Source Code (Python)

def password(text, key):
    result = ""

    for char in text:
        if char.isalpha():
            base = ord('a') if char.islower() else ord('A')
            result += chr((ord(char) - base + key) % 26 + base)
        else:
            result += char

    return result


def main():
    message = input("Enter the message: ")
    key = int(input("Enter the key (0-25): "))

    encrypted = password(message, key)
    print("Encrypted message:", encrypted)

    decrypted = password(encrypted, -key)
    print("Decrypted message:", decrypted)


if __name__ == "__main__":
    main()