How can you put a backdoor into an encryption algorithm? Are there any technique

Question

How can you put a backdoor into an encryption algorithm? Are there any techniques that can be used to reduce the time it takes to break a key?

I am looking for practical examples encryption schemes that have backdoors, because they were designed to or new techniques exposed them. It is my understanding that you can use a lack of entropy and diffusion in block ciphers which will reduce the necessary time that it takes to break a key. This is especially true with s-boxes, which can be designed to not increase the strength of the key. As for one-way algorithms, using none-primes can make it easier to reverse keys. What are other decisions that can be made to make a broken encryption algorithm?

The goal is find examples that can be used for encryption pedagogy, by collecting a sample of weak ciphers to practice on.

Explanation / Answer

Schneier's "A Self-study Course in Block-Cipher Cryptanalysis" is an excellent resource for what you are looking for. In particular, FEAL-4 would be a promising cipher to look at as it is breakable by almost every cryptanalysis technique.

One thing you will notice when going through Schneier's course (or when looking at cryptanalysis research in general) is that a lot of times it is helpful to restrict the block cipher by decreasing the number of rounds. Decrease the number of rounds enough, and every block cipher is probably breakable. This will teach you a lot.

Once you are familiar with standard cryptanalysis techniques, it shouldn't be too hard to come up with a cipher that is breakable (where you'll know how to break it).

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