The document describes a C++ program that implements a simple substitution cipher for encrypting plaintext messages. It includes function definitions for error handling and character encryption, and outlines the main execution flow that reads from 'plain.txt' and writes encrypted messages to 'cipher.txt'. The program uses arrays to store substitution characters and limits the maximum length of messages.

1. Lab11.cppLab11.cpp//**********************************
*****************************************************
***********************
// FILE: Lab11.cpp
//
// DESCRIPTION: Implements a simple substitution cipher to e
ncrypt secret messages.
//
// AUTHORS: your-name (your-email-address)
// your-partner's-name (your-partners-email-address)
//
// COURSE: CSE100 Principles of Programming with C++, F
all 2015
//
// LAB INFO: Lab 11 Date/Time: your-lab-date-and-
time TA: your-lab-ta
//***************************************************
*****************************************************
******
???// For exit()
???// For ifstream, ofstream
???// For cout, endl
usingnamespace std;
// Define an int constant named MAX_MSG_LEN which is the
maximum number of characters in the plaintext message.
// Initialize MAX_MSG_LEN to 50.
???
//-------------------------------------------------------------------------
-------------------------------------

2. // FUNCTION: error()
//
// DESCRIPTION
// This function is called when an non-
recoverable error occurs. We display the message in p_err_msg
on the out-
// put window and terminate the program with an exit code of -
1.
//
// PSEUDOCODE
// Function error(In: p_err_msg : string) Returns Nothing
// Send to cout p_err_msg followed by ". Terminating..." foll
owed by endl
// Call exit(-1)
// End Function
//-------------------------------------------------------------------------
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???
//-------------------------------------------------------------------------
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// FUNCTION: encrypt_plain_char()
//
// DESCRIPTION
// This function has two parameters,
//
// p_plain_char - is a plaintext character that is being encrypted.
// p_index -
is the index of p_plain_char in the plaintext character array.
//
// This function determines what the encrypted character should
be and returns it. Because our plaintext charac-
// ter set is limited to uppercase letters, which have ASCII value
s 65 (A), 66 (B), 67 (C), ... 90 (Z), we can
// subtract 65 from p_plain_char to form the index of the substit
ution character in either sub_even or sub_odd.

3. // It may seem weird that we are subtracting an integer from a c
haracter variable, but remember that the values
// of character variables are in fact integers, so something like p
_plain_char - 'A' is perfectly legal since
// 'A' evaluates to 65.
//
// PSEUDOCODE
// Function encrypt_plain_char(In: p_plain_char : char; In: p_in
dex : int) Returns char
// Define and initialize sub_even array as discussed in the lab
project document
// Define and initialize sub_odd array as discussed in the lab
project document
//
// Define int sub_index = p_plain_char - 'A'
// Define char cipher_char
//
// If p_index is even Then
// cipher_char = sub_even[sub_index]
// Else
// cipher_char = sub_odd[sub_index]
// End If
//
// Return cipher_char
// End Function
//-------------------------------------------------------------------------
-------------------------------------
???
//-------------------------------------------------------------------------
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// FUNCTION: encrypt()
//
// DESCRIPTION
// This function has two parameters,
//

4. // p_plain - a C-
string (an array of characters with a null character at the end) co
ntaining the plaintext
// message. This is an input parameter.
// p_cipher - a C-
string containing the ciphertext. This is an output parameter.
// Given the plaintext message p_plain, this function encrypts it
(character by character) and returns the
// ciphertext message in p_cipher.
//
// PSEUDOCODE
// Function encrypt(In: p_plain : char[]; Out: p_cipher : char[])
Returns Nothing
// Define int index
//
// --
This is a vary loop where we are varying the value of index so
we can access the characters of p_plain
// --
in order. Note the conditional expression: we continue looping
until index has been advanced to point
// --
to the null character that follows the last valid character of p_p
lain.
// For index = 0; p_plain[index] != '0'; ++index Do
// Define char plain_char = p_plain[index]
// Define cipher_char = encrypt_plain_char(plain_char, ind
ex)
// p_cipher[index] = cipher_char
// End For
//
// --
Note, we have to place the null character at the end of p_cipher
in order to form a valid C-string.
// p_cipher[index] = '0'

5. // End Function
//-------------------------------------------------------------------------
-------------------------------------
???
//-------------------------------------------------------------------------
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// FUNCTION: main()
//
// DESCRIPTION
// This is where execution begins. We open "plain.txt" for readi
ng and "cipher.txt" for writing. For each plain-
// text message in "plain.txt" we encrypt it to form a ciphertext
message, which is written to "cipher.txt".
//
// PSEUDOCODE
// Function main() Returns int
// Open "plain.txt" for reading using an ifstream object name
d fin
// Open "cipher.txt" for writing using an ofstream object nam
ed fout
//
// If "plain.txt" could not be opened for reading Then
// Call error("Could not open plain.txt for reading")
// Else If "cipher.txt" could not be opened for writing Then
// Call error("Could not open cipher.txt for writing")
// End If
//
// Define char arrays plain and cipher, both of size MAX_MS
G_LEN
//
// --
This is an EOF loop. We read strings from fin until fin fails be
cause we have reached the end of file.
// While fin >> plain Do
// Call encrypt(plain, cipher)

6. // Send cipher to fout followed by endl
// End While
//
// Close fin
// Close fout
// End Function
//-------------------------------------------------------------------------
-------------------------------------
???