For this homework, you will be working on string manipulation in Python. Your task is to create a program that stores accounts as usernames and passwords within a single string. The program encrypts passwords before storing them. The program will consist of various functions for retrieving passwords for a given username, verifying the correct passwords, and adding accounts.

You will get a starter file with functions and some variables $($ALPHABET$,$ ENCRYPTION$\_$KEY, and users$\_$passes$)$ initialized. In the users$\_$passes string, there is a $$#$$ between user names and passwords; there is a $*$ between every username#password pair.

users$\_$passes $=$ "donald#xgfc$*$mickey#zygon$*$snow#smbpn$"$

User names and $($encrypted$)$ passwords in the users$\_$passes string are as follows:

duck mouse snow white

The program must also have the following functions:

$1.$ is$\_$lower$\_$english:

$$ This function takes a string variable as a parameter.

$$ It checks if all characters in the string variable are English lowercase alphabetic

characters, i$.$e$.$ all characters must be in the ALPHABET global variable.

$$ It returns a Boolean value $($True if all characters are English lowercase alphabetic

characters, False if not$).$ User name

donald

mickey

Password

$$ You have to add the parameters and return values to the functions defined in the starter code.

$$ The $$pass$$ statement in Python is a placeholder statement which is used when the syntax requires a statement, but you don't want to execute any code. It is a null operation $--$ when it is executed, nothing happens. It's often used as a placeholder indicating where code will eventually go$,$ essentially leaving a function empty.

$2.$ encrypt$\_$decrypt:

$$ This function receives three string parameters from the calling program: the password, key$1$ and key$2.$

$$ This function encrypts$/$decrypts the password as follows: Each letter in the password is located in key$1.$ Then the letter at the same position in key$2$ is found. This is the encrypted counterpart of that letter. All such letters are put together to construct the encrypted password. The resulting string is then sent back to the calling program as the result of this function.

$$ For example, assume that ALPHABET is passed to key$1,$ ENCRYPTION$\_$KEY is passed to key$2$ and the password is $$duck$.$

key$1=$ "abcdefghijklmnopqrstuvwxyz"

key$2=$ "qjfxnawmbucezhyiklopgrstvd"

$$ The first letter in the password is $$d$,$ which is the fourth letter in key$1.$ Therefore we find the fourth letter in key$2,$ which is $$x$$; this is the encrypted version of $$d$.$ We repeat this procedure for all other letters of the password $$duck$$ to construct the encrypted string $$xgfc$.$

$$ While calling this function, if ALPHABET is passed to key$1$ and ENCRYPTION$\_$KEY is passed to key$2,$ encryption is accomplished. Conversely, if ENCRYPTION$\_$KEY is passed to key$1$ and ALPHABET is passed to key$2,$ decryption is accomplished.

$3.$ add$\_$account:

$$ This functions takes $3$ parameters: users$\_$passes, new$\_$username, and new$\_$password.

$$ It encrypts the new password $($using the encrypt$\_$decrypt function$)$ before

concatenating the username and password to the users$\_$passes string.

$$ It sends back the new users$\_$passes string as its result.

$4.$ retrieve$\_$password:

$$ This function takes $2$ parameters: users$\_$passes and username.

$$ It decrypts the password that is stored in users$\_$passes for the specific username by

calling the encrypt$\_$decrypt function.

$$ It returns the decrypted password.

$5.$ get$\_$valid$\_$input:

$$ This function takes a message string $($question to be asked to the user$)$ as a parameter from the calling program.

$$ It asks the user for a string input x$,$ which will be either a username or a password.

$$ It checks if x comprise of lowercase English alphabetic characters by calling the is$\_$lower$\_$english function. As long as it is not, it prints out an error statement and asks the user for x again.

$$ It sends back x as its result.

$6.$ main:

$$ Here, the program asks the user for a valid username by calling the get$\_$valid$\_$input

function.

$$ If the username exists in the users$\_$passes variable, the program asks the user for the

password by calling the get$\_$valid$\_$input function again.

i$.$ If the password given by the user and the real password $($obtained by calling

the retrieve$\_$password function$)$ are identical, the user will have

successfully logged in$.$

ii$.$ Otherwise, the user will be told that the password is incorrect.

$$ If the username does not exist in the users$\_$passes variable, the program asks the user if s$/$he wants to add an account as either $$Y$$ or $$N$.$

i$.$ If the user answers with a string that starts with $$Y$$ after converting the first letter to uppercase, the program asks the user for the username and password by calling the get$\_$valid$\_$input function and adds the account to the users$\_$passes string by calling the add$\_$account function and prints the resulting users$\_$passes.

The program output should be exactly as follows $($bold entries are sample input$)$:

Sample Output $1$

Enter your username: donald Enter your pas