int key : It is used to take the value of key.If it is +ve value then means right shift and if it is -ve value means left shift.

int ch : It is used for choice whether you want to perform encryption or decryption.

char c[] : It is used to store stanard Alphabets with their index values.

string num : It represent input string i.e. it can be plain-text or cipher-text.

int l : It is used to calculate the length of input string.

int a[] : It is an integer array used to store the index values of the input string(num).

char cte[] : It is a character array used to store the encrypted values of the input string(num).

char[] num1 :  It stores the valuse of string num.

int k : It is used to increase the index value of array a according to the input string(num).

int ct : It is variable where we calculate the encrypted value of the input string(num).

int a1[] : It is an integer array used to store the index values of the input string(num).

char cte1[] : It is a character array used to store the decrypted values of the input string(num).

int k1 : It is used to increase the index value of array a according to the input string(num).

int pt : It is variable where we calculate the decrypted value of the input string(num).