters, this particular cipher is stronger as it uses character pairs.
nmtrfsbsaaryntbsaauozgpxeumtdytdw
xoutnpomngipttrbsaasfueaabffsmnazeumY
You can validate your puzzle solution with certitude.