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

You'd be able to try guess the algorithm if you know how to decrypt it. I can create many algorithms that would result in "A751CD9E1F99" for some input.

Now, if you have many inputs / outputs available, you could try changing only a little your input to see what happens to the output, for instance. Good encryption algorithms usually result in major output changes for minor input changes.