How would I reverse engineer a cryptographic algorithm?

Question

I wrote an application that encrypts text in this way:

1. Get the input text

2. Reverse the text

3. Convert to hexadecimal

4. <

Answer 1

If you have access to a black box which does the encryption, you can get a lot of information by feeding it particular input values.

As a simple example, if the black box does "one time pad" style encryption, if you feed it all zeroes you get the one time pad. (In fact, feeding it any input value will get you the one time pad with an additional xor.)

Note that good cryptosystems are resistant to such attacks, even if the cryptosystem is already known (but the key is not).