A cryptanalyst is a specialist in analysing and breaking any type of hidden information, codes and cyphers. Cryptanalyst can be used as a noun since it denotes a person.
Moreover, you will know that cryptanalyst is a noun in the sentence because it determines a person.
For example: He has been the first cryptanalyst at the army during World War II. Cryptanalyst was used as a common noun because it is identified as a name of class of people.