Since the revelations of Edward Snowden in 2013, the topic of cryptographic backdoors has been popular in both cyber security and popular culture. However, such backdoors were well known in the cryptology community well before 2013. There are a variety of mechanisms for modifying cryptographic primitives so that the resultant modified algorithm is more easily compromised, but the change in the algorithm is not immediately obvious. This paper provides a review of several modifications to traditional cryptographic primitives, that result in backdoors. Then general principles and counter measures are extrapolated from those studies to provide a generalized understanding of cryptographic backdoors and counter measures.