For figuring out the type of cipher, consider how each cipher works.
Simple substitution ciphers (cryptogram) would have a letter distribution where some letters are a lot more frequent than others.
Playfair and Vigenere would have "flat" letter frequency tables.
Also, from the way Playfair is composed you can look for things like:
does the code have an odd number of letters? (if so then it can't be Playfair)
if it has an even number of letters, break the code into pairs and check
does the code have double letter pairs (if so then it can't be Playfair)
and of course since Playfair has letters in a 5x5 table one letter is left out
such as Q but sometimes I/J are combined so it may or may not be Playfair
The author seems to be introducing ciphers in each chapter with increasing difficulty.
Hope that helps,
-Jack
A fool who truly believes there is no 3 in three
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