To create 1 file: decoder-ring.c

You should use the getchar library function to read console input one character at a time. The use of any other library functions for input is not recommended at this time.

Envision our decoder ring as a disk with the 26 upper-case alphabet letters arrayed around its edge in order, with a 27th place between 'Z' and 'A' containing a space. Each of the 27 slots has a number associated with it: the space is 0, 'A' is 1, 'B' is 2, and so on up to 'Z' = 26.

Assume a key value of zero, and a message of "Hello world!". The first letter 'H' has value 8. The key is 0, and there is no previous value, so 'H' will be encoded as 'H' (8 + 0 + 0 = 8 = 'H'). The second letter is 'e', which has value 5. The previous value plus 'e' plus the key is 'M' (8 + 5 + 0 = 13 = 'M'.) The third letter of the message is 'L'. (Note that we immediately convert lower-case letters to upper-case for this code.) Previous value 13 plus 'L' (12) plus key (0) = 25 = 'Y'. The fourth character is also 'L'. Previous value (25) plus 'L' (12) plus key (0) = 37, which when wrapped around the decoder ring (modulo 27) is 10, or 'J'.

The enciphered result for "Hello world!" is thus "HMYJYYUI LPP" with a key of 0, or "IOANCDAQIV A" with a key of 1, or "JQDRHJHYREKM" with a key of 2, and so on.

Newline characters (' ') are transmitted unencoded. Note that we treat spaces -- and all other non-alphabetic characters -- as value 0. They will be decoded as blank spaces.

This assignment can be completed using simple loop and conditional constructs, along with basic integer operations like addition and modulus. Your encoder-ring and decoder-ring programs should take an optional key value as a command-line argument. If no argument is given, the key is assumed to be 0. You may use the C library function atoi, or parse the command-line integer yourself. The encoder-ring reads in plaintext messages line by line and outputs ciphertext encrypted with the specified key. The decoder-ring reads in ciphertext as input and outputs the plaintext message decrypted with the specified key.