I’ve written two previous posts (here and here) about how cribs can be used in solving ciphers, especially by those who program computers. However, I omitted a rather obvious use that doesn’t require computer coding knowledge. That method is to determine the cipher type of an unknown cipher.
Some ciphers are presented as puzzles but the cipher type is not given. Many geocaching puzzle caches are prime examples. If a crib is given, or can be guessed, it can be used to confirm or eliminate the cipher type. If you are a computer programmer, you can write code that will “drag” a crib across the ciphertext to determine if it could possibly fit there, although that’s not easy for some types. If you don’t program there is still an easy way to do this. There are free websites by programmers who will perform this function for you. Here are two that I use sometimes, although I have my own crib draggers for many cipher types.
The Scytale This one even shows you a worksheet with all letter placements.
BION’s gadgets This one checks many periods at once for some types and shows the exact placement. For some types it also shows the ciphertext and plaintext matched up.
Even if you can’t positively determine a cipher type by crib dragging, you may be able to eliminate one or several types, thus reducing the types you have to consider. For some ciphers this can be done by eye, without a computer or website. For example, suppose the ciphertext shows word divisions and the crib is TOMORROW. There are two 8-letter words in the ciphertext, but neither of them has the doubled letters in positions 5 and 6 or the repeats where the O’s should be. You can eliminate simple substitution (Aristocrat), Key Phrase and Tridigital (if it’s numerical). Could it be a Ragbaby? If the key digit is 0 somewhere in either of those two ciphertext words, you may be able to confirm or eliminate one or both of them as possible placements. For Condi and Sequence you probably can’t confirm the type, but if the ciphertext doesn’t have a W, say, you can eliminate Sequence, which is a transposition type. There are often easier ways to determine a cipher type, but cribs can be useful in this way.