Getting Input Key for Atbash Cipher
For one thing, it is very inadvisable to use the += operator with strings, because it's slow and it requires the string to be relocated in the memory. This isn't much of a problem with higher-level languages like Python, but with C and also Java, it's not a good idea, and it's not good practice. Since you know that there are 26 letters in the alphabet, you can make a length 26 array to store the key instead. Otherwise, the rest of the code looks good! :)